ANSI-AGMA 2004-B89-Gear Materials and Heat Treatment Manual

ANSI/AGM ANSI/AGMA A 2004---B89 B89 (Rev (Revisiono ision of AGMA AGMA 240.01)  J anuary19 y1989 Reaffirme affirmed October 1995

AMERI MERICA CAN N NATION TIONA AL STANDARD Gear Materia rials andH nd Heat Trea reatmentM nt Manua nual

G ear Ma terials terials and Hea t Treatment Treatment Manual

Gear Mate Materials rialsAn And d HeatTre at TreatmentMan nt Manua ual AGMA AGMA 2004---B89 B89 (Rev (Revisiono ion of AGMA AGMA 240.01) [Tables or ot her self-self---supporting sections may be q uote d or extract ed in their entiret y. C redit lines should aterials and H eat Tre Treatme atment nt M anual, with the permission of the rea d: E xtracte d fro m AG MA 20042004---B 89, 89, Gear M aterials publish publisher, er, t he American G ear Ma nufacturers Association, Association, 1500 500 King Street, Suite 201 201, Alexandria Alexandria , Virginia Virginia 22314.] AG MA Sta ndards a re subject subject to consta nt improvement, improvement, revision revision or withdra wal a s dicta dicta ted by experienc experience. e. Any person person who refers to a n AG MA Technical echnical Publicat Publicat ion should should be sure sure that the publication publication is the latest available from the Association on the subject matter.

ABSTRACT Treatment nt M anual  provides The  Gear Materials and H eat Treatme   provides information pertaining to engineering engineering materials and materia l treat ments used used in gear manufa cture. Topics incl included uded a re definitions, selection selection guidelines, guidelines, hea t treat ment, qua lity control, life life considerations and a bibliography. bibliography. The The materia l selec selection tion includes includes ferrous, nonferrous and nonmetallic materials. Wrought, cast, and fabricated gear blanks are considered. The heat treatment section includes data on through hardened, flame hardened, induction hardened, carburized, carbonitrided, and nitrided gears. Quenching, Quenching, distortion, and shot shot peeninga re discus discussed. sed. Qua lity control is discuss discussed ed as related t o gea r blanks, blanks, process control, control, a nd meta llurgi llurgical cal testing on the final products.

Copyright  E , 1989 Rea ffirmed O ctober 199 1995

American American G ear Ma nufacturers Ass Association ociation 1500 1500 King Stree t, Suite 201 201 Alexan Alexan dria , Virginia Virginia 2231 22314 4

February 1989

I SB N: 1 --- 5558955589 --- 524524 --- 7

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FOREWORD [The foreword, foo tnotes, and a ppendices, ppendices, if if any, are provided for informat informat ional purposeso nly and should not be construed as pa rt o f AG MA St an da rd 20042004---B 89 (Formerly 240 240.01 .01), ),  G ear M aterials aterials and H eat Treatme Treatment  nt  Manual .] .] The Standa rd provides provides a broad range of information information on gear mat erials erials and their heat treatment. It is intended to a ssist ssist the designer, designer, processengineer, processengineer, manufacturer and hea t treat er in the selec selection tion and processing processing of materialsfor gearing. gearing. D ata contained contained herein herein representsa representsa consens consensusfrom usfrom metallurg metallurgic ical al repres representativ entatives es of member companies companies of AG MA. This Standa rd replaces AG MA 240.01 0.01, October 197 1972. The f irst irst draf t of AG MA 240.0 40.01 1,   Gear Materials  Manual , was prepared in October 19 1966. It wa s approved by the AG MA membership membership in in March 19 1972. Reprinting of AG MA 240.01for distribution distribution was discontinued discontinued in 19 1982 because it it ha d been decided in 197 1979 9 by the Met allurgy and Ma teria ls C ommitt ee to revise its for mat . The initia l draft of AG MA 20042004---B 89 (for merly 240 240.01 .01)) was completed completed in April, April, 198 1983. Work continued on t he Sta ndard with numerous additiona l revised revised draft s within the Metallurgy Metallurgy and Ma terials terials Committee until it it was balloted balloted in 19 1988. It was completed completed and approved approved by the AG MA Technical echnical D ivisi ivision on Executive Executive Co mmittee in September September 1988 and o n Ja nuary 23, 19 1989 it was approved as an Americ American an National Standard. Suggestions Suggestions for t he improvement improvement of this standard will be welcome. welcome. They should should be sent to the American American G ea r Ma nufa cturers Associatio Associatio n, 1500 1500 King Stree t, Suite 201, 201, Alexan Alexan dria , Virginia 2231 22314. 4.

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PERSONNEL of theAGMA Committeefor MetallurgyAndMaterials Cha irman: L. E. Arnold (Xtek, Inc.) Vice C hairman: G . J . Wiskow (Falk)

ACTIVE MEMBERS M. Abney (Fairfield Manufacturing) R . J . Andreini (Earle M. J orgensen) E. S. B erndt (C and M of Indiana) J. B on net (WesTech) N. K. Burrell (Metal Improvement Co. Inc.) R . J. C unningham (B oeing) P. W. E arly, J r. (G leason) A. G iammarise (G eneral Electric) J. P. H orvath (G . M . C hevrolet --- M uncie) J. B ruce Kelly (G eneral Motors) D . R . M cVittie (The G ea r Works --- Sea ttle)

N. P. M ilano (Regal Beloit C orporation) A. G . M ilburn (The G ea r Works --- Sea ttle) P. Rivart (CLECIM) R . H. Shapiro (Arrow G ear) W. L. Shoulders (Reliance E lectric) (D eceased) M. Starozhitsky (Outboard Ma rine) A. A. Swiglo (IPSEN) S. Tipton (C at erpillar) D. Vukovich (Eaton) L. L . Witte (G eneral Motors)

ASSOCIATE MEMBERS T. B ergq uist (Western G ea r) J. D . B lack (General Motors) E. R. Ca rrigan (Emerson E lectric) P. E. Cary (Metal Finishing) H. B . G ayley (IMO D elaval) J. F. Craig (Cummins Engine) T. C . G lew (Prager) D . K. G uttshall (IMO D elaval) W. H . H eller (Pee rless Winsmith) D. L. Hillman (Westinghouse, Air Brake) B. A. Hoffmann (Dresser) L. D . H ouck (Ma ck Trucks) A. J. Lemanski (Sikorsky)

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R. L. Leslie (SPECO Corporation) B. L. Mumford (Alten Foundry) G . E . Olson (Cleveland) J. R . Partridge (Lufkin) E. M . R ickt (Auburn G ear) H. I. Sanderow (Supermet) R . L. Schwettman (Xtek, Inc.) L. J . Smith (Invincible G ear) Y . Sueyoshi (Tsubakimoto C ha in) M. Tana ka (Nippon G ear) R. E. Vaglia (Farrel Connecticut) T. L . Winterro wd (C ummins Engine)

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Tableof Contents Section Title

Page

1.

Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2.

References and Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2.1 2.2

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Information Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3.

D efinitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

4.

Materials Selection G uidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12

5.

5 6 7 7 7 8 9 9 19 19 25 25

H eat Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10

6.

Mecha nical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G rade and H eat Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C leanliness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D imensional Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C ost and Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H ardenability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Machina bility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ferrous G earing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Select io n C riteria f or Wrought , C a st , o r Fa brica ted St eel G e aring . . . . . . . . . . . . C opper B ase G ea ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Non ---Ferrous Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non ---Metallic Materia ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Through H ardening Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fla me and I nduction H a rdening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C arburizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C arbonitriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other H eat Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D istortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shot Peening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Residual Stress Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

26 28 34 38 39 41 42 42 47 51

Metallurgical Quality C ontrol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8

Incoming Material Quality C ontrol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Incoming Ma terial H ardness Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Incoming Material Mechanical Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H eat Treat Process C ontrol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Part C haracteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metallurgical, Mechanical and Non---Destructive Tests and Inspections . . . . . . . . Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mecha nical Property Test B ar C onsiderations . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

52 52 53 53 55 56 61 63

B ibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

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Appendices Appendix A Appendix B

P lastic G ea r Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Approximate Maximum Controlling Section Size Considerations for Through H a rdened G earing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C ase H a rdena bility of C arburizing Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Service Life C onsiderations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

65

6

Table 4---6 Ta ble 4---7 Table 4---8 Table 4---9 Table 4---10 Table 4---11 Table 4---12 Table 4---13

Typical G ear Materials --- Wrought Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical Brinell H ardness Ranges and Strengths for Annealed, Normalized & Tempered Steel G earing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical Brinell Ha rdness Ra nges and Strengths for Q uenched and Tempered Steel G earing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Machina bility of C ommon G ear Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mecha nical Pro perty Req uirements --- Cold Dra wn, Stress Relieved Steel B ars (Special C old D rawn, H igh Tensile) . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical Chemical Analyses for Though Hardened Cast Steel G ears . . . . . . . . . . . Tensile P ro pert ies o f Thro ugh H a rde ned C a st St eel G e a rs . . . . . . . . . . . . . . . . . . . Minimum Ha rdness and Tensile Strength Requirements for G ray Ca st Iron . . . Mecha nical Properties of D uctile I ron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C hemical Analyses of Wrought B ronze Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical Mechanical Properties of Wrought Bronze Alloy Rod and B ar . . . . . . . . . C hemical Analyses of C ast B ronze Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Properties of C ast B ronze Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11 14 14 16 17 22 22 23 24

Table Table Table Table Ta ble Ta ble

Test B ar Size for C ore H ardness D etermina tion . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical Effective Case Depth Specifications for Carburized G earing . . . . . . . . . . Approximate Minimum Core Hardness of Carburized G ear Teeth . . . . . . . . . . . . Approximate Minimum Surface Hardness --- Nitrided Steels . . . . . . . . . . . . . . . . . C ommonly U sed Q uencha nts f or Ferro us G e a r M at eria ls . . . . . . . . . . . . . . . . . . . Typica l Shot Size a nd Intensity for Shot Peening . . . . . . . . . . . . . . . . . . . . . . . . . . .

35 38 39 41 43 50

Appendix C Appendix D

67 69 70

Table s Table 4---1 Table 4---2 Table 4---3 Table 4---4 Table 4---5

5---1 5---2 5---3 5---4 5---5 5---6

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Tableof Contents Section Title

Page

Figures Fig 4---1 Fig 4---2

Typical D esign of C ast Steel G ears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 D irectionality of Forging Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Fig 5---1

F ig 5---4 Fig 5---5 Fig 5---6 F ig 5--- 7 F ig 5--- 8

Variation in Ha rdening Patterns Obtainable on G ear Teeth by Flame H ardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Variations in Ha rdening Patterns Obtainable on G ear Teeth by Induction H ardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recommended Maximum Surface Ha rdness and Effective Ca se D epth Hardness Versus Percent Carbon for Flame and Induction Hardening . . . . . . . G e nera l D e sign G u id elines f or B la nks f or C a rburized G e a ring . . . . . . . . . . . . . . . Typica l D ist ortio n C ha ra ct erist ics of C a rburized G e a ring . . . . . . . . . . . . . . . . . . . . Shot Peening Intensity C ontrol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R esid ua l S tre ss by P ee nin g 1045 St ee l a t 62 H R C w it h 330 Sh ot . . . . . . . . . . . . . . D e pt h o f C o mpr essive St re ss Ve rsus Alme n I nt en sit y f or S te el . . . . . . . . . . . . . . .

33 45 46 48 49 50

Fig 6---1 Fig 6---2 Fig 6---3 Fig 6---4

C ircular (H ead Shot) Magnetic Particle I nspection . . . . . . . . . . . . . . . . . . . . . . . . . C oil Shot Magnetic Particle I nspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . U ltrasonic Inspection Oscilloscope Screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distance---Amplitude Reference Line f or Ultrasonic Inspection . . . . . . . . . . . . . .

58 59 61 62

Fig 5---2 Fig 5---3

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1. Scope

ASTM A290---82,   Carbon and Alloy Steel Forg-  ings for Rings for Reduction G ears 

This Manual wa s developed t o provide basic information and recommend sources of additional information pertaining to gear materials, their treatments, and other considerations related to the manufacture and use of gearing.

ASTM A310---77,   Methods and Definitions for  M echanical Testing of Steel Products 

ASTM A311---79, Specifi cation for StressRelieved  Cold D rawn Carbon Steel B ars Subject to M echanical  Property Requir ements 

Metallurgical aspects of gearing as related to rating (a llowable  s ac  and  s at  values) are not included, but, are covered in AG MA rating standa rds.

ASTM A356---84,   H eavy--- Walled Carbon, L ow  Al loy, and Stain less Steel Castin gs for Steam Turbin es 

ASTM A370---77,   Methods and Definitions for 

2. ReferencesandInformation

M echanical Testing of Steel Products 

ASTM 388---80,   Recommended Practice for Ul- 

2.1References.

trasonic E xaminati on of H eavy Steel Forgings 

Abbreviations a re used in the references to specific documents in this Stand ard. The a bbreviations include: AGMA, American G ear Ma nufacturers Association; ASNT, American Society of Nondestruct ive Testin g; ASTM, Ame rica n Socie ty fo r Testing Materials; SAE, Society of Automotive Engineers.

ASTM A400---69(1982),  Recommended Practice  for Selection of Steel Bar Compositions According to  Section 

ASTM A534---87,  Standard Specification for Car-  burizing Steels for An ti --- Friction Bearings 

ASTM A535---85,  Standard Specifi cation for Spe-  cial --- Quali ty Ball and Roll er B earing Steel 

The following documents contain provisions which, through reference in this Stand ard, constitute provisions of this document. At the time of publication, the editions were valid. All publicationsa re subject to revision, and the userso f this Stand ard a re encouraged to investigate the possibility of applying the most recent editions of the publicat ions listed. AG MA

ASTM A536---80,  Specification for Ductile Iron  Castings 

ASTM A833---84,  Indentation H ardnessof M etal-  lic M aterials by Comparison H ardness Testers 

ASTM A609---83,  Specification for Steel Castings, Carbon and L ow Alloy Ultrasonic Examinations  Thereof  

141.01---1984,   Plastics Gearing

---  Molded, Machined, And Other Methods, A Report on  theState of the Art 

ASTM B 427---82,   Specification for Gear Bronze  Al loy Castings 

AG MA 2001---B 88, Fundamental Rating Factors 

ASTM B 505---84,  Specifi cation for Copper--- Base 

and Calculation M ethodsfor InvoluteSpur and H elical  G ear Teeth 

All oy Continuous Castings 

ASTM E8---83,  M ethods of Tension Testin g of M e- 

AG MA 6033---A88, Standard for M arine Propul- 

tallic M aterials 

sion Gear Units, Part 1 Materials 

ASTM E10---78,  Test M ethod for Bri nell H ardness 

ANSI /AG MA 6034---A88, Practicefor Single and  D ouble Reduction Cylindri cal --- Worm and H elical ---  Worm Speed Reducers 

of M etallic M aterials 

ASTM E18---79,  Test M ethods for Rockwell H ard-  ness and Rockwell Superficial H ardness of M etalli c  Materials 

ASNT---TC ---1A (J une 80),   Recomm ended Prac-  tice by American So ciety fo r Non destructive Testing

ASTM E54---80,  M ethod for Chemical A nalysis of   Special Brasses and Bron zes 

ASTM A48 ---83, S pecification for Gray I ron Cast-  ings 

ASTM E112---84,  M ethodsfor D eterminin g Aver-  age G rain Size 

ASTM A148---84, Steel Castin gs, H igh  Strength, for Struct ural Purposes 

SAE J434---J une 86,  Automotive Ductile (Nodu-  lar) Iron Castings 

ASTM A220---76,  Specification for Pearlitic Mal- 

SAE J 461---Se pt 81,  Wrought and Cast Copper 

leable Ir on Castings 

Alloys 

ASTM A255---67,  M ethod for E nd --- Qu ench Test 

SAE J462---S ept 81,  Cast Copper Alloys 

for H ardenability of Steel 

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SAE J 463---S ept 81, Wrought Copper and Copper 

American So ciety for Testing and Ma terials ASTM Standards

Alloys 

Society of Automotive E ngineers, I nc. SAE Handbook

SAE J808a ---SAE H S 84,  Manual on Shot Peen-  in g 

American Iron and Steel Institute AISI Steel Products Ma nuals

MIL ---S ---13165 B (31 D ec 66 Amendmen t 2---25 June 79),  Shot Peenin g of M etal Parts 

American National Standards Institute ANSI Sta ndards

MIL ---STD ---271F, Requirements for Non destruc-  tive Testing M ethods 

Naval Publications and Forms Center Military Standards and Specifications

ASTM E709---80,  Magnetic Particle Examination 

Metal Powder Industries Federation MPIF Standard 35

ASTM E125,   Reference Photographs for Magnet-  ic Particle In dications on Ferrous Castings 

Copper Development Association CDA Data books

ASTM E186---8,

Standard Reference Radio-  graphs for Heavy Walled (2 to 4 1/2 inch)(51 to 114  mm) Steel Castings 

Iron Castings Society G ray and D uctile Iron Ca stings Ha ndbook

ASTM E280---81,   Standard Reference Radio- 

Steel Founders’ Society Steel Castings Handbook

graphs for H eavy Walled (4 1/2 to 12 inch) (114 to 305  mm) Steel Castings 

3. Definitions

ASTM E399---83,   Test M ethod for Pl ain --- Strain  Fracture Toughness of Metallic Materials 

Annealing --- Full.   Full annealing consists of hea ting steel or o ther f errous alloys to 1475---1650 _ F (802---899 _ C ) and furnace cooling to a prescribed temperature, generally below 600 _ F (316 _ C ). This treatment forms coarse lamellar pearlite, the best microstructure f or ma chinability of low a nd medium carbon steels. Unless otherwise stated, annealing is assumed to mean full annealing. Annealing --- Spheroidizing.   Spheroidize annealing is a process of heating and cooling steel tha t produces a globular carbide in a ferritic mat rix. This heat t reatment results in the best machinability for high carbon (0.60percent carbon or higher) and alloy steels. Austempering. Austempering is a heat treat process consisting of quenching a ferrous alloy (steel or ductile iron) from a temperature above the transformat ion range in a mediumha ving a rate of cooling sufficiently high t o prevent high te mperature tra nsformation products, and maintaining the alloy temperature within the bainitic range until desiredt ransformation is obtained. The bainitic transformation range is below the pearliticra nge, but above the martensitic range. Austempering is applied to steels and, more recently in the development stage for ductile iron gearing (refer to 4.8.4.3). Austenite. Austenite in ferrous alloys is a microstructural pha se consisting of a solid solution o f carbon and alloying elements in fa ce---centered cubic crystal structured iron.

ASTM E446---81,   Standard Reference Radio-  graphs for Steel Castings U p to 2 inch (51 mm) in  Thickness 

ANSI /SAE AMS 2300 F,  Magnetic Particle In-  spection , Premi um Ai rcraft --- Qu ali ty Steel Cleanlin ess  ANSI/SAE AMS 3201 G ,   Magnetic Particle In-  spection , A ircr aft --- Qu ali ty Steel C leanli ness 

2.2Information Sources. Design of gears is concerned with the selection of materials and metallurgical processing. This Manual cannot substitute for metallurgical expertise, but is intended t o be a basic tool to a ssist in the selection a nd meta llurgical processing of gear ma terials. The material information and metallurgical processes conta ined herein a re based o n established data and practices which can be found in the appropriate publications. It is necessary that the designer use a source of metallurgical knowledge of materials a nd processing. Material specifications are issued by agencies, including the government, large industrial users, an d technical societies, some of whom a re: ASM International ASM Metals Ha ndbooks ASM Hea t Treate rs G uide ASM Metals Reference B ook ASM Standard

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AustenitizingTemperature. The temperat ure at which ferrous alloys undergo a complete microstructural phase transformation to austenite.

case. Ha rdness survey is preferred for contra l purposes. (3) Tota l case depth. The total case depth is the depth to which the carbon level of the case has decreased to the carbon level of the ba se material. This is approximat ely 1.5 times the effective case dept h.

Bainite.   Bainite is a microstructural phase resulting from the transformation of austenite, and consists of an aggregate of ferrite a nd iron carbide. Its appearance is feathery if formed in the upper portion of the bainite transformation range, and acicular if formed in the lower portion.

(4) C ase depth to 0.40 percent carbon. Ef fective case depth is less frequently referred to a s the depth to 0.40 percent carbo n. This depth may be mea sured by analyzing the carbon content or estimating based on microstructure. E stimating based on microstructure ignores the hardenability of the base material and isnot as accurate a measurement as directly analyzing the carbo n level. There is poor correla tion between microstructure readings and material strength gradients using this method.

Carbon.  C arbon is the principal ha rdening element in steel, and it’s amount determines the maximum hardness obtainable. G enerally as carbon is increased, t ensile strength a nd wear resistance increase; however, ductility a nd welda bility decrease. Carbonitriding.  A modified form of gas carburizing, in which steel (typically plain carbon and very low alloy) is heated between 1450---1650 _ F (788---899 _ C ) in a n ammonia enriched carburizing at mosphere. This results in simultaneous absorption of carbon and nitrogen, which results in the formation of complex nitrides in a high carbon case.

CaseDepthofFlameorInductionHardenComponents.  This is defined as the depth at which the hardness is10H RC pointsbelow the minimumspecified surface ha rdness. CaseDepth of Nitrided Components.   Nitrided case depth is defined as the depth a t which the hardness is equivalent to 105 percent of the measured core hardness. The case depth is determined by a microhardness tester and measured normalto theto oth surface at 0.5 tooth height and mid face width.

Carburizing---Gas.   G as carburizing consists of heating a nd holding low carbon or a lloy steel (less than 0.30 percent carbon) at 1650---1800 _ F (899---982 _ C ) in a controlled carbonaceous atmosphere, which results in the diffusion of carbon into the part (0.70---1.00 percent carbo n is t ypically obta ined a t the surface). Temperatures a bove 1800 _ F (982 _ C ) may be ultilized in specialized equipment such as vacuum carburizers. After carburizing, parts are e ith er cooled t o 1475---1550 _ F (802---843 _ C ) a n d held at this temperature to stabilize and then direct q ue nch ed ; o r slo w co ole d a nd r ehe a te d t o 1475---1550 _ F (802 ---843 _ C ) a nd quenched.

CaseHardness.  C ase H ard ness is the micro --hardness measured perpendicular to the tooth surfa ce a t a depth of 0.002 to 0.004 inches (0.05 to 0.10 mm) at 0.5 tooth height and mid face width. Cementite. Cementite is a hard microstructure phase otherwise known as iron carbide (Fe 3C) and characterized by a n ort horhombic crysta l structure.

Case Depth of Carburized Components. The case depth for carburized gearing may be defined in several ways including effective case depth, etched case depth, total case depth, and depth to 0.40 percent carbon. The carburized case depth referred to in this Manua l will be effective case depth. Ca rburized case depth terms are defined as follows:

Combined Carbon.   The amount of carbon in steelor cast iron that ispresent in other than elemental form.

(1) Effective case depth. The effective case depth is the hardened depth to HRC 50 at 0.5 tooth height a nd mid face width, normal to the tooth surface.

D.I. (Ideal Critical Diameter).  Ideal critical diameter is the diameter which, when quenched in an infinite quench severity (such as ice brine), will result in a microstructure consisting of 50 percent martensite of the center of the bar.

Core Hardness.   Core Hardness for AGMA toot h design purposes is the hardness at the intersection of the root diameter and the centerline of the tooth a t mid face width on a finished gear.

(2) Et ched case depth. Et ched case depth is determined by etching a sample cross---section with nitric acid, a nd measuring the depth of the da rkened area. The etched case a pproximates the effective

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Decarburization.  D ecarburizat ion is the reduction in surfa ce carbon content o f a gea r or test piece during thermal processing.

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Ferrite (alpha). Ferrite is a microstructural phase consisting of essentially pure iron, and is characterized with a body centered cubic structure. Flame Hardening.   Flame Hardening of steel gearing involves oxyf uel burner heating to 1450---1650 _ F (788 ---899 _ C ) followed by quenching and tempering.

sorbed into the surface of a ferrous materia l at a temperature below the austenitizing temperature [1000---1150 _ F (538---621 _ C )], while submerged in a gas stirred and activated molten chemical salt bath. These processes are used mainly for improved wear resistance and fatigue strength.

Nitriding (Gas).   Surface hardening process in which alloy steel, after machining following quench and tempering, is subjected to a cracked ammonia fur na ce a tmo sphere at 950---1060 _ F (510 ---571 _ C ) causing nitrogen to be absorbed into the surface, forming hard iron nitrides.

Grain Size. G rain size is specified as either coarse (grain size 1 through 4) or fine (grain size 5 through 8), determined according to ASTM E112. Graphite.   G raphite is carbon in the free state with a shape described as either flake, nodule, or spheroid. The graphite shape classifies the type of cast iron a s either gray, ductile, or malleable.

Nitrocarburizing. Nitrocarburizing is a gaseous heat treatment in which both nitrogen and carbon are absorbed into the surface of a ferrousmaterial at a temperature below the austenitizing temperature [1000---1150 _ F (538---621 _ C )]. Nitrocarburizing is done ma inly for a ntiscuffing and to improve surface fatigue properties.

Hardenability.   An indication of the depth to which a steel will harden during heat trea tment (see 4.6). Hardening. The process of increasing hardness, typically through heat ing and cooling.

Normalizing.   Normalizing consists of heating steel or other ferrous alloys to 1600---1800 _ F (871---982 _ C ) and cooling in still or circulated air. Normalizingis used primarily to obtain a uniformmicrostructure.

H---BandSteels. H ---B an d steels ar e steels which are produced and purchased t o a specified Jo miny hardenability range. Induction Hardening.   Induction hardening of gearing is the selective heat ing of gea r teet h profiles to 1450---1650 _ F (788 ---899 _ C ) by electrical inductance thro ugh the use of a coil or single to oth inductor to obtain the proper heat pattern and temperature, fo llowed by q uenching and tempering.

Pearlite. Pearlite is a microstructure consisting of lamellar layers of ferrite and cementite, with a body centered cubic crystal structure. Quench and Temper.   The quench and temper process on ferrous alloys involves heating a part to the auste nite tra nsforma tion stat e a t 1475---1650 _ F (802---899 _ C ), fo llowed by rapid cooling (quenching). The part is then reheated (tempered) to a specifict emperature generallybelow 1275 _ F (690 _ C ) t o achieve the desired mechanical properties for the gear application.

 J ominy End Quenching Hardenability Test. The standard method for determining the hardenability of steel. The test consists of heating a sta ndard one inch (25 mm) diameter test bar to a specified temperature, placing the specimen in a fixture so that a stream of water impinges on one end, cooling the specimen to room temperature, grinding flats, and measuring the hardnessa t 1/16 inch (1.6 mm)intervals starting at the quenched end.

StressRelief. Stress relief is a thermal cycle used to relieve residual stresses created by prior heat treatments, machining, cold working, welding, or other fa bricat ing techniques. Ma ximum stress relief is achieved at 1100 _ F (593 _ C) minimum.

Martensite. Martensite is the diffussionless transforma tion of austenite to a body centered tetra gonal structure, characterized by an acicular nee dle ---like appea ra nce.

Surface Hardness. Surface H ardness is the hardness measured directly on the surface. To obta in accurate results on shallow case hardened parts, a superficial test must be used.

Microstructure. Microstructure is the material structure observed on a sample polished to a mirror finish, etched, and viewed at 100X or higher magnif ication.

Tempering. Tempering is reheat ing a hardened part to a specified temperature, generally below  1275 _ F (690 _ C ) to reduce hardness and increase toughness.

Nitriding (Aerated Salt Bath).   This term includes a number of heat treat processes in which nitrogen and carbon in varying concentrations are ab-

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TestCoupon. A test coupon is an appropriately sized sample(often a bar) used generally for surface hardening treat ments. It should be of the same specified material grade, with regard to composition and hardenability limits, as the gear it represents. The test coupon should be heat treated along with the gear(s) it represents.

crostructure, material cleanliness, surface conditions and residual stresses.

4.1.3TensileStrength. Tensile stren gth pre dict s the stress level above which fracture occurs. It is not recommended for use in gear manufacturing specifications. 4.1.4Yield Strength. Yield strength determines the stress level above which permanent deforma tion occurs.

Through Hardening.   Through hardening is a term used to collectively describe methods of heat treatment of steelother than surface hardening techniques. These include: annea ling, normalizing (or normalizing and t empering) and q uenchinga nd tempering (refer to 5.1). D epth of hardening is dependent upon hardenability, section size and heat treat considerations.

4.1.5 Toughness. Toughness is de termined by impact strength, tensile ductility and/or fracture toughness testing. Although not directly considered in gear rating, toughness may be important for high impact or low temperature applications or both. Toughness of stee l gearing is ad versely af fecte d by a variety of factors such as:

NOTE:  Through ha rdening does not imply that the part has equivalent hardness throughout the entire cross section.

