ANSI-AGMA 2004-B89-1995 Gear Materials and Heat Treatment Manual

ANSI/AGMA 2004---B89 (Revision of AGMA 240.01) January 1989 Reaffirmed October 1995

AMERICAN NATIONAL STANDARD Gear Materials and Heat Treatment Manual

Gear Materials and Heat Treatment Manual

Gear Materials And Heat Treatment Manual AGMA 2004---B89 (Revision of AGMA 240.01) [Tables or other self---supporting sections may be quoted or extracted in their entirety. Credit lines should read: Extracted from AGMA 2004---B89, Gear Materials and Heat Treatment Manual, with the permission of the publisher, the American Gear Manufacturers Association, 1500 King Street, Suite 201, Alexandria, Virginia 22314.] AGMA Standards are subject to constant improvement, revision or withdrawal as dictated by experience. Any person who refers to an AGMA Technical Publication should be sure that the publication is the latest available from the Association on the subject matter.

ABSTRACT The Gear Materials and Heat Treatment Manual provides information pertaining to engineering materials and material treatments used in gear manufacture. Topics included are definitions, selection guidelines, heat treatment, quality control, life considerations and a bibliography. The material selection 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, distortion, and shot peening are discussed. Quality control is discussed as related to gear blanks, process control, and metallurgical testing on the final products.

Copyright E, 1989 Reaffirmed October 1995

American Gear Manufacturers Association 1500 King Street, Suite 201 Alexandria, Virginia 22314

February 1989

ISBN: 1---55589---524---7

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FOREWORD [The foreword, footnotes, and appendices, if any, are provided for informational purposes only and should not be construed as part of AGMA Standard 2004---B89 (Formerly 240.01), Gear Materials and Heat Treatment Manual.] The Standard provides a broad range of information on gear materials and their heat treatment. It is intended to assist the designer, process engineer, manufacturer and heat treater in the selection and processing of materials for gearing. Data contained herein represents a consensus from metallurgical representatives of member companies of AGMA. This Standard replaces AGMA 240.01, October 1972. The first draft of AGMA 240.01, Gear Materials Manual, was prepared in October 1966. It was approved by the AGMA membership in March 1972. Reprinting of AGMA 240.01 for distribution was discontinued in 1982 because it had been decided in 1979 by the Metallurgy and Materials Committee to revise its format. The initial draft of AGMA 2004---B89 (formerly 240.01) was completed in April, 1983. Work continued on the Standard with numerous additional revised drafts within the Metallurgy and Materials Committee until it was balloted in 1988. It was completed and approved by the AGMA Technical Division Executive Committee in September 1988 and on January 23, 1989 it was approved as an American National Standard. Suggestions for the improvement of this standard will be welcome. They should be sent to the American Gear Manufacturers Association, 1500 King Street, Suite 201, Alexandria, Virginia 22314.

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PERSONNEL of the AGMA Committee for Metallurgy And Materials Chairman: L. E. Arnold (Xtek, Inc.) Vice Chairman: G. J. Wiskow (Falk)

ACTIVE MEMBERS N. P. Milano (Regal Beloit Corporation) A. G. Milburn (The Gear Works --- Seattle) P. Rivart (CLECIM) R. H. Shapiro (Arrow Gear) W. L. Shoulders (Reliance Electric) (Deceased) M. Starozhitsky (Outboard Marine) A. A. Swiglo (IPSEN) S. Tipton (Caterpillar) D. Vukovich (Eaton) L. L. Witte (General Motors)

M. Abney (Fairfield Manufacturing) R. J. Andreini (Earle M. Jorgensen) E. S. Berndt (C and M of Indiana) J. Bonnet (WesTech) N. K. Burrell (Metal Improvement Co. Inc.) R. J. Cunningham (Boeing) P. W. Early, Jr. (Gleason) A. Giammarise (General Electric) J. P. Horvath (G. M. Chevrolet --- Muncie) J. Bruce Kelly (General Motors) D. R. McVittie (The Gear Works --- Seattle)

ASSOCIATE MEMBERS R. L. Leslie (SPECO Corporation) B. L. Mumford (Alten Foundry) G. E. Olson (Cleveland) J. R. Partridge (Lufkin) E. M. Rickt (Auburn Gear) H. I. Sanderow (Supermet) R. L. Schwettman (Xtek, Inc.) L. J. Smith (Invincible Gear) Y. Sueyoshi (Tsubakimoto Chain) M. Tanaka (Nippon Gear) R. E. Vaglia (Farrel Connecticut) T. L. Winterrowd (Cummins Engine)

T. Bergquist (Western Gear) J. D. Black (General Motors) E. R. Carrigan (Emerson Electric) P. E. Cary (Metal Finishing) H. B. Gayley (IMO Delaval) J. F. Craig (Cummins Engine) T. C. Glew (Prager) D. K. Guttshall (IMO Delaval) W. H. Heller (Peerless Winsmith) D. L. Hillman (Westinghouse, Air Brake) B. A. Hoffmann (Dresser) L. D. Houck (Mack Trucks) A. J. Lemanski (Sikorsky)

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Table of Contents Section Title

Page

1.

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

2.

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

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

3.

Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

4.

Materials Selection Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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

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

6.

Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Grade and Heat Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cleanliness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dimensional Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cost and Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hardenability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Machinability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ferrous Gearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selection Criteria for Wrought, Cast, or Fabricated Steel Gearing . . . . . . . . . . . . Copper Base Gearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Non---Ferrous Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non---Metallic Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Through Hardening Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flame and Induction Hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carburizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbonitriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Heat Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shot Peening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Residual Stress Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

26 28 34 38 39 41 42 42 47 51

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

Incoming Material Quality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Incoming Material Hardness Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Incoming Material Mechanical Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heat Treat Process Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Part Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metallurgical, Mechanical and Non---Destructive Tests and Inspections . . . . . . . . Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Property Test Bar Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

52 52 53 53 55 56 61 63

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

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Appendices Appendix A Appendix B

Plastic Gear Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Approximate Maximum Controlling Section Size Considerations for Through Hardened Gearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Case Hardenability of Carburizing Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Service Life Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

65

6

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

Typical Gear Materials --- Wrought Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical Brinell Hardness Ranges and Strengths for Annealed, Normalized & Tempered Steel Gearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical Brinell Hardness Ranges and Strengths for Quenched and Tempered Steel Gearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Machinability of Common Gear Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Property Requirements --- Cold Drawn, Stress Relieved Steel Bars (Special Cold Drawn, High Tensile) . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical Chemical Analyses for Though Hardened Cast Steel Gears . . . . . . . . . . . Tensile Properties of Through Hardened Cast Steel Gears . . . . . . . . . . . . . . . . . . . Minimum Hardness and Tensile Strength Requirements for Gray Cast Iron . . . Mechanical Properties of Ductile Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Analyses of Wrought Bronze Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical Mechanical Properties of Wrought Bronze Alloy Rod and Bar . . . . . . . . . Chemical Analyses of Cast Bronze Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Properties of Cast Bronze Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11 14 14 16 17 22 22 23 24

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

Test Bar Size for Core Hardness Determination . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical Effective Case Depth Specifications for Carburized Gearing . . . . . . . . . . Approximate Minimum Core Hardness of Carburized Gear Teeth . . . . . . . . . . . . Approximate Minimum Surface Hardness --- Nitrided Steels . . . . . . . . . . . . . . . . . Commonly Used Quenchants for Ferrous Gear Materials . . . . . . . . . . . . . . . . . . . Typical Shot Size and Intensity for Shot Peening . . . . . . . . . . . . . . . . . . . . . . . . . . .

35 38 39 41 43 50

Appendix C Appendix D

67 69 70

Tables Table 4---1 Table 4---2 Table 4---3 Table 4---4 Table 4---5

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Page

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

Typical Design of Cast Steel Gears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Directionality of Forging Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Fig 5---1

Fig 5---4 Fig 5---5 Fig 5---6 Fig 5---7 Fig 5---8

Variation in Hardening Patterns Obtainable on Gear Teeth by Flame Hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Variations in Hardening Patterns Obtainable on Gear Teeth by Induction Hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recommended Maximum Surface Hardness and Effective Case Depth Hardness Versus Percent Carbon for Flame and Induction Hardening . . . . . . . General Design Guidelines for Blanks for Carburized Gearing . . . . . . . . . . . . . . . Typical Distortion Characteristics of Carburized Gearing . . . . . . . . . . . . . . . . . . . . Shot Peening Intensity Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Residual Stress by Peening 1045 Steel at 62 HRC with 330 Shot . . . . . . . . . . . . . . Depth of Compressive Stress Versus Almen Intensity for Steel . . . . . . . . . . . . . . .