(1) Low temperature (2) I mproper he at trea tment or microstruc --ture (3) High sulfur (4) H igh phosphorus and embrittling type residual elements (5) No nmeta llic inclusions (6) L arge gra in size (7) Absence of alloying elements such as nickel.

TransformationTemperature. The temperature at which a change in microstructure phase occurs.

4. Material SelectionGuidelines Ma ny facto rs influence the selection of materials for gears, and the relative importance of each can vary. These factors include: (1) (2) (3) (4) (5) (6) (7)

Mechanical Properties G rade and H eat Treatment Cleanliness Dimensional Stablility Availability and C ost Hardenability and Size Effects Machinability and Other Manufacturing Characteristics

NOTE: G ear toughness is adversely affected by design or manufacturing considerat ions (such as notches, small fillet ra dii, too l marks, material defects, etc., which act as stress concentrators). 4.1.6 Heat Treatment.   Most wrought ferrous materials used in gearing are heat treated to meet ha rdness and/or mechan ical property req uirements. Round a nd flat stock can be purchased in numerous combinations of mechanical and thermal processing, such a s hot rolled, cold rolled, cold drawn, stress relieved, pickled, annealed, and quenched and tempered. G ear blanks are generally given an a nnealing or normalizing heat treatment, which homogenizes the micro --- structure f or machinability and mechanical property uniformity. G ear blanks can also be quenched and tempered.

4.1 Mechanical Properties. It is necessary fo r the gear designer to know the application and design loadsa nd to calculate the stressesbefore the material selection can begin. 4.1.1 Hardness.   The strength properties are closely related to material hardness, which is used in AG MA gear ra ting practice. Surface hardness is an important consideration for gear wear. Core hardness is an important consideration for bending and impact strength.

4.1.7 Stock Removal.   All rough ferrous gear castings, forgings and barstock have a surface layer containing decarburization, nonmetallic inclusions, seams, and other surface imperfections. This layer should be removed from critical gearing surfaces. The minimum surface stock removal varies with stock size and t ype of mechanical working. Minimum

4.1.2FatigueStrength. Conta ct and bending fatigue strengths are used to predict, at a given stress level, the number of cycles that gearing can be expected to endure before pitting or fracture occurs. Contact and bending fatigue strengths are influenced by a variety of f actors such as ha rdness, mi-

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stock removal tables can be found in most machining and materials handbooks.

quired as a function of subsequent heat treatment; such as quench and temper or case hardening. See Tables 4 ---1, 4 ---2, and 4---3 f or gra des and reco mmended heat treatments.

4.2 GradeandHeat Treatment.   The specific gear design will usually dictate the grade of material re-

Table4---1 Typical Gear Materials --- WroughtSteel Common Alloy Steel G rades

Common Heat 1 Trea t P ra ctice

G enera l Rema rks/Applicat ion

1045 4130 4140 4145 8640 4340

T---H , I ---H , F ---H T---H T---H , T---H &N, I ---H , F ---H T---H , T---H &N, I ---H , F ---H T---H , T---H &N, I ---H , F ---H T---H , T---H &N, I ---H , F ---H

Low H ardenability Marginal H ardena bility Fa ir H a rdena bility M edium H a rdena bilit y M edium H a rdena bilit y G o o d H a rd ena bilit y in H ea vy Se ct io ns

Nitralloy 135 Mod. Nitralloy G 4150

T---H &N T---H &N I ---H , F ---H , T---H , TH &N

4142

I ---H , F ---H , T---H &N

4350 @

T---H, I ---H, F---H

Special H eat Treatment Special H eat Treatment Q uench C ra ck Sensitive G ood Hardenability U sed when 4140 exhibits Marginal H ardenability Quench Crack Sensitive, Excellent Hardenability in Heavy Sections

1020

C ---H

Very L ow H ardena bility

4118 4620 8620

C ---H C ---H C ---H

Fair C ore H a rdenability G ood C ase H ardenability Fair C ore H a rdenability

4320 8822

C ---H C ---H

G ood C ore H ardenability G ood C ore H ardenability in H ea vy Sections

3310 @ 4820 9310

C ---H C ---H C ---H

Excellent H ardenability (in H ea vy Sections) for all three grades

1 C ---H = Ca rburize Ha rden T---H = Through Ha rden

F ---H = Fla me H arden I ---H = I nduction H arden T---H&N = Through H arden then nitride

2 Recognized, but not current standard grade.

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Table4---2 Typical Brinell HardnessRangesandStrengthsfor Annealed, NormalizedandTempered Steel Gearing Norma lized & Tempered #

Annealed H eat Treat ment @ Typica l Alloy Steels 1 Specified

1045

Brinell Hardness Range HB

Tensile Strength min ksi (MPa)

Y ield Strength min ksi (MPa)

Brinell Hardness Range HB

Tensile Strength min ksi (MPa)

Y ield Strength min ksi (MPa)

159---201

80 (550)

50 (345)

159---201

80 (550)

50 (345)

156---197

80 (550)

50 (345)

167---212

90 (620)

60 (415)

187---229

95 (655)

60 (415)

262---302

130 (895)

85 (585)

197---241

100 (690)

60 (415)

285---331

140 (965)

90 (620)

212---255

110 (760)

65 (450)

302---341

150 (1035)

95 (655)

4130 8630 4140 4142 8640 4145 4150 4340 4350 Type

1. Steels shown in order o f increased harde nability. 2. Ha rdening by quench and tempering results in a combination of properties generally superior to t hat achieved by a nneal or no rmalize and t emper; i.e., impact, ductility, etc. See Table 4---3 for q uench a nd tempered gea ring. 3. Ha rdness and strengths able to be obtained by normalize and tempering are a lso a function of controlling section size a nd t empering temperat ure considerations.

4.3Cleanliness. Alloy steel manufa ctured with electric furnace practice for barstock and forged steel gear applications is commonly vacuum degassed, inert a tmosphere (argon) shielded and botto m poured to improve cleanliness and reduce objectionable gas content (hydrogen, oxygen and nitrogen). Improved cleanliness (reduced nonmetallic inclusion content) results in improved transverse ductility and impact strength, but machinability may be reduced; for example, with sulfur content less than 0.015 percent. Vacuum degassed steel may be further refined by vacuum a rc remelting (VAR) or electroslag remelting (ESR ) of the steel. These refining processes further reduce gas and inclusion size and content for improved fat igue strength to produce the highest q uality steel for critical gearing applications. Significant

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increase in cost and reduced machinability, however, must be fully evaluated with respect to the need for improved properties for other than critical gearing applications.

NOTE:  For more informat ion see ASTM A534 and A535, and AMS 2301 and 2300. 4.4 Dimensional Stability.   The process to achieve the blueprint design ma y require ma terial considerations such as: added stock, die steps, restricted hardenability, etc. to minimize distortion and possible cracking (see 5.8). 4.5 Cost and Availability.   The specific material selection is often determined by cost and a vailability fa ctors such as sta ndard industry alloys and procurement time.

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Table4---3 Typical Brinell HardnessRangesand Strengthsfor QuenchedandTemperedAlloySteel Gearing Alloy Steel * Grade 4130 8630 4140 8640

4350

Y ield Strength minimum ksi (MPa)

Hea t Treatment Wa ter Quench & Temper

212---248 up to 302---341

100

(690)

75

(515)

145

(1000)

125

(860)

Oil Quench & Temper

241---285] up to 341---388

120

(830)

95

(655)

341---388

170

(1170)

277---321 up to 363---415w

135

(930)

180

(1240)

4142 4145 4150 4340

Tensile Strength minimum ksi (MPa)

Hardness Range HB [

Oil Quench & Temper

150 (1035) 110

(760)

145 (1000)

*

Steels shown in order of increased hardenability, 4350 being the highest. These steels can be ordered to “H” Band hardenability ranges. [ Ha rdness range is dependent upon controlling section size (refer to a ppendix B ) and q uench severity. ] It is difficult to cut t eeth in 4100 Series steels above 341 H B an d 4300 Series steels above 375 HB . (4340 and 4350 provide advantage due to higher tempering temperatures and microstructure considerations) w High specified ha rdness is used f or special gea ring, but costs should be evaluated d ue to reduced machinability.

The standard wrought carbon and alloy steels such as 1020, 8620, 4320, 4820, 9310, 4140, 4150 and 4340 are available from service centers and steel mills. Service centers can usually furnish these materials in small q uantities and with short d elivery time from t heir inventories. Steel mill purchases require “ millq uantities” (several thousand pounds)a nd long delivery time. Ho wever, the mill qua ntity cost may be substant ially lower, a nd non ---sta nda rd steels can be supplied on special request.

by quenching from the austenitizing temperature. The a s quenched surface hardness is dependent primarily on the carbon content of the steel part and cooling rate. The depth to which a particular hardnessis achieved with a given quenching condition isa function of the ha rdenability, which is largely determined by the alloy content of the steel grade.

4.6.1 Determination. Hardenability is normally determined by the Jo miny End Q uench Test (ASTM A255) or can be predicted by the Ideal Diameter (DI) concept.

When specifying parts with small quantity requirements, standard alloys should be specified or engineering dra wings should allow optiona l materials. In the case of steel and iron castings and no nferrous materials, SAE and ASTM designations should be used wherever possible.

4.6.1.1 J ominy Test Method.   A one inch (25 mm) diamete r bar, f our inches (102 mm) in length is first normalized then uniformilyhea ted to a standard austenitizing temperature. The bar is placed in a fixture, then quenched by spraying room temperature water a gainst one end fa ce.

4.6Hardenability. Hardenabilityof steelistheproperty that determines the hardness gradient produced

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4.6.1.2 J ominyAnalysis.  Rockwell C hardness measurements are made along the length of the bar on ground flats in one sixteenth o f a n inch (1.6 mm) intervals. J ominy hardena bility is expressed in H RC obtained at each interval starting at the water quenched end face.

(4) C hara cteristics of the cutting fluid used. There is abundant material published on machinability. The mechanics of the cutting operation will not be considered here. Only metallurgical fa ctors will be discussed. Chemical composition and microstructure of steel have major influences on machinability, since they af fect properties and structures. Meta llic oxides like alumina and silica form hard oxide inclusions and contribute to poor machinability. E lementssuch as sulfur, lead, selenium, a nd t ellurium form soft inclusions in the steel mat rix and can benefit machining. Calcium additions (in steel making) form hard, irregular inclusions and can a lso benefit ma chining. Ho wever, sulfur, lead and calcium inclusions which improve machinability can decrease mechanical properties, particularly in the transverse direction. Ca lcium treated steel, when used in high stress gear and shaft applications, may significantly reduce fatigue life compared to conventional steelmaking practices. Ca rbon content over 0.30 percent decreases machinability due to increased hardness. D ependent on carbon and sulfur levels, higher manganese also decreases machinability. In genera l, alloys which increase hardness and to ughness decrease machinability. The more common gear materials are listed in Table 4---4 on the basis of good, fa ir, a nd poor machinability. With good machinability as a base, a fair rating would add 20 to 30 percent to the machining cost, and poor would add 40to 50percent.

Example: J5 = 40 is interpreted as a hardness of 40 H RC at a d istance o f 5/16 inch (8 mm) from the wa ter q uenched end.

4.6.1.3H---BandSteel. Jominy hardenability has been applied to stand ard steels. For a given composition the Jominy hardenability data fallswithin a predicted range. Ste els purchased to predicted ha rdenability ranges are called H ---Ba nd steels. These B ands are published by ASTM, AISI, and SAE. Steels can be purchased to H ---B and, or restricted H ---B an d, specification s. 4.6.1.4 IdealCriticalDiameter. The Ideal CriticalD iameterMethod (DI ) isbased on chemical analysis described in AISI, SAE,  M odern Steels and Th eir  Properties by B ethlehem Steel, and other hardenability ref erence publications. 4.6.2 Application.  Hardenability is constant for a given steel composition; however, hardness will vary with the cooling rate. Therefore, the hardness obta ined at any location on a part will depend on carbon content, hardenability, part size, configuration, quench media, and quenching conditions. Typically a steel composition is selected with a hardenability characteristic tha t willyield an as quenched hardness above the specified hardness so that toughness and machinability can be a ttained through a ppropriate tempering. As the section thickness increases, the steel hardenability must be increased in order to maintain a given hardness in the part section.

4.8 FerrousGearing.  Ferrous materials for gearing include carbon and alloy wrought and cast steels, cast iron and ductile irons. G earing of a lloy and carbon steel is manufactured from different forms of rough stock depending upon service, size, design, quantity, availability, and economic considerations. These forms include wrought steel, weld fabrications and castings.

4.7Machinability. Several f actors influence t he machinability of materia ls and in turn aff ect the economy and feasibility of manufacturing. These fa ctors must be considered at the design stage, particularly when high strength levels are being specified. Factors influencing machinability a re: (1) M at erial being cut, including composition, microstructure, ha rdness, shape, and size. (2) C utting speeds, f eeds and cutting tools. (3) C ondition o f ma chine to ols, including rigidity, precision, power, etc.

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4.8.1 WroughtSteel.   Wrought steel is the generic term applied to carbon and alloy steels which are mechanically worked into form for specific applications. The standard wrought steel forms are round stock, flat stock and fo rgings. Forgings reduce machining time, and are available in a wide range of sizes and grades.

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Table4---4 Machinabilityof Common Gear Materials Material Grades

Low---Carbon CarburizingSteel Grades --- Remarks

1020

G ood ma china bility, a s rolled, a s forged, or norma lized.

4118 4620 8620 8822

G o od ma china bilit y, a s rolled, or a s f orged. H owever, no rma lized is preferred. I na deq ua te coo ling during no rma lizing ca n result in gummy ma teria l, reduced t oo l lif e a nd po or surfa ce f inish. Q uench a nd temper a s a prior trea tment ca n a id ma china bility. The economics of the pretreatments must be considered.

3310 4320 4820 9310 Material Grades

Fa ir to goo d ma china bility if norma lized a nd t empered, a nnea led o r q uenched a nd t empered. No rma lizing wit hout t empering result s in reduced machinability.

1045 1141 1541

G ood machinability if normalized.

4130 4140 4142

G o od ma china bility if a nnea led, or norma lized a nd tempered to a pproxima tely 255 H B or q uenched a nd tempered to a pproxima tely 321 H B . O ver 321 H B, ma china bility is fa ir. Above 363 H B , machinability is poor. Ina deq uate (slack) quench with subsequent low  tempering temperature may produce a part which meets the specified hardness, but produces a mixed microstructure which results in poor machinability.

4145 4150 4340 4345 4350

R ema rks f or medium ca rbon a llo y st eel (a bo ve) a pply. H owever, t he higher ca rbon results in lo wer ma china bility. Sulf ur a ddit io ns a id the ma china bilit y o f t hese gra des. 4340 ma china bilit y is goo d up t o 363 H B . The higher ca rbo n leve l in 4145, 4150, 4345, a nd 4350 ma kes them more difficult to ma chine a nd should be specified only for heavy sections. Ina dequa te (slack) q uench can seriously affe ct machinability in these steels.

MediumCarbon Through HardenedSteel Grades --- Remarks

NOTE:   Co arse grain steels are more machinable than fine grain. Ho wever, gear steels are generally used in the fine gra in condition since mechanical properties are improved, a nd distortion during hea t treat ment is reduced. Increasingly cleaner steels are now a lso being specified for gea ring. However, if sulfur content is low, less than 0.015 percent, machinability may decrease appreciably. Material Grades G ra y I ro ns

Other Gear Material --- Remarks G r a y ca st iro ns ha ve go od ma china bilit y. H igher st rengt h gra y ca st iro ns [above 50 ksi (345 MPa) tensile strength] have reduced machinability.

Ductile Irons

Annealed or normalized ductile cast iron has good machinability. The “as cast” (not heat treated) ductile iron has fair machinability. Quenched and tempered ductile iron has good machinability up to 285 HB and fair machinability up to 352 HB . Above 352 HB , ma chinability is poor.

G e ar B r on ze s a nd B r a sse s

All ge a r br onze s a nd bra ss h ave go od ma ch ina bilit y. The ve ry h igh st re ngt h h ea t t re at ed bro nze s [a bo ve 110 ksi (760 M Pa ) t ensile st re ngt h] have fa ir machinability.

Aust en it ic Stainless Steel

All a ust enit ic st a in le ss st ee l gr ad es o nly h ave f a ir ma chin abilit y. B e ca use of work hardening tendencies, f eeds and speeds must be selected to minimize work hardening.

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4.8.1.1 Round Stock. Round bars can be purchased in various diameters for sta ndard carbon and alloy grades. They are typically available as hot rolled, hot rolled ---cold dra wn, hot rolled ---cold finished a nd f orged rounds. Cold drawing produces a close tolerance ba r with improved mechanical properties (higher hardness and yield strength). Low to medium carbon steels are no rmally a vailable a s cold dra wn bar for gear ing. Hot rolled ---cold finished bars are machined (turned, ground and/or polished) fo r improved size control, but show no improvement in mechanical properties over hot rolled or annealed bar. Hot rolled bars are mechanically worked at a ppro xima te ly 2100---2400 _ F (1150 ---1315 _ C ) a n d may be subsequently annealed, straightened and stress relieved. Forged round bars are forged ro und under a press or ha mmer a t the same a pproximate temperature as hot rolled bars (higher temperature for lower carbon content carbon or alloy steel) a nd

are manufa ctured to a size larger than can be formed with rolling dies or rolls. Forged round bars can be purchased in a variety of heat treat conditions depending upon applicat ion. Ho t rolled bars are also now manufactured from continuouscast steelbar manufactured with continuous casters. Cont inuous cast bar is subsequently hot rolled with sufficient reduction in cross sectional area (7 to 1 minimum) during hot deformation to produce densification and quality bar for many gearing a pplications. Approximate ma ximum diameter of the various types of round stock, depending upon steel mill capacity, is a s follows: Hot Rolled: Cold Drawn: Co ld Finished: Forged Round:

8.0 inch (205 mm) 4.0 inch (100 mm) 5.0 inch (125 mm) 16.0 inch (405 mm)

Table4---5 Mechanical PropertyRequirements --- Cold Drawn,StressRelievedSteel Bars (Special Cold Drawn,High Tensile) Size included inch (mm)

 

Steel Designation

Mechanical Properties for Rounds, Squares and Hexagons Minimum Tensile Strength ksi (MPa)

Minimum Yield Strength ksi (MPa)

Elongation in 2 inches (50 mm) percent, min

Nominal Hardness H RCw

1137 SR * 1045 SR 1141 SR 1144 SR 1144 SS [ 4145 SS ]

95 115 115 115 140 150

(655) (795) (795) (795) (965) (1035)

90 100 100 100 125 130

(620) (690) (690) (690) (860) (895)

11 10 11 10 10 w 10 w

24 24 24 24

3.001 (76.1) to 3.500 (89)

4145 SS ]

150

(1035)

130

(895)

10 w

32

3.001 (76.1) to 4.000 (102)

1045 SR 1141 SR 1144 SR

105 105 105

(725) (725) (725)

90 90 90

(620) (620) (620)

0.375 (10) to 3. 000 (76)

9 9 9

30 32

24 24 24

* Stress Relieved. [ Special steel. Additional requirements: H ardness, Rockwell C 30, min. 1144 SS not available above 2.5 in (64 mm). ] Special steel. Additional requirements: Ha rdness Rockwell C 32, min. 4145 SS not a vailable above 3.5 in (89 mm). w Typical value, not a requirement.

NOTE:  So me cold finish steel companies furnish many of the a bove steels under various trade na mes.

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4.8.1.2Flator Plate.   Commercial flat or plate steel of numerous carbon and alloy grades is available in standa rd thicknesses in a wide range of widt hs and lengths. Flat stock is typically available in hot rolled or hot rolled and annealed conditions.

(3) Rolled Ring Forging. This method produces a do nut ---shaped work piece. Typicallyt he process involves piercing a pa ncake ---shaped billet wit h a mandrel and shaping the ring by a hammer action between the mandrela nd the press anvil. Large diameter rings are rolled on a roller press fro m circular billets conta ining a centra l hole.

4.8.1.3 Forgings. Forgings are made by hot mechanical deformation (working of a steel billet into a specific form) which densifiest he structure, and ma y provide improved inclusion orientat ion. Typically, deformation is done while the billet is at temperatures generally above 1900 _ F(1038 _ C ).

For additional information on wrought steel manufacture and steelmaking refining practices, reference should be made to the f ollowing sources: American Society for Metals (ASM International),  M etal H andbooks 

Cast ingots, from which blooms and billets are manufactured prior t o forming forgings a nd barstock, are now also bottom poured as well as conventional top poured. Bottom poured ingots are poured with a bottom ingate and runner which provides molten steel to the ingot mold, much like steel castings are produced. B otto m poured ingots show improved macro ---cleanliness and ingot yield (more usable ingot metal after conventional cropping or removal of the top pipe cavity and bott om discard of to p poured ingots).

American Iron and Steel Institute (AISI),  Steel  Products M anual  Forging Industry H andbook,  by the Forging In-

dustry Association

4.8.2 Weld Fabrications.  Weld fa bricat ed gears generally consist of rolled or forged rings, formed plate or castings for the rim (tooth) section, a forged or cast hub and mild steel plate f or the web or arm support sections. The rim or tooth section is heat treated to obta in specified ha rdness (mechanical properties) prior to weld assembly. Afte r weld assembly, using appro priate preheat and postheat temperatures, welded a ssemblies a re fur na ce str ess relieved a t 950---1250 _ F (510---675 _ C ) depending upon the previous tempering temperature used to obtain the specified hardness of th e rim section. ASTM A290 should be referenced for ring forgings for fabricated gears.

Alloy steel, manufactured by electric furnace practice using part or all of the cleanlinesstechniques discussed in 4.3, can result in improved transverse ductility and impact strength. Forging stock is always fully killed steel to minimize the occurrence of fissures due to dissolved ga ses during the fo rging process. The sta ndard forging classifications are:

4.8.3CastSteels. Ca rbon and alloy steelcastings are used for a wide variety of through hardened gearinga nd, to a lesser degree, for case hardened application s. The size of cast gea ring varies fro m 10.0 inch (254 mm) outside diamet er with a 2.0 inch (51 mm) face width for solid rimgears, to split ring gearsa bout 480inch (12 192 mm) outside diameter with a 40inch (1016 mm) face. Sma ller gea rs generally have a solid web and hub design, with possible cored holes in the web or flange for weight reduction. Larger gears are usually solid hub, split hub, or split hub and rim design, which incorporate cast arms rather than the heavier solid web design used for smaller gears. Still larger ring gears are solid or split ringdesign withbolt holes at the splits and on the inside diameter flange for gear assemblya nd mounting purposes. Split gears are cast in two or four segments. Typical cast gear designs a re shown in F ig 4 ---1. 4.8.3.1 Manufacture.Ca st steelis manufactured by the open hearth, electric arc, or induction furnace

(1) Open D ie Forging. This method produces a rough dimensioned piece by mechanical deformation between an upper and lower die (hammer and anvil) in an open frame press or hammer. Open die forgings may be specified to be upset forged to increase center densification. An upset forging is produced when the billet is initially hot worked in one direction, and then is rotated 90 degrees and hot worked again. U pset forgings are often used for critical high speed gearing, greater than 30,000 f eet /minut e (152 m/sec) pitch line velocity, which develop high centrifugal stress at the center. (2) Closed D ie Forging. This method produces a closer toleranced piece, generally smaller than an open die fo rging. The upper and lower dies trap the steel billet in a closed (confined) cavity and the press action deforms the metal to fill the die cavity, producing a more exact contoured forging.

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melting processes, using both acid or basic lined furnace steel making practices. Secondary refining processes can be used for reducing the gas, phosphorus, and sulfur levels of cast steel.

type steels. Ca rburizing grades a re usually 1020, 8620 and 4320 types. As with wrought steel, care must be taken to ensure that the specified cast analysis for through hardened gearing has sufficient hardena bility to obta in the specified minimum hardness.

4.8.3.2 Material GradesofCastSteel.  The material grades used for cast gearing are generally modifications (silicon, etc) of standa rd AISI or SAE designations. Through hardened gearing applications generally use 1045, 4135, 4140, 8630, 8640, and 4340

Typical chemical a nalyses and tensile propert ies of through ha rdened cast steels are shown in Tables 4---6 a nd 4---7, respectively.

S OLID WEB

C ORED WEB

SMALLER GEARS

S OLID RING

S OLID HUB

S P LIT RING

S P LIT HUB

S P LIT HUB AND RING

LARGER GEARS INCLUDING OPEN GEARING (NOTE:   Each design above can be made by forging or weld fabrication.)

Fig4---1 Typical Design of Cast Steel Gears

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Table4---6 Typical Chemical Analysesfor Through Hardened Cast Steel Gears Alloy Percent fo r C ast St eel Types

Element

1045 Type

4140 Type

8630 Type

8642 Type

4340 Type

C a rbon

0.40---0.50

0.37---0.43

0.27---0.37

0.38---0.45

0.38---0.43

M a nga nese

0. 60---1. 00

0.70---1. 00

0.70---1. 00

0. 70---1. 00

0.70---1.00

P hosphorus, max.

0.050

0.030

0.030

0.030

0.030

Sulfur, max.

0.060

0.040

0.040

0.040

0.040

Silicon, max.

0.60

0.60

0.60

0.60

0.60

0.60---0.90

0.60---0.90

1.65---2.00

Nickel

--- ---

--- ---

C hromium

--- ---

0.80---1.10

0.60---0.90

0.60---0.90

0.70---0.90

M olybdenum

--- ---

0.15---0.25

0.30---0.40

0.40---0.50

0.20---0.30

GENERAL NOTES: 1. Type designations indicate non ---conformance to exact AISI ana lysis requirements. 2. When basic steel making practice, ladle refining or AOD (argon oxygen decarburization) processing are used, lower phosphorus and sulfur content s to less tha n 0.020 percent are commonly a chieved. 3. Van ad ium cont ent of 0.06---0.10 percent may be specified for gra in refinement . 4. Aluminum content o f 0.025 percent maximum may be specified fo r low a lloy cast steel (per ASTM A356) for ladle deoxidation to improve toughness, cleanliness and machinability. 5. Ot her AISI Type and proprieta ry chemical ana lyses are used for carbon and low alloy cast gears according to ASTM A148 or customer specifications, depending upon specified hardness (mechanical properties), type of heat treat ment a nd controlling section size (hardena bility) considerations. 6. Source: AG MA 6033---A88,   Standard for M arine Propulsion G ear Uni ts, Part 1 Materials .

Table4---7 TensilePropertiesof Through Hardened Cast Steel Gears! Minimum Yield Strength 0.2 percent O ffset ksi (MPa)

Percent Minimum Elongation in 2 in (50 mm)

Percent Minimum Reduction in Area

100 (690)

75 (480)

15.0

35.0

241---285

110 (760)

80 (550)

13.0

31.0

C

262---311

118 (810)

90 (620)

11.0

28.0

D

285---331

130 (900)

100 (690)

10.0

26.0

E

302---352

140 (970)

115 (790)

9.0

24.0

F

321---363

145 (1000)

120 (830)

8.0

20.0

G

331---375

150 (1030)

125 (860)

7.0

18.0

Brinell Hardness Range

Minimum Ten sile Strength ksi (MPa)

A

223---269

B

AGMA@ 6033---A87 Class

 

NOTES: 1. Above tensile requirements for seven classes are modifications of three grades of ASTM A148 (G ra de s 105---85 th rough 150---135). 2. Sour ce: AG MA 6033---A88,  Standard for M arine Propulsion G ear Un its, Part 1 M aterials .

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4.8.3.3 Repair Welding of Cast Steel.   Repair welding of castings prior to heat t reatment is routinely performed by the casting producer. Repairs in the rim (tooth) portion and other critical load bearing locations should be performed only prior to heat trea tment. H eat treat able electrodes (4130, 4140a nd 4340 Types) should be used f or repairing prior t o heat trea tment in order to produce hardness equivalent to the base metal after heat treatment. Repair welding, if allowed aft er heat t reatment, shall be followed by reheat trea tment, whenever possible. I f reheat treat ment is not possible, localized preheat a nd post heat are recommended to avoid or minimize unfa vorable residual t ensile stress or high ha rdness in the heat a ffected zone. All welds should be inspected to the same qua lity standa rd used to inspect the casting.

Recommended ASTM specifications for nondestructive inspection test procedures are:

NOTE:  Weld repair in the too th portion ma y require notification of the purchaser.

4.8.3.6 Additional Information for Cast Steel. Information is available in: ASM Handbook series, Volume 5, 8th edition, Steel Founder’s Society of America (SFSA) Publication

ASTM E709---80,  Magnetic Particle Examination  ASTM E 125---63 (1980),  Reference Photographs  for M agnetic Particle Indi cations on Ferrous Castings 

ASTM A609---83, Ul trasonic Examination of    Carbon and L ow A lloy Steel Castings  ASTM E186---80,   Standard Reference Radio-  graphs for H eavy Walled [ 2 to 41/2 i nch) (51 to 114  mm )] Steel Castings 

ASTM E280---81,   Standard Reference Radio-  graphs for H eavy Walled [ 4 1/2 to 12 in ch(114 to 305  mm )] Steel Castings 

ASTM E446---81,   Standard Reference Radio-  graphs for Steel Castings U p to 2 inch (51 mm) in  Thickness 

4.8.3.4 HeatTreatmentofCast Steel.   Castings are heat treated to either a specified hardness or to specified hardness and minimum mechanical properties. The minimum number of hardness tests required o n both rim fa ces of gear ca stings is generally based on t he outside diameter. The number of tests increases with O D size. M echanical property tests (tensile and impact) a re generally required only when specified. Reference should be made to 6.2and 6.3 for additional information.

ASM Handbook, Volume 11, 8th edition,  Non-  destructive I nspection and Qu ality Control 

4.8.4 CastIron. Ca st Iron is the generic term for the fa mily of high carbon, silicon, iron alloys. The fa mily of cast irons is classified by the following categories. 4.8.4.1 GrayIron.  G ray iron conta ins (typically over 3.0 percent) carbon, which ispresent as graphite flakes. It is chara cterized by the gra y color occurring on a fracture surface. Refer to Grayand DuctileIron  Castings H andbook  for additional information. (1) Material considerations. C ast ironsf or gears are made by t he electric arc furnace, cupola, or induction practice and should be free of shrink, porosity, gas holes, entrapped sand and hard a reas in the tooth portion.

4.8.3.5 QualityofCastSteel. C astings should be furnished free of sand, scale, extraneous appendages, a nd ha rd a reas resulting from arc---airing, gas cutting, and repair welding which could adversely af fect machining. Casting should also be free of cracks, hot tears, chills, and unfused chaplets in the rim section. C astings must meet the nonde structive test requirements in the rim section. The q uality specified in other than the rim (tooth) section is often less stringent. Minor discontinuities in finish machined teeth, if present, are often contour ground for removal, in preference to cosmetic weld repair. Approval by the customer may be required.

Repair welds in areas to be machined should have machinability eq uivalent to the casting. Repair welds in the t ooth portion should only be performed with the approval of the gear purchaser.

D ry or wet fluorescent magnetic pa rticle inspections are routinely performed t o meet specified surface quality requirements. Other nondestructive testing, such as radiograph and ultrasonicinspection, is performed to evaluat e internal integrity of the rim (tooth) section when specified. Metho ds of testing, test locations, and acceptance standards are established between the purchaser and manufacturer.

ANSI/AG MA

(2) Hea t Treat ing. Ca st iron castings are generally furnished as cast unless otherwise specified. Stress relieving may be deemed necessary to hold close dimensional to lerances. It is recommended that castings be heated to 1000 to 1100 _ F (538---593 _ C ), holding at temperature up to one hour per inch of maximum section and furnace cooled to below 600 _ F (315 _ C ).

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(3) Chemical Analysis. U nless otherwise specified, t he chemical a nalysis is left to the discretion of the casting supplier as necessary to produce castings to the specification.

1

(4) Mechanical Properties. Ca st iron gears are rated a ccording to AG MA practice based on ha rdness. Therefore, hardness determines the rating of the gear. Minimum hardness requirements for t he classes of cast iron are sho wn in Table 4---8.

20 (140) 30 (205) 35 (240) 40 (275) 50 (345) 60 (415)

(2) Hea t Treating. D uctile iron castings shall be heat treated by annealing, normalizing and tempering or quenching a nd tempering or as ---cast a s required t o meet the specified mechanical properties. These heat treatments produce ferritic, pearlitic or martensitic structures. (3) Chemical Analysis. U nless o therwise specified, t he chemical a nalysis is left to the discretion of the casting supplier as necessary to produce castings to the specification.

A B

(4) Mechanical Properties. Typical mechanical propert ies are shown in Table 4---9. Ot her pro perties may be as agreed upon by the gear manufacturer and casting producer.

C

NOTE:  See ASTM A48 for tolerances on a s cast and machined diameter and retest considerations if bar fails to meet requirements.

Tensile test coupo ns should be poured fro m the same ladle or heat and be given the same heat treatments a s the castings they represent. Test coupon mold design shallbe in accordance with ASTM A536. Size of the Y ---block mold, if used, is a t the optio n of the producer unless specified by the gear manufacturer.

Table4---8 MinimumHardness and TensileStrength Requirements for GrayCast Iron ANSI/AG MA

155 180 205 220 250 285

Repair welds in areas to be machined should have eq uivalent ma chinability as the casting. Repair welding in the tooth portion should only be performed with the approval of the gear purchaser.

As Cast Machined Diameter, Diameter,   ASTM A48 Test B ar , in (mm) in (mm) 0.50 (12. 7) 0.750 (19.0) 1.25 (31.8)

20 30 35 40 50 60

(1) Material Considerations. Ductile iron castings are made by the electric arc furnace, cupola or induction practice and should be free of shrink, porosity, gas holes and entrapped sand and hard areas in the tooth portion.

Tensile test coupons are cast in separate molds in a ccordan ce with the provisions of ASTM A48. The size of the cast test coupon is dependent upon the thickness of the tooth portion of the casting as follows:

0.88 (22. 4) 1.20 (30.5) 2.00 (50.8)

Ten sile Strength ksi (MPa)

4.8.4.2DuctileIron. D uctile iron, sometimes referred to as nodular iron, is characterized by the spheroidal shape of the graphite in the meta l matrix, produced by innoculation with ma gnesium and rare eart h elements. A wide ra nge of mechanical properties a re produced through control of the alloying elements and subsequent heat treatments. (Refer to Gray and D uctile Iron H andbook .)

Tensile tests should only be required when specified. Tensile t est requirements a re shown in Table 4---8, an d t esting should be perfo rmed in accord ance with ASTM A48,   Standard Specifications for G ray  Iron Casting .

0.25---0.50 (6. 4--- 12. 7) 0.51---1.00 (12.8---25.4) 1.01---2 incl. (25.5---50.8)

Brinell Hardness

1 See ASTM A48 for additional information.

Hardness tests should be made in accordance with ASTM E 10. Ha rdness tests should be made on the mid rim thickness or mid face width of the too th portion diameter. At least one hardness test should be made o n each piece, and sufficient hardness tests should be made to verify that the part meetsthe minimum hardness specified. Specified minimum hardness must be maintained to the finish machined dimensions fo r a cceptance.

Thickness of Too th Section, in (mm)

ASTM Class Number

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Tensile tests should be perf ormed in accorda nce with ASTM D esignation E8, Standard M ethod of Ten-  sion Testing of M etallic M aterials . The yield strength is normally determined by the 0.2 percent offset method. For required retesting, if tensile bar fa ils to meet requirements, refer to ASTM A536.

When eight hardnesst ests are specified, they shallbe made 90 degrees apart on both cope and drag side. For solid cylindrical pieces, with length over diameter of one or more, the number of ha rdness tests should be as follows: Diameter of Toot h Po rtion , in(mm) To 3 (76) incl. O ver 3 (76) t o 6 (152) incl. Over 6 (152)

Hardness tests should be performed in accordance with ASTM D esignat ion E 10,  Standard Meth-  od of Test for Brin ell H ardness of M etalli c M aterials . H ardness tests should be made on the mid rim thickness or mid fa ce width of the tooth portion diameter. Number of hardness tests per piece is based on the diameter of the casting as follows: Outside D iameter of Casting, in(mm) To 12 (305 ) O ver 12 (305) to 36 (915) O ver 36 (915) t o 60 (1525) O ver 60 (1525)

Number o f H a rdness Tests 1 2 4

NOTE:  The hardness tests shall be spaced uniformly a round the circumference. When many smallpieces are involved, all poured from the same ladle or heat, and heat treated in a single furnace load , a sample testing plan is generally used with the approval of the gear manufacturer.

Number o f H ard ness Tests 1 2 4 8

4.8.4.3 Austempered Ductile Iron.   Austempered D uctile Iro n (ADI ) is a d uctile iron with higher strength and hardness tha n conventional ductile irons. The higher properties of AD I a re a chieved by closely controlled chemistry a nd an austempering heat treatment. This treatment results in a unique microstructure of bainitic ferrite and larger amounts of carbon stabilized austenite. With variation in austempering temperature and transformation time, several ranges of engineering properties can be achieved.

When two hardness tests are required, one should be mad e on t he cope side over a riser and the other on the drag side approximately 180 degrees away between risers. When four hardness tests are required, t wo tests should be ma de on the cope side, one over a riser and the ot her approximately 180degrees away between risers, and two testson the drag side 90 degrees away from the testson the copeside.

Table4---9 Mechanical Propertiesof DuctileIron 1

ASTM Grade Designation 60 --- 40--- 18 65---45---12 80---55---06 100---70---03 120--- 90--- 02

Former AGMA Class

Recommended Hea t Treat ment

Mi n. Tensile Brinell Strength Ha rdness Ra nge ksi (MPa)

A --- 7--- a An ne ale d Fe rrit ic A---7---b As---Cast or Annealed Ferrit ic---Pe arlitic A---7---c Normalized Ferritic---Pearlitic A---7---d Quench & Tempered Pearlitic A --- 7--- e Q ue nch & Te mpe re d Martensitic

Min. Yield Strength ksi (MPa)

Elongation in 2 inch (50 mm) percent min

170 ma x. 156---217

60 (415) 65 (450)

40 (275) 45 (310)

18. 0 12.0

187---255 241---302 R a nge Specified

80 (550) 100 (690) 120 (830)

55 (380) 70 (485) 90 (620)

6.0 3.0 2. 0

1 See ASTM A536 or SAE J 434 for further info rmation. NOTE: Ot her tensile properties and hardnesses should be used only by agreement between gear manufa cturer and casting producer. ADI has been utilized in several significant applications, such as automotive ring gears and pinions, but is still an emerging technology. AD I permits low-

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er machining and hea t trea t cost and replacement of more costly forgings for certa in a pplications.

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Test programs ar e currently underwa y which will more clearly define operational properties of ADI.

rately determined using special microhardness measurement techniques.

4.8.4.4 MalleableIron.  M alleable iron is a heat treated white (chilled) iron which can be produced with a range of mechanical properties depending on the alloying practice and heat treatment. This has generally been replaced by ductile iron. (Refer to ASTM A220.)

Parts can be heat treated after sintering, but must be processed in a controlled at mosphere to prevent changes in surface chemistry. Ca rburizing and carbonitriding can be performed, but products with a den sity u nde r 6.8 g/cm# will not develop a definite case due to the ease of diffusion through the more porouslower density material. Penetration hardness testing cannot be correlat ed to mat erial strength, but parts will achieve a file hard surfa ce. Salt bat hs and wat er q uench systems should be a voided.

4.8.5Powder Metal (P/M). Powder metal parts are formed by compressing metal powders in a die cavity and heat ing (sintering) the resulta nt compact to metallurgically bond the powder particles. Secondary operationssuch as repressing or sizing may be used to obta in precise control of shape and size or to improve mechanical properties. The powd er meta l process is used to re duce cost by eliminat ing machining operat ions, provide a ccurate dimensional control over large production runs, and obta in characteristics and shapes difficult to obtain by other methods. However, because of molding die costs, high production qua ntities are usually necessary to realize savings.

Further improvements in strength can be achieved by the use of hot forming powder metal. Powder metal preforms are heated to forging temperature and finished forged to final shape and density. Parts processed in this manner have strengths and mechanical properties approa ching the properties of wrought materials. Although this process is much more costly than the conventional powdermetal process, it can still be cost effective for high production parts requiringhighermechanical properties tha n a chievable using the sta ndard process.

Although several powder metal materials are available, a lloy steel is usually specified for gear a pplications.

The controlled porosity in powder metal parts permits their impregnation with oil to provide a self lubricating part, especially f or the internal type of gears.

“As sintered” alloy steels have a tensile strength ra nge o f 40---80 ksi (275---550 MP a), wit h a n elon ga tion of 4.0 percent or less and an apparent hardness of H R B 60---85. Hea t trea ted powder meta l alloys ha ve t ensile stre ngt hs o f 100 to 170 ksi (690---1170 MPa) with elongationso f 1.0percent or less, depending on d ensity a nd a lloy selected.

The powde r met al process is well---suited to the production of gears for several reasons: (1) C arbide dies provide consistent pa rt a ccuracy over long runs. (2) Retention of some porosity contributes to quietly running gears and allows for self ---lubrication.

Density is the most significant characteristic of powder metal materials. For a given composition, mechanical properties are proportional to density; i.e., higher strengths are achieved at higher density levels. In recent years, powder metal processes have improved to the point where a typical density of 7.0 to 7.4 g/cm#  can be a chieved using seconda ry operations.

(3) Powder metal gears can be made with blind corners, thus eliminating undercut relief tha t is needed with cut gears, and have extra support strength a t the blind end. (4) Powder metal gears can be combined with other parts such as cams, ratchets, other gears, and assorted components.

The ductility of powder metal parts is substantially lower than for wrought steels. Ha rdnessspecifications can be developed for powder metal parts, but must be specified as “apparent hardness” since the hardness value obtained using a standard tester (either HRB or HRC) is a combination of the powder particle hardness and porosity. The actua l hardness of the powder meta l material will be higher than the apparent hardness reading and can be more accu-

ANSI/AG MA

Spur gears are the easiest to produce out of powder metal because of the vertical action of the press and ea se of ejection of t he preform from the die cavity before sintering. Bevel, miter, helical, and other special gear forms are, however, possible in powder metal with sufficient development. True involute gears a re less difficult and may be less costly to pro-

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duce in sufficient quantities than by other methods because tooth configuration is not a limitation.

a nd no n ---destructive inspection (magnet ic part icle and ultrasonic or radiograph) pra ctices.

4.8.6 Other FerrousMaterials.   In addition to materials used for gears which are described in this Manual there are other ferrous materials used for gears. These include hot work t ool steel (H series), high speed steels, a ustenitic, ma rtensitic and precipitation hardening sta inless steels, etc. Special gea r analyses are freq uently used in applications with very high strength req uirements.

Fabricated (welded) gears are generally manufactured when they are more economical than forged or cast gears. G ear rims are normally forged or rolled rings, formed alloy plate, or, less freq uently, cast. Hardenability of the gear rim steel must be adequate to enable a 1000 _ F (540 _ C ) minimum tempering temperature to obta in hardness. The welded assembly should, therefore, be stress relieved at 950 _ F(510 _ C ) minimum [50 _ F(28 _ C ) below the tempering temperat ure]. G ear rims used in the annealed condition can be stress relieved a t 1250 _ F (675 _ C ).

4.9 SelectionCriteria for Wrought, Cast, or FabricatedSteel Gearing. Selection of the gear blank producing method for most applications is primarily a matter of economics, with quality becoming increasingly important as tooth loads, down time costs and safety considerations increase. Critical application gearing, such as f or a erospace and special high speed, is commonly manufa ctured of vacuum degassed alloy steel, further refined at premium cost by vacuum arc remelt (VAR) or electroslag remelt (ESR) processing. These and other more economical refining processes (AOD, ladle refined, etc.) improve cleanliness and produce higher qua lity steel.

Forged or hot rolled die generated gear teeth, with the direction of inclusion (metal) flow parallel to the profile of teeth, result in the optimum direction of inclusions fo r gea ring. Application is limited because quantities or critical application considerations must justify the increased development and die costs.

4.10 Copper BaseGearing.  No n ---fe rro us gea rs are made from alloys of copper, aluminum, and zinc. Alloys of copper are in wide use for power tra nsmission gearing. Most of these are used in worm gearing where the reduced coefficient of friction between dissimilar materials and increased malleability are desired.

Wrought or forged steel is generally considered more sound than castings because the steel is hot worked. Wrought steel is anisotropic, however, meaning that the mechanical properties (tensile ductility and fa tigue and impact strength) vary according to t he direction of ho t working or inclusion f low during f orming (see F ig 4---2). Impro ved ste el cleanliness has the effect of improving the transverse and tangential properties of f orged steel in order to approach, but not equal, the longitudinal properties. Inclusions in wrought steel forgings, barstock, rolled rings and plate are perpendicular to the root radius or profile of machined gear teeth.

4.10.1 Gear Bronzes.  A family of four bronzes accounts for most of the nonferrous gear materials, mainly because of their “ wear resista nce” chara cteristics for withstanding a high sliding velocity with a steel worm gear. (1) Phosphor or Tin Bronzes. These bronzes are tough and have good corrosion resistance. They possess excellent rubbing characteristics and wear resistance which permitsuse in gears and worm wheels for severe wear a pplications. This alloy is the basic gear alloy and is commonly designated as SAE C90700 (obsolete SAE 65) and is referred to as tin bronze. (2) Ma nganese B ronzes. This is the na me given to a fa mily of high strength yellow brasses. They are characterized by high strength and ha rdness and a re the toughest materials in the bronze family. They achieve mechanical properties through alloying without heat treatment. These bronzes have the same strength and ductility as annealed cast steel. They have good wear resistance but do not possess the same degree o f corrosion resistance, weara bility

NOTES:Mechanical propertiesin the transverse direction will vary with inclusion type and material form. Mechanical property data is normally measured in the longitudinal direction. C astings genera lly being isot ropic (no n ---directiona lity of properties), when sound in the rim tooth section, can provide comparable mechanical properties to those of forgings. Ca sting qua lity involves controlled steel making, molding, casting, heat treating

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or bearing quality as phosphor and aluminum bronzes. (3) Aluminum B ronze. Aluminum bronze materials are similar to the ma nganese bronzes in toughness, but are lighter in weight and attain higher mechanical properties through heat treat ment. As the strength of aluminum bronze isincreased, ductility is reduced. This bronze has good wear resistance and

has low coefficient of friction against steel. Bearing characteristics are better tha n for manganese bronze but are inferior to the phosphor bronzes. (4) Silicon Bro nzes. Silicon bronzes are commonly used in lightly loaded gearing for electrical applications because of their low cost and nonma gnetic properties.

DIREC TION OF METAL AND INC LUS ION FLOW

ROLLED RING FORG ING

LONG ITUD INAL TENS ILE TES T B AR OR P ROP ERTIES

TRANS VER S E TENS ILE TES T B AR

DIRE C TION OF METAL AND INC LUS ION FLOW

PINION FORGING

TRANS VERS E TENS ILE TES T B AR

LONG ITUD INAL TENS ILE TES T B AR

TANG ENTIAL TENS ILE TES T B AR

NOTE: AS TM E399 may be use d if impa ct tes ting is required .

Fig4---2Directionalityof ForgingProperties 4.10.2 Gear Brasses andOther Copper Alloys. G ear bra sses are selected f or their corrosion resista nt properties. The most common gear brass is yellow brass, used because of its good machinability. Other brass materials are used because of their high-

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er strength, but they are more difficult to machine. Wear resistance of these brasses is somewhat lower than for the higher strength manganese bronzes.

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4.10.3 WroughtCopper Base.  Wrought copper base materials is a general term used to describe a group of mechanically shaped gear materials in which copper is the ma jor chemical component. This group of gear materials includes bronzes, brasses, an d o ther copper a lloys. Table 4---10 presents chemical ana lyses of common wrought bronze alloys, while Table 4---11presents typical mechanica l propert ies of these wrought bronze alloys in rod and bar form. 4.10.4 CastCopper Base. Copper base castings are specified by melting method, heat treatment, ana lysis or type, ha rdness and t ensile properties. 4.10.4.1CastWormBronzes. Specifications describe type of bronzes according to chemical ana lysis. Re fer to Table 4---12 for chemical a na lyses o f common cast copper bronze alloys, including phosphor or tin bronze, leaded tin bronze (improved machinability) and higher strength manganese bronze and aluminum bronze. Mechanical properties of separa te cast te st specimen s ar e sho wn in Table 4---13.

as agreed to by the gear manufacturer and casting producer. The chemical analysis shall be determined from a sample obtained during pouring of the heat. The gear manufacturer may perform a product ana lysis for chemistry. In t he event of disagreement in chemical analysis, ASTM Designation E54,  Stan-  dard M ethods of Ch emical An alysis of Special B rasses  and Bronzes, may be used as the referee method.

(4) Casting Hardness. Hardness tests are normally made in accordance with ASTM E10,  Method  of Test for B rinell H ardness of M etalli c M aterials. The load in kilograms force listed in Table 4---13 should be used. Hardness tests are to be made on the tooth portion of thepart after final heat treatment, if required. The number of hardness tests made should be specified by the gear manufacturer. (5) C asting Tensile Pro perties. Tensile t ests a re only req uired when specified. Tensile tests when specified are made in accordance with ASTM E8,  Ten-  sion Testing of Metallic M aterials. Tensile test bars fo r sand castings may be atta ched to casting or cast separate ly. Tensile test bars for static chill castingsma y be cast separately with a chill in the bottom of the test bar mold. Tensile test ba rs for centrifugal castings may be cast in a separate centrifugal mold for test bars or cast in a chill test bar mold.

4.10.4.2 General Informationfor Copper Castings. Additional information regarding manufacturing, chemical a nalysis, hea t t reat ing, tensile properties, hardness and hardness control, cast structure and supplementary data for cast copper alloys is as follows: (1) Ca sting Ma nufacture. C ast copper base gear materials may be melted by any commercially recognized melting method f or t he composition involved. Ca stings should be free of shrink, porosity, gas holes and entrapped sand in the tooth portion. Ca stings should also be furnished free of sand and extraneous appendages.

NOTE: An integral or separately cast test bar does not necessarily represent the properties obta ined in the casting. The properties in the casting are dependent upon t he size a nd design of the casting and foundry practice.

Repair welding in other t han the tooth portion may be performed by the casting supplier. Repair welds in the tooth a rea should be performed only with the a pproval of the gear manufacturer.

Three test coupons shall be poured from each melt o f meta l or per 1000 lbs (454 kg) of melt except where the individual casting weighs more than 1000 lbs (454 kg).

(2) Ca sting Heat Treat ing. Copper B ase castings are heat treated a s required to obtain the specified mechanical properties.

Heat treated castings should have the test coupons heat treated in the same furnace loads as the casting they represent.

(3) Ca sting Chemical Analysis. Chemical analysissha ll be in conforma nce with the type specified or

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Table4---10 Chemical Analysesof WroughtBronzeAlloys Bronze 1 Alloy U NS NO.

Former AGMA Type

Co mposition, Percent Ma ximum (unless shown as a range o r minimum) Cu (incl Ag)

Pb

Fe

Sn

Zn

Al

As

Mn

Si

Ni (incl C o)

C 62300

--- ---

R em.

--- ---

2.0 to 4.0

0. 60

--- ---

8. 5 to 11.0

--- ---

0. 50

0.25

C 62400

--- ---

R em.

--- ---

2. 0 to 4.5

0. 20

--- ---

10. 0 to 11.5

--- ---

0. 30

0. 25

C 63000

ALB R 6

R em.

--- ---

2.0 to 4.0

0.20

0.30

9.0 to 11.0

--- ---

1.50

0.25

4.0 to 5.5

C 64200

ALB R 5

Rem.

0.05

0.30

0.20

0.50

6.3 to 7.6

0.10

1.5 to 2.2

0.25

C 67300

--- ---

58.0 to 63.0

0.40 to 3.0

0.50

0.30

R em.

0.25

2.0 to 3.5

0.50 to 1.5

0.25

0.15

--- ---

1.0

--- ---

1 U nified Numbering System. For cross reference to SAE, fo rmer SAE & ASTM, see SAE Inf ormation Report SAE J461. For added copper alloy information, a lso see SAE J463.

Table4---11 Typical Mechanical Properties! of Wrought BronzeAlloyRod and Bar Bronze2Alloy UNS NO. C 62300 C 62400 C 63000 C 64200 C 67300

Former AGMA Type

Tensile St rengt h ksi (MPa )

 

Yield Strength ksi (MPa )

Elongation in 2 in (50 mm) percent, min.

Hardness HB and HRB

--- ---

90

(620)

45

(310)

25

180H B (1000kgf)

--- ---

95

(655)

50

(345)

12

200H B (3000kgf)

ALB R 6

90

(620)

45

(310)

17

100 H R B

ALB R 5

93

(640)

60

(415)

26

90 H R B

70

(485)

40

(275)

25

70 H R B

--- ---

1 Typical mechanical properties vary with form, temper, a nd section size considerations. 2 U nified Numbering System. For cross reference to SAE, former SAE & ASTM, see SAE Information Report SAE J 461. For added wrought copper alloy informat ion, a lso see SAE J 463.

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Table4---12 Chemical Analysesof Cast BronzeAlloys

Bronze Former Alloy * AGMA UNS NO. Type

Co mposition, P ercent Ma ximum (unless shown a s a range o r minimum) Cu

Sn

Pb

Zn

Fe

Ni Sb (incl Co ) S

P

Al

Si

Mn

C 86200

MNB R 3

60.0 to 66.0

0.20

0.20

22.0 to 28.0

2.0 to 4.0

--- ---

1.0

--- ---

--- ---

3.0 to 4.9

--- ---

2.5 to 5.0

C 86300

MNB R 4

60.0 to 66.0

0.20

0.20

22.0 to 28.0

2.0 to 4.0

--- ---

1.0

--- ---

--- ---

5.0 to 7.5

--- ---

2.5 to 5.0

C 86500

MNB R 2

55.0 to 60.0

1.0

0.40

36.0 to 42.0

0.4 to 2.0

--- ---

1.0

--- ---

--- ---

0.5 to 1.5

--- ---

0.10 to 1.5

C90700

MNBR 2

88.0 to 90.0

10.0 to 12.0

0.50

0.50

0.15

0.20

0.5

0.05

0.30{

0.005

0.005 --- ---

C92500

MNBR 5

85.0 to 88.0

10.0 to 12.0

1.0 to 1.5

0.50

0.30

0.25

0.8 to 1.5

0.05

0.30{

0.005

0.005 --- ---

C 92700

M NB R 3

86. 0 to 89.0

9.0 to 11.0

1.0 to 2.5

0.70

0.20

0.25

1.0

0.05

0.25{

0.005

0.005 --- ---

C92900

--- ---

82.0 to 86.0

9.0 to 11.0

2.0 to 3.2

0.25

0.20

0.25

2.8 to 4.0

0.05

0.25{

0.005

0.005 --- ---

C95200

ALBR 1

86.0 min

--- ---

--- ---

--- ---

2.5 to 4.0

--- --- --- ---

--- ---

0.50{

8.5 to 9.5

--- --- --- ---

C 95300

ALB R 2

86.0

--- ---

--- ---

--- ---

0.8 to 1.5

--- --- --- ---

--- ---

--- ---

9.0 to 11.0

--- ---

--- ---

min C95400

ALBR 3

83.0 min

--- ---

--- ---

--- ---

3.0 to 5.0

--- ---

2.5

--- ---

--- ---

10.0 to 11.5

--- ---

0.5

C95500

ALBR 4

78.0 min

--- ---

--- ---

--- ---

3.0 to 5.0

--- ---

3.0 to 5.5

--- ---

--- ---

10.0 to 11.5

--- ---

3.5

* U nified Numbering System. For cross reference to SAE, former SAE & ASTM, see SAE Informa tion Report SAE J461. For added copper alloy information, a lso see SAE J462.

{

For cont inuous castings, phosphorus shall be 1.5 percent maximum.

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Table4---13 Mechanical Propertiesof Cast BronzeAlloys! Copper Alloy UNS.2 NO .