33 45 46 48 49 50

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

Circular (Head Shot) Magnetic Particle Inspection . . . . . . . . . . . . . . . . . . . . . . . . . Coil Shot Magnetic Particle Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ultrasonic Inspection Oscilloscope Screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distance---Amplitude Reference Line for 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 Forgings for Rings for Reduction Gears ASTM A310---77, Methods and Definitions for Mechanical Testing of Steel Products

This Manual was developed to 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 A311---79, Specification for Stress Relieved Cold Drawn Carbon Steel Bars Subject to Mechanical Property Requirements

Metallurgical aspects of gearing as related to rating (allowable sac and sat values) are not included, but, are covered in AGMA rating standards.

ASTM A356---84, Heavy---Walled Carbon, Low Alloy, and Stainless Steel Castings for Steam Turbines ASTM A370---77, Methods and Definitions for Mechanical Testing of Steel Products

2. References and Information

ASTM 388---80, Recommended Practice for Ultrasonic Examination of Heavy Steel Forgings

2.1 References. Abbreviations are used in the references to specific documents in this Standard. The abbreviations include: AGMA, American Gear Manufacturers Association; ASNT, American Society of Nondestructive Testing; ASTM, American Society for 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 Carburizing Steels for Anti---Friction Bearings ASTM A535---85, Standard Specification for Special ---Quality Ball and Roller Bearing Steel

The following documents contain provisions which, through reference in this Standard, constitute provisions of this document. At the time of publication, the editions were valid. All publications are subject to revision, and the users of this Standard are encouraged to investigate the possibility of applying the most recent editions of the publications listed.

ASTM A536---80, Specification for Ductile Iron Castings ASTM A833---84, Indentation Hardness of Metallic Materials by Comparison Hardness Testers ASTM A609---83, Specification for Steel Castings, Carbon and Low Alloy Ultrasonic Examinations Thereof

AGMA 141.01---1984, Plastics Gearing --Molded, Machined, And Other Methods, A Report on the State of the Art

ASTM B427---82, Specification for Gear Bronze Alloy Castings

AGMA 2001---B88, Fundamental Rating Factors and Calculation Methods for Involute Spur and Helical Gear Teeth

ASTM B505---84, Specification for Copper---Base Alloy Continuous Castings ASTM E8---83, Methods of Tension Testing of Metallic Materials

AGMA 6033---A88, Standard for Marine Propulsion Gear Units, Part 1 Materials

ASTM E10---78, Test Method for Brinell Hardness of Metallic Materials

ANSI/AGMA 6034---A88, Practice for Single and Double Reduction Cylindrical ---Worm and Helical --Worm Speed Reducers

ASTM E18---79, Test Methods for Rockwell Hardness and Rockwell Superficial Hardness of Metallic Materials

ASNT---TC---1A (June 80), Recommended Practice by American Society for Nondestructive Testing

ASTM E54---80, Method for Chemical Analysis of Special Brasses and Bronzes

ASTM A48---83, Specification for Gray Iron Castings

ASTM E112---84, Methods for Determining Average Grain Size SAE J434---June 86, Automotive Ductile (Nodular) Iron Castings

ASTM A148---84, Steel Castings, High Strength, for Structural Purposes ASTM A220---76, Specification for Pearlitic Malleable Iron Castings

SAE J461---Sept 81, Wrought and Cast Copper Alloys

ASTM A255---67, Method for End---Quench Test for Hardenability of Steel ANSI/AGMA

SAE J462---Sept 81, Cast Copper Alloys

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SAE J463---Sept 81, Wrought Copper and Copper Alloys

American Society for Testing and Materials ASTM Standards Society of Automotive Engineers, Inc. SAE Handbook American Iron and Steel Institute AISI Steel Products Manuals American National Standards Institute ANSI Standards Naval Publications and Forms Center Military Standards and Specifications Metal Powder Industries Federation MPIF Standard 35 Copper Development Association CDA Data books Iron Castings Society Gray and Ductile Iron Castings Handbook Steel Founders’ Society Steel Castings Handbook

SAE J808a---SAE HS 84, Manual on Shot Peening MIL---S---13165 B (31 Dec 66 Amendment 2---25 June 79), Shot Peening of Metal Parts MIL---STD---271F, Requirements for Nondestructive Testing Methods ASTM E709---80, Magnetic Particle Examination ASTM E125, Reference Photographs for Magnetic Particle Indications on Ferrous Castings ASTM E186---8, Standard Reference Radiographs for Heavy Walled (2 to 4 1/2 inch)(51 to 114 mm) Steel Castings ASTM E280---81, Standard Reference Radiographs for Heavy Walled (4 1/2 to 12 inch)(114 to 305 mm) Steel Castings

3. Definitions

ASTM E399---83, Test Method for Plain ---Strain Fracture Toughness of Metallic Materials

Annealing --- Full. Full 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). This treatment forms coarse lamellar pearlite, the best microstructure for machinability of low and 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 that produces a globular carbide in a ferritic matrix. This heat treatment results in the best machinability for high carbon (0.60 percent 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 transformation range in a medium having a rate of cooling sufficiently high to prevent high temperature transformation products, and maintaining the alloy temperature within the bainitic range until desired transformation is obtained. The bainitic transformation range is below the pearlitic range, 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 phase consisting of a solid solution of carbon and alloying elements in face---centered cubic crystal structured iron.

ASTM E446---81, Standard Reference Radiographs for Steel Castings Up to 2 inch (51 mm) in Thickness ANSI/SAE AMS 2300 F, Magnetic Particle Inspection, Premium Aircraft ---Quality Steel Cleanliness ANSI/SAE AMS 3201 G, Magnetic Particle Inspection, Aircraft ---Quality Steel Cleanliness 2.2 Information Sources. Design of gears is concerned with the selection of materials and metallurgical processing. This Manual cannot substitute for metallurgical expertise, but is intended to be a basic tool to assist in the selection and metallurgical processing of gear materials. The material information and metallurgical processes contained herein are based on 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 and processing. Material specifications are issued by agencies, including the government, large industrial users, and technical societies, some of whom are: ASM International ASM Metals Handbooks ASM Heat Treaters Guide ASM Metals Reference Book ASM Standard ANSI/AGMA

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Austenitizing Temperature. The temperature at which ferrous alloys undergo a complete microstructural phase transformation to austenite.

case. Hardness survey is preferred for contral purposes. (3) Total 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 base material. This is approximately 1.5 times the effective case depth. (4) Case depth to 0.40 percent carbon. Effective case depth is less frequently referred to as the depth to 0.40 percent carbon. This depth may be measured by analyzing the carbon content or estimating based on microstructure. Estimating based on microstructure ignores the hardenability of the base material and is not as accurate a measurement as directly analyzing the carbon level. There is poor correlation between microstructure readings and material strength gradients using this method.

Bainite. Bainite is a microstructural phase resulting from the transformation of austenite, and consists of an aggregate of ferrite and 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. Carbon. Carbon is the principal hardening element in steel, and it’s amount determines the maximum hardness obtainable. Generally as carbon is increased, tensile strength and wear resistance increase; however, ductility and weldability 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 an ammonia enriched carburizing atmosphere. This results in simultaneous absorption of carbon and nitrogen, which results in the formation of complex nitrides in a high carbon case.

Case Depth of Flame or Induction Harden Components. This is defined as the depth at which the hardness is 10 HRC points below the minimum specified surface hardness. Case Depth of Nitrided Components. Nitrided case depth is defined as the depth at which the hardness is equivalent to 105 percent of the measured core hardness. The case depth is determined by a microhardness tester and measured normal to the tooth surface at 0.5 tooth height and mid face width.

Carburizing--- Gas. Gas carburizing consists of heating and holding low carbon or alloy 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 carbon is typically obtained at the surface). Temperatures above 1800_F (982_C) may be ultilized in specialized equipment such as vacuum carburizers. After carburizing, parts are either cooled to 1475---1550_F (802---843_C) and held at this temperature to stabilize and then direct quenched; or slow cooled and reheated to 1475---1550_F (802---843_C) and quenched.

Case Hardness. Case Hardness is the micro--hardness measured perpendicular to the tooth surface at 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 (Fe3C) and characterized by an orthorhombic crystal 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 Manual will be effective case depth. Carburized case depth terms are defined as follows:

Combined Carbon. The amount of carbon in steel or cast iron that is present 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 and 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 tooth design purposes is the hardness at the intersection of the root diameter and the centerline of the tooth at mid face width on a finished gear.