Former AG MA Type

C86200

MNB R 3

C86300

MNBR 4

C86500 C 86500

Ca sting Method &Condition #

Minimum Typical H ard ness % Minimum Minimum Percent 4 4 Tensile Str engt h Yield Strength Elongation HB HB ksi (MPa) ksi (MPa) in 2 inch 500 3000 (50 mm) kgf kgf

Sand, Centrifugal Continuous Sand, Centrifugal Continuous

90

(620)

45 (310)

18

--- ---

180

110 110

(760) (760)

60 (415) 62 (425)

12 14

--- ----- ---

225 225

MNBR 2 MNBR 2

Sand, Centrifugal Continuous

65 70

(450) (485)

25 (170) 25 (170)

20 25

112 112

--- ----- ---

C90700 C90700 C 90700

BRONZ E 2 BRONZ E 2 BRONZ E 2

Sand Continuous C entrifugal

35 40 50

(240) (275) (345)

18 (125) 25 (170) 28 (195)

10 10 12

70 80 100

--- ----- ----- ---

C92500 C92500

BRONZ E 5 BRONZ E 5

Sand Continuous

35 40

(240) (275)

18 (125) 24 (165)

10 10

70 80

--- ----- ---

C92700 C92700

BRONZ E 3 BRONZ E 3

Sand Continuous

35 38

(240) (260)

18 (125) 20 (140)

10 8

70 80

--- ----- ---

C 92900

--- ---

Sand, C ontinuous

45

(310)

25 (170)

8

90

--- ---

C95200 C95200

ALBR 1 ALBR 1

Sand, Centrifugal Continuous

65 68

(450) (470)

25 (170) 26 (180)

20 20

--- ----- ---

125 125

C95300 C95300

ALBR 2 ALBR 2

Sand, Centrifugal Continuous

65 70

(450) (485)

25 (170) 26 (180)

20 25

Sand, Centrifugal Co ntinuous (HT)

80 80

(550) (550)

40 (275) 40 (275)

12 12

---------

140 140

C95300 ALBR 2 C 95300 ALB R 2

---------

160 160

C95400 C95400 C95400 C95400

ALBR ALBR ALBR ALBR

3 3 3 3

Sand, Centrifugal (HT) Continuous Sand, Centrifugal(HT) Continuous (HT)

75 85 90 95

(515) (585) (620) (655)

30 32 45 45

(205) (220) (310) (310)

12 12 6 10

---------

---------

160 160 190 190

C95500 C95500 C95500 C95500

ALBR ALBR ALBR ALBR

4 4 4 4

Sand, Centrifugal Continuous Sand, Centrifugal (HT) Continuous (HT)

90 95 110 110

(620) (655) (760) (760)

40 45 60 62

(275) (290) (415) (425)

6 10 5 8

---------

---------

190 190 200 200

1 For rat ing of worm gears in a ccorda nce with AG MA 6034---A87, the M at erials Factor,  k s , will depend upon the particular casting method employed. 2 U nified Numbering System. For cross reference to SAE, fo rmer SAE & ASTM, see SAE I nformat ion Report SAE J461. For added copper alloy information, a lso see SAE J462. 3 Refe r to ASTM B 427 for sand a nd centrifugal cast C90700 alloy and sand cast C 92900. 4 Minimum tensile strength and yield strength shall be reduced 10% for continuous cast bars having a cross section of 4 inch (102 mm) or mo re (see ASTM B 505, Table 3 foot not e). 5 B HN a t ot her load levels (1000 kgf or 1500 kgf) may be used if a pproved by purchaser.

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One test specimen should be tested from each groupof three test coupons cast. If this bar meetsthe tensile requirements, the lot should be accepted. If the first bar fails to meet the specified requirements, the two rema ining specimens shall be tested. The average properties of these two ba rs must meet specified requirements for acceptance of the lot. (6) Casting Hardness Control. The gear manufacturer can select at random any number of castingsfrom a given lot to determine the hardness at or within 1 inch (25mm) of the cast OD or as indicated on gear manufacturer’s drawing. The lot should consist of all gears produced from one melt of metal. Determination of hardness at or near the root diameter is optional and should be agreed upon by the purchaser and gear manufacturer. The minimum hardness, using a 500 kg loa d, shall be 80 HB for static chill and centrifugal chill castings, a nd 70 HB for sa nd castings. The minimum hardness at or near the root diameter shall be agreed upon bythe purchaser and the casting producer. Failure of any gear to meet hardness requirements specified is subject to rejection.

castings and, in particular, the tooth section. It may be a dvisable to specify by use of phot omicrographic standards both acceptable and non---acceptable phase distributions in the gear rim section.

4.11Other Non---FerrousMaterials.  In addition to the more common non ---ferrous ma terials used f or gears, several wrought aluminum and beryllium copper alloys are occasionally used. Specifications are specialized and should be resolved between the user and supplier. 4.12Non---MetallicMaterials.  Many gears, particularly those used to transmit motion rather tha n power, a re produced from non ---metallic materia ls. B ecause of the wide range of non ---metallic materials, engineering dat a on t he various types of non ---metals is usually most ea sily available fro m the producers. Plastics are being used at a rapidly increasing rate as gear materials in the fine pitch range. Improved materials, advances in gear mold design and molding technology, development of engineering dat a, and the successful use of plastic gears in many applications have all contributed to the establishment of certa in plastics as engineering mat erial suitable for fine pitch gears.

(7) Ca st Structure. When required, the producer should furnish specified microspecimens or photomicrographs for each melt with the certificate of hardness, chemistry, a nd mechanical properties.

Non---metallic gea rs are usually selected for properties such as low friction, ability to operate with no lubricant, resistance to water absorbtion, a nd quietness of operation. (See Appendix A and AG MA 141.)

(8) Supplemental Data. The following supplementary req uirement should apply only when specified by contractual agreement. D etails of this supplementary requirement should be agreed upon by the casting producer and gear manufacturer. (a) With proper foundry technique, the properties of sta tic chilled and centrifugal cast separate test bars should be t he same.

5. Heat Treatment Heat treatment is a heating and cooling process used to a chieve desired properties in gear ma terials. Ferrous gearing may be through hardened or surface hardened when gear rating or service requirements warrant higher hardness and strength for improved fatigue strength or wear resistance. Common heat treatments for ferrous materials include:

(b) An integral or a separa te test bar simply signifies the melt q uality poured into the mold to ma ke the casting. It d oes not express the specific propertiesa nd characteristics of the castingwhich are greatly dependent on design, size, and foundry technique.

(1) P rehea t trea tment s--Anneal Normalize and temper Quench and temper Stress relief

(c) The gra in size of cast copper ba se alloys varies as a function of cooling rate and section thickness. R ecommended maximum grain size for centrifugal ca stings is 0.035 mm in the rim, 0.070mm in t he web a nd 0.120mm in the hub. The grain size fo r copper base alloysis determined per ASTM E 112 at 75X magnification.

(2) H ea t trea tment s--Through harden (anneal, normalize, or normalize and temper, and quench and temper).

(d) The grain size of sta tic cast copper base alloys should be mutually agreed upon by the consumera nd producer with reference to the varioussections of the

ANSI/AG MA

Surface harden profile heated (flame and induction harden) and profile chemistry

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modified (carburize, carbonitride, and nitride)

Typical specified hardnessra nges for normalized and t emper ed stee ls are shown in Table 4---2.

(3) Po st hea t trea tment --Stress re lieve

5.1.3 Normalizingand Annealingfor Metallurgical Uniformity.   The normalizing and annealing processes are frequently used, either singularly or in combination, as a homogenizing heat t reatment for alloy steels. These processes are used in wrought steel to reduce meta llurgical non ---uniformity such as segregated alloy microstructures (banding) a nd distorted crystaline microstructures from mechanical working.

Specialized heat treatment for nonferrous materials should be recommended by the producer.

5.1 Through Hardening Processes. Through hardened gearsare heated to a required temperature and cooled in the furnace or quenched in air, gas or liquid. Through hardening may be used before or af ter the gear teeth a re formed.

Cycle annealing is a term applied to a special normalize/temper processin which the parts are ra pidly cooled to 800---1000 _ F (427---538 _ C ) a f te r nor ma lizing at 1600---1750 _ F (871---954 _ C ), followed by a 1200 _ F (649 _ C ) temper with controlled cooling to 600 _ F (316 _ C ).

There are generally three methods of heat treating through hardened gea ring. In ascending order of hardness for a particular type of steel they are; annea ling, normalizing (or normalizing a nd tempering), and quenching and tempering. Modifications of q uench hardening, such as austempering and martempering, occur infrequently for steel gearing and are, therefore, not discussed. Austempering is used, however, for through hardened (approximately 300 to 480 HB ) ductile cast iron gea rs.

5.1.4 QuenchandTemper. The quench and temper processon ferrous alloysinvolves heating to form austenite a t 1475---1600 _ F (802 ---871 _ C ), followed by rapid quenching. The rapid cooling causes the gear to become harder and stronger by formation of martensite. The gear is then tempered to a specific temperature, generally below 1275 _ F(691 _ C ) , t o achieve the desired mecha nical proper ties. Tempering reduces the material hardness and mechanical strength but improves the material ductility a nd toughness (impact resistance). Selection of the tempering temperature must be based upon the specified hardness range, material composition, and the as quenched hardness. The tempered hardness varies inversely with tempering temperature. Pa rts are normally air cooled from tempering temperatures. Table 4---3 gives ha rdn ess guide lines fo r so me st eel grades.

NOTE:   Through hardening does not imply equal hardness through all sections of the part. See 4.6 for discussion of hardena bility. 5.1.1 Annealing.  Annealing consists of heating steel or other ferrous alloys to 1475---1650 _ F (802---899 _ C ), and furnace cooling to a prescribed temperature [generally below 600 _ F (316 _ C)]. Annealing may be the final treatment (when low  hardness requirements permit) or is typically a pretreatment applied to the cast or wrought gear blank in the “rough.” It results in low hardness and provides improved machinability and dimensional stability (minimumresidual stress). Typical hardness for a nnea led g ea ring is sho wn in Table 4---2.

The hardness and mechanical properties achieved from the quench and temper process are higher than those achieved from the normalize or anneal process.

5.1.2 Normalizing. Normalizing consists of hea ting steel or ot her fe rrous alloys to 1600---1800 _ F (871---982  _ C ) and cooling in still or circulated air. Normalizing results in higher hardness tha n a nnealing, with hardness being a function of grade of steel and the part section thickness. However, with plain carbon steels containing up to a bout 0.4 percent carbon, no rmalizing does not increase ha rdness significantly more than annealing, regardless of section size.

5.1.4.1 Applications.   The quench and temper process should be specified for the following conditions: (1) When the gea r applicat ion stress ana lysis indicates that the ha rdness and mechanical properties for the specified mat erial grade can best be achieved by the q uench and t emper process.

Allo y st ee ls a re no rma lly t empe re d a t 1000---1250 _ F (538 ---677 _ C ) after normalizing for uniform hardness, dimensional stability and improved machinability.

ANSI/AG MA

(2) When the hardness and mechanical properties required for a given gear application can be achieved more economically by quench and temper

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of a lower alloy steel, than by normalizing or annea ling.

with the tempering embrittlement phenomen on f ro m t empe rin g in a lo we r r an ge (500---600 _ F ) often referred to as “500 _ F or A ---E mbrit tleme nt .”

(3) When it is necessary to develop mechanical properties (core properties) in sections of the part which will not be altered by subsequent heat treatments (for example nitriding, flame hardening, induction hardening, electron beam hardening, and laser hardening).

5.1.4.4 Designer Specification. The designer should specify the following on the drawing. (1) G rade of steel (2) Quench and temper to a hardness range. The hardness range should be a 4 HRC or 40 HB point ra nge. The designer should not specify a tempering temperature range on the drawing. It is best to specify a hardness range and allow the heat trea ter to select the tempering temperature to obtain the specified ha rdness. Specifying both tempering temperatures and hardness ranges on a drawing causes an impractical situat ion for the heat treater. Tempering below 900 _ F(482 _ C ) should be approved by the purchaser.

5.1.4.2 ProcessingConsiderations.  The major factors of the quench and temper process that influence hardness and material strength are: (1) (2) (3) (4)

Ma terial chemistry and hardena bility Quench severity Section size Time at temperat ure

The steel carbon content determines the maximum surface hardness which can be achieved, while the alloy composition determines the hardness gradient which can be achieved through the part . Refer to 4.6 for more information on hardenability.

(3) Any testing required. For example, hardness tests, o r a ny non ---destructive t ests such as magne tic particle inspection or dye penetrant inspection, including the freq uency of testing.

5.1.4.3 Tempering. Tempering lowe rs hard ness and strength, which improves ductility and toughness or impact resistance. The tempering temperature must be caref ully selected based upon the specified hardness range, the quenched ha rdness of the part, and t he material. The optimum tempering temperature is t he highest tempera ture possible while mainta ining the specified hardness range. H ardness after tempering varies inversely with the tempering temperatureused. Partsa re normally air cooled from the tempering temperature.

5.1.4.5 SpecifiedHardness. The specified ha rdness of through hardened gearing is generally measuredon the geartooth end face and rim section. Historically, this has been interpreted to mean that the specified hardness must be met at this locat ion. D esigners often interpret this to mean that minimum hardness is to be obtained at the roots of teeth for gear rating purposes. Since depth of hardening depends upon grade of steel (hardenability), controlling section size (refer to Appendix B ) and heat t reat practice, a chieving specified ha rdness on these surfaces may not necessarily insure hardnessat the roots of teet h. If gea r root hardness is critical to a specific design criteria, the gear tooth root hardness should be specified. H owever, care should be ta ken to a void needlessly increasing material costs by changing to a higher hardena bility steel where service life has been successful.

Tables in the a ppropriat e reference a re available as guidelinesf or the effect of tempering temperature on hardness.

CAUTION:   Some steels can become brittle and unsuitable for service if tempered in the temperature range of 800---1200 _ F (425---650 _ C ). This phenomenon is called “temper brittleness” and is generally considered to be caused by segregation of alloying elements or precipitation of compounds at ferrite and prior austenite grain boundaries. If the part under consideration must be tempered in this range, investigate the specific material’s susceptibility to temper brittleness and proceed accordingly. Molybdenum content of 0.25---0.50 percent ha s been shown to eliminate temper brittleness in most steels. Temper brittleness should not be confused

ANSI/AG MA

5.1.4.6 MaximumControllingSectionSize. The maximum controlling section size is based upon the hardenability of alloy steel for through hardened gear blanks. Appendix B illustrates the controlling section for various gear configurations whose teeth are machined aft er heat treat ment. 5.1.4.7 Additional Information.   For more informat ion, consult t he fo llowing: The ASM Handbook, Volume 4,  H eat Treating , 8th or 9th edition.

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Milita ry specifica tion M IL ---H ---6875 and Mil--STD ---1684.

the gea r element within the heat source (flame or induction coil) which envelopes the entire fa ce width. G earing is removed f rom the heat source a nd immediately hardened by the quenchant. Shafting and gearing can also be progressively spin hardened by spinning the shaft or tooth section within the heat source a nd fo llowing quench hea d. The hea t source and quench head tra verse axially along the length to be hardened.

5.1.5StressRelief. Stressrelief isa therma l cycle used to relieve residual stresses created by prior heat treatments, machining, cold working, welding, or other f abricating techniques. The idea l temperature range for full stress relieving is 1100---1275 _ F (593---691 _ C ). Lower temperatures are sometimes used when 1100 _ F (593 _ C ) temperatures would reduce hardness below t he specified minimum. Lower temperatures with longer holding times are sometimes used.

G earing can also be tooth to too th, progressively hardened by passing the flame or inductor and following quench head between the roots of teeth. Inductor or f lame heads or burner may be designed either to pass in the root diameter between flanks of adjacent teeth, to heat the root diameter and opposite flanks of adjacent teeth, or may fit or encompass the top land to heat the top land and opposite flanks of each tooth.

NOTE: Stress relief below 1100 _ F(593 _ C )reduces the effectiveness. Stress relief below  900 _ F(482 _ C ) is not recommended. 5.1.6 HeavyDraft,ColdDrawn,StressRelieved Steel Bars.  Heavy draft, cold drawn, stress relieved bars may be used as an alternative to quench a nd tempered steel. However, fatigue properties of this steel may not be equivalent to q uench and tempered steel with the same tensile properties. Size limitations a nd mechanical properties are listed in Table 4---5. For f urt her de ta ils see ASTM A---311.

Hea t sources designed to pa ss between a djacent teeth followed by quenching are desirable from bot h endurance or bending strength and wear considerations, because both the flanks of teeth and root diameter are hardened. Only t he non---critical top lands of teeth are not hardened. An inductor or flame head which encompasses only top lands of teeth and adjacent flanks followed by quenching provide wear resistance to the flanks, but endurance or bending strength in the roots is not enhanced. Residual tensile stress in the roo ts of t eeth ma y also prove detrimental. It is, therefore, recommended that both the designer and heat treater know what type of hardening pattern is desired.

5.2 Flameand InductionHardening.  Flame or induction hardening of gearing involveshea ting of gear te et h to 1450---1600 _ F(788---871 _ C ) followed by quench and tempering. An oxyfuel burner is used for flame hardening. An encircling coil or toot h by toot h inductor is used for induction hardening. These processes develop a ha rd wear resistant case on the gear teeth. When only the surface is heated to the required depth, only the surface is hardened during q uenchin g (see Figs 5---1 and 5---2). Ma te ria l selection and heat trea t condition prior to flame or induction ha rdening significant ly affects the ha rdness and uniformity of properties which can be obtained.

G earing may also be tooth to tooth, progressively hardened by passing the inductor between the roots of adjacent teeth, while the gear element issubmerged in a synthetic quench (termed “Delapena Process”). This process, like other to oth to tooth hardening t echniques, is time consuming and is not economical fo r small, finer pitch gearing (finer tha n 10 D P). Spin hardening is more economical for smaller gears.

5.2.1Methodsof FlameandInductionHardening. B oth of these methods of surfa ce hardening can be done by spin hardening, or by tooth to tooth hardening. Spin hardening of gearing involves heat ing all of the teeth across the f ace simulta neously by spinning

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SPIN FLANK FLAME HARDENING

FLAME HEAD

FLAME HEAD

FROM THIS

TO THIS

FLANK FLAME HARDENING

FLAME HEAD

FLAME HEAD

FROM THIS

TO THIS

FLANK AND ROOT FLAME HARDENING

FLAME HEAD

FLAME HEAD

FROM THIS

TO THIS

FLAME HEAD

FLAME HEAD

FROM THIS

TO THIS

THE HARDENING PATTERNS S HOWN ARE NOT P OS S IBLE FO R ALL SIZES AND DIAMETRAL P ITC HES OF G EARING, AND ARE DE P ENDENT UP ON THE C APACITY OF THE EQ UIPMENT.

Fig5---1 Variationin HardeningPatternsObtainableon Gear Teethby FlameHardening ANSI/AG MA

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SPIN HARDENING

INDU C TION C OIL OR FLAME HEAD

INDU C TION C OIL OR FLAME HEAD

FLANK HARDENING

INDUC TOR OR FLAME HEAD

INDUC TOR OR FLAME HEAD

FLANK AND ROOT HARDENING

INDUCTOR OR FLAME HEAD

Fig5---2Variationsin HardeningPatternsObtainableon Gear Teethby InductionHardening ANSI/AG MA

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Three basic gases are used for flame heating, which include MAPP, acetylene and propane. These gases are ea ch mixed with air in particular rat ios and are burned under pressure to generate the flame which the burner directs on t he work piece.

used in place of more costly nitriding which cannot economically generat e some of the deeper ca ses required. Co ntour induction is preferred over flame when root hardness and closer control of case depth is required. Contour flame hardening of the flanks and roots is not generally available. The general application of flame hardening is to the flanks only, except when spin flame hardening is applied. The spin flame process generally hardens below the roo ts, but ha rdens teeth through the entire cross section, reducing core ductility of t eeth a nd increasing distortion (see Fig 5---2).

Simple torch type flame heads are also used to manually harden t eeth. Since there is no a utomatic controlof thisprocess, high operator skill isrequired. Induction hardening employs a wide variety of inductorsranging from coiled copper tubing to forms machined from solid copper combined with laminated materialsto a chieve the required induced electrical currents.

If high root hardnessis not required, flame hardening ismore available and more economical than induction hardening for herringbone and spiral bevel gearing.

Coarser pitch teeth generally require inductors powered by medium frequency motor generato r sets or solid state units. Finer pitch gearing generally utilizes encircling coils with power provided by high frequency vacuum tube units.

NOTE:  AG MA q uality level will be reduced approximately one level (from the green condition) after f lame or induction hardening unless subsequent finishing is performed.

Wide faced gearing is heated by scanning type equipment while more limited areas can be heated by stationary inductors. Parts are rota ted when encircling coils are used.

5.2.3Material.  A wide variety of materials can be flame or induction ha rdened, including (cast a nd wrought) carbon and alloy steels, martensitic stainless steels, ductile, malleable and gray cast irons. G enerally, steels with carbon content of approximat ely 0.35---0.55 percent ar e suitable f or flame or induction hardening. Alloy steels of 0.5 percent carbon o r higher a re susceptible to cracking. The higher the alloy content with high carbon, the greater the tendency for cracking. Cast irons also have a high tendency fo r cracking.

Induction heating depth and pattern are controlled by frequency, power density, shape of the inductor, workpiece geometry and workpiece a rea being heated. Contour o r profile hardened tooth patterns for 4---12 D .P. gearing can be obta ined by dual freq uency spin coil induction heating using both low (audio) freq uency (AF) of 1---15 kHz a nd higher (ra dio) fre quency (RF ) of approximat ely 350---500 kHz. Initially low audio frequency is used to preheat the root area, followed by high radio frequency to developt he profile heated pattern, followed by quenching.

Selection o f the material condition of the gearing can affect the magnitude and repeatability of flame and induction hardening. Hot rolled material exhibitsmore dimensional change and variation than hot rolled, cold drawn mat erial because of densification from cold working. A quench and tempered ma terial condition or preheat treatment, however, provides the best hardening response and most repeatable distortion.

Quenching after flame or induction heating can be integral with the heat source by use of a separa te following spray, or separate by using an immersion quench tank. Oil, water or polymer solutions can be used, in addition to a ir, depending upon hardena bility of the steel and hardening requirements.

5.2.4PriorHeatTreatment. For more consistent results, it is recommended tha t coarser pitched gears of leaner alloy steels receive a quench a nd t emper pretreatment; for example, 4140 steel with teeth coarser than 3 DP.

5.2.2Application. Flame and induction hardening have been used successfully on most gear types; e.g., spur, helical, bevel, herringbone, etc. These processes are used when gear teeth require high surface hardness, but size or configuration does not lend itself to carburizing and quenching the entire part. These processes may also be used when the maximum contact a nd bending strength achieved by carburizing is not required. These processes are also

ANSI/AG MA

In both carbon and alloy steels, normalized or annealed structures can be hardened. These structures do, ho wever, require longer heating cycles and a more severe quench which increase the chance of

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5.2.6 Process Considerations.   Several areas must be considered when processing. Some of the more critical req uirements a re outlined below.

cracking. The a nnealed structure is the least receptive to flame or induction hardening. Successful induction ha rdening of either gray or ductile cast iron is dependent on the a mount of carbon in t he ma trix. The combined carbon in pea rlite willrea dilyd issolve at the austenitizing temperature. Pearlite microstructures are desirable. Pearlite promoting a lloy a dditions such a s copper, tin, nickel or molybdenum may be necessary to form this microstructure.

5.2.6.1 Repeatability.  Repeatable process control is essential for acceptable results. With induction, thisisusuallynot a problemwith properlymainta ined eq uipment since electrical power chara cteristics, inductor movement and integral quench intensity can be readily controlled. Repea ta biltiy becomes more difficult with flame hardening. Equipment varies from hand held torches to tailor made machine tools with well controlled movement of burner heads. E quipment must be such that heating rates across the burner face are consistent from cycle to cycle. G as pressure and mixing of heating gases must be uniform. B urner head location must be precise from cycle to cycle.

5.2.5HardeningPatterns.  There are two basic methods of flame or induction ha rdening gears, spin hardening and tooth to tooth hardening. See Figs 5---1 a nd 5---2 fo r varia tion s o f these processes a nd the resultant hardening patterns. The hardening patterns shown are not possible for all sizes and diametral pitches. For coarser pitches, requirements should be worked out with the supplier. For induction hardening, the kW or power capacity o f the equipment limits the pattern which can be attained. Roo t flame hardening by the tooth by too th process is difficult and should be specified with care.

5.2.6.2HeatingwithFlameor Induction.  Accurate heating to the proper surface temperature is a critical step. Burner or inductor design, heat input and cycle time must be closely controlled. U nderheating results in less than specified hardness and case depth. Overheating can result in cracking. Flame ha rdening may also cause burning or melting of tooth surfaces.

The induction coil method is generally limited to gears of approximately 5 DP and finer. The maximum diameter and fa ce width of gears capable of being single shot induction coil hardened is determined by the area of the outside diameter and the kW capacity of the eq uipment. Long slender parts can be induction hardened with lower kW capacity equipment by having the coils scan the length of the part while the part is rotating in the coil.

5.2.6.3 Quenching.   Heat must be removed quickly and uniformly to obtain desired hardness. The quenchant should produce acceptable as quenched hardness, yet minimize cracking. Quenchants used are: water, soluble oil, polymer, oil and air. Parts heated in an induction coil are usually quenched in an integral quench ring or in an agitated quench media. When the part is scanned while rotat ing in a coil, a spray quench usually follows behind the coil.

Flank or root and flank induction scan hardening (contour) can be a pplied to a lmost any to oth size with appropriate supporting equipment and kW capacity. However, for pitches of approximately 16 D P and finer, these methodsa re not recommended. Spin hardening in an induction coilis recommended. Spin hardening of finer pitchesis also required when using flame burners.

Flank hardened teeth usually have an integral quench following the inductor, or the gear is submerged in liquid during heat ing. Quench time and temperature are critical and in---spray q uenching, pre ssure velocity an d d irection of the quench media must be considered. When localized or air quenching is used, a coolant is used on a portion of the metal away from the heating zone to maintain the base metal near ambient temperature so the part mass cana bsorb heat fromthe heated zone.

The allowable durability and root strength rating for the different hardening patterns should be obtained from a ppropriate AG MA ra ting practices. These bending strength ratings are lower at t he roots of t eeth when only the tooth flanks are hardened.

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5.2.6.4Tempering. Tempering is manda tory only when specified. However, for particular processes, judgment should be exercised befo re omitting tempering. I t is good practice to temper af ter quenching to increase to ughness and reduce residual stress and crack susceptibility. Tempering should be fo r a sufficient time to insure that hardened teeth reach the specified tempering temperature. Flame hardened parts which are air quenched are self tempered, and separate tempering is unnecessary.

depth does not apply. When root is also to be hardened, depth of case at the root may be specified.

5.2.7 Rating Considerations.   Designers should be aware t hat AG MA decreases load ra tings for gears which do not ha ve hardened roo ts. AG MA gear rating standards should be consulted for appropriate stress numbers. 5.2.7.1HeatAffectedZone.  In flame hardening, the heat affected zone (HAZ) isa region that is heatedto 1300---1400 _ F, (704 _ C ---760 _ C ) but does not get hardened and thus has lower strength. This zone should be located either a minimum of 1/8inch up the flank from the critical root fillet or well below the root diameter.

5.2.6.5 SurfaceHardness.   Surface hardness is the hardness measured on the immediate surface and is primarily a function of the carbon content (see Fig 5---3). H a rdness may be lower as a result of prior heat treatment, alloy content, depth of hardening, heat ing time, mass and q uenching considerat ions.

Contour induction hardening results in case depth at the root to be approximately 60 percent of the depth at the pitchline due to mass quench and hardenabiltiy effect. Profile hardening of fine pitched gearing using a submerged q uench decreases the difference between pitchline and root case depth.

5.2.6.6 Effective Case Depth. Effective case depth for flame and induction hardened gears isnormally defined as the dista nce below the surface a t the 0.5 tooth height where hardness drops 10 H RC points below the surface hardness (see Fig 5---3). When a tooth is through hardened, effective case

60 MAXIMUM S URFAC E HARDNE S S

50

!

40

H=  " 0

EFFEC TIVE C AS E DEP TH HARDNES S

30 0.20

0.30

0.40 0.50 0.60 C ARB ON C ONTENT --- P ERC ENT

0.70

0.80

Fig5---3 Recommended MaximumSurfaceHardnessand EffectiveCaseDepth HardnessVersusPercentCarbon for Flameand InductionHardening

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5.2.7.2 Case Depth Evaluation (HardnessPattern). Although it is not always practical, particularly on larger gearing, the only positive way to check case depth is by sectioning an a ctual part. For tooth by tooth hardening, a segment of a gear can be hardened and sectioned. Case depth should be determined on a no rmal tooth section, using an appropriat e superficial o r micro ---hardness t ester. When a gear cannot be sectioned, hardness pattern and depth can be checked by polishing end fa ces of teeth and nitric acid etching. G rit blasting is also occasionally used. Ha rdnessca n also be checked on end fa ces at flank and root areas.

(3) Results of ma gnetic particle inspection, if required.

5.3Carburizing.  G as carburizing consists of heat ing an d holding low carbon alloy steel (0.07---0.28 percent Carbon) at normally 1650---1800 _ F (899---982 _ C ) in a controlled atmosphere which causes additional carbon to diffuse into the steel (typica lly 0.70---1.10 percen t ca rbo n a t t he surf a ce). G ear blanks to be carburized a nd hardened are generally preheated a fter the initial annea l by a subcritical anneal at 1100 _ F ---1250 _ F (590---675 _ C ), normalize, normalize and temper or quench and temper to specified hardness before ca rburize hardening. This is done for machinability, dimensional stability and possible grain refinement considerat ions. An intermediate stress relief before final machiningbefore carburizing may be used to remove residual stress fro m rough machining.

NOTE:   During tooth by tooth induction hardening, power is lowered and travel is sometimes increased as the inductor approaches the end faces. This is to prevent edge burning and cracking. In these instances, hardnessma y be lower at the ends, particularly at the root a rea. In thiscase, existence of a hardness pattern can be demonstrated by a cid etching, but actual depth cannot be accurately measured.

After carburizing for the appropriate time, gearing will usually be cooled to 1475---1550 _ F (802---843 _ C ), held a t temperat ure to stabilize while maintaining the carbon potential, and direct quenched. G earing may be at mosphere cooled af ter carburizing to below approximately 600 _ F (315 _ C ) and then reheated in controlled atmosphere to 1475---1550 _ F (802 ---843 _ C ) and quenched. After quenching, gearing is usually tempered at 300---375 _ F (149 ---191 _ C ). G earing may be subsequently given a refrigeration trea tment to tra nsform retained austenite and retempered.

5.2.8Specifications. The drawing, order, or written specification should include the following information: (1) Chemical analysis range of the material or designation. (2) Prior heat treatment. (3) Ha rdening pattern required. (4) Minimum surface hardness required. (Maximums may be specified f or induction ha rdened parts). (5) Those areas where the surface hardness is to be measured and the frequency of inspection. (6) Depth of hardening required and the location(s) at which the depth is to be obtained. (7) Whether destructive tests a re to be used for determining the depth of hardening and the frequency of such inspection. (8) Tempering tempera ture, if req uired. (9) Ma gnetic particle inspection, if required.

5.3.1 Applications.   Carburized and hardened gearing is used when optimum properties are required. High surface hardness, high case strength, fa vorable compressive residual stress in the hardened case, and suitable core properties based on selection of the appropriate carburizing grade of steel, result in the highest AG MA gear tooth rat ings for contact stress, pitting resistance a nd root strength (bending). Ca rburized gear ratings are higher than the ratings for through hardened and other types of surface hardened gearing because of higher fatigue strength. Improved load distribution can be obta ined by subsequent hard gear finishing. Conventional hard gear finishing (skiving an d grinding) results in some sacrifice of beneficial compressive stress at the surface and substantia lly increases costs.

5.2.9Documentation.   The heat treater should submit the f ollowing information: (1) Surface hardness range obtained a nd the number of pieces inspected.

Ca rburized gearing is used in enclosed gear units for general industrial use, high speed a nd aero space precision gear units and also large open gearing for mill applicat ions. Ca rburized gearing is also used for

(2) D epth of hardening obtained at each location specified when destructive tests are required, and the number of pieces inspected.

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improved wear resista nce. Specified finish operationsa fter hardening depend upon accuracya nd contact requirements for all applications.

carburized helical and spur gea ring to 41/2 D P. The test bar should ha ve minimum dimensions of 5/8 inch (16 mm) diameter by 2 inch (50 mm) long. One inch (25 mm) diameter  ¢ 2.0 inch (50 mm) long ba r may be used for coarser pitch carburized gearing to 1.5 D P. The size of the bar for coa rser than 1.5 D P gearing should be mutually a greed upon, and should approximat e the inscribed diameter at mid height of the toot h cross section. The bar length should be 2---3 time s th e d ia met er.

Carburizing technology is well established and the available equipment and controls make it a reliable process. Surfa ce hardness, case depth, and core hardness can be specified to reasonably close tolerances, a nd the q uality can be audited. Some gearing does not lend itself to carburize hardening because of distortion. G earing which distorts a nd cannot be straightened without cracking, rack gears, thin sections, complex shapes, parts not designed for finishing or where finishing is cost prohibitive, present manufacturing problems. Press quenching af ter ca rburizing can be used t o minimize distortion. Selected areas of gearing can be protected f rom carburizing (masked) to permit ma chining after ha rdening, or can be machined af ter carburizing and slow cooling before ha rdening.

When specified, core ha rdness and core microstructure can be determined at the center of the round bar size shown in Table 5---1 accord ing t o diametral pitch.

Table5---1 Test Bar Sizefor CoreHardness Determination

G ea ring beyond 80 inch (2032 mm) diamet er is difficult to carburize due to the limited number of available f urnaces for processing. M aximum size of carburize gearing is currently in the 120 inch (3048 mm) diameter range. Most of this large gearing req uires too th f inishing (skiving and /or grinding) a fte r carburizing and hardening.

5.3.2Materials. Material selection isa n integral part of the design process. Selection should be made on the basis of mate rial hardness and ha rdenability, chemistry, cleanliness, performance, and economical considerations. Performance criteria include, but are not limited t o, t he fo llowing: toughness, not ch sensitivity, f atigue strength, bending strength, pitting wear resistance, and operational characteristics. Re feren ce should be mad e t o Table 4---1 for a list o f typical carburizing materials a nd Appendix C for case ha rdenability considerations.

BAR SIZE

4 1/2 D P and finer

1.25 inch (32.0 mm) D . ¢   3.0 inch (76 mm) long

2 1/2 D P to less than 4 1/2 D P

2.25 inch (57 mm) D . ¢ 5.0 inch (130 mm) long

1 1/2 D P to less than 2 1/2 D P

3.0 inch (76 mm) D . ¢ 7.0 inch (180 mm) long

1 1/2 D P and coarser

3.5 inch (89 mm) D . ¢ 8.0 inch (205 mm) long

Test discs or plates may a lso be used whose minimum thickness is 70 percent of the appropriate test bar diameter. The minimum inscribed diameter on a test disc (or plate dimensions) should be a minimum of three times its thickness. The recommended test bar diameter for bevel gearing is to be approximately equa l to t he inscribed diameter of the normal tooth thickness at mid fa ce width.

5.3.3Control WithTestBars. Test ba rs ar e used to show that the case properties and, when required, core properties meet specificat ions. Test bars should be of the same steeltype as the gear(s), but not necessarily the same heat. B ars should accompany gearing through a ll heat trea tments, including all post hardening treatments. Consideration should be given to evaluation of tha t portion of the case that is not removed during too th finishing.

When disagreement exists as to the properties obtained on the test bar and the parts, an actual part may be sectioned fo r ana lysis.

5.3.3.1CaseHardness. Ca se hardness should be measured with microhardness testers which produce small shallow impressions, in order that the hardness values obta ined are representa tive of the surfaces or area being tested. Those testers which produce Dia mond Pyramid or Knoop hardness numbers (500 gram load) are recommended. When measuring di-

A section, with a ground and polished surface (normal, a t mid length of a test bar), is considered satisfactory for determining effective case depth of

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rectly on the surface of a case hardened part or test bar, superficial or standard Rockwell A or C scale may be used. Other instruments such as Scleroscope or Equotip are also used when penetration hardness testers can not be used. Consideration must be given to t he case depth relat ive to the depth of t he impression made by the tester.

(0.13 mm) is used. Care should also be exercised in establishing the perpendicular to the mid too th point when starting the traverse. Effective case depth at roo ts ar e typically 50---70percent o f mid toot h height case depths, and tips may be 150percent of mid tooth height case depths.

NOTE: See definition of case depth o f carburized components, Section 3.

Low readingscan be obtained when the indentor penetrates entirely or partially through the case. Microhardness testsf or surface hardness should be made on a mounted and polished cross---section at a depth of 0.002 to 0.004 inch (.05 to . 10 mm) below  the surfa ce. Ca re must be taken during grinding and polishing not t o round the edge being inspected a nd not to temper or burn the ground surface.

When steels of high hardenability such as 4320, 4327, 8627, 4820, 9310, and 3310 are used for fine pitches, the high through hardening characteristics of the steel may prevent obtaining a hardness less than 50 H RC across the tooth section. The case depth should then be determined in the following manner: Measure the base material hardness at mid tooth height at the mid face.For each one HRC point above 45 HRC , one H RC point should be added to the 50 HR C ef fective case depth criterion (example, core hardness equa ls 47 HR C, effective case depth should be measured at 52H RC ). Ca se depth in these instances may also be measured on a test bar, if bar size has been previously correlat ed to the gea r too th section (refer to 5.3.3).

NOTE: D irect surface ha rdness readings (ASTM E18---79) o r f ile checks at the too th tip or flank will generally confirm the case hardness. However, if secondary transformation products are present below the first several thousandths of the case, direct surface checks will not necessarily indicat e t heir presence. Microhardness inspection 0.002 to 0.004 inch (.05 to .10 mm) from the edge on a polished cross section o f t he to oth is more a ccurate. This type of inspection may be necessary for accurate micro ---hardness readings near the surface.

NOTE: Through carburized fine pitch teeth have several disadvantages. Favorable compressive surfa ce stresses are lowered. E xcessive too th distortion a nd a loss of core ductility can a lso o ccur. Part s of this type should be carefully reviewed for case depth specifications and f or use of lower ha rdenability steels such a s 4620 a nd 8620.

5.3.3.2 Core Hardness.   When required, core hardness may be determined by a ny hardness tester, giving consideration to the size of the specimen as discussed in 5.3.3.

5.3.3.4 CaseCarbon Content.   Surface carbon content may be determined from a round test bar by taking turnings to a depth of 0.005 inch (0.13 mm). Spectrographic techniquesha vea lso beendeveloped for this purpose. Carbon gradient can also be determined on the bar by machining chips at 0.002 to 0.010 inch (0.05 to 0.25 mm) increments thro ugh the case, depending on accuracy desired and depth of case. G rinding in steps through the case would be used with spectrographic t echniques.

NOTES:  See definition of core hardness, Section 3. Occasionally banding, which results from the steel melting practice, can cause variationsin coreha rdness during testing with a microhardness tester. These variations should not f all below the minimum, when core hardness is specified.

5.3.3.3 CaseDepth--- Effective. The procedures used to prepare the cross sectioned specimen for case hardness (refer to 5.3.3) should be used to prepare the specimen for case depth evaluation. The microha rdness tra verse should be start ed 0.002 to 0.004 inch (.05t o .10 mm) below the surface and extend to at least 0.01inch (.25mm) beyond the depth a t which 50H RC iso btained. Usually an interval of 0.005inch

ANSI/AG MA

Test specimens should be carburized with the parts. Care should be exercised to maintain surface integrity during cooling or in tempering for subsequent machining. B ar should be straightened to within 0.0015 inch (0.038 mm) (TIR ) bef ore machining. Test specimens must be clean a nd ma chined dry. Ca re must be ta ken to ensure that the turnings are

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free of any extraneous carbonaceous materia ls prior to ana lysis.

continuous at mosphere control is preferred, but other a pproved method s may be used.

5.3.3.5Microstructure. The microstructure may be determined on a central normal section of the test bar or tooth, preferably mounted, a fter being properly polished a nd et ched.

(3) Subzero Treatment (Reta ined Austenite C onversion Trea tment ). When the surfa ce hardness is low due to excessive retained austenite in the case microstructure, it may be necessary to refrigerate the parts to transform the retained austenite to martensite. The refrigeration treatment may vary from 20 _ F (---7 _ C ) to ---120 _ F (---84 _ C ). To minimize microcracking, parts should be tempered before and after refrigeration.

Microstructure will vary with the core hardness as related to steel hardenability, section size and quench severity.

5.3.4 Specifications.   To aid in obtaining the above chara cteristics, the hea t treat er should be given the following as a minimum:

NOTE:  C aution should be exercised in the use of refrigeration treatment on critical gearing. Microcracks can result which can reduce fatigue strength to a moderate degree. Use of refrigeration may require agreement between the customer and supplier.

(1) Material. (2) Ca se dep th r ange (re fe r to Table 5---2). (3) Surface ha rdness range. When additional characteristics are required, the following additional items may be specified in whole or part:

(4) C arbide C ontrol. When high surface carbon results in a heavy continuous carbide netwo rk in the outer portion of the case, parts should be reheated to typically 1650 _ F(900 _ C )in a lower carbon potential at mosphere, typically 0.60 percent carbon, to diff use and break up the excess carbide. Carbide networks should be avoided whenever possible as they tend to reduce fatigue strength of the material.

(1) Core hardness. Approximate minimum tooth core hardness, which can be obtained from some typicalcarburizing gradesof steel and good agita ted oil q uenching, a re sho wn in Table 5---3. (2) Core microstructure. (3) Case microstructure. (4) Surface carbon content. (5) Subzero trea tment. (6) Areas to be free of ca rburizing by appropriat e masking by copper plating or use of commercial stop---of f comp oun ds.

(5) D ecarburization. Surfa ce decarburization as defined for carburized gearing is a reduction in the surface carbon in the outer 0.005 inch (.13 mm) below the specified minimum. This is characterized by an increase in carbon content with increasing depth; for example, when the peak carbon content is subsurface.

5.3.5 Carburizing Process Control.  Precision carburizing requires close control of many factors including:

G ross decarburization can be readily detected microscopically as a lighter shade of martensite and clearly defined ferrite grains. Hardness in this area will be substantially lower.

(1) Temperature C ontrol. Furnace equipment with temperature uniformity, close temperature control, and accuracy of temperature recording and control instruments. Controls should be checked and calibrat ed a t regular intervals.

Partial decarburization will result in a lighter shade of martensite, but may not show discernible ferrite. It will result in reduced hardness if the carbon content fa lls below approximat ely 0.60 percent.

(2) Atmosphere Co ntrol. Furnaces should be capable of maintaining a carburizing atmosphere with controllable carbon potential. Instrumentation for

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Table5---2 Typical EffectiveCaseDepth Specificationsfor CarburizedGearing Normal Diametral 1 Pitch

Normal Too th 2 Thickness

16 14 12 10 8 7 6 5 4 3.5 3.0 2.75 2.5 2.25 2.0 1.75 1.5 1.25 1.0 0.75

0.098 0.112 0.131 0.157 0.198 0.224 0.251 0.314 0.393 0.449 0.523 0.571 0.628 0.698 0.785 0.897 1.047 1.256 1.570 2.094

Range of Normal Diametral Pitch 17.5 --- 13. 7 17.5 --- 13. 7 13.7 --- 10. 5 10. 5 --- 8. 5 8.5 --- 7.5 7.5 --- 6.5 6.5 --- 5.2 5.2 --- 4.3 4.3 --- 3.7 3. 7 --- 3. 1 3. 1 --- 2. 8 2. 8 --- 2. 6 2. 6 --- 2. 3 2. 3 --- 2. 2 2. 2 --- 1. 9 1. 9 --- 1. 6 1. 6 --- 1. 3 1.3 --- 1. 1 1.1 & less 1.1 & less

Range of Normal Circular Pitch 0. 180 --- 0.230 0. 180 --- 2.300 0. 230 --- 0.300 0. 300 --- 0.370 0.370 --- 0.480 0.370 --- 0.480 0.480 --- 0.600 0.600 --- 0.728 0.728 --- 0.860 0. 860 --- 1.028 1. 026 --- 1.200 1. 026 --- 1.200 1. 200 --- 1.400 1. 200 --- 1.400 1. 428 --- 1.676 1. 676 --- 1.976 1. 976 --- 2.400 2. 400 --- 2.828 2.828 & more 2.325 & more

Effective Case Depth (inches) to RC 50 Spur, Helical 6 Bevel &Mitre 0.010 --- 0.020 0.010 --- 0.020 0.015 --- 0.025 0.020 --- 0.030 0.025 --- 0.040 0.025 --- 0.040 0.030 --- 0.050 0.040 --- 0.060 0.050 --- 0.070 0.060 --- 0.080 0.070 --- 0.090 0.070 --- 0.090 0.080 --- 0.105 0.080 --- 0.105 0.090 --- 0.125 0.105 --- 0.140 0.120 --- 0.155 0.145 --- 0.180 0.170 --- 0.205 0.170 --- 0.205

3, ,4 5

Worms with G round 7 Threads 0.020 --- 0. 030 0.020 --- 0. 030 0.025 --- 0. 040 0.035 --- 0. 050 0.040 --- 0.055 0.040 --- 0.055 0.045 --- 0.060 0.045 --- 0.060 0.045 --- 0.060 0.060 --- 0. 075 0.075 --- 0. 090 0.075 --- 0. 090 0.075 --- 0. 090 0.075 --- 0. 090 0.075 --- 0. 090 0.075 --- 0. 090 0.075 --- 0. 090 0.075 --- 0. 090 0.075 --- 0.090 0.075 --- 0.090

1 All case depths are based on normal diametral pitch. All other pitch measurements should be converted before specifying a case depth. 2 G ears with thin top lands may be subject to excessive case depth at t he tips. La nd width should be calculated bef ore a case is specified. 3 Ca se at ro ot is typically 50---70 percent of ca se at mid to oth. 4 The case depth for bevel and mitre gea rs is calculated f rom the thickness of the toot h’s small end. 5 For gearing requiring maximum performance, deta iled studies must be made of the applicat ion, loading and manufacturing procedures to determine the required effective case depth. For further det ails refe r to AG MA 2001---B 88. 6 To convert a bove dat a to metric, multiply values given by 25.4 to det ermine mm equivalent. 7 Worm and ground---thread case depths allow for grinding. U n---ground worm gear cases may be decreased accordingly. For very heavily loaded coarse pitch ground threa d worms, hea vier case depth than shown in table may be required.

5.4 Carbonitriding. The purpose of this Section is to establish method s for specifying carbonitrided gearing. Info rmation in 5.3 on carburizing will generally apply to carbonitriding, with noted exceptions. Typically ca rbonitriding is carried out at lower te mpera tures, 1550---1650 _ F (843 ---899 _ C ), and for shorter times than gas carburizing. Shallower case depths a re generally specified for carbonitriding tha n isusual for production carburizing. Its effect on steel is similar to liquid cyaniding and has replaced cyaniding because of cyanide disposal problems.

ANSI/AG MA

Normally 2.5 to 5 percent anhydrous ammonia is added to the carburizing atmosphere when carbonitriding. Specified case depths are usually from 0.003 to 0.030 inch (0.076 to 0.76 mm) maximum.

5.4.1 Applications (Advantages and Limitations). Use of carbonitriding is more restricted than carburizing. It is limited to shallower cases for finer pitch gearing since the process must be conducted at lower temperatures than carburizing. D eep case depths require prohibitive time cycles. One o f the

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advantages of carbonitriding is better case hardenability in lower alloy or plain carbon steels. The carbonitrided case has better wear and temper resista nce than a straight carburized case. Ca rbonitriding can be used to minimize distortion in finer pitch gearing because lower austenitizing and quenching temperatures can be used along with less severe quench techniques and still achieve hardness. These fa cts, along with lower a lloy steels, result in the lower core hardness mentioned previously, thus reducing toot h growth and distortion. H owever, if higher core hardness and deeper case depths are required for bending resistance, carbonitriding may no t be a pplicable.

spection of nitrided gearing. This section covers the selection and processing of mat erials, hardnesses obtainable, and definitions and inspection of depth of hardening. Conventional gas nitride hardening of gearing, which has had a quench and temper pretreatment and is usually finish machined, involves heating a nd holding at a temperature between 950---1060 _ F (510---571 _ C ) in a controlled cracked ammonia atmosphere (10 to 30 percent d issociat ion). Nitride hardening can also be achieved with the ion nitriding process. During nitriding, nitrogen atoms are absorbed into the surface to formha rd iron and alloy nitrides. The practical limit on case depth is about 0.040inch (1.0 mm)ma ximum, which requires a thorough stress analysis (for other than wear applications) of the effectiveness of the case for coarse pitch gearing.

Table5---3 ApproximateMinimumCoreHardnessof  Carburized Gear Teeth Grade

Ha rdness HRC Minimum Pitch 2---3

3316 9315 3310 9310 4820 8822 4320 8620 4620 1020

34 32 31 28 27 25 23 18 -----

1

4

5---6

7 &UP

36 34 33 31 33 30 27 24 18 14

37 36 35 33 35 32 30 26 22 16

38 37 36 34 36 34 33 28 25 18

NOTE:  The above processes (5.4 and 5.5) should not be confused with aerated salt bath nitriding or nitrocarburizing in which nitrogen is absorbed into the steel surface at approximat ely 1060 _ F(570 _ C ) f or sh ort cycles of 2.5 to 4.0 hours in an aerated salt bath or atmosphere. These processes result in a wear resistant surface layer of 0.001inch (0.025 mm) or less, with a nitrogen compoun d la yer t o a dep th of 0.015---0.020 inch (0.38---0.50 mm) which enha nces fa tigue strength.

1 D epending upon the Jominy curve of the particular materia l, maximum ha rdness will typica lly be 8---10 poin ts high er t ha n th e minimums listed. U se of H band steel is the normal method of hardenability control.

5.5.1 Applications.   Nitrided gears are used when gear geometry and tolerances do not lend themselves to other case hardening methodsbecause of distortion, and when through hardened gears do not provide sufficient wear and pitting resistance. Nitrided gears are used on applications where thin, high hardness cases can withstand applied loads. Nitrided gea rs should not be specified if shock loading is present, due to inherent brittleness of t he case.

5.4.2 Materials.  Typically ca rbon a nd low a lloy steels such as 1018, 1022, 1117, 4022, 4118 and 8620 steels are used for carbonitriding. 5.4.3 SpecificationandInspection.  C ase depth, microstructure, hardness, etc. for carbonitrided parts can all be specified and evaluated as prescribed in the section for carburized gearing. Ca se depth is specified and measured as effective or total, depending upon application. Cases shallower than 0.010 inch (0.25 mm) are generally specified a s to tal case depth. The advantages and limitations as described herein should be fully understood before specifying carbonitriding for industrial gearing.

5.5.2 Materials.   Steels containing chromium, vanadium, aluminum, and molybdenum, either singularly or in combination, a re required in order to form stable nitrides at the nitriding temperature. Typical steels suita ble for nitriding are 4140, 4150, 4340, the Nitralloy grad es, an d steels with chromium contents of 1.00 to 3.00 percent. Aluminum containing grades such as Nitralloy 135 and Nitralloy N will develop higher case hardness.

5.5 Nitriding.  The purpose of this section is to provide information, means of specifying, and in-

ANSI/AG MA

5.5.3Pre---treatments. Part s to be nitrided must be quenched and tempered to produce the essential-

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ly tempered mart ensitic microstructure required fo r case diffusion. Microstructure must be free of primary ferrite, such as is produced by a nnealing a nd normalizing, which produces a brittle case prone to spalling. The nitriding process will cause a slight uniform increase in size. However, residual stresses from quench and tempering may be relieved at the nitriding temperature, causing distortion. This should be avoided by tempering at approximately 50 _ F (28 _ C ) minimum above the intended nitrided temperature af ter quenching. In order to minimize distortion of certain gearing designs, intermediate stress relieving a ft er rough machining at 25---50 _ F (14---28 _ C ) below the tempering temperature may also be req uired prior to finish machining to relieve machining stresses before nitriding.

part by dimensional analyses both prior to and after nitriding.

5.5.4NitridingProcessProcedures. Variablesin the nitriding processa re the combined effects of surface condition, degree of ammonia dissociation, temperature, a nd time of nitriding. Nitrogen a dsorption in the steel surface is aff ected by oxide a nd surface contamination. In order t o guarantee nitrogen adsorption it may be necessary to remove surface oxidation by chemical or mechanical mea ns. The nitriding process aff ects the rate of nitrogen adsorption and the thickness of the resultant brittle white layer on the surface. A two stage nitriding process (two temperat ures with increased percent of ammonia dissociation at the second higher temperature) generally reduces the thickness of the white layer to 0.0005---0.001inch (0.013---0.026 mm) ma ximum. The wh ite laye r t hickness is also dependent upon the ana lysis of steel.

In a lloys such a s series 4140 a nd 4340 steels, nitrided hardness is lessened appreciably by decreased core hardness prior to nitriding. Thismust be considered when selecting tempering or stress relieving temperatures.

The ion nitride process usesionized nitrogen ga s to effect nitrogen penetration of the surface by ion bombardment. The process can provide flexibility in determining the type of compound produced. The process can also be tailored to better control nitriding of geometric problems, such as blind holes and small orifices.

If distortion control is verycritical, the newer ion nitriding process should be considered. Nitriding can be a ccomplished at lower temperatures with ion nitriding than those used for conventional gas nitriding.

5.5.5 SpecificCharacteristicsof NitridedGearing. Nitriding does not lend itself to every gear application. The nitride processis restricted by and specified by case depth, surface ha rdness, core hardness and materia l selection constraints.

Nitridingover decarburized steel causes a brittle case which may spallunder load. Therefore, nitrided surfaces subject to stress should be free of decarburization. Sharp corners or edges become brittle when nitrided and should be removed to prevent possible chipping during handling and service.

5.5.5.1 Material Selection.   Selection of the grade of steel is limited to those alloys that contain metal elements that form ha rd nitrides as discussed in 5.5.2.

Where it is desired t o selectively nitride a part, the surfaces to be protected from nitriding can be plated with dense copper 0.0007 inch (0.018 mm) minimum thickness, tin plate 0.0003 to 0.005 inch (0.008 to 0.13 mm) thick, or by coating with proprieta ry pa ints specifically designed fo r t his purpose.

5.5.5.2 CoreHardness.  C ore hardness obtained in the quench and temper pretreatment must provide sufficient strength to support the case under load and tooth bending and rim stresses. Core hardness requirements limit material selection to those steels that can be tempered to the core hardness range with a tempering temperature that is at least 50 _ F (28 _ C ) above the nitriding temperature. Approximate core ha rdness

Nitrided parts will distort in a consistent manner when allmanufacturing phasesand t he nitriding process are held constant. The amount a nd direction of growth or movement should be determined for ea ch

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obtained on typical nitrided steels are as follows: Steel Type

most specifications only specify a minimum case depth requirement.

Minimum Surface H a rdness, H RC

4140 4150 4340 Nitralloy 135

Case depth should be determined using a microhardness tester. At least t hree ha rdness tests should be made beyond the depth a t which core ha rdness is obtained to assure that the case depth has been reached.

28 30 32 34

A test bar, for example 1/2 to 1 inch (13 to 25 mm)diameter with a length 3 ¢ the diameter, disc or plate section, can be used for determining case depth of nitrided parts.The test section must be of the same specified chemical analysis range and must be processed in the same manner as the parts it represents.

5.5.5.3 SurfaceHardness. Surface hardness is limited by the concentration of ha rd nitride forming elements in the alloy and the core hardness of the gear. Lower coreha rdness does not support the hard, thin case as well as higher core hardness. Lower core hardness will result from less alloy, larger section size, reduced quench severity and a greater degree of martensite tempering. Lower core hardness results in a microstructure which causes a lower surface hardness nitrided case, since it limits the ability to form high concentration of hard metallic nitrides. Surface hardness will also increase with increasing nitride case depth.

Sectioning of an actual part to determine case depth need only be performed when theresults of the test bar are cause for rejection, or the surface hardness of the part(s) is not within 3 HRC points of the surface hardness of the test bar.

5.5.6 Specifications.  Parts which are to be nitrided should have t he f ollowing specified:

Approximate minimum surface hardness which can be obta ined on nitrided steel is shown in Table 5---4.

(1) (2) (3) (4) (5)

Material grade Prehea t t reat ment (see 5.5.5.2) Minimum surface ha rdness Minimum total case depth Ma ximum thickness of white layer, if required (6) Areas to be protected from nitriding by masking, if required (7) Nitriding temperature (8) Meta llurgical test coupons

Table5---4 ApproximateMinimumSurfaceHardness --- Nitrided Steels St eel Type 4140 4150 4340

Minimum Surface Hardness R 15N H RC ! 85 48 85 48 84 46

Nitralloy (contains Al)

90

60

2 1/2 percent C hrome (EN 40B & 40C a nd 31CrMoV9) @

89

58

5. 6 Other Heat Treatments.   G earing may also be heat treated by other means, including laser heat treating and electron beam heat treating. B oth laser and electron beam surface hardening of gea rs are selective in nature a nd are genera lly applied to gears smaller than those routinely hardened by other methods. The production quantity of any gear must be sufficient to justify the cost of capital equipment and set---up to surface hardened by either process, such as qua ntity production for the automotive industry. These processes are not available from commercial heat treaters. Thermal energy for heating the surface to the austenitizing temperature is supplied by either the laser (light amplification by stimulated emission o f ra diation) o r electron (kinetic energy of electrons) beam, while the underlying mass provides the heat sink to quench harden the surface. U se of electron beam heat treat ing for gear

1 Converted to H RC 2 British and G erman a nalyses, respectively NOTE: D ata infers a 269HB minimum core hardness.

5.5.5.4CaseDepth. The specified case dept h for nitrided gearing is determined by the surface and sub---surface stress gradient of the design application. Surface hardness and core hardness will influence the design’s minimum required case depth. Since the diffusion of nitrogen is extremely slow,

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teeth is restricted, however, to full gear tooth contours, and is better suited for flat than curved surfa ces. This is true because the stream o f electrons must have line of sight access to the surface to be hardened with a beam impingement angle of at least 25 degree (25---90 degrees impingement angle range). D ual laser beam optics have been developed, however, for flank and root contour surface hardening of gear teeth.

por bubbles and restrict the flow of quenchant should be avoided. There are a varietyo f quenchants to choosef rom such as: oil, polymer, molten salt, wat er, brine and gases. Each variety is available with a wide range of q uench chara cteristics. Table 5---5 associat es some material grades and their normally used quenchants. Agitation is externally produced movement of the quenchant past the part. The degree and uniformityof a gitation greatly influences its rate of heat removal. Agitation can be provided by propellers or pumps in the quench tank or by moving the parts through the quenchant.

Reference should be made to the   ASM M etals  Handbook, 9th Ed ition, Volume 4 on H eat Treating for additional information on laser a nd electron beam hea t trea ting, as well as other modifications of heat treatments applied to gearing.