(2) Etched case depth. Etched case depth is determined by etching a sample cross---section with nitric acid, and measuring the depth of the darkened area. The etched case approximates the effective ANSI/AGMA

Decarburization. Decarburization is the reduction in surface carbon content of a gear 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 oxyfuel burner heating to 1450---1650_F (788---899_C) followed by quenching and tempering.

sorbed into the surface of a ferrous material 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 furnace atmosphere at 950---1060_F (510---571_C) causing nitrogen to be absorbed into the surface, forming hard iron nitrides.

Grain Size. Grain size is specified as either coarse (grain size 1 through 4) or fine (grain size 5 through 8), determined according to ASTM E112. Graphite. Graphite 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 as 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 ferrous material at a temperature below the austenitizing temperature [1000---1150_F (538---621_C)]. Nitrocarburizing is done mainly for antiscuffing and to improve surface fatigue properties.

Hardenability. An indication of the depth to which a steel will harden during heat treatment (see 4.6). Hardening. The process of increasing hardness, typically through heating 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. Normalizing is used primarily to obtain a uniform microstructure.

H--- Band Steels. H---Band steels are steels which are produced and purchased to a specified Jominy hardenability range. Induction Hardening. Induction hardening of gearing is the selective heating of gear teeth profiles to 1450---1650_F (788---899_C) by electrical inductance through the use of a coil or single tooth inductor to obtain the proper heat pattern and temperature, followed by quenching 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 austenite transformation state at 1475---1650_F (802---899_C), followed by rapid cooling (quenching). The part is then reheated (tempered) to a specific temperature generally below 1275_F (690_C) to achieve the desired mechanical properties for the gear application.

Jominy End Quenching Hardenability Test. The standard method for determining the hardenability of steel. The test consists of heating a standard 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 hardness at 1/16 inch (1.6 mm) intervals starting at the quenched end.

Stress Relief. Stress relief is a thermal cycle used to relieve residual stresses created by prior heat treatments, machining, cold working, welding, or other fabricating techniques. Maximum stress relief is achieved at 1100_F (593_C) minimum.

Martensite. Martensite is the diffussionless transformation of austenite to a body centered tetragonal structure, characterized by an acicular needle---like appearance.

Surface Hardness. Surface Hardness is the hardness measured directly on the surface. To obtain 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 magnification.

Tempering. Tempering is reheating 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 abANSI/AGMA

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crostructure, material cleanliness, surface conditions and residual stresses.

Test Coupon. A test coupon is an appropriately sized sample(often a bar) used generally for surface hardening treatments. 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.

4.1.3 Tensile Strength. Tensile strength predicts the stress level above which fracture occurs. It is not recommended for use in gear manufacturing specifications. 4.1.4 Yield Strength. Yield strength determines the stress level above which permanent deformation occurs.

Through Hardening. Through hardening is a term used to collectively describe methods of heat treatment of steel other than surface hardening techniques. These include: annealing, normalizing (or normalizing and tempering) and quenching and tempering (refer to 5.1). Depth of hardening is dependent upon hardenability, section size and heat treat considerations.

4.1.5 Toughness. Toughness is determined 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 steel gearing is adversely affected by a variety of factors such as:

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

(1) Low temperature (2) Improper heat treatment or microstruc--ture (3) High sulfur (4) High phosphorus and embrittling type residual elements (5) Nonmetallic inclusions (6) Large grain size (7) Absence of alloying elements such as nickel. NOTE: Gear toughness is adversely affected by design or manufacturing considerations (such as notches, small fillet radii, tool marks, material defects, etc., which act as stress concentrators).

Transformation Temperature. The temperature at which a change in microstructure phase occurs.

4. Material Selection Guidelines Many factors 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 Grade and Heat Treatment Cleanliness Dimensional Stablility Availability and Cost Hardenability and Size Effects Machinability and Other Manufacturing Characteristics

4.1.6 Heat Treatment. Most wrought ferrous materials used in gearing are heat treated to meet hardness and/or mechanical property requirements. Round and flat stock can be purchased in numerous combinations of mechanical and thermal processing, such as hot rolled, cold rolled, cold drawn, stress relieved, pickled, annealed, and quenched and tempered. Gear blanks are generally given an annealing or normalizing heat treatment, which homogenizes the micro--- structure for machinability and mechanical property uniformity. Gear blanks can also be quenched and tempered.

4.1 Mechanical Properties. It is necessary for the gear designer to know the application and design loads and to calculate the stresses before the material selection can begin. 4.1.1 Hardness. The strength properties are closely related to material hardness, which is used in AGMA gear rating 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 type of mechanical working. Minimum

4.1.2 Fatigue Strength. Contact 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 factors such as hardness, miANSI/AGMA

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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 for grades and recommended heat treatments.

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

Table 4---1 Typical Gear Materials --- Wrought Steel Common Alloy Steel Grades

Common Heat 1 Treat Practice

General Remarks/Application

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 Hardenability Marginal Hardenability Fair Hardenability Medium Hardenability Medium Hardenability Good Hardenability in Heavy Sections

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 Heat Treatment Special Heat Treatment Quench Crack Sensitive Good Hardenability Used when 4140 exhibits Marginal Hardenability Quench Crack Sensitive, Excellent Hardenability in Heavy Sections

1020

C---H

Very Low Hardenability

4118 4620 8620

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

Fair Core Hardenability Good Case Hardenability Fair Core Hardenability

4320 8822

C---H C---H

Good Core Hardenability Good Core Hardenability in Heavy Sections

3310 @ 4820 9310

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

Excellent Hardenability (in Heavy Sections) for all three grades

1 C---H = Carburize Harden T---H = Through Harden

F---H = Flame Harden I---H = Induction Harden T---H&N = Through Harden then nitride

2 Recognized, but not current standard grade.

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Table 4---2 Typical Brinell Hardness Ranges and Strengths for Annealed, Normalized and Tempered Steel Gearing Normalized & Tempered #

Annealed Heat Treatment @ Typical Alloy Steels 1 Specified

1045 4130 8630 4140 4142 8640 4145 4150 4340 4350 Type

Brinell Hardness Range HB

Tensile Strength min ksi (MPa)

Yield Strength min ksi (MPa)

Brinell Hardness Range HB

Tensile Strength min ksi (MPa)

Yield 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)

1. Steels shown in order of increased hardenability. 2. Hardening by quench and tempering results in a combination of properties generally superior to that achieved by anneal or normalize and temper; i.e., impact, ductility, etc. See Table 4---3 for quench and tempered gearing. 3. Hardness and strengths able to be obtained by normalize and tempering are also a function of controlling section size and tempering temperature considerations. 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.

4.3 Cleanliness. Alloy steel manufactured with electric furnace practice for barstock and forged steel gear applications is commonly vacuum degassed, inert atmosphere (argon) shielded and bottom 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 arc remelting (VAR) or electroslag remelting (ESR) of the steel. These refining processes further reduce gas and inclusion size and content for improved fatigue strength to produce the highest quality steel for critical gearing applications. Significant ANSI/AGMA

NOTE: For more information see ASTM A534 and A535, and AMS 2301 and 2300. 4.4 Dimensional Stability. The process to achieve the blueprint design may require material 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 availability factors such as standard industry alloys and procurement time.

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Table 4---3 Typical Brinell Hardness Ranges and Strengths for Quenched and Tempered Alloy Steel Gearing Alloy Steel * Grade 4130 8630 4140 8640 4142 4145 4150 4340 4350

Tensile Strength minimum ksi (MPa)

Yield Strength minimum ksi (MPa)

Heat Treatment

Hardness Range HB [

Water 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)

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. [ Hardness range is dependent upon controlling section size (refer to appendix B) and quench severity. ] It is difficult to cut teeth in 4100 Series steels above 341 HB and 4300 Series steels above 375 HB. (4340 and 4350 provide advantage due to higher tempering temperatures and microstructure considerations) w High specified hardness is used for special gearing, but costs should be evaluated due 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 quantities and with short delivery time from their inventories. Steel mill purchases require “mill quantities” (several thousand pounds) and long delivery time. However, the mill quantity cost may be substantially lower, and non---standard steels can be supplied on special request. When specifying parts with small quantity requirements, standard alloys should be specified or engineering drawings should allow optional materials. In the case of steel and iron castings and nonferrous materials, SAE and ASTM designations should be used wherever possible.

by quenching from the austenitizing temperature. The as quenched surface hardness is dependent primarily on the carbon content of the steel part and cooling rate. The depth to which a particular hardness is achieved with a given quenching condition is a function of the hardenability, which is largely determined by the alloy content of the steel grade. 4.6.1 Determination. Hardenability is normally determined by the Jominy End Quench Test (ASTM A255) or can be predicted by the Ideal Diameter (DI) concept. 4.6.1.1 Jominy Test Method. A one inch (25 mm) diameter bar, four inches (102 mm) in length is first normalized then uniformily heated to a standard austenitizing temperature. The bar is placed in a fixture, then quenched by spraying room temperature water against one end face.