The temperature of the quenchant may af fect its ability to extract heat. Ea ch quenchant should be used within its appropriate range of temperature. The temperature of a water quenchant is more critical than that of an oil.

5.7Quenching.   Quenching is the rapid cooling of steel from a suitable elevated temperature. The quenching process is one of the major operations that influences the microstructure, hardness, mechanical properties a nd residual stress distribution, assuming the gear has been properly heated before the quench. The preferred microstructure after quenching is primarily martensite.

5.8 Distortion.   Distortion of gearing during heat trea tment is inevitable and varies with the hardening process. The part design and manufacturing process must consider movement during heat treat ment. Tolerancing must consider these changes. Section size modification may be required along with added stock for grinding or machining after heat treatment.

The designer’s or heat treater’s responsibility is to select the q uench variables to obta in the required properties in the gear. The quench needs to be fast enough to avoid secondary transforma tion products, but slow enough to reduce distortion and avoid cracking. The materia l ha rdenability will determine how severe the q uench has to be f or a pa rticular part geometry.

5.8.1 Causes. D imensional changes of gearing resulting from heat treat ment occur principally when steel is q uenched. These changes o ccur in both quenched and tempered and surface hardened gears. D istortion is due to mechanical and thermal stresses and pha se transformation. P rocess varia bles and design considerat ions have a significant effect upon the amount o f distortion. High induced stress can result in quench cracking. Thermal processes such as annealing, normalizing, and diffusion controlled surface hardening processes such as nitriding, which do not require liquid quenching, result in less distortions tha n processes that require liquid q uenching.

Quench cracks usually originate at sharp corners or substant ial section size changes. Ho wever, even with perfectlyunifo rm sections, parts can easily crack if mad e of high---carbo n, high ---ha rdena bility stee ls and the quench is too severe. D elayed quench cracks can occur hours or days af ter quenching, especially if improperly tempered or stress relieved. It is good practice to immediately temper after quenching if quench crack problems are a concern.

5.8.2QuenchingandTempering. Quenched and tempered gearing changes size and distorts due to mechanical and thermal stresses and microstructural transformations. Quenchingt he structure to martensite prior to tempering results in steelgrowing in size. Tempering of the hardened structure reduces the volume, but the combined effects of quenching and tempering still result in a volume and size increase.

The main fa ctors which control the q uench rate are: part geometry, type of quenchant, degree of agitation and quench temperature. The geometry will af fect how q uickly and unifo rmly the q uenchant willcirculat e around the part. Pockets which trap va-

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Table5---5 CommonlyUsedQuenchantsfor FerrousGear Materials Material G rade

Quenchant

Rema rks

1020

Wa ter o r B r ine

C a rburized a nd q uenched wit h go od q uench a git at io n.

4118 4620 8620 8822 4320

Oil

C arburized and q uenched in well agitated conventional oil at 80---160 _ F(27---71 _ C ) is normally required. For finer pitched gearing, hot oil at 275---375 _ F(135---190 _ C ) may be used to minimize distortion. Some loss in core hardness will also result from hot oil quench.

3310

Oil

C arburized and q uenched in hot oil at 275---375 _ F (135---190 _ C ). This is the preferred quench. In larger sections, conventional oil can be used.

1045 4130 8630

Wa t er, O il o r Polymer

Type o f q ue nch ant d epe nd s upo n ch emist ry a nd se ct io n size. L arge sections norma lly req uire wa ter or low   concentration polymer. Smaller sections can be processed in well agitat ed o il.

1141 1541

O il o r P olymer

G o o d respo nse in well a git at ed co nvent io na l o il o r polymer. I nduction or flame hardened pa rts normally quenched in polymer.

4140 4142 4145

O il o r P olyme r

S ame a s a bo ve ; ho we ve r, t hin se ct io ns o r sha rp co rne rs can represent a crack hazard. H ot oil should be considered in these cases. With proper equipment, air quench can be used for flame hardened parts.

9310

These are high hardena bility steels which can be crack sensitive in moderate to thin sections. Hot oil is often used. High concentrat ion polymer should be used with caution. 4150 4340 4345 4350

O il o r P olyme r

Gray or D uctile Iron

O il, P olymer or Air

I f co nve nt io na l o il is use d, pa rt s a re o ft en r emo ve d w ar m and tempered promptly after quench. C rack sensitivity a pplies a lso to flame or induction hardened parts with high concentration polymer being the usual quenchant. Oil is sometimes used and air quench can be applied for flame hardening with proper equipment. Q uench med ia depend s upo n a llo y co nt ent . H igh a llo y irons ca n be a ir q uenched to modera te ha rdness levels. U nalloyed or low a lloy irons require oil or polymer. In this section parts and flame or induction hardened surfaces can be crack sensitive.

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5.8.3.1 CarburizedGearing. D istortion of carburized gearing makes it one of the least repeatable of surface hardened processes. Lackof repeatability is due to the greater number of varia bles which af fect distortion. Close control is, therefore, required. D istortion results from microstructural tra nsformation, and residual stress (from t hermal shock, uneven cooling, etc.) considerations. Transformat ion in the case results in growth which sets up residual surface compressive stress. This stress is balanced by corresponding residual tensile stress beneat h t he case.

Distortion of quenched and tempered gearing occurs generally as follows: (1) G ears (a) Outside and bore diameters grow larger and go out of round. (b) Side faces become warped, a nd exhibit runout. (2) Pinions. Pinions become bowed, with the a mount o f bow ing increasing with higher length/diameter ratios and smaller journal diameters; amount of bowing or ra dial runout is often confined to journal diameters and shaft extensions for integral shaft pinions.

Principal variables affecting the amount of growth, distortion, a nd residual stress include: (1) G eometry.

Normally, ro ugh gear blanks (forging, ba rstock, or ca sting) have sufficient stock provided so distortion can be a ccommodat ed by machining. High L/D ratio pinions may require straightening and a thermal stress relief prior to finish machining. In some exceptional instances, straightening, thermal stress relief, rough machining, and a second stress relief prior to finish machining may all be necessary to keep the pinion dimensionally stable during finish machining. Sequence of manufacture is dependent upon design considerations a nd the temperature used for stress relief. Stress relief temperature is dependent upon specified hardness and temper resistance of the steel.

(2) Hardenability (carbon and alloy content) of the base material. Higher hardenability increases growth and distortion. (3) Fixturing techniques in the furnace and during q uenching. (4) Carbon potential of the carburizing atmosphere. (5) Ca rburizing temperature and temperature prior to quenching. (6) Time between quench and temper for richer alloys. (7) Quenchant type, temperature and a mount of agitation.

Modified methods of q uench hardening, such as austempering of ductile iron, reduces distortion a nd forms a modified hardened structure at higher quenchant temperatures than those conventionally used (refer to 4.8.4.3).

(8) Resulta nt metallurgical characteristics of the case, such as carbon content, case depth, amount of retained austenite, carbides, etc.

NOTE: D irect quenching generally results in less distortion than slow cooled, reheated and quenched gea rs, providing gears are properly cooled from the carburizing temperature to the quench temperature before hardening.

5.8.3 Surface Hardened Gearing.   Distortion must be minimized, controlled and made predictable to minimize costly stock removal (lapping, skiving, or grinding), when tooth accuracy requirements dictate.

Once a component is designed t o minimize distortion, processing techniques should be optimized to make distortion consistent. At times, redesign of components may be required to reduce distortion.

Selective surface hardening of gear teeth by flame a nd induction ha rdening results essentially in onlydistortion of the teeth because only the teeth are heated and quenched. Amount of distortion increases with case pattern depth and increases as more of the tooth cross section is hardened, compared to profile hardened tooth patterns. Distortion is not limited to gear teeth, however, when the entire gear ishea ted and quenched as with carburizing.

ANSI/AG MA

Stock removal by grinding after carburize hardening should be limited to approximately 0.007 inch (0.18mm) per tooth surface or 20 percent of t he case depth, whichever is less. Exception may be made for coa rser pitch gea ring with cases 0.080 inches (2 mm) or greater. Surfaces other than the tooth flanks and roots may tolerate greater stock removal.

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G eneral design considerations of carburized gearing related to distortion include the following (re fe r t o F ig 5---4):

D istortion of carburized gearing also exhibits the following t ypical cha ra cteristics (refer to Fig 5 ---5): (1) Reduction in tooth helix angle (“helix unwind” ), which often requires an increased helixa ngle to be machined into the element prior to carburizing (more prevalent in pinions). Teeth o n larger dia meter, smaller face width gears may exhibit “ helix wind ---up” a fte r ha rdening.

(1) Larger teeth (lower D P) distort more. (2) R im thicknessshould be the same at both end faces. (3) Radial web support section under the rim should be centrally located. Web support section thickness under the rim is recommended to be not less than 40---50 percent of the fa ce width fo r precision gears. Near solid “ pancake” gear blanks, designed with modera te recess on both sideso f the web section, distort less. The recess is provided to ena ble clean ---up grinding of the rim and hub end faces af ter hardening.

(2) End growth on gear teeth at both ends of the face due to increased case depth (carburizing from two directions, 90 degrees apart, followed by improved quench action for the same reason) may appear as reverse tooth crowning on narrow face gearing. Teeth a re oft en crown cut prior to ha rdening to compensate for reverse crown or are chamfered a t the ends of teeth. Teeth ma y also be both crown cut and chamfered.

(4) Ho les in the web section close to t he rim, to reduce the weight or provide holes for lifting, may cause collapsing of the rim section o ver the ho les.

(3) Eccentricity (rad ial run ---out) of gears a nd their bores is dependent upon ho w they a re fixtured in the furnace.

(5) H igh length/diameter ratio pinions distort more. Jo urnals may be req uired to be ma sked in order to prevent carburizing and then be finish machined after hardening with sufficient stock for clean ---up. Masking can also be used for ease of straightening.

(4) Taper a cross the fa ce (tapered t eeth), bore ta per a nd “ hour---glassing” of t he gea r bore can occur due t o non ---uniform growth of teeth across the fa ce and non ---unifo rm shrinking of th e bores.

(6) Ca ntilever pinions, with teeth on the end o f the shaft, a nd “ blind ended” teeth on pinions, where the adjacent diameter is larger than the root diameter, present problems from both distortion and finishing standpoints.

(5) B owing of the integral shaft pinions. Integral shaft pinions should, whenever possible, be hung or f ixtured in the vertical position (a xes vertical) to minimize bowing.

C ANTILE VE R P INIO N

B LIND E ND E D TE ETH

HIG H L/D RATIO CONCENTRIC BLANKS

Fig5---4General Design Guidelinesfor Blanksfor Carburized Gearing

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S TRAIG HT HELICAL UNWIND

TAPER

HOURG LAS S ING

B OWING

END G ROWTH (REVERSE CROWN)

EC C ENTRIC ITY

Fig5---5 Typical DistortionCharacteristicsof Carburized Gearing G ears may be fixtured vertically through the bores or web holes on a support rod (axes horizonta l), or fixtured horizontally (individually or stacked) to minimize distortion, depending on size and face width. Larger ring gears are positioned horizontally with sufficient stockf or clean ---up of the teeth. B ores and web sections can be masked to prevent carburizing, and enable subsequent machining.

(2) Increased growth of t he teeth (greater than for carburized gearing) because the entire tooth crosssection may be hardened in finer pitch gearing. (3) Cro wning or reverse crowning of the t eeth across the face dependent upon the heat pattern. Crowning is more desirable f rom a tooth loading standpoint. (4) Taper of teet h due to varied heat pa ttern and case depth across the face.

Thin section gea rs, such as bevel ring gears, ma y be press quenched to minimize distortion.

5.8.3.2FlameandInductionHardenedGearing. Flame and induction hardened gearing generallydistort less than carburized gearing because only the teeth a re heated and subsequently quenched. Contour induction hardening of tooth profiles produce less distortion and growth tha n spin hardening methods.

D istortion of the teeth from spin induction hardening is often considered more repeata ble than with spin flame ha rdening, because of fewer human error fa ctors involved during ma chine a nd ind uctor set --ups with induction hardening. Spin flame hardening involves more ma nua l set ---up fa ctors, which include positioning of the flame, gas flows, etc. However, spin flame hardening can be engineered with special flame heads and fixtures for required control.

D uring both spin flame and spin induction ha rdening, the entire tooth cross section is often hardened to t he specified depth below the roots of te eth.

CAUTION: D eep spin hardening of gear teeth may cause excessive tooth growth and may affect bore size.

For high bending strength applicat ions, it is not desirable to ha ve the hardening pattern terminat e in the roots of the teeth because of residual tensile stress considerations. D istortion increases as a greater cross---section of a t ooth is hardened. Spin flame and spin induction hardening generally produce the following distortion chara cteristics:

5.8.3.3 Nitrided Gearing.  Nitriding of gearing results in less distortion, compared to carburize, flame, and induction hardening. Prior quench and temper heat trea tment, which results in distortion, is done before machining and nitriding. Parts are also not heated above the transformation temperature or previous tempering temperat ure of the steel during nitriding, and are not quenched, as occursduringcarburizing, flame or induction hardening. Therefore, nitrided gear teeth are not generally required to be

(1) Helical unwinding of the gear teeth, as with carburized pinions.

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5.9.2 ProcessControl.  Because it is difficult to directly measure the effects of shot peening on a part, a high degree of process control is essential to assure repeata bility.

ground or lapped after hardening to meet dimensional tolerance requirements. Bea ring diameters of shaft extensions are often ground af ter nitriding with only minimum stock provided. Surfa ces can also be masked for subsequent machining.

5.9.2.1 IntensityControl.  I ntensity refers to the kinetic energy with which the peening media strikes the part. Thisenergy controls the depth of the peening effect. It is measured by shot peening a flat, hardened steel strip called an Almen Strip, in the same manner a s the pa rt will be peened. The strip is held flat o n an Almen block placed in the representative location during the peening operation. When released from the block, the strip will bow convexly on the peened surface. The amount of bow is measured in incheswith a ga uge and iscalled the arc height (see Fig 5---6). There are three classifica tions of Almen Strips, N, A, and C, which have thicknesses of 0.031 inch (0.8 mm), 0.051 inch (1.3 mm) and 0.0938 inch(2.4 mm) respectively. St rips a re SAE 1070 cold rolled spring steel, ha rdened an d te mpered t o 40---50 H R C . F la t ne ss t ole ra nce is +---0.0015 inch ( +---0.04mm). Figure 5 ---6 also shows t he dime nsions for the Almen strips and holding fixture. An intensity determination must be made at the beginning, at intervals of no more than four hours and at the end of each production run.

When close tolerances are required, gea ring can be r ough ma ch in ed a nd st re ss r elie ve d a t 50 _ F(28 _ C ) below the prior t empering temperature to relieve rough machining residual stress prior to finish machining a nd nitriding. During nitriding, outer surfaces grow approxima te ly 0.0005---0.001 inch (0.013---0.025 mm). B or es size may shrink up to 0.0015 inch (0.04 mm) depending upon size.

5.9 Shot Peening.   Shot peening is a cold working process performed by bombarding the surface of a part with small spherical media which results in a thin layer o f high magnitude residual compressive stress at the surface. This stress may improve the bending fatigue strength of a gear tooth as much as 25 percent. It is becoming an accepted practice to specify shot peening on carburized and other heat treated gears. B ecause the process increases bending fa tigue strength, it may be used either to salvage or upgrade a gear design. Conta ct fat igue strength may a lso be improved in some instances by shot peening, but quantitative data to substantiate this condition is limited. Shot peening should not be confused with grit and shot blasting, which a re cleaning operations.

Whenever a processing procedure is developed for a new part, an intensity curve must be developed which esta blishes the time req uired to rea ch peening saturat ion of the Almen strip. This is accomplished by shot peening several strips at various times of exposure to the shot stream and plotting the resulting arc heights. Saturation is defined as that point at which doubling the time of exposure will result in no more tha n a 10 percent increase in a rc height.

5.9.1 Equipment. Machinery used for shot peening should be auto matic and provide means for propelling shot by air pressure or centrifugal force against the work. Mechanical means for moving the work through the shot stream by either translation or rotation, or both, should be provided. Machinery must be capa ble of consistently reproducing the shot peening intensity and coverage required.

5.9.2.2 ShotControl. Shot size and shape must be caref ully controlled during the shot peening process, to minimize the number of fra gmented particles caused by fracturing of the shot. These fragmented particles can cause surface d ama ge. Also, a s a result of lower mass, fragmented shot particles will lengthen the time to reach a specifiedpeeningintensity. Periodic inspection of the shot is required to control shot size and shape within specification limits. When these limits ar e reached , the shot should be classified and separated to restore size and shape integrity as sho wn in M IL ---S ---13165B. 5.9.2.3 Coverage Control.   Coverage refers to the percentage of indentation tha t occurs on the surfa ce of the part. O ne hundred percent coverage isde-

Regardless of the type of equipment used, the gear must be rota ted on its axis while exposed to the shot stream. For optimization of shot peening of gears, nozzle type equipment is generally preferred because of the ability to vary the angle of shot impingement and, therefore, achieve more uniform intensity along the too thform. This type of equipment is generally used for high performance gearing, although centrifugal wheel equipment is of ten used f or very high volume production.

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fined as uniform dimpling of the original part surface as determined by either visual examination using a 10X magnifying glass or by using a fluorescent tracer dye in a scanning process. In t he latter process, full coverage has beena chieved when no tracesof the dye remain when viewed under ultraviolet light. A minimum of 100 percent coverage is required on any shot peened part.

quired to obt ain multiples of 100 percent coverage is tha t multiple timest he time to reach 100 percent coverage (200 percent, 300 percent, etc.).

5.9.3Design Consideration.  The following sections describe items tha t the d esigner should include in a shot peening specification. 5.9.3.1GoverningProcess Specification.Acommonly referenced shot peening specification is MI L ---S ---13165B which id ent ifie s ma te ria ls, eq uipment requirements, procedures, and quality control requirements for effective shot peening. The SAE Manual on Shot Peening , SAE ---J 808a ---SAE H S84, may a lso be used.

Co verage must be related to t he part, not the Almen strip. The actual part must be examined for complete coverage in all areas specified to be shot peened. The peening time required to obtain 100 percent coverage should be recorded. The time re-

3.0 +---0.015 in (76+--- 0.4mm)

+--0.031 +--0.001 in (0.79 0.02mm) +--0.051 +--0.001 in (1.30 0.02mm) -

N S TRIP

+--0.0938 +-0.001 in (2.38 0.02mm) --

A S TRIP

PEENING NOZZLE

C S TRIP

0.745 to 0.750 in (18.9 to 19.0 mm)

ALMEN S TRIPS

SHO TS TREAM MEAS URING D IAL 4 to 6 in (102 to 152 mm)

10 ---3 2 SCREWS ALMEN TES TS TRIP

HARDENED B ALL SUP PO RTS

0.75 in (19.0 mm)

3.0 in (76 mm) 1.5 in (38.1mm)

3.0 in (76 mm)

ARC HEIGHT

0.75 in (19.0 mm) HOLDING FIXTURE P EENING TES T (a )

S TRIP REMOVED, RES IDUAL S TRES SE S INDUC E ARCHING (b )

S TRIP MOU NTED FO R HEIGHTMEASUREMENT (c)

Fig5---6 Shot PeeningIntensityControl 5.9.3.2 ShotSizeand Type.   Shot type and size selection depends upon the material, hardness, and geometry of the part to be peened. Shot types available are cast steel (S), conditioned cut wire (CW), glass bead, and ceramic. Most shot peening of fer-

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rous materials is accomplished with cast steel shot. Ca st steel shot is a vailable in two hardness ranges: 45---55 H RC , a nd 55---62 HR C . When pe enin g gea rs higher in hardness than 50 HRC, the harder shot

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5.9.3.5Masking. At times, it isdesirable to mask finished machined areas of the part from shot impingement. Typical masked a rea s would be finished bores or bearing surfaces. If masking is required, this should be stated in the shot peening requirements and defined on the drawing, with masked area tolerances given. 5.9.3.6 DrawingExample.  A typical example of drawing or blueprint specification for shot peening would be a s follows:

should be specified to achieve higher ma gnitudes of compre ssive stress (re fer to Fig 5 ---7).

5.9.3.3 Intensity. The intensity governs the depth o f the compressive layer and must be specified as the arc height on the A, C, or N strip (see 5.9.2.1). The ra nge o f a rc height is genera lly 0.004 inch (0.10 mm) wide, but it can be specified to a closer tolerance fo r mo re re pea ta ble results. Figure 5---8 illustrat es the depth of the compressive layer on steel at 31 and 52 HR C hardness according to intensity.

Shot peen area (s) indicated with S170 cast steel shot to an intensity of 0.010---0.014A per MIL ---S ---13165B ; Mask area(s) indicated (if necessary). O ther areas optional. U se 55---62 H RC shot, 100 percent minimum coverage.

5.9.3.4 Coverage.   In most cases, 100 percent coverage is adequate. In some instances, it ma y be desirable to specify multiples of 100 percent in an a ttempt to achieve more blending of a poorly machined surface. A typical statement in a blueprint specification is “ 100 percent minimum coverage.”

0

0 HRC 46 SHOT

--- 50 --- 500 ---100 ---1000

---150 HRC 61 S HOT ---200

---1500

---250 0

0.004

0.008

0.012

0.016

DEP TH IN INCHES

Fig5---7Residual StressbyPeening1045Steel at62HRC with 330Shot

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1.0

.040 HRC 31 .035

.75

.030

.025

.50

.020 HRC 52 .015

.25

.010

.005

0 0

.002

.004

.006

.008

.010C

0

INTENS ITY 0

 

.005

 

.010

.015

.020

 

.025 A

Fig5---8 Depth of CompressiveStressVersusAlmen Intensityfor Steel Table 5---6 gives shot size a nd in te nsity fo r var ious diametra l pitches.

The plastic flow of the surface a s a result of peening will tend to obscure minute cracks.

Table5---6 Typical Shot Sizeand Intensityfor Shot Peening

(2) All heat treating operations must be performed prior to shot peening as high temperatures [over 450 _ F(232 _ C )] will thermally stress relieve the peening effects.

Diametral Pitch 8 --- 16 4 --- 7 2 1/2 --- 3 1/2 1 3/4 --- 2 3/4 --- 1

Shot Size S110 S170 S230 S330 S550

(3) G enerally a ll machining of areas to be peened are complete prior to shot peening. It is possible to restore surface finish in peened areas (and retain benef icial eff ects) by lapping, honing, or polishing, if mat erial removal is limited to 10 percent of the depth o f compressive layer.

Intensity 0.006 0.010 0.014 0. 016 0.006

-----------

0.010A 0.014A 0.018A 0. 020A 0.008C

(4) Compressive residual stress levels produced by shot peening can be quantitatively measured by X ---ra y diffra ction. C urrently this must be measured on a cut sample in a laborat ory X ---ray diff raction unit. Portable units are under development.

NOTE:  The values for shot size and intensity should be considered typical and not mandatory. Variables such as gear geometry, hardness, and surface condition in the root may make ot her specifications more desirable.

(5) When there are significant machining marks in the tooth roots, it isdesirable to achieve an intensity sufficient to produce a depth of compressive stress to negate the stress riser effect of the machining mark. H owever, shot dia meter should not exceed 50 percent of the fillet radius.

5.9.3.7 General Comments.   Additional comments for shot peening include the following: (1) All magnetic particle or dye penetrant inspections should be perfo rmed befo re shot peening.

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5.10 Residual StressEffects. Residual stresses play an important role in the manufacture and performance of gears. Residual stressescreat ed byma chining and heat treating operations are responsible for much of the distortion that occurs during manufacture. The residual stress distribution in finished gears can determine whether or not the gears will survive in service. Residual stresses (either fa vorable or unfavorable) are induced mechanically, thermally, by phase transformation, or by modification of surfa ce chemistry (such as by nitriding). Ea ch of these, singularly and in combination (such as by carburizing), can a ffe ct the degree o f in ---process disto rtion and the residual stress state present in the finished parts. The following sections briefly discuss t he causes of ea ch type of induced residual stress.

induced. Thermal, phase transforma tion and modification of surfa ce chemistry stresses result from hea t treatment of steel.

5.10.2.1 Thermal and Phase Transformation Stresses. Thermal stresses result from the heating and cooling of materials. Quenching, one type of thermal stress, can a lso be considered a phase tra nsformation stress. Quenching, particularly fast quenching to form martensite, generates both thermal and phase transformat ion stresses. For example, two types of residual stress patterns can form on quenching of a round bar. The most common type of residual stress pattern in smalldiameter bars is a tensile stress at t he surface a nd a compressive stress at the center. This stress pattern results from the surface of a bar coolingf aster than the center. The phase transforma tion to martensite creates volume expansion producing tensile stress at the surface. This in turn creates a compressive stress at the center.

5.10.1MechanicallyInducedResidual Stresses. There are two types of mechanically induced residual stresses, machining stresses and finishing opera tion stresses. Ma chining stresses are created by the cutting of the gear shape and can be either beneficial or detrimental. Parts given a final heat treat ment after finish machining may have the grossresidual stresses from milling, turning, and hobbing minimized by intermediate stress relief heat treatments in order to prevent significant distortion during the final heat treatment. Machining cuts taken just prior to final heat treatment must be light enough so as not to create significant residual stresses. G rinding after final hea t trea tment must be performed very carefully since it can crea te residual tensile stresses in the surface of the gear which can adversely affect performance. La pping, honing or caref ul grinding of gears after final heat treatment maintains beneficial compressive residual stresses. Finishing operations such as shot peening (refer to 5.9) and roller burnishing also impart beneficial compressive residual stresses when properly controlled. These operations are t ypically performed on finished gears to improve the pitting and surfa ce bending fat igue resistance.

The second a nd o pposite type of residual stress pattern occurs during quenching of large diameter bars. In this situation, the surface hardens but the center remains at an elevated temperature for some extended period of time. The thermal contraction exceeds the expansion of the t ransformat ion to ma rtensite, setting up residual tensile stressa t the center and residual compressive stress at the surface. These two types of stress patterns are determined by two variables, size of the bar and speed of the q uench. When the sum of t hese two variables is large, for example large diameter bar with a fast quench, the stress pattern will be of the second type with residual tensile stress at t he center a nd residual compressive stress at the surface. When the cooling rates of the surface and center are similar, the thermal contraction can not overcome the expansion from the martensitic formation and residual tensile stress will form at the surface, while the center will consist of residual compressive stress.

5.10.2.2 Residual Stresses by Modification of  SurfaceChemistry. This type of residua l stress must a lso be considered in conjunction with therma l residual stress because modification of surface chemistry requires heating, and heating can introduce thermal stresses, which must be taken into account. Ca rburizing, the most common type of surface chemistry modification, will serve as a good example of these types of residual stresses. In q uenched carburized steels, the transformation temperature of austenite to martensite in the core occurs at a much higher

Use of cubic boron nitride (CBN) grinding may have a fa vorable effect on the residual stresses in the finished gear. Under extreme grinding conditions, however, CB N grindingmay also induce surface tempering residual tensile stresses. O ther ha rd gea r finishing methods (e.g. skiving) will need to be individually evaluated as to effect on residual stress levels.

5.10.2 MetallurgicallyInducedResidualStress. The other types of residual stress, although quite different, can allbe categorized as being metallurgically

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temperature tha n t he case, a nd a s discussed in the previous section, the austenite to martensite transformat ion creat es a volume expansion. Therefore, as the pa rt is cooling, transformation begins in the core and moves outward toward the case setting up tensile stresses in the core. The expansion of the case is opposed by the previously transformed core imparting beneficial compressive stresses in the case. C ompressive stresses in the case help reduce surfa ce pitting caused by tooth contact stress above and below the pitchline. They help counteract tensile stresses caused by bending in the root.

ASTM A370, a re normally surface hardness tests made using: (1) Rockwell (2) B rinell (3) Rebound Tests (Eq uotip & Shore) Hardness testing, using any method or instrument, must be made with calibrated instruments with data substantiated and documented to insure reliability. Statistical process control (SPC) is an accepted method of control. Minimum number of hardness tests on both rim or edge faces of through hardened cast and forged gear blanks is generally based on the outside diameter and increases with size. Hardness tests are made on the rim edge at mid rim thickness af ter fina l heat treatment.

6. Metallurgical QualityControl Metallurgical information should be available regarding: (1) incoming material grade information (2) incoming material ha rdness and mechanical tests (3) heat treat process control (4) pa rt characteristics (5) metallurgical testing (final product) (6) microstructure (7) test coupon considerations

6.2.1 Cast Gears.   Recommended number of hardness tests are a s follows: Outside Diameter, inches (mm) 0 --- 40 (1020) Over 40 _ 80 (1020 to 2030) Over 80 _ 120 (2030 to 3050) Over 120 (3050)

Refer t o Appendix D on Service Life Co nsiderations.