4.6 Hardenability. Hardenability of steel is the property that determines the hardness gradient produced ANSI/AGMA

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4.6.1.2 Jominy Analysis. Rockwell C hardness measurements are made along the length of the bar on ground flats in one sixteenth of an inch (1.6 mm) intervals. Jominy hardenability is expressed in HRC obtained at each interval starting at the water quenched end face. Example: J5 = 40 is interpreted as a hardness of 40 HRC at a distance of 5/16 inch (8 mm) from the water quenched end. 4.6.1.3 H--- Band Steel. Jominy hardenability has been applied to standard steels. For a given composition the Jominy hardenability data falls within a predicted range. Steels purchased to predicted hardenability ranges are called H---Band steels. These Bands are published by ASTM, AISI, and SAE. Steels can be purchased to H---Band, or restricted H---Band, specifications. 4.6.1.4 Ideal Critical Diameter. The Ideal Critical Diameter Method (DI) is based on chemical analysis described in AISI, SAE, Modern Steels and Their Properties by Bethlehem Steel, and other hardenability reference publications. 4.6.2 Application. Hardenability is constant for a given steel composition; however, hardness will vary with the cooling rate. Therefore, the hardness obtained 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 that will yield an as quenched hardness above the specified hardness so that toughness and machinability can be attained through appropriate tempering. As the section thickness increases, the steel hardenability must be increased in order to maintain a given hardness in the part section.

(4) Characteristics 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 factors will be discussed. Chemical composition and microstructure of steel have major influences on machinability, since they affect properties and structures. Metallic oxides like alumina and silica form hard oxide inclusions and contribute to poor machinability. Elements such as sulfur, lead, selenium, and tellurium form soft inclusions in the steel matrix and can benefit machining. Calcium additions (in steel making) form hard, irregular inclusions and can also benefit machining. However, sulfur, lead and calcium inclusions which improve machinability can decrease mechanical properties, particularly in the transverse direction. Calcium treated steel, when used in high stress gear and shaft applications, may significantly reduce fatigue life compared to conventional steelmaking practices. Carbon content over 0.30 percent decreases machinability due to increased hardness. Dependent on carbon and sulfur levels, higher manganese also decreases machinability. In general, alloys which increase hardness and toughness decrease machinability. The more common gear materials are listed in Table 4---4 on the basis of good, fair, and 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 40 to 50 percent. 4.8 Ferrous Gearing. Ferrous materials for gearing include carbon and alloy wrought and cast steels, cast iron and ductile irons. Gearing of alloy 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.

4.7 Machinability. Several factors influence the machinability of materials and in turn affect the economy and feasibility of manufacturing. These factors must be considered at the design stage, particularly when high strength levels are being specified. Factors influencing machinability are: (1) Material being cut, including composition, microstructure, hardness, shape, and size. (2) Cutting speeds, feeds and cutting tools. (3) Condition of machine tools, including rigidity, precision, power, etc.

ANSI/AGMA

4.8.1 Wrought Steel. 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 forgings. Forgings reduce machining time, and are available in a wide range of sizes and grades.

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Table 4---4 Machinability of Common Gear Materials Material Grades

Low--- Carbon Carburizing Steel Grades --- Remarks

1020

Good machinability, as rolled, as forged, or normalized.

4118 4620 8620 8822

Good machinability, as rolled, or as forged. However, normalized is preferred. Inadequate cooling during normalizing can result in gummy material, reduced tool life and poor surface finish. Quench and temper as a prior treatment can aid machinability. The economics of the pretreatments must be considered.

3310 4320 4820 9310 Material Grades

Fair to good machinability if normalized and tempered, annealed or quenched and tempered. Normalizing without tempering results in reduced machinability.

1045 1141 1541

Good machinability if normalized.

4130 4140 4142

Good machinability if annealed, or normalized and tempered to approximately 255 HB or quenched and tempered to approximately 321 HB. Over 321 HB, machinability is fair. Above 363 HB, machinability is poor. Inadequate (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

Remarks for medium carbon alloy steel (above) apply. However, the higher carbon results in lower machinability. Sulfur additions aid the machinability of these grades. 4340 machinability is good up to 363 HB. The higher carbon level in 4145, 4150, 4345, and 4350 makes them more difficult to machine and should be specified only for heavy sections. Inadequate (slack) quench can seriously affect machinability in these steels.

Medium Carbon Through Hardened Steel Grades --- Remarks

NOTE: Coarse grain steels are more machinable than fine grain. However, gear steels are generally used in the fine grain condition since mechanical properties are improved, and distortion during heat treatment is reduced. Increasingly cleaner steels are now also being specified for gearing. However, if sulfur content is low, less than 0.015 percent, machinability may decrease appreciably. Material Grades Gray Irons

Other Gear Material --- Remarks Gray cast irons have good machinability. Higher strength gray cast irons [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, machinability is poor.

Gear Bronzes and Brasses

All gear bronzes and brass have good machinability. The very high strength heat treated bronzes [above 110 ksi (760 MPa) tensile strength] have fair machinability.

Austenitic Stainless Steel

All austenitic stainless steel grades only have fair machinability. Because of work hardening tendencies, feeds and speeds must be selected to minimize work hardening.

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are manufactured 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 application.

4.8.1.1 Round Stock. Round bars can be purchased in various diameters for standard carbon and alloy grades. They are typically available as hot rolled, hot rolled---cold drawn, hot rolled---cold finished and forged rounds. Cold drawing produces a close tolerance bar with improved mechanical properties (higher hardness and yield strength). Low to medium carbon steels are normally available as cold drawn bar for gearing. Hot rolled---cold finished bars are machined (turned, ground and/or polished) for improved size control, but show no improvement in mechanical properties over hot rolled or annealed bar. Hot rolled bars are mechanically worked at approximately 2100---2400_F (1150---1315_C) and may be subsequently annealed, straightened and stress relieved. Forged round bars are forged round under a press or hammer at the same approximate temperature as hot rolled bars (higher temperature for lower carbon content carbon or alloy steel) and

Hot rolled bars are also now manufactured from continuous cast steel bar manufactured with continuous casters. Continuous 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 applications. Approximate maximum diameter of the various types of round stock, depending upon steel mill capacity, is as follows: Hot Rolled: Cold Drawn: Cold Finished: Forged Round:

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

Table 4---5 Mechanical Property Requirements --- Cold Drawn, Stress Relieved Steel Bars (Special Cold Drawn, High Tensile) Size included inch (mm)

Steel Designation

Mechanical Properties for Rounds, Squares and Hexagons Minimum Minimum Elongation in Nominal Tensile Strength Yield Strength 2 inches (50 mm) Hardness percent, min ksi (MPa) ksi (MPa) HRCw

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 30 32

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)

9 9 9

24 24 24

0.375 (10) to 3.000 (76)

* Stress Relieved. [ Special steel. Additional requirements: Hardness, Rockwell C 30, min. 1144 SS not available above 2.5 in (64 mm). ] Special steel. Additional requirements: Hardness Rockwell C 32, min. 4145 SS not available above 3.5 in (89 mm). w Typical value, not a requirement. NOTE: Some cold finish steel companies furnish many of the above steels under various trade names. ANSI/AGMA

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

(3) Rolled Ring Forging. This method produces a donut---shaped work piece. Typically the process involves piercing a pancake---shaped billet with a mandrel and shaping the ring by a hammer action between the mandrel and the press anvil. Large diameter rings are rolled on a roller press from circular billets containing a central hole. For additional information on wrought steel manufacture and steel making refining practices, reference should be made to the following sources: American Society for Metals (ASM International), Metal Handbooks American Iron and Steel Institute (AISI), Steel Products Manual Forging Industry Handbook, by the Forging Industry Association

4.8.1.3 Forgings. Forgings are made by hot mechanical deformation (working of a steel billet into a specific form) which densifies the structure, and may provide improved inclusion orientation. Typically, deformation is done while the billet is at temperatures generally above 1900_F(1038_C). Cast ingots, from which blooms and billets are manufactured prior to forming forgings and 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. Bottom poured ingots show improved macro---cleanliness and ingot yield (more usable ingot metal after conventional cropping or removal of the top pipe cavity and bottom discard of top poured ingots).

4.8.2 Weld Fabrications. Weld fabricated 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 for the web or arm support sections. The rim or tooth section is heat treated to obtain specified hardness (mechanical properties) prior to weld assembly. After weld assembly, using appropriate preheat and postheat temperatures, welded assemblies are furnace stress relieved at 950---1250_F (510---675_C) depending upon the previous tempering temperature used to obtain the specified hardness of the 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 cleanliness techniques 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 gases during the forging process. The standard forging classifications are:

4.8.3 Cast Steels. Carbon and alloy steel castings are used for a wide variety of through hardened gearing and, to a lesser degree, for case hardened applications. The size of cast gearing varies from 10.0 inch (254 mm) outside diameter with a 2.0 inch (51 mm) face width for solid rim gears, to split ring gears about 480 inch (12 192 mm) outside diameter with a 40 inch (1016 mm) face. Smaller gears 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 ring design with bolt holes at the splits and on the inside diameter flange for gear assembly and mounting purposes. Split gears are cast in two or four segments. Typical cast gear designs are shown in Fig 4---1. 4.8.3.1 Manufacture. Cast steel is manufactured by the open hearth, electric arc, or induction furnace

(1) Open Die 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. Upset forgings are often used for critical high speed gearing, greater than 30,000 feet/minute (152 m/sec) pitch line velocity, which develop high centrifugal stress at the center. (2) Closed Die Forging. This method produces a closer toleranced piece, generally smaller than an open die forging. 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. ANSI/AGMA

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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. Carburizing grades are 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 hardenability to obtain the specified minimum hardness.

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

Typical chemical analyses and tensile properties of through hardened cast steels are shown in Tables 4---6 and 4---7, respectively.

SOLID WEB

CORED WEB

SMALLER GEARS

SOLID RING

SOLID HUB

SPLIT RING

SPLIT HUB

SPLIT HUB AND RING

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

Fig 4---1 Typical Design of Cast Steel Gears

ANSI/AGMA

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Table 4---6 Typical Chemical Analyses for Through Hardened Cast Steel Gears Alloy Percent for Cast Steel Types

Element Carbon Manganese Phosphorus, max. Sulfur, max. Silicon, max. Nickel Chromium Molybdenum

1045 Type

4140 Type

8630 Type

8642 Type

4340 Type

0.40---0.50 0.60---1.00 0.050 0.060 0.60 --- ----- ----- ---

0.37---0.43 0.70---1.00 0.030 0.040 0.60 --- --0.80---1.10 0.15---0.25

0.27---0.37 0.70---1.00 0.030 0.040 0.60 0.60---0.90 0.60---0.90 0.30---0.40

0.38---0.45 0.70---1.00 0.030 0.040 0.60 0.60---0.90 0.60---0.90 0.40---0.50

0.38---0.43 0.70---1.00 0.030 0.040 0.60 1.65---2.00 0.70---0.90 0.20---0.30

GENERAL NOTES: 1. Type designations indicate non---conformance to exact AISI analysis requirements. 2. When basic steel making practice, ladle refining or AOD (argon oxygen decarburization) processing are used, lower phosphorus and sulfur contents to less than 0.020 percent are commonly achieved. 3. Vanadium content of 0.06---0.10 percent may be specified for grain refinement. 4. Aluminum content of 0.025 percent maximum may be specified for low alloy cast steel (per ASTM A356) for ladle deoxidation to improve toughness, cleanliness and machinability. 5. Other AISI Type and proprietary chemical analyses 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 treatment and controlling section size (hardenability) considerations. 6. Source: AGMA 6033---A88, Standard for Marine Propulsion Gear Units, Part 1 Materials.

Table 4---7 Tensile Properties of Through Hardened Cast Steel Gears! Brinell Hardness Range

Minimum Tensile Strength ksi (MPa)

Minimum Yield Strength 0.2 percent Offset ksi (MPa)

Percent Minimum Elongation in 2 in (50 mm)

Percent Minimum Reduction in Area

A B C

223---269 241---285 262---311

100 (690) 110 (760) 118 (810)

75 (480) 80 (550) 90 (620)

15.0 13.0 11.0

35.0 31.0 28.0

D E

285---331 302---352

130 (900) 140 (970)

100 (690) 115 (790)

10.0 9.0

26.0 24.0

F G

321---363 331---375

145 (1000) 150 (1030)

120 (830) 125 (860)

8.0 7.0

20.0 18.0

AGMA@ 6033---A87 Class

NOTES: 1. Above tensile requirements for seven classes are modifications of three grades of ASTM A148 (Grades 105---85 through 150---135). 2. Source: AGMA 6033---A88, Standard for Marine Propulsion Gear Units, Part 1 Materials.

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4.8.3.3 Repair Welding of Cast Steel. Repair welding of castings prior to heat treatment 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 treatment. Heat treatable electrodes (4130, 4140 and 4340 Types) should be used for repairing prior to heat treatment in order to produce hardness equivalent to the base metal after heat treatment. Repair welding, if allowed after heat treatment, shall be followed by reheat treatment, whenever possible. If reheat treatment is not possible, localized preheat and post heat are recommended to avoid or minimize unfavorable residual tensile stress or high hardness in the heat affected zone. All welds should be inspected to the same quality standard used to inspect the casting.

Recommended ASTM specifications for nondestructive inspection test procedures are: ASTM E709---80, Magnetic Particle Examination ASTM E125---63 (1980), Reference Photographs for Magnetic Particle Indications on Ferrous Castings ASTM A609---83, Ultrasonic Examination of Carbon and Low Alloy Steel Castings ASTM E186---80, Standard Reference Radiographs for Heavy Walled [2 to 41/2 inch) (51 to 114 mm)] Steel Castings ASTM E280---81, Standard Reference Radiographs for Heavy 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) in Thickness 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 ASM Handbook, Volume 11, 8th edition, Nondestructive Inspection and Quality Control 4.8.4 Cast Iron. Cast Iron is the generic term for the family of high carbon, silicon, iron alloys. The family of cast irons is classified by the following categories. 4.8.4.1 Gray Iron. Gray iron contains (typically over 3.0 percent) carbon, which is present as graphite flakes. It is characterized by the gray color occurring on a fracture surface. Refer to Gray and Ductile Iron Castings Handbook for additional information. (1) Material considerations. Cast irons for gears are made by the electric arc furnace, cupola, or induction practice and should be free of shrink, porosity, gas holes, entrapped sand and hard areas in the tooth portion. Repair welds in areas to be machined should have machinability equivalent to the casting. Repair welds in the tooth portion should only be performed with the approval of the gear purchaser. (2) Heat Treating. Cast iron castings are generally furnished as cast unless otherwise specified. Stress relieving may be deemed necessary to hold close dimensional tolerances. 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).

NOTE: Weld repair in the tooth portion may require notification of the purchaser. 4.8.3.4 Heat Treatment of Cast 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 on both rim faces of gear castings is generally based on the outside diameter. The number of tests increases with OD size. Mechanical property tests (tensile and impact) are generally required only when specified. Reference should be made to 6.2 and 6.3 for additional information. 4.8.3.5 Quality of Cast Steel. Castings should be furnished free of sand, scale, extraneous appendages, and hard areas resulting from arc---airing, gas cutting, and repair welding which could adversely affect machining. Casting should also be free of cracks, hot tears, chills, and unfused chaplets in the rim section. Castings must meet the nondestructive test requirements in the rim section. The quality 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. Dry or wet fluorescent magnetic particle inspections are routinely performed to meet specified surface quality requirements. Other nondestructive testing, such as radiograph and ultrasonic inspection, is performed to evaluate internal integrity of the rim (tooth) section when specified. Methods of testing, test locations, and acceptance standards are established between the purchaser and manufacturer. ANSI/AGMA

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(3) Chemical Analysis. Unless otherwise specified, the chemical analysis is left to the discretion of the casting supplier as necessary to produce castings to the specification.

1 ASTM Class Number 20 30 35 40 50 60

(4) Mechanical Properties. Cast iron gears are rated according to AGMA practice based on hardness. Therefore, hardness determines the rating of the gear. Minimum hardness requirements for the classes of cast iron are shown in Table 4---8.

4.8.4.2 Ductile Iron. Ductile iron, sometimes referred to as nodular iron, is characterized by the spheroidal shape of the graphite in the metal matrix, produced by innoculation with magnesium and rare earth elements. A wide range of mechanical properties are produced through control of the alloying elements and subsequent heat treatments. (Refer to Gray and Ductile Iron Handbook.) (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. Repair welds in areas to be machined should have equivalent machinability as the casting. Repair welding in the tooth portion should only be performed with the approval of the gear purchaser. (2) Heat Treating. Ductile iron castings shall be heat treated by annealing, normalizing and tempering or quenching and tempering or as---cast as required to meet the specified mechanical properties. These heat treatments produce ferritic, pearlitic or martensitic structures.

Tensile tests should only be required when specified. Tensile test requirements are shown in Table 4---8, and testing should be performed in accordance with ASTM A48, Standard Specifications for Gray Iron Casting. Tensile test coupons are cast in separate molds in accordance 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:

0.25---0.50 (6.4---12.7) 0.51---1.00 (12.8---25.4) 1.01---2 incl. (25.5---50.8)

As Cast Machined Diameter, Diameter, ASTM A48 Test Bar, in (mm) in (mm) 0.88 (22.4) 1.20 (30.5) 2.00 (50.8)

0.50 (12.7) 0.750 (19.0) 1.25 (31.8)

(3) Chemical Analysis. Unless otherwise specified, the chemical analysis is left to the discretion of the casting supplier as necessary to produce castings to the specification. (4) Mechanical Properties. Typical mechanical properties are shown in Table 4---9. Other properties may be as agreed upon by the gear manufacturer and casting producer.

A B C

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

Tensile test coupons should be poured from the same ladle or heat and be given the same heat treatments as the castings they represent. Test coupon mold design shall be in accordance with ASTM A536. Size of the Y---block mold, if used, is at the option of the producer unless specified by the gear manufacturer.

Table 4---8 Minimum Hardness and Tensile Strength Requirements for Gray Cast Iron ANSI/AGMA

155 180 205 220 250 285

Tensile Strength ksi (MPa) 20 (140) 30 (205) 35 (240) 40 (275) 50 (345) 60 (415)

1 See ASTM A48 for additional information.

Hardness tests should be made in accordance with ASTM E10. Hardness tests should be made on the mid rim thickness or mid face width of the tooth portion diameter. At least one hardness test should be made on each piece, and sufficient hardness tests should be made to verify that the part meets the minimum hardness specified. Specified minimum hardness must be maintained to the finish machined dimensions for acceptance.

Thickness of Tooth Section, in (mm)

Brinell Hardness

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When eight hardness tests are specified, they shall be made 90 degrees apart on both cope and drag side.

Tensile tests should be performed in accordance with ASTM Designation E8, Standard Method of Tension Testing of Metallic Materials. The yield strength is normally determined by the 0.2 percent offset method. For required retesting, if tensile bar fails to meet requirements, refer to ASTM A536.

For solid cylindrical pieces, with length over diameter of one or more, the number of hardness tests should be as follows: Diameter of Tooth Portion, in(mm) To 3 (76) incl. Over 3 (76) to 6 (152) incl. Over 6 (152)

Hardness tests should be performed in accordance with ASTM Designation E10, Standard Method of Test for Brinell Hardness of Metallic Materials. Hardness tests should be made on the mid rim thickness or mid face width of the tooth portion diameter. Number of hardness tests per piece is based on the diameter of the casting as follows: Outside Diameter of Casting, in(mm) To 12 (305 ) Over 12 (305) to 36 (915) Over 36 (915) to 60 (1525) Over 60 (1525)

Number of Hardness Tests 1 2 4

NOTE: The hardness tests shall be spaced uniformly around the circumference. When many small pieces 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 of Hardness Tests 1 2 4 8

4.8.4.3 Austempered Ductile Iron. Austempered Ductile Iron (ADI) is a ductile iron with higher strength and hardness than conventional ductile irons. The higher properties of ADI are achieved by closely controlled chemistry and 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 made on the cope side over a riser and the other on the drag side approximately 180 degrees away between risers. When four hardness tests are required, two tests should be made on the cope side, one over a riser and the other approximately 180 degrees away between risers, and two tests on the drag side 90 degrees away from the tests on the cope side.

Table 4---9 Mechanical Properties of Ductile Iron 1 ASTM Grade Designation 60---40---18 65---45---12 80---55---06 100---70---03 120---90---02

Former AGMA Class

Recommended Heat Treatment

Min. Tensile Brinell Strength Hardness Range ksi (MPa)

A---7---a Annealed Ferritic A---7---b As---Cast or Annealed Ferritic---Pearlitic A---7---c Normalized Ferritic---Pearlitic A---7---d Quench & Tempered Pearlitic A---7---e Quench & Tempered Martensitic

Min. Yield Strength ksi (MPa)

170 max. 156---217

60 (415) 65 (450)

40 (275) 45 (310)

187---255 241---302 Range Specified

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

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

Elongation in 2 inch (50 mm) percent min 18.0 12.0 6.0 3.0 2.0

1 See ASTM A536 or SAE J434 for further information. NOTE: Other tensile properties and hardnesses should be used only by agreement between gear manufacturer and casting producer. ADI has been utilized in several significant applications, such as automotive ring gears and pinions, but is still an emerging technology. ADI permits lowANSI/AGMA

er machining and heat treat cost and replacement of more costly forgings for certain applications.

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Test programs are currently underway which will more clearly define operational properties of ADI. 4.8.4.4 Malleable Iron. Malleable 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.)

rately determined using special microhardness measurement techniques. Parts can be heat treated after sintering, but must be processed in a controlled atmosphere to prevent changes in surface chemistry. Carburizing and carbonitriding can be performed, but products with a density under 6.8 g/cm# will not develop a definite case due to the ease of diffusion through the more porous lower density material. Penetration hardness testing cannot be correlated to material strength, but parts will achieve a file hard surface. Salt baths and water quench systems should be avoided.

4.8.5 Powder Metal (P/M). Powder metal parts are formed by compressing metal powders in a die cavity and heating (sintering) the resultant compact to metallurgically bond the powder particles. Secondary operations such as repressing or sizing may be used to obtain precise control of shape and size or to improve mechanical properties. The powder metal process is used to reduce cost by eliminating machining operations, provide accurate dimensional control over large production runs, and obtain characteristics and shapes difficult to obtain by other methods. However, because of molding die costs, high production quantities 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 approaching the properties of wrought materials. Although this process is much more costly than the conventional powder metal process, it can still be cost effective for high production parts requiring higher mechanical properties than achievable using the standard process.

Although several powder metal materials are available, alloy steel is usually specified for gear applications.

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

“As sintered” alloy steels have a tensile strength range of 40---80 ksi (275---550 MPa), with an elongation of 4.0 percent or less and an apparent hardness of HRB 60---85. Heat treated powder metal alloys have tensile strengths of 100 to 170 ksi (690---1170 MPa) with elongations of 1.0 percent or less, depending on density and alloy selected.

The powder metal process is well---suited to the production of gears for several reasons: (1) Carbide dies provide consistent part accuracy 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 achieved using secondary operations.

(3) Powder metal gears can be made with blind corners, thus eliminating undercut relief that is needed with cut gears, and have extra support strength at 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. Hardness specifications 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 actual hardness of the powder metal material will be higher than the apparent hardness reading and can be more accuANSI/AGMA

Spur gears are the easiest to produce out of powder metal because of the vertical action of the press and ease of ejection of the 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 are 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.

and non---destructive inspection (magnetic particle and ultrasonic or radiograph) practices.

4.8.6 Other Ferrous Materials. 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 tool steel (H series), high speed steels, austenitic, martensitic and precipitation hardening stainless steels, etc. Special gear analyses are frequently used in applications with very high strength requirements.

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

4.9 Selection Criteria for Wrought, Cast, or Fabricated Steel 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 for aerospace and special high speed, is commonly manufactured 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 quality 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 for gearing. Application is limited because quantities or critical application considerations must justify the increased development and die costs. 4.10 Copper Base Gearing. Non---ferrous gears are made from alloys of copper, aluminum, and zinc. Alloys of copper are in wide use for power transmission 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 fatigue and impact strength) vary according to the direction of hot working or inclusion flow during forming (see Fig 4---2). Improved steel cleanliness has the effect of improving the transverse and tangential properties of forged 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 resistance” characteristics 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 permits use in gears and worm wheels for severe wear applications. 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) Manganese Bronzes. This is the name given to a family of high strength yellow brasses. They are characterized by high strength and hardness and are 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 of corrosion resistance, wearability

NOTES: Mechanical properties in the transverse direction will vary with inclusion type and material form. Mechanical property data is normally measured in the longitudinal direction. Castings generally being isotropic (non---directionality of properties), when sound in the rim tooth section, can provide comparable mechanical properties to those of forgings. Casting quality involves controlled steel making, molding, casting, heat treating ANSI/AGMA

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or bearing quality as phosphor and aluminum bronzes. (3) Aluminum Bronze. Aluminum bronze materials are similar to the manganese bronzes in toughness, but are lighter in weight and attain higher mechanical properties through heat treatment. As the strength of aluminum bronze is increased, ductility is reduced. This bronze has good wear resistance and

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

DIRECTION OF METAL AND INCLUSION FLOW

ROLLED RING FORGING

LONGITUDINAL TENSILE TEST BAR OR PROPERTIES

TRANSVERSE TENSILE TEST BAR DIRECTION OF METAL AND INCLUSION FLOW

PINION FORGING

TRANSVERSE TENSILE TEST BAR

LONGITUDINAL TENSILE TEST BAR

TANGENTIAL TENSILE TEST BAR

NOTE: ASTM E399 may be used if impact testing is required.