When four hardness tests are specified, two tests shall be on the cope side, (one over a riser and the other approximately 180 degree awa y between risers) and the other two tests shall be on the dra g side 90 degrees away from the tests on the cope side.

Spectrographic Analysis X ---R ay Ana lysis Atomic Absorption Wet Che mistry

When eight hardness tests are specified, four tests shall be on the cope side, (two over risers approximate ly 180 degrees apa rt, two between risers also a pproximately 180 degrees apart, 90 degrees away from tests over the risers) and the other four tests shall also be on the drag side, 90 degrees apart.

Iron casting grades are identified by their mechanical properties such as tensile strength, yield strength, and elongation. Ha rdnessma y be specified but cannot be used to identify grade. Bronze material grades are normally qualified using chemical ana lysis and hardness tests.

When sixteen hardness tests are specified, eight tests shall be on the cope side (four over risers and four between risers around the gear), and the other eight tests shall be on the drag side equally spaced around the gear. La rge segmented gears shall be hardness inspected on the cope and dra g rim edge of each segment per agreement between the customer and supplier.

B rass materia l grades are identified by chemical analysis.

NOTE:  So urce certification is commonly accepted for ana lysis certificat ion. 6.2 Incoming Material Hardness Tests.   Material hardness tests, often specified in accordance with

ANSI/AG MA

2 4 8 16

When two ha rdness tests are specified, one shall be on the cope side, preferably over a riser; the other on t he dra g side, approximately 180 _    away.

6.1 Incoming Material Quality Control.  Ma terial grade is certified by chemical test. G enerally this isa destructive process. The following types of tests are commonly used and are listed in ascending order of cost for ferrous materials: (1) (2) (3) (4)

Number of Tests Recommended (R im Face)

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6.2.2ForgedPinionsandGears. Forged pinions and gears include cylindrical shapes, d isc shapes and rings.

each ring edge, 90 degrees apart from one edge to the other. (3) When a tota l of six hardness tests are specified, they shall be 120 degrees apart on ea ch rim edge.

6.2.2.1 Cylindrical ShapedForgings. (1) A minimum of four ha rdness tests shall be taken on the major (tooth) diameter of forgings up to fifteen inches. Two readings, 180 degrees apart, shall be taken at t he center of the length of the major diameter (center of to oth section a t mid fa ce). O ne reading shall be ta ken approximat ely 1 inch (25 mm) from each end of the major diameter, 180 degrees apart.

(4) When a tota l of eight hardnesst estsa re specified, they shall be made 90 degrees apart on each rim edge.

6.3IncomingMaterial MechanicalTests. Mechanical property test ba rs, for t ensile testing and less frequently impact testing, are only required when specified. Refer to 6.8 for merits and limitations of mechanical test bars.

(2) A minimum of five hardness tests shall be taken on the major diameter of forgings over 15 inches (380 mm) in dia meter. Three re a dings, 120 degrees apart, shallbe taken at the center of the length of the major diameter (center of the tooth section at mid face). One reading shall be taken approximat ely 2 inches (50 mm) from each end of the major diameter, 180 degrees apart .

Test bar stock for gearing manufactured from forgings and bar stock are normally obtained from a prolongation or extension of the rough stock, in the axial or longitudinal direction with respect to the component and the direction of metal flow during forging. Refer to ASTM A291 for mechanical test certification of forged gearing.

6.2.2.2 DiscShapeForging.

Test bar sto ck, approximat ely 1.5 ¢ 5 ¢   6.0inch (38 ¢ 127 ¢ 152 mm) long, are normally at ta ched to the dra g (bottom) rim edge of t he casting or are cast as separate test blocks from the same heat of steel. Refer to ASTM A148 for mechanical test certification of cast gearing.

(1) A minimum of two hardness tests, 180 degrees apart with one on each side, shall be taken at the mid radius on forgings of up to 18.0 inches (457 mm) in diameter, inclusive. (2)A minimum of four hardness tests, two on each side 180 degrees apart, shall be taken at the mid ra dius on for gings over 18.0 inches (457 mm) in diameter.

Test bar stock should remain a tta ched to or a ccompany the rough stock until all thermal trea tment is completed.

6.2.3 Forged Rings (Reference ASTM A290). Recommended number of hardness tests is as follows: Diameter of Ring, in (mm) U p to 40 (1016) Over 40 to 80 (1016 to 2032) Over 80 to 120 (2032 to 3048) Over 120 (3048)

 

Minimum tensile properties for steel gearing are shown in Tables 4 ---2, 4---3 and 4---7, a nd also i n ASTM A290, A291 and A148.

Number of Tests Recommended

6.4 HeatTreatProcessControl. The many variables involved in the heat treatment of gear materials makesprocess controlcomplex. Process variablesinclude: time, temperature, rate o f heating and cooling, heating media , cooling media, types of controls, base mat erial composition, condition of process equipment, evaluation techniques, and part geometry.

2 (180 _    apart) 4 (180 _    apart) 6 (120 _    apart) 8

(90 _    apart)

Heat treat processes change the microstructure and mechanical properties of the gea r materia l. Any dimensional change, such as distortion or part growth, a nd a ny cosmetic change, such as coloration or surface texture, a re characteristics of a specific heat treat process, but are not primary factors for process control. Process parameters used to control the heat treatment of gear materials are a s follows:

(1) When a tota l of t wo ha rdness tests are specified, they shall be made 180 degrees apart, one on the ringedge and the otheron the opposite ringedge. (2) When a tota l of four hardness tests are specified, they all shall be made 180 degrees apart on

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6.4.1Temperature. Tempera ture selection and control is an important parameter in the heat treatment of gear ma terials. In carburizing and nitriding, the rate of diffusion into steel is dependent on temperature. The carbon concentration in the furnace at mosphere is also temperature d ependent. Specific temperature ranges are required to harden the various grades of steel. Hardness and mechanical properties of a materia l grade a re dependent on the tempering temperature after hardening.

commonly used methods for measuring and controlling carbon potential in a furnace atmosphere: (1) Water Vapor Concentration. For a given temperature, the carbon concentration on the surface of the part is related to the water vapor concentration (dew point) in a furnace at mosphere. The water vapor concentration is measured using a dew  cell or dew pointer. The water vapor concentration is expressed a s the a tmosphere dew point mea sured in degrees fahrenheit.

6.4.1.1 Temperature Uniformity. Since the properties obtained in gear mat erials are dependent on the temperatures at which they are treated, the uniformity of the temperature within the working dimensions of the furnace equipment should be measured. The amount of variation allowed is dependent on the type of heat treatment and the material properties d esired.

(2) Ca rbon Dioxide Co ncentration. The concentrations of carbon dioxide and carbon monoxide in a furnace atmosphere at a given temperature are related to the carbon concentration on the surface of the part. The carbon dioxide concentration is measured with an external infrared ga s ana lyzer and expressed as a percentage.

6.4.1.2 Thermal History.  It is advisable to make a time temperature plot of the heat trea t processesa s a monitoring device and as process documentation. This is usually accomplished with strip chart recorders.

(3) Oxygen Concentration. The concentration of carbon on the part surface is related to the oxygen concentration in the furnace atmosphere at a given temperature a nd carbon monoxide level. The oxygen concentrat ion is measured with an o xygen probe positioned in the furnace heat chamber.

6.4.2Time. The duration of each segment of the heat treat process is critical to achieving the desired material properties. For example, the depth of carbon penetra tion during carburizing is dependent on how long the part was held at the carburizingt emperature.

6.4.5QuenchControl. Control of the quenching operation involves monitoring the variables which affect the rate and uniformity of part cooling. Thisincludes inspecting the condition, cleanliness and concentration (if applicable) of the quenchant; t he proper operation of any device used for agitation; and ensuringtha t the q uenchant stays at the proper temperat ure (refer to 5.7).

When the furnace tempera ture instrument indicates that the furnace chamber has recovered its heat, the part in the chamber may not be up to temperature. It is important tha t the part be held at temperature long enough for the entire part to be at temperature. Time at temperature for through hardening is genera lly 0.75 hour per inch (25.4 mm) of section.

There a re several methods a vailable t o monitor and quantify the cooling rate of the q uenching process. These include the standard nickel ball test, magnetic test, hot wire test and interval test. Sample parts or test coupons can also be used as long as the test piece hardena bility is accounted for (refer to 5.7 on quenching).

6.4.3 Rate. The rates of hea ting and cooling are important considerations. For example, if a n induction hardened part is heat ed too slowly, the core material will get too hot and lose its mechanical properties. If a steel gear is cooled too q uickly, it will have high internal stresses and possibly crack.

6.4.6 Tempering Temperatures. It is important that the tempering temperature be controlled to achieve the desired hardness. It is prudent to select an initial tempering temperature which is on the low  side of the tempering range. It is easier and more cost effective to retemper a part that is too hard, than reharden and retemper a soft part. 6.5 Part Characteristics.

6.4.4 Atmosphere Control. The composition of the furnace atmosphere is an important part of process control. Control of carbon potential in the furnace a tmosphere is critical to carburizing and t he protection of surfaces from carbon pickup or depletion during the hardening process. There are three

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6.5.1.4 Carburize and Harden Examination. Surface hardness and core hardness measurements are used to monitor the carburizing process. If the core hardness of a part is within the expected range regardless of the o ther hardness measurements, the part was satisfactorily quenched. If the part hardness is low, this isa n indication of decarburization, inadequate quenching, excessive retained austenite, undissolved carbides, too high tempering temperature, inadequate case depth, or low surface carbon.

Part characteristics such as hardness, micro --structure and test coupon results can provide valuable information.

6.5.1 Hardness. H ardness is the most common characteristic used to measure results of the heat trea t process. There are numerous types of hardness testing devices which can be used, but ea ch type has its own a pplication limits and must be used correctly. Statistical process control (SPC) is an accepted method to insure reliability using hardness testing.

6.5.1.5 Case Depth Examination.   Carburized case depth is typically measured by making a microhardness traverse across a sectioned part or test coupon to find the depth from the surface where the hardness is equivalent t o Rockwell C 50. 6.5.1.6RetainedAusteniteExamination.  If the surface hardness of a carburized part is low, it may be due to t he presence of ret ained a ustenite in the carburized case. Reta ined austenite can be transformed to martensite by freezing the carburized part. If the surface hardness improves after freezing, there was reta ined austenite in the carburized case which is an indicat or of high surface carbon concentration or too high of a quench temperature.

6.5.1.1 As Quenched Hardness.   As quenched hardness of a part isa good indicator of the heat treat process. Many factors determine the as quenched hardness sucha s decarburization and retained austenite. High as quenched hardness is the result of good heat treatment. L ow as quenched hardness usually results from one or more of many factors such as deteriorat ing quenchant , malfunctioning quench agita tors, or too low an austenitizing temperature. 6.5.1.2 Decarburization.  If a surface has been decarburized, hardness will be low. If the surface hardness is low, it is advisable that two hardness checks be made on a qualifying test part to insure tha t the hardness below the decarburized zone meets blue print requirements. The two hardness checks should be made using the following sequence: grind surface for hardness measurement, regrind surface until the hardness indentation is removed, and then make another ha rdness measurement near the original location. If both measurements are the same, there is no decarburization. If the hardness increases, th ere is possible decar burizat ion. To det ermine the depth of decarburization, a test coupon or part that was run with the load should be sectioned, mounted, polished a nd etched. I t should be noted, however, tha t in most cases decarburization is not permissible. 6.5.1.3 Post Temper Hardness Examination. Tempering pa rts reduces hardness. As tempering temperature increases, hardness decreases. Tempering temperature is determined by many factors, mainly type of steel and as quenched hardness. A hardness measurement technique can be used t o monitorfurnace soak time and uniformity.If the part hardness is great er in a heavy section compared t o a light section, or if the hardness increases as surface metal is removed, these are good indicators of insufficient soak time. If the part hardness varies from the specified range between piecesin a furnace load , this is a goo d indication o f a processing problem.

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6.5.2 Microstructure.  The composition of the various phases in the microstructure of a gear willtell a lot about the hea t treat process. It is recommended that a trained metallographer or metallurgist perform the microstructure ana lysis. 6.5.2.1TemperedMartensite. If a hardened gear has been correctly hardened and tempered, the microstructure will be composed primarily of tempered martensite provided that the hardenability of the steel was adeq uate. 6.5.2.2 Bainite.  If a gear has been improperly quenched, the microstructure might be interspersed with bainite, which is characterized by a fea thery appearance if severely under quenched, or a darker acicular pattern for marginal quenching. 6.5.2.3 RetainedAustenite.  All carburized case microstructures will contain some retained austenite, usually less than 5 to 30 percent by volume. However, if the carbon content of the carburized case is high, a larger percentage of retained austenite will be present and will reduce the case hardness. Retained austenite is characterized by a white background in a matrix of other structures (see 6.5.1.6). 6.5.2.4 Undissolved Carbides.   If a carburized part has an excessively high carbon concentration,

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the microstructure will contain undissolved carbides usually populating the case. Undissolved carbides are chara cterized by blocky white regions in a matrix of martensite and retained a ustenite. A normal structure will consist of light, scattered pinpoint carbides, while a structure of excessively high carbon concentration will have carbides contained in a network at the grain boundary. Continuousintergranular carbide network is not desirable for gearing.

table Brinell and Rockwell test machines provided that the following are met: (1) Surface to be inspected provides access and has the required surfa ce finish, generally 64 microinches (5 microns), or: (2) If t he size of the hardness impression on the test surface is permitted, or: (3) Ma ss of t he test surface will support the test load.

6.5.3TestCoupons. Test coupons o f repre senta tive geometry are frequently used for destructive testing in lieu of destroying gearing. Microstructure and hardness testing of test coupons can be correlated to gearing characteristics.

Through hardened gearing is commonly inspected on the faces of gear rims, top lands of teeth where size permits, gap of herringbone (double helical) gearing and on adjacent diameters of pinions other than bearing journals. Through hardened gearing is rarely inspected for hardness on the flanks of teeth or in root radii because hardenability of the steel selected should insure o btaining the specified hardness at these locations. When hardness testers are not available for accurate measurement at roots of teeth, destructive sectioninga nd testingmay be required.

6.6 Metallurgical, Mechanical and Non---Destructive Tests and Inspections. Tests a nd inspections which may be made on the final or near final product are fatigue testing, hardness testing, surface temper inspection, magnetic particle inspection, and ultrasonic inspection. 6. 6.1 FatigueTesting.  Fatigue (life) testing of the fina l product is the proof of the suitability of the design fo r the intended purpose.

Other portable hardness testing instruments are available (ASTM A833). One tester uses a hammer to simultaneously impact a known hardness test bar and t he unknown workpiece with a hardened ba llbetween the two test surfaces. Comparison is made of the ba ll diameter on each to determine hardness of the unknown. Other portable instruments measure the recoil or rebound height or velocity of a dropped hardened ball, or use a high ultrasonic frequency activated indenter to measure hardness.

It is desirable to expedite thistesting while mainta ining validity of the t est data . This can be done by running the test at some overload rat io and evaluating the damage with time for the test conditions. D amage can be compared with that for the product design conditions. This comparison must be made for both the bea m strength and the surface durability of the teeth. Miner’s Rule is a widely accepted method o f ma king these comparisons.

It is desired that surface hardened gearing be ha rdness inspected, non ---destructively, so a s not to leave an objectionable impression. Portable testers which measure the rebound height or velocity of a dropped hardened ball or use a high ultrasonic frequency activated hardness indenter, may be used.

When damage value accumulated on the test equa ls the da mage value of the design, the test specimen survived the minimum specified product life. Due to the statistical nature of fatigue failure there is a wide distribution of data . In low cycle fatigue where most high overload and damage fractures occur, this scatter ba nd from the lower threshold to the upper threshold is approximately 100 to 1 wide. Since the distribution may be considered a log function, it is necessary for about half the test unitsto run at ten times the threshold life to validate the product design. This would constitute a Miner’s Rule damage of ten.

Conventional Rockwell test machines can be used to hardness inspect surface hardened gearing when size of t he gearing permits and where a visible impression is permitted. Ha rdened files, including tho se tempered to lower har dness tha n 60---64H RC , can also be used to approximate hardness by the scratch te st (Ref erence SAE J ---864). Inspectio n of the hardness on the flanks of surface hardened coa rse gearing wit h non ---destructive port able ha rdness testers can be improved when the instrument can be fixed for perpendicularity to the test surface. Ha rdness measurement in the roots of teeth may not

6.6.2 Hardness Testing on the Gear Product. Through hardened finish machine gearing can be conventionally hardness tested by standa rd a nd por-

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be reliable due to accessibility in the radius of curvature a nd surface roughness. roughness.

be used in some instances. Caution should be exercised cised if t he hea vier vier load C scale scale is used.

For improved accuracy and where permitted, through ha rdened steel and cast iron gearing should should be hardness inspected directly in Brinell (not converted). erted). Ha rdnes rdnesss of surface urface hardened hardened gearin gearingg shoul should d be directly directly measured in Rockwel Rockwelll (C or A scale) scale) or converted converted to Rockwell with with suitable porta ble instruments.

6.6.4 Magnetic Particle Inspection.   Magnetic particle inspection inspection is a non ---destructive -destructive testing method fo r locating surface surface and nea r surface discondiscontinuities in ferromagnetic materia l. When a magnetmagnetic field is introduced introduced into the part, disconti discontinui nuities ties laying approximat approximat ely transverse transverse to the magneticf ield will cause a leakage field. Finely divided ferromagnetic particles particles,, dry or in an oil base or water base sus sus-pension, pension, a re applied over the surfa surfa ce of the materia l under test. Thes Thesee particle particless will will gather and hold at the leakage field making the discontinuities discontinuities visib visible le to the naked eye.

Portable instruments vary in accuracy and reliability. Users, therefore, should take precautions to insure insure a ccurate ccurate calibration calibration and test results. results. Hardness Hardness testin testingg equipm equipment ent manufactu manufacturer rerss should should be contacted and literat literat ure searched for additiona l information on principl principles es of hardness inspecinspection, a vailable vailable test eq uipment uipment a nd their capabilities. capabilities. Sta tistical tistical process control control is a useful tool to be used with ha rdness testing. testing.

Use of electric current is, by far, the best means for magnetizing parts for magnetic particle inspection. Either longitudinal or circular fields may be introduced into parts. There are ba sically sically two types of electric current in common use, and both are suitable for magnetizing purposes in magnetic particle testing. The two types of current are direct current and alternating current. The magnetic fields produced by direct direct a nd by a lternating currents currents differ in many cha racteristics. racteristics. The The ma in difference, which which is of prime importance in magnetic particle testing, is tha t fields produced by direct current generallypenetrat e the entire cross cross sec section tion of the part, whereas the fields produced by alternating current are confined to the metal at or near the surface surface of the part under test. From this, it is evident evident that when deep penetration of field into the part is required, direct current must must be used as the source source of magnetizing force. B y fa r, the most most satisfactory satisfactory source source of D .C . is is the rectifirectification of alternating current. Both single phase and three phase A.C. are furnished commerci commercially ally.. B y the use of rectifiers, rectifiers, reversing reversing A.C. is rectified and the delivered delivered d irect irect current current is entirely entirely the equivalent of straight D .C. for ma gnetic particle particle testing purposes purposes..

6.6.3 SurfaceTe SurfaceTemper Inspection.  Surface temper inspection inspection is used used to detect a nd classify classify localized localized overheating on ground surfa ces by use use of a chemical etch method. method. D etails of t he process process are covered covered in Temper I nspectio nspectio n Proces Process . AG MA 230.01 230.01,,  Surface Temper Inspection criteria includes a class designation fo r critical a nd no n ---critical a rea s. To e valuat e the severity of surface temper, grinding burns are classified by intensity intensity of color fro m light light gray to brown to black. Severe burning or re ---ha rdening is indicate d by patches of white in the darkened areas. Cracking may a lso be present. R e ---ha rdening o r cracking are cause for rejection. rejection. Tables I and II in AG MA 230.01 cover cover temper classes classes ranging from C lass A (Light (Light temper) to Cla ss D (Heavy temper). temper). C lass lass C (Moderate temper) temper) for a limited area and hardness reduction may be permitted. Rework for excessive temper is generally permitted by mutual agreement between customer and supplier.

Sources of alternating current are single phase stepped do wn to 115, 115, 230 230,, or 460 460 volts. This This is accomplished plished by means of tra nsformers to the low voltages required. At these low voltages, magnetizing currents up to several several thousand thousand a mperesa mperesa re often used. used. The trend in E urope is to use A.C. current for ma gnetic particle particle testing because the intent of their testingis location location of surface surface disconti discontinui nuitieso tieso nly. nly. SubsurSubsurfa ce discontinuities discontinuities are best detected by radiogra phy or ultrasonic non ---destructive -destructive test methods. A.C. currents tends to give better particle mobility, and

Case depth shall be determined on a normal too th section. section. Ha rdness testers which which produce small shallow impressions should be used in order that the hardness values obtained will be representative of the surface surface area being tested. Microhardness testers which produce Diamond Pyramid or Knoop Hardness number are recommended, although other testers such such as Rockwellsupe Rockwellsuperfic rficial ial A or 15N scales scales can

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demagnetization is more complete than with a D.C. field.

(7) (7) For prod magnetization with direct current, current, a minimum minimum of 60 60 amperes per inch of prod spacing spacing will will produce a minimumma minimumma gnetizing force of 20o 20o ersteds at the midpoint o f t he prod line for plat e 3/4 inch inch thick or less. less. A safer figure to use, however, however, is 200 200 amps per inch, unless this current strength produces an interferin interferingg surface power patt ern. P rod spacing spacing for practical inspection inspection purposes purposes is limited limited to about eight (8) inches maximum, except in special cases.

There are two essential essential componentso f magnetic particle testing, each of equal importance for reliable reliable results. results. The first is the proper magnetizat ion of t he part to be tested, with with proper field strength in the appropriate direction for the detection of defects. The The second sec ond is the use use of the proper proper magnetic magnetic particles particles type to secure secure the best possib possible le defect indicat indicat ions under prevailing conditions.

(8) (8) All parts should be demagnetized a fter ma gnetic pa rticle rticle inspection. inspection.

6.6.4.1Gene .1 General Principles Principles.  Some general principl iples and rul rules on magne magneti tizi zing ng means means,, fiel field d strength, current distribution and strength requirement s ar e liste d be low (ref er t o Figs 6-6---1 a nd 6 ---2).

FIELD

(1) (1) Fields Fields should should be at 90 degrees to the direction o f def ects. This This may may req uire magnetizing in two directions.

HEAD B ATH

(2) (2) Fieldsgenera ted by electric electric currents currents are at 90 degrees degrees to the dire directi ction on of curr current ent flow. flow. (3) (3) When magnetizing with electric electric currents, pass the current in a direction parallel to the direction of expected expected discontinuities. discontinuities.

CURRENT

DIS C ONT ON TINUITY

(4) Circular magnetization has the advantage over longitudinal magnetization in that there are few, if any, local poles to cause confusion in pa rticle rticle patterns, and it is preferred when a choice of methods is permissible. permissible.

HEAD SHOT C IRC ULAR MAG MAG NETIZAT NETIZATION LOC ATES DIS C ONTINUIT ONTINUITIES O C C URR ING 45 --- 90 DEG REES TO THE THE D IREC TION OF THE THE FIELD.

(5) (5) C ircular ircular ma gnetization specific specificat at ions generally require from 100 to 100 1000 amps per inch o f part diameter. Amperage requirements should should be incorincorporated into the magnetic particle procedure.

INS INS P EC TFO R PARTICLE INDICAT INDICATIONS S HOWING LONG ITUDINAL DISC ONTINUIT ONTINUITIES --- MARK MARK DIS C ONTINUT ONTINUTIES IES .

Fig6Fig 6---1 Circu Circular (Head Shot) Magnetic Parti Particle cleIInspection

(6) For coil magnetization, a widely used formula fo r a mperage calculations calculations is: NI  =

45 000 L / D 

6.6.4.2 Magnetic Particles.  The particles used are finely finely divided divided ferromagnetic ferromagnetic material. material. Properties Properties vary over a wide range for different applications including cluding magnetic properties, properties, size, size, shape, density, mobility and visibility or contrast. Varying requirements ments for varying arying condit condition ionss of test test and varying arying properties of suitable suitable materia lsha ve led to the develdevelopment of a large number of different t ypes of a vailvailable mat erials. The The choice of which one t o use is an important one, since the appearance of the particle

(Eq 6.1)

where N I   = L /D 

ampere ampere turns turns required, required, = leng length th to diameter diameter ratio.

NOTE:  The 45 000 constant may vary with specifications.

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patterns at discontinuities will be affected, even to the point of whether whether or not a pattern is formed. formed. F IE LD

methods is in the range of 60 60 to 40 40 microns. microns. P articles larger than this tend to settle out of susp suspensi ension on rapidly. ly. In general, wet method materia lsexhibit a greater sensitivity than dry powders. Fluorescent particles have the greatest contrast of the wet method ma terials. als. Although Although fluorescen fluorescentt wet particles particles have the greatest sensitivity and contrast, they can provide a confusing confusing background background on surfaces with a finishgrea finishgrea ter than 250 RMS.

C U R RE NT THROUG H COIL

B ATH

6.6.4.3 DocumentedProce Procedures. Written procedures for ma gnetic particle particle testing should as a minimum include: (1) (1) Which ASTM, ASTM, ASNT or agency specificaspecifications the procedure meets. (2) Q ua lifica tio ns-ns---

DIS C ONTINUIT ONTINUITY COIL SHOT LONG ITUDINA UD INAL L MAG MAG NETIZAT NETIZATION LOC ATES TRAV RAVERS E DIS C ONTINUIT ONTINUITIES .

(a) Indicate that the operators are qualified an d tested to ASNT---TC -TC ---1A Level II , MI L --STD ----271F, et c.

INS INS P EC TFO R PARTICLE INDICAT INDICATIONS S HO WING TRANSVERS E DISC ONTINUIT ONTINUITIES . NOTE:

(b) Indicate type of equipment used for inspection, spection, A.C . and D .C. f ull wave rectified, etc. etc.

EFFEC TIVE IVE LENGTH LENGTH MAG MAG NETIZED NETIZED BY CO IL SHOT IS A FEWINCHES FEWINCHES ON EITHER SIDE OF CO IL. IL. MAXIMUM IMUM LENGTH LENGTH OF ARTICLE CO VERED BY ONE SHOT IS 18 INC INC HES (46 CM). ON LONG LONG ARTICLES RTICLES , REP EATS HOTS AND AND B ATHS DO WN THE LENGTH LENGTH O F ARTICLE. P LAC LAC E ARTICLES C LOSE TO THE THE C OIL BO DY.

(c)I ndicate type of particlesused for inspecinspection, fluorescent or black visible, wet or dry particle. For t he wet method, particle particle concentration concentration should should also be indicated. (3) G ene ra l-l--(a) State when insp inspecti ection on ist o be done;a fter heat treat, finish machining, etc. (b) State what the surface will will be; for example, 250 250 RM S, black forge, etc.

Fig6Fig 6---2 Coil Shot MagneticP tic Particle rticle Inspection (1) Dry Powders. It is evident that size plays an important part in the behavior of magnetic particles. particles. A large, large, heavy particle is not likel likelyy to be arrested and held by a weak f ield when such such particles are moving over the surface of the part. On the other hand, very fine powders will be held by very weak fields, since their massis very very small small.. Extremel Extremelyy fine particl particles es may also adhere t o the surface where there are no discondiscontinuities, especially if it is rough, and form confusing backgrounds. backgrounds. M ost dry f erromagnetic powders used for detecting discontinuities discontinuities a re careful mixtures of par ticles of a ll sizes. sizes. The The smaller ones a dd sensitivity and mobility, mobility, while while the larger ones not only aid in locating large defects, but by a sweeping sweeping a ction, counteract the tendency of fine powders to leave a dusty background. Thus, Thus, by including the entire size range, a balanced powder with sensitivity over most of the range of sizes sizes of discontinuities discontinuities is produced. produced.

(c) State amps per inch of diameter for circular magnetization and the formula used for calculation of longitudinal magnetization. (d) State what what method method will will be used used for determining mining field magitude; such such a s pie gage, et c. (e) State demagnetization, demagnetization, if required, and level of demagnetization required. (4) (4) Standa rd of Acceptance Acceptance (a) Indicate ma ximum size size and density of indications indications permitted. permitted. (b)Indicate (b)Indicate reporting procedures if needed. For further information on magnetic particle testing, refer to: Principles of M agnetic agnetic Particle Particle Testing , C.E. Betz

Metals H andbook andbook Volum olumee II Eighth Eighth Edition Edition Non des destructive I nspe nspection and Quality Control 

(2) (2) Wet Wet Method Ma terials. When When the ferromagnetic particles are applied as a suspension in some liquid medium, much much finer part icles icles can be used. used. The The upper limit of particle size in most commercial wet

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6.6.5 Ultrasonic Inspection. U ltrasonic inspection is a nondestructive test method to determine the internal soundness and cleanliness of gearing by passing sound (ultrasound) through the material. Very short sound waves of a frequency greater than 20,000 cycles per second (audible limit) are voltage generated and transmitted into the part by a transducer. In the method most often used, returning sound waves are tra nsformed into volta ge and monitored on an oscilloscope screen.