Fig 4---2 Directionality of Forging Properties er strength, but they are more difficult to machine. Wear resistance of these brasses is somewhat lower than for the higher strength manganese bronzes.

4.10.2 Gear Brasses and Other Copper Alloys. Gear brasses are selected for their corrosion resistant properties. The most common gear brass is yellow brass, used because of its good machinability. Other brass materials are used because of their highANSI/AGMA

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4.10.3 Wrought Copper Base. Wrought copper base materials is a general term used to describe a group of mechanically shaped gear materials in which copper is the major chemical component. This group of gear materials includes bronzes, brasses, and other copper alloys. Table 4---10 presents chemical analyses of common wrought bronze alloys, while Table 4---11 presents typical mechanical properties of these wrought bronze alloys in rod and bar form. 4.10.4 Cast Copper Base. Copper base castings are specified by melting method, heat treatment, analysis or type, hardness and tensile properties. 4.10.4.1 Cast Worm Bronzes. Specifications describe type of bronzes according to chemical analysis. Refer to Table 4---12 for chemical analyses of 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 separate cast test specimens are shown 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 analysis for chemistry. In the event of disagreement in chemical analysis, ASTM Designation E54, Standard Methods of Chemical Analysis of Special Brasses 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 Brinell Hardness of Metallic Materials. The load in kilograms force listed in Table 4---13 should be used. Hardness tests are to be made on the tooth portion of the part after final heat treatment, if required. The number of hardness tests made should be specified by the gear manufacturer. (5) Casting Tensile Properties. Tensile tests are only required when specified. Tensile tests when specified are made in accordance with ASTM E8, Tension Testing of Metallic Materials. Tensile test bars for sand castings may be attached to casting or cast separately. Tensile test bars for static chill castings may be cast separately with a chill in the bottom of the test bar mold. Tensile test bars 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 Information for Copper Castings. Additional information regarding manufacturing, chemical analysis, heat treating, tensile properties, hardness and hardness control, cast structure and supplementary data for cast copper alloys is as follows: (1) Casting Manufacture. Cast copper base gear materials may be melted by any commercially recognized melting method for the composition involved. Castings should be free of shrink, porosity, gas holes and entrapped sand in the tooth portion. Castings should also be furnished free of sand and extraneous appendages.

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

Repair welding in other than the tooth portion may be performed by the casting supplier. Repair welds in the tooth area should be performed only with the approval of the gear manufacturer.

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

(2) Casting Heat Treating. Copper Base castings are heat treated as 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) Casting Chemical Analysis. Chemical analysis shall be in conformance with the type specified or

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Table 4---10 Chemical Analyses of Wrought Bronze Alloys Bronze 1 Alloy UNS NO.

Former AGMA Type

Composition, Percent Maximum (unless shown as a range or minimum) Cu (incl Ag) Pb

Fe

Sn

Zn

Al

As

Mn

Si

Ni (incl Co)

C62300

--- ---

Rem.

--- ---

2.0 to 4.0

0.60

--- ---

8.5 to 11.0

--- ---

0.50

0.25

C62400

--- ---

Rem.

--- ---

2.0 to 4.5

0.20

--- ---

10.0 to 11.5

--- ---

0.30

0.25

C63000

ALBR 6

Rem.

--- ---

2.0 to 4.0

0.20

0.30

9.0 to 11.0

--- ---

1.50

0.25

4.0 to 5.5

C64200

ALBR 5

Rem.

0.05

0.30

0.20

0.50

6.3 to 7.6

0.10

1.5 to 2.2

0.25

C67300

--- ---

58.0 to 63.0

0.40 to 3.0

0.50

0.30

Rem.

0.25

2.0 to 3.5

0.50 to 1.5

0.25

0.15

--- ---

1.0

--- ---

1 Unified Numbering System. For cross reference to SAE, former SAE & ASTM, see SAE Information Report SAE J461. For added copper alloy information, also see SAE J463.

Table 4---11 Typical Mechanical Properties! of Wrought Bronze Alloy Rod and Bar Bronze2 Alloy UNS NO. C62300 C62400 C63000 C64200 C67300

Former AGMA Type

Tensile Strength ksi (MPa)

Yield Strength ksi (MPa)

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

Hardness HB and HRB

--- ---

90

(620)

45

(310)

25

180HB (1000kgf)

--- ---

95

(655)

50

(345)

12

200HB (3000kgf)

ALBR 6

90

(620)

45

(310)

17

100 HRB

ALBR 5

93

(640)

60

(415)

26

90 HRB

70

(485)

40

(275)

25

70 HRB

--- ---

1 Typical mechanical properties vary with form, temper, and section size considerations. 2 Unified Numbering System. For cross reference to SAE, former SAE & ASTM, see SAE Information Report SAE J461. For added wrought copper alloy information, also see SAE J463.

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Table 4---12 Chemical Analyses of Cast Bronze Alloys Bronze Former Alloy * AGMA UNS NO. Type

Composition, Percent Maximum (unless shown as a range or minimum) Cu

Sn

Pb

Zn

Fe

Ni Sb (incl Co) S

P

Al

Si

Mn

C86200

MNBR 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

C86300

MNBR 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

C86500

MNBR 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 --- ---

C92700

MNBR 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

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

C95300

ALBR 2

86.0 min

--- ---

--- ---

--- ---

0.8 to 1.5

--- ---

--- ---

--- ---

--- ---

9.0 to 11.0

--- ---

--- ---

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

* Unified Numbering System. For cross reference to SAE, former SAE & ASTM, see SAE Information Report SAE J461. For added copper alloy information, also see SAE J462. {

For continuous castings, phosphorus shall be 1.5 percent maximum.

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Table 4---13 Mechanical Properties of Cast Bronze Alloys! Copper Alloy UNS.2 NO.

Former AGMA Type

C86200 MNBR 3

Casting Method & Condition #

Minimum Typical Hardness % Percent Minimum Minimum 4 4 HB HB Tensile Strength Yield Strength Elongation in 2 inch ksi (MPa) 500 3000 ksi (MPa) (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

C86500 MNBR 2 C86500 MNBR 2

Sand, Centrifugal Continuous

65 70

(450) (485)

25 (170) 25 (170)

20 25

112 112

--- ----- ---

C90700 BRONZE 2 C90700 BRONZE 2 C90700 BRONZE 2

Sand Continuous Centrifugal

35 40 50

(240) (275) (345)

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

10 10 12

70 80 100

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

C92500 BRONZE 5 C92500 BRONZE 5

Sand Continuous

35 40

(240) (275)

18 (125) 24 (165)

10 10

70 80

--- ----- ---

C92700 BRONZE 3 C92700 BRONZE 3

Sand Continuous

35 38

(240) (260)

18 (125) 20 (140)

10 8

70 80

--- ----- ---

C92900

Sand, Continuous

45

(310)

25 (170)

8

90

--- ---

C95200 ALBR 1 C95200 ALBR 1

Sand, Centrifugal Continuous

65 68

(450) (470)

25 (170) 26 (180)

20 20

--- ----- ---

125 125

C95300 C95300 C95300 C95300

ALBR 2 ALBR 2 ALBR 2 ALBR 2

Sand, Centrifugal Continuous Sand, Centrifugal Continuous (HT)

65 70 80 80

(450) (485) (550) (550)

25 26 40 40

(170) (180) (275) (275)

20 25 12 12

---------

---------

140 140 160 160

C95400 C95400 C95400 C95400

ALBR 3 ALBR 3 ALBR 3 ALBR 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 4 ALBR 4 ALBR 4 ALBR 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

C86300 MNBR 4

--- ---

1 For rating of worm gears in accordance with AGMA 6034---A87, the Materials Factor, k s , will depend upon the particular casting method employed. 2 Unified Numbering System. For cross reference to SAE, former SAE & ASTM, see SAE Information Report SAE J461. For added copper alloy information, also see SAE J462. 3 Refer to ASTM B427 for sand and centrifugal cast C90700 alloy and sand cast C92900. 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 more (see ASTM B505, Table 3 footnote). 5 BHN at other load levels (1000 kgf or 1500 kgf) may be used if approved by purchaser. ANSI/AGMA

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One test specimen should be tested from each group of three test coupons cast. If this bar meets the tensile requirements, the lot should be accepted. If the first bar fails to meet the specified requirements, the two remaining specimens shall be tested. The average properties of these two bars must meet specified requirements for acceptance of the lot. (6) Casting Hardness Control. The gear manufacturer can select at random any number of castings from 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 load, shall be 80 HB for static chill and centrifugal chill castings, and 70 HB for sand castings. The minimum hardness at or near the root diameter shall be agreed upon by the 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 advisable to specify by use of photomicrographic standards both acceptable and non---acceptable phase distributions in the gear rim section. 4.11 Other Non--- Ferrous Materials. In addition to the more common non---ferrous materials used for gears, several wrought aluminum and beryllium copper alloys are occasionally used. Specifications are specialized and should be resolved between the user and supplier. 4.12 Non--- Metallic Materials. Many gears, particularly those used to transmit motion rather than power, are produced from non---metallic materials. Because of the wide range of non---metallic materials, engineering data on the various types of non---metals is usually most easily available from 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 data, and the successful use of plastic gears in many applications have all contributed to the establishment of certain plastics as engineering material suitable for fine pitch gears.

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

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

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

5. Heat Treatment Heat treatment is a heating and cooling process used to achieve desired properties in gear materials. 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 separate test bar simply signifies the melt quality poured into the mold to make the casting. It does not express the specific properties and characteristics of the casting which are greatly dependent on design, size, and foundry technique.

(1) Preheat treatments--Anneal Normalize and temper Quench and temper Stress relief

(c) The grain size of cast copper base alloys varies as a function of cooling rate and section thickness. Recommended maximum grain size for centrifugal castings is 0.035 mm in the rim, 0.070 mm in the web and 0.120 mm in the hub. The grain size for copper base alloys is determined per ASTM E112 at 75X magnification.

(2) Heat treatments--Through harden (anneal, normalize, or normalize and temper, and quench and temper).

(d) The grain size of static cast copper base alloys should be mutually agreed upon by the consumer and producer with reference to the various sections of the ANSI/AGMA

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

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modified (carburize, carbonitride, and nitride)

Typical specified hardness ranges for normalized and tempered steels are shown in Table 4---2.

(3) Post heat treatment--Stress relieve

5.1.3 Normalizing and Annealing for Metallurgical Uniformity. The normalizing and annealing processes are frequently used, either singularly or in combination, as a homogenizing heat treatment for alloy steels. These processes are used in wrought steel to reduce metallurgical non---uniformity such as segregated alloy microstructures (banding) and 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 gears are heated to a required temperature and cooled in the furnace or quenched in air, gas or liquid. Through hardening may be used before or after the gear teeth are formed.

Cycle annealing is a term applied to a special normalize/temper process in which the parts are rapidly cooled to 800---1000_F (427---538_C) after normalizing 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 gearing. In ascending order of hardness for a particular type of steel they are; annealing, normalizing (or normalizing and tempering), and quenching and tempering. Modifications of quench 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 gears.

5.1.4 Quench and Temper. The quench and temper process on ferrous alloys involves heating to form austenite at 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), to achieve the desired mechanical properties. Tempering reduces the material hardness and mechanical strength but improves the material ductility and 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. Parts are normally air cooled from tempering temperatures. Table 4---3 gives hardness guidelines for some steel grades.

NOTE: Through hardening does not imply equal hardness through all sections of the part. See 4.6 for discussion of hardenability. 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 (minimum residual stress). Typical hardness for annealed gearing is shown 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 heating steel or other ferrous alloys to 1600---1800_F (871---982 _C) and cooling in still or circulated air. Normalizing results in higher hardness than annealing, with hardness being a function of grade of steel and the part section thickness. However, with plain carbon steels containing up to about 0.4 percent carbon, normalizing does not increase hardness 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 gear application stress analysis indicates that the hardness and mechanical properties for the specified material grade can best be achieved by the quench and temper process.

Alloy steels are normally tempered at 1000---1250_F (538---677_C) after normalizing for uniform hardness, dimensional stability and improved machinability. ANSI/AGMA

(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 annealing.

with the tempering embrittlement phenomenon from tempering in a lower range (500---600_F) often referred to as “500_F or A---Embrittlement.” 5.1.4.4 Designer Specification. The designer should specify the following on the drawing. (1) Grade of steel (2) Quench and temper to a hardness range. The hardness range should be a 4 HRC or 40 HB point range. The designer should not specify a tempering temperature range on the drawing. It is best to specify a hardness range and allow the heat treater to select the tempering temperature to obtain the specified hardness. Specifying both tempering temperatures and hardness ranges on a drawing causes an impractical situation for the heat treater. Tempering below 900_F(482_C) should be approved by the purchaser. (3) Any testing required. For example, hardness tests, or any non---destructive tests such as magnetic particle inspection or dye penetrant inspection, including the frequency of testing. 5.1.4.5 Specified Hardness. The specified hardness of through hardened gearing is generally measured on the gear tooth end face and rim section. Historically, this has been interpreted to mean that the specified hardness must be met at this location. Designers 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 treat practice, achieving specified hardness on these surfaces may not necessarily insure hardness at the roots of teeth. If gear root hardness is critical to a specific design criteria, the gear tooth root hardness should be specified. However, care should be taken to avoid needlessly increasing material costs by changing to a higher hardenability steel where service life has been successful.

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

Material chemistry and hardenability Quench severity Section size Time at temperature

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. 5.1.4.3 Tempering. Tempering lowers hardness and strength, which improves ductility and toughness or impact resistance. The tempering temperature must be carefully selected based upon the specified hardness range, the quenched hardness of the part, and the material. The optimum tempering temperature is the highest temperature possible while maintaining the specified hardness range. Hardness after tempering varies inversely with the tempering temperature used. Parts are normally air cooled from the tempering temperature. Tables in the appropriate reference are available as guidelines for 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 has been shown to eliminate temper brittleness in most steels. Temper brittleness should not be confused ANSI/AGMA

5.1.4.6 Maximum Controlling Section Size. 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 after heat treatment. 5.1.4.7 Additional Information. For more information, consult the following: The ASM Handbook, Volume 4, Heat Treating, 8th or 9th edition.

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Military specification MIL---H---6875 and Mil--STD---1684.

the gear element within the heat source (flame or induction coil) which envelopes the entire face width. Gearing is removed from the heat source and 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 and following quench head. The heat source and quench head traverse axially along the length to be hardened.

5.1.5 Stress Relief. Stress relief is a thermal cycle used to relieve residual stresses created by prior heat treatments, machining, cold working, welding, or other fabricating techniques. The ideal 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 the specified minimum. Lower temperatures with longer holding times are sometimes used.

Gearing can also be tooth to tooth, progressively hardened by passing the flame or inductor and following quench head between the roots of teeth. Inductor or flame 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 Heavy Draft, Cold Drawn, Stress Relieved Steel Bars. Heavy draft, cold drawn, stress relieved bars may be used as an alternative to quench and tempered steel. However, fatigue properties of this steel may not be equivalent to quench and tempered steel with the same tensile properties. Size limitations and mechanical properties are listed in Table 4---5. For further details see ASTM A---311.

Heat sources designed to pass between adjacent teeth followed by quenching are desirable from both endurance or bending strength and wear considerations, because both the flanks of teeth and root diameter are hardened. Only the 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 roots of teeth may also prove detrimental. It is, therefore, recommended that both the designer and heat treater know what type of hardening pattern is desired.

5.2 Flame and Induction Hardening. Flame or induction hardening of gearing involves heating of gear teeth to 1450---1600_F(788---871_C) followed by quench and tempering. An oxyfuel burner is used for flame hardening. An encircling coil or tooth by tooth inductor is used for induction hardening. These processes develop a hard wear resistant case on the gear teeth. When only the surface is heated to the required depth, only the surface is hardened during quenching (see Figs 5---1 and 5---2). Material selection and heat treat condition prior to flame or induction hardening significantly affects the hardness and uniformity of properties which can be obtained.

Gearing may also be tooth to tooth, progressively hardened by passing the inductor between the roots of adjacent teeth, while the gear element is submerged in a synthetic quench (termed “Delapena Process”). This process, like other tooth to tooth hardening techniques, is time consuming and is not economical for small, finer pitch gearing (finer than 10 DP). Spin hardening is more economical for smaller gears.

5.2.1 Methods of Flame and Induction Hardening. Both of these methods of surface hardening can be done by spin hardening, or by tooth to tooth hardening