Scanning sensitivity and indication limitations are often determined using test blocks by establishing a dista nce---a mplitude referen ce line on the oscilloscope screen as illustrat ed in Fig 6---4. As an example, sensitivity may be adjusted to esta blish the specified indica tion he ight [2 1/2 inch (63 mm)] fro m the flat bottom hole (FB H) in the 4inch (102mm) block, and at the same sensitivity, the indication from the same size FBH in the 12 inch (305 mm) block isno ted on the oscilloscope screen. A straight line is drawn between the two points. Any indication noted must not e xceed th e det ermined dista nce---a mplitude reference line. Also, indications are often specified not to exceed a certain magnitude a nd length on the scanning surface or result in loss in back reflection height exceeding specified limits, both expressed in a percent of the back reflection height established during calibration for scanning sensitivity.

There are two test methods used, depending upon the media, for couplingthe ultrasonictra nsducer to the heat treated work piece. U ntreated coarse grained structures do not lend themselves to ultrasonic testing. Surfacesto bescanned, such as the outside diameter a nd ends or end fa ces of cylindrical or disc shaped rough stock are generally machined to 125---250 micro ---in ch maximum surf ace r oughness. This provides improved contact for the transducer with the work piece. One method uses a couplant: oil, glycerin or a commercial paste spread evenly on the surfa cesto be inspected. The second method uses water as the couplant, with the transducer and work piece submerged in a ta nk.

Reference can be made to the equipment manufacturer’s literature, or to the American Society for Metals (ASM) Metals H andbook, Volume 11 on “ Non---D estructive Testing” (SNDT), for additional information. Important considerations include appropriate transducer frequency, operator requirements and qualification, application limitations, work piece requirements (grain size), instrument calibration, t est block requirements, test specifications and interpretation of test results.

With t he most common t echnique of ultrasonic inspection, namely, the pulse echo technique, the transducer both emits sound wa ves and receives the returning signals from the back surface and possible defects. The returning signals are subsequently monitor ed on an oscilloscope screen as shown in F ig 6---3. The indicat ion to the left of the oscilloscope screen in Fig 6---3 is caused by the sound wave entering the steela nd is called “initial pulse” or “ contact interference.” The indicat ion to the right is caused by sound reflecting off of the back surface and in the middle is the signal reflecting from any defects shown. The horizontal line, called the “ sweep line,” provides a measure of dista nce or depth in the work piece, as related to the rate of travel of sound in the material. The sweep line can be calibrated by use of a test block or section of known thickness in the work piece in order tha t ea ch marker shown on the sweep line represents a standard distance or depth. Depth of the defect from the transducer contact point on t he scanning surface can, therefore, be determined.

The American Society fo r Testing Ma teria ls and AG MA specifications which follow may be used for ultrasonic inspection of wrought a nd cast gea ring. Forgings and bar stock: (1) AG MA 6033---A88, Sect ion 10. (2) ASTM A388,   Ultrasonic Examination of    H eavy Steel Forgings.

Castings: (1) AG MA 6033---A88, Sect ion 11. (2) ASTM A609,  Steel Castings, Carbon and  L ow Al loy, Ultr asonic Examination Thereof.

6.7 Microstructure. The major function of the material selection and heat treat ing process is to a chieve the desired microstructure at the critical locations so that the part will have the desired contact and bending strength capa city. Ha rdened steel gearing microstructure should be tempered martensite at the entire tooth surface. The microstructure will vary around the gear tooth flank and throughout t he tooth cross section. The tooth mass will have a significant effect on the resulting microstructure a nd hardness throughout the tooth section. The heat treatment variables will

Before testing, the instrument must be calibrated a ccording to the test specification. Scanning sensitivityis often established as either the sensitivity to just obt ain a specified back reflection height, o r at the sensitivity to obtain an indication of specified height from a flat botto m hole drilled into test blocks.

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significantly effect the microstructure achieved. G ear toot h qua lity control must include microstruc-

ture considerations a s well as hardness control.

TRANS DU C ER S UITAB LE C OUP LANTON S URFAC E

X Y DEFECT

B AC K REFLECTING S URFAC E

INITIAL P ULS E

B AC K REFLEC TION

Y

3 in (76 mm)

X

DEFECT

MARKERS

Fig6---3Ultrasonic Inspectionwith OscilloscopeScreen

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INDICATION FROM FB H IN 4 in (102 mm) BLOCK

INDIC ATION D ---A REFE RENC E LINE FROM FBH IN 12 in (306 2 1/2” mm) B LOCK (63 mm )

3 in (76 mm )

11 in (279 mm)

TES T BLOCKS: 12 AND 4 in (306 AND 102 mm) TES T BLOCKS C ONTAINING S AME S IZE FLAT B OTTOM HOLE DRILLED TO A DE P TH OF 1 in

Fig6---4 Distance--- AmplitudeReferenceLinefor Ultrasonic Inspection

Control of the microstructures in flame and induction hardened steel gears must also consider the width and location of heat effected zones which will always exist at the ends of the hardened pattern.

austenite. Some research has shown that microcracks are produced by subzero trea ting. In carburized and hardened steel gears, carbide forms and distribution are a n area of microstructure concern. Continuous network carbide is generally considered to be una cceptable microstructure. D iscontinuous carbide network is generally allowed within limits.

Microstructure evalua tion must include the existence of structures other than tempered martensite at the gear tooth surface and at core positions. In carburized and hardened steel gears, retained austenite will exist in the case after the heat treating operations. D ata and opinions vary as to the a llowable limits for reta ined a ustenite. Subzero treatment is specified for some applications to reduce retained

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B ainite, pearlite, and ferrite a re undesirable a t the gear tooth surface of surface hardened gearing. These structures will exist in core microstructures of coarse tooth gearing.

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6.8 Mechanical Property Test Bar Considerations. Test coupons a re specified by compa ny an d industry standards for evaluating mechanical properties of wrought and cast steel a nd non ---ferrous mat erials used for gearing.

(e.g. shaft extension), has an effect on mechanical properties. This variance is due mainly to the increased degree of mechanical working and increased response to heat treating, as compared to larger forged sections. G enerally, smaller section test bars and sections show improved mechanical properties. (2) C astin gs--(a) Mass effect. Small section of the test bar being tested, such as standard impact test bars, results in improved properties compared to larger cast sections. Also, the smaller section of the sta ndard integral or separa te cast test coupons, and its effect related to improved solidification mechanism (reduced micro ---segre ga tio n a nd mi cro ---unso undn ess) and increased response to heat treating, causes mechanical property variance compared to larger cast sections. (b) Loca tion o f the t est coupon. Test coupon may be better located during heat treat ment, causing increased response to heat treating and improved mechanical properties.

NOTE:   It should be realized, however, that mechanical properties obtained from test coupons for wrought and cast steel, cast iron and non ---ferro us alloys are n ot eq uivalent to the actual properties of gearing from which the test coupons were obtained or associated. Smaller section test coupons are typically specified for economicconsiderationsa nd instrument t esting limitations. 6.8.1 Reasons for Mechanical Property Variance.   The reasons for mechanical properties obtained from test coupons not being equivalent to those o f gearing include the following considerations: (1) Wrought Forgings an d B ar stock--(a) Test coupon orientation and location. Mechanical properties of forgings and bar stock are anisotropic (refer to 4.9) which means that properties vary in the longitudinal and transverse (or tangential) directions. These directionsa re defined with respect to direction of metal flow and inclusion orientation induced by mechanical working. Unless otherwise specified, test results from shaft extensions in the longitudinal direction are those typically reported by forging manufa cturers for solid on shaft gearing. The longitudinal direction, however, provides optimum properties compared to properties from the transverse (or tangential) direction. The transverse (or tangential) direction is more representative of gear teeth depending upon helix angle.

6.8.2Mechanical PropertiesAffected. Mechanical properties obta ined from test coupons, especially tensile ductility (percent elongation a nd reduction of area measured afte r tensile testing), impact strength and fatigue strength, are generally higher for test coupons than for actual fo rged or ca st gearing. Tensile and yield strengths of test coupons, however, better represent actual corresponding properties of gearing, provided hardness of the test coupons is within the specified ra nge. 6.8.3Interpretation.  Mechanical properties obtained fro m test coupons should be considered as an indication of the qua lity of gear materia ls, but should not be interpreted as representing the precise mechanical properties of gearing for the reasonscited in 6.8.1 and 6.8.2. Specified mechanical properties for test coupons should be minimum properties, not typical properties. D esigners should incorporate a ppropriate factors of safety based on experience fo r design of gea ring to accommodate variance between measured and actual properties of gearing. In addition to test coupons providing indications as to the metallurgical q uality of gea r materia ls, test coupons provide a comparison of steel qua lity between different orders and can of ten help identify problems in steel making and heat treating.

Location or depth of the test coupon from the fo rged section (e.g. from the outside diamet er, mid--section or from the center) and its effect with respect to the degree of mechanical working and segregation, causes varia nce in mechanical properties. Segregation isincreased and degree of mechanical working is reduced towa rds the center of hot worked or wrought sections. (b) Ma ss effect. Small section of the test bar being tested, and the smaller section of the gearing from which the test coupon may ha ve been obtained

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Bibliography ASTM A148---83,

Specifications for Steel C astings for H igh Strength Structural Purposes 

ASTM A291---82,

Specification for Carbon and Alloy Steel Forgings for Pinions and Gears for Reduction  Gears 

ASTM A356---83,

Specifi cation f or Steel Castings, Carbon and L ow Al loy, H eavy--- Walled, for Steam  Turbines 

ASTM E 125---63 (1980),   Reference Photographs for Magnetic Particle I ndication s on Ferrous Castings  ASTM E186---80,

Standard Reference Radiographs for H eavy Walled (2 to 4 1/2 inch) (51 to 114 mm)  Steel Castings 

ASTM E280---81,

Standard Reference Radiographs for H eavy Walled (4 1/2 to 12 inch) (114 to 305 mm)  Steel Castings 

ASTM E446---81,

Standard Reference Radiographs for Steel Castings Up to 2 inch ( 51 mm) i n Th ickness 

ASTM E609---83,

Ul trasonic Examination of Carbon and L ow All oy Steel Castings 

ASTM E709---80,

M agnetic Particle Examination 

MIL ---H ---6875G (Fe b 86),  Process for H eat Treatment o f Steel 

ReferenceAddresses American Society for Metals Meta ls Park, OH 44073 (216) 338---5151 Metals Ha ndbooks Hea t Treat ers G uide Metals Reference B ook

American Iron and Steel Institute 1000 16th St reet, NW Washingto n, D. C . 20036 (202) 452---7100 AISI Steel Products Ma nuals Naval Publications and Forms Center 5801 Tabo r Avenue Ph iladelphia , PA 19120 (215) 697---3321 Military Sta ndards

American Society for Testing and Ma terials 1916 Race Street P hiladelphia , PA 19103 (215) 299---5400 ASTM Standards

Metal Powder Industries Federation 105 College Ro ad Ea st\Princeto n, NJ 080540 (609) 542---7700 MPIF Standard 35

Society of Automotive E ngineers, I nc. 400 Co mmonwealth D rive Warren da le, PA 15096 (412) 776---4841 SAE Handbook AMS Standards

ANSI/AG MA

Other: G ray and D uctile Iron Ca stings Ha ndbook Cast Steel Handbook Modern Plastics Encyclopedia

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AppendixA PlasticGear Materials [This Appendix is provided for informational purposes only and should not be construed as part of AG MA St andard 2004---B 89,   Gear Materials and H eat Treatment M anual .]

A1. Purpose. The purpose of this Appendixis to provide information on plastic materials which have been used fo r gea ring. For physical properties, refer to appropriate product standards.

moplastic materia l are used, with the la tter being by far the most prevalent.

A5.1 Phenolic(T/S --- indicates thermosetting). Phenolics are invariably compounded with various fillers such as woodflour, mineral, glass, sisal, chopped cloth, and such lubricants as PTFE (polytetraf luorethylene) and graphite. Phenolics are generally used in applicat ions requiring stability, and when higher temperatures are encountered.

A2.Tolerances. Under certain operating conditions, the tolerances for plastic gears may be less critical than for metal gears for smooth a nd q uiet performance. Ordinarily, however, the same care in manufa cturing, testing, measuring, a nd q uality level specifications should be utilized in plastic gearing as in metal gearing. The inherent resiliency of some of the plastic used may result in better conjugate action. The resiliency of many plastic gears gives them the ability to better dampen moderate shock or impact type loads within the capabilities of the particular plastics materials.

A5.2 Polyimide (T/S). Polyimide is usually 40---65 percent fiber glass reinfo rced a nd ha s goo d strength retention when used at high operating temperatures. A5.3 Nylon(T/P --- indicatesthermoplastic). Nylon is a family of thermoplastic polymers. The most widely used of any molded gea ring material is nylon 6/6, but nylon 6 an d nylon 12a re a lso used. So me nylons absorb moisture which may cause dimensional insta bility. Nylon may be compounded with various types and amounts of glass reinforcing materials, mineral fillers, and such lubricants as PTFE and MoS 2 (molybdenum disulfide). A5.4 Acetal (T/P).  Acetal has a lower water absorption rate than nylon and, therefore, is more stable after molding or machining. Acetal polymers are used unfilled or filled, with glass and minerals with and without lubricants, such as PTFE and MoS 2, a s well as one version with f ibrous PTFE. A5.5Polycarbonate(T/P). Polycarbonate isgenerally used with the addition of glass fiber a nd/or PTFE lubricant a nd is a fine, low shrinkage material for producing consistently a ccurate molded gears.

A3. Operating Characteristics.   G enerally, plastic gearing materials are noted for low coefficient of friction, high efficiency performance, and quiet operation. Many plastic gearing materialsha ve inherent lubricity so that gears require little or no external lubrication. They can perform satisfactorily when exposed to many chemicals which have a corrosive effect on metal gears. Plastic gearing, when operating at low stress levels in certain environments, have been known to outwear equivalent metal gears.

A4. LoadCarryingCapacity. The maximum loa d carrying capacity of most plastic gears decreases as the temperature increases more than with metal gears. The upper temperature limit of most thermoplastic gears is 250 _ F(121 _ C ) at which point they lose approximately 50 percent of their rated strength. The upper operating temperature limit of thermosetting gears now exceeds 400 _ F(250 _ C ). Very little degradation of mechanical properties in certain thermosetting materials occurs at temperatures up to 450 _ F(232 _ C ).

A5.6 Polyester (T/P). Polyesters are both unfilled and with glass fiber, and are finding their way into more markets as a molded gearing material in competition with nylon and acetal. A5.7 Polyurethane(T/P). Polyuretha ne is generally noted for its flexibility and, therefore, has the ability to absorb shock and deaden sound. A5.8SAN(Styreneacrylonitrile)(T/P).  SAN isa stable, low shrinkage material and is used in some lightly loaded gear applicat ions.

A5. Plastic Materials. Ma ny different plastics are now used for gearing. Both thermosetting and ther-

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A5.9 Polyphenylene Sulfid (T/P).   When compounded with 40 percent glass fiber with or without internal lubricants, it ha s been found in certa in gear applications to have much greater strength, even at elevated temperatures, than most materials previously available.

A8.3 Burrs. Feather edge burrs, if not eliminated by backup discsor subsequent removal by other means, will impair inspection of gearing and possibly contribute to noise during operation. A9. LaminatedPhenolicsPlastics.

A5.10 Polymer Elastomer (T/P).   Polymer elastomer is a newcomer to the gearing field, and ha s excellent sound deadening qualities and resistance to flex fatigue, impact, and creep, among other advanta geous chara cteristics.

A9.1 Industrial Laminated Thermosetting Products. These products, whether in sheet or rod form, contain laminations or plies of fibrous sheet materials such as cellulose, paper, asbestos, cotton fa bric, glass fabric, or mat. These materia ls are impregnated or coated with a phenolic resin and consolidated under high pressures and temperatures into various grades which have properties useful for gearing.

A6. PartCombinations.  Several plasticgears can be molded together as a gear cluster. Co mbinations of gears, pulleys, sprockets, a nd ca ms can a lso be produced as a single part.

Fabric base grades are chosen to withstand severe shock loads and repeated bending stresses, a nd to resist wear. Fabric base grades are tougher and less brittle tha n paper base grades. The linen grades made with finer textured lightweight fabrics will machine with less trouble. G ears of linen base phenolic are abrasive, and thus may require a hardened steel mate a nd adeq uate lubrication.

A7.GearBlanks. Many of these plasticmat erials, nota bly unfilled nylon and a cetal, a re available in sta ndard extruded shapes, such as rounds, squares, and rectangles of various sizes from which gears can be machined. G ears can be molded a t less cost if large quantity warrants the cost of the mold. A8. Machined Plastics Gears.  The quality of machined gears may be generally better than their molded counterparts, but the molded too th surface is superior to the ma chined surface in smoothness and toughness. Final tooth strength is generally better in a molded gear, than an equivalent machined gear, because of the flow of the material into the toot h cavity of the mold. G ear cutting is done on standard machinesa nd with standa rd tools. The following considerations will a ssist in obta ining higher quality machined parts.

Asbestos---pheno lic grad es ha ve excellent thermal a nd dimensional stability. The glass fa bric base grades have good hea t resista nce and very high tensile a nd impact strength. A9.2 Performance Characteristics. Pheno lics are used for fine pitch gears due to economy, high resiliency, and high wear resistance. Lower density tha n metals often provides higher strength to weight ratios. It should be noted tha t a ll grades have some dimensional change due to humidity. A9.3 Chopped Fabric Molding Compound. Chopped fabric impregnated with phenolic resin is capable of being molded a s a gear but may require finish machining to meet most commercial quality requirements.

A8.1Inspection.  The modulus of elasticity is so low in plastics that errors in measurements are very difficult to control. The use of controlled load checking equipment isa lmost mandat ory to avoid errors in measurements.

A10. Plastic GearingReferences. AG MA 141.01, Plastics G earing --- M olded,M achined 

A8.2 Tools.  Sha rp cutting tools are necessary to avoid tooth profile and size variation due to deflection.

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and O ther M ethods.

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AppendixB ApproximateMaximumControllingSection SizeConsiderations for Through Hardened Gearing [This Appendix is provided for informational purposes only and should not be construed as part of AG MA St andard 2004---B 89,   Gear Materials and H eat Treatment M anual .]

B1. Purpose. This Appendix presents approximate maximum controlling section size considerat ions for through hardened (quench and tempered) gearing. Also presented are factors which affect maximum controlling size, illustrations as to how  maximum controlling section size is determined f or gearing, and recommended maximum controlling section sizes for several low alloy steels from AG MA 6033---A88, M arinePropulsion Gear Units,Part 1,M a- 

pally on hardenability, specified hardness, depth of desired hardness, quenching and tempering temperat ure considerations. Reference should be made to 4.6 of the Sta ndard for hardenability considerations.

B3. Illustrations. Figure B ---1 illustra te s co ntr olling sections for quenched gear configurations whose teeth are machined af ter heat t reatment. NOTE: Evaluation o f the controlling section size for the selection of an appropriate type of steel and/or specified ha rdness need not include consideration of standard rough stock machining allowances. Other special stock allowances such as t hose used to minimize distortion during heat treatment must be considered.

terials.

B2. Definition. The controlling section of a part isdefined as that section which has the greatest effect in determining the rate of cooling during quenching at the location (section) where the specified mechanical properties (hardness) are req uired. The maximum cont rolling section size for steel is based princi-

TableB---1 ApproximateMaximumRecommended ControllingSectionSize* Specified B rinell H ardness 223---262 248---293 262---311 285 --- 311 302---352 321---363 341---388 w 363---415 w

**

Alloy C ontrolling Section Size, in (mm) AISI 4140

AISI 4340 No rest rict io n  ] No restriction No restriction To 25. 0 (640) includ ed To 15.0 (380) includ ed To 12.0 (305) included To 8.0 (203) included To 3. 75 (95) includ ed

To 8. 0(203) includ ed To 5.5(140) included To 4.5(115) included To 4. 0(102) in clud ed To 3.0 (76) includ ed No t reco mmended Not recommended No t reco mmend ed

4350 Type [ No restriction  ] No restriction No restriction No r est rict io n No rest rict io n No restrict io n No restriction To 23.0 (585) incl.

NOTES: * Ma ximum controlling section sizes higher than tho se above can be recommended when substantia ted by test data (heat treat practice). Ma ximum recommended controlling section sizes for nitrided gearing a re less than those a bove for the same hardness range because of higher tempering temperature required for nitriding gearing (refer to 5.5). Ma ximum recommended sizes for flame o r induction hardening gearing would be same a s above, dependent upon specified core ha rdness. [ 4350 Type Steel is genera lly considered eq uivalent to AI SI 4340 fo r chemica l ana lysis, except tha t carbo n is 0.48---0.55 perce nt . ] “ No restriction” indicat es maximum controlling section size is not a nticipate d to provide any restrictions fo r conventional size gea ring w 900 _ F(482 _ C ) minimum temper may be req uired to meet these hardness specificat ions. ** H igher spe cifie d ha rdne sses (e.g. 375---415 HB , 388---321 HB and 401---444 HB ) a re used fo r specia l gearing, but costs should be evaluated d ue to reduced machinability.

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B4.Recommendations. Table B ---1 pro vides approximate recommended maximum controlling section sizes for o il quenched and tempered gearing (H = 0.5) of several low alloy steels based on specified hardness range, normal stock allowance before hardening, minimum tempering temperature of 900 _ F(482 _ C ) and obtaining minimum hardness at the roots of teeth.

and published tempering response/hardena bility data. Maximum controlling section sizes for rounds greater than 8.0 inch (205 mm) O.D. generally req uire in---house hea t trea t experiments of la rger sections followed by sectioning and transverse hardness testing. Normalized and tempered heavy section gearing may also require maximum controlling section size considerations if the design does not permit liquid quenching. Specified hardnesses able to be obta ined with t he same t ype steel (hardenability) is considerably lower, however, and higher hardenability steel may be req uired. In ---house normalized and t empered/har dness testing experiments are req uired.

B5. General Comments. Maximum controlling section sizes versus specified hardness for section sizes to 8.0 inch (203 mm) diameter rounds can also be approximated by use of the “C hart Predicting Approximate Cross Section Hardness of Quenched Ro und B ar s from Jominy Test Results” published in Practical D ata f or Metallurgists by Timkin Steel Co.,

TEETH

TEETH 2 inch (50)

--- --- --- ---   --- --1.5 inch (38)

8 inch (203)

--- --- --- --- --- ---

10 inch (254)

6 inch (152)

ControllingSection: 8in (203mm) Diameter

ControllingSection: 2in (50mm) Facewidth

TEETH

TEETH

--- --- --- --- --- --- --- --4 inch (102) 8 inch (203)

36 inch (914)

--- --- --- --- --- --- --- ---

32 inch (813) 36 inch (914)

12 inch (304)

ControllingSection: 2in (50mm) Wall Thickness(If theborediameter is less than20%of thelengthof thebore, then the outsidediameter)

ControllingSection: 2in (50mm) RimThickness

FigB---1Illustrationsof ControllingSection Size

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AppendixC CaseHardenabilityof CarburizingSteels [This Appendix is provided for informational purposes only and should not be construed as part of AG MA St andard 2004---B 89,   Gear Materials and H eat Treatment M anual .]

C1. Purpose.  This Appendix assists in the selection of a grade of carburizing steel to insure that the carburized case has sufficient ha rdenability to be capa ble of hardening roots of teet h to meet specified surface hardness requirements. The method used is based on steel hardenabilityconsiderations and standard hardening proceduresused for carburized gearing. It may be used in conjunction with design and other considerations to select the appropriate grade of steel.

can be used for carburized gearing considerations without regard to the fact that gear teeth are machined prior to carburize hardening. The controlling section size in both instances is the section related t o the location of gear teeth which governs the ra te of heat removed during quench hardening. C3. SelectionofSteel. To ensure that the stee l under considerat ion ha s sufficient case hardena bility to be capable of satisfactorily hardening the case in the roo ts of te et h, F ig C ---1 should be used . F igure C ---1 is based on hardena bility and controlling section size considerations. Steels a re presented in order of hardena bility on the ordinate of Fig C ---1. Steels not shown on Fig C ---1, t herefore, can be evaluated by comparing hardenability to tho se steels presented to determine the approximate maximum recommended controlling section size (a s indicat ed by the solid line in F ig C ---1).

C2. Method.  The controlling section size of carburized gearing can be determined using the same general principles described in Appendix B for through hardened gearing. Figure B ---1 in Appendix B describesexamples of how the controllingsection size is determined for through hardened gearing when the teeth a re cut af ter heat trea ting. The same examples

0

Approximate Controlling Section Size, mm 400 600 800 1000

200

1200

1400

AISI 9310 AISI 4820 CASE

ADEQUATE HARDE NABILITY

AISI 4320 CASE MAY OR M AY N O T HARDEN

AISI 8822 AISI 8620

NO C ASE HARDENABILITY Source: The Influence of Mi crostructure on the  of Case---Carburized Components 

by G eoff rey Pa rrish, ASM Text (1980) AISI 4118 0

5

10

15

20 25 30 35 40 45 Approximate Cont rolling Section Size, inch

50

55

60

FigC---1 Effect of ControllingSection on theCaseHardenabilityof CarburizingGrades ofSteel

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AppendixD ServiceLifeConsiderations [This Appendix is provided for informational purposes only and should not be construed as part of AG MA St andard 2004---B 89,   Gear Materials and H eat Treatment M anual .]

D1. Purpose. G ears are generallyremoved fromservice due to wea r, pitting, plastic flow, or breakage. If the service life is less tha n expected, a n in ---dept h investigation should be initiated. This Appendix deals briefly with the causes of gear fa ilures and the t ypes of failures encountered.

tion criticals in the system causing vibration, ina dequate grounding, etc.

D2.7MaterialCauses. Although materialsrarely are the principal cause of failure, they can contribute to fa ilure if mate rial selection results in less tha n the required combination of properties compatible with the design and a pplication. Improper selection of material can result in inadequate hardness (surface or subsurface) and toughness, or improper microstructure af ter heat trea tment. Wrought mat erials such a s hot ro lled ba rs can ha ve serious banding, which is a lloy and carbon segregation in banded form. B anding can affect properties, particularly in a carburized case and core. D2.7.1 Forging Defects.   Forging defects which can contribute to premature failure include excessive forging temperature, inadequate reduction, improper grain flow, f lakes, a nd bursts from insufficient forging temperature.

D2. Causes of Lower than Expected Life.  When shorter than expected life is obtained, a number of fa ctors should be reviewed. These factors are gear design, manufacture, heat treatment, assembly and installation, maintenance, service conditions and materia l causes. D2.1 Gear Design.  Failures related to gear design may be due to improper geometry or tolerances; i.e., pressure angle, tooth thickness, gear class or type, etc. D2.2 Manufacture.  Ma nufacturing practices which could shorten service life include grinding burns, insufficient or excessive stock removal after heat treatment, straightening, cracks, stress risers (tool marks and surface finish), poor radii, etc.

D2.7.2 Casting Defects.   Casting defects which can contribute to premature failure include shrinkage, porosity, slag, chemical deviation, cracks, sand, improper weld repair, core shift, cold shuts, etc.

D2.3 HeatTreatment.  H eat treat factors which could affect service life include under or over heating, secondary transformation products, surface decarburization, inadequate quench, improper hardness, microstructure, case depth, decarburization, and quench cracks.

D2.7.3 Inclusions. An infrequent cause of fra cture initiation is internal non ---metallic inclusions which relate to melting practices. Steels can be specified to varying cleanliness levels. Inadequate stock removal can leave undesirable surface defects. D3. TypesofGearFailures. Types of gear fa ilure are pictured in AG MA 110, Nom enclature of G ear Tooth 

D2.4 Assembly and Installation.  Improper assembly and installation are major contributors to premature failures and ma nifest themselvesin excessive loa ding, wear, and misalignment.

Failure M odes.

D3.1 Wear.   The most common wear failure modes are a dhesion, abrasive scoring, corrosion, and flaking. These usually occur at or a bove the pitch line. Wear is influenced by surface hardness and microstructure.

D2.5 Maintenance.   Failures related to inadequate maintenance include: contamination of the system; improper lubrication; vibration due to inadequa te rigidity, faulty gaskets, seals, and bea rings; and corrosion.

D3.2 Pitting.   Pitting modes are initial pitting, destructive pitting, a nd spalling, a nd result from excessive sliding and rolling contact stresses. Pitting resistance is influenced by surface finish, surface hardness, surface residual stress, microstructure, case depth, and core hardness.

D2.6 Service Conditions.   Service conditions which could adversely effect gear life are excessive temperatures, overload, shock or impact loading, contaminants, loss of lubrication, corrosion, vibra-

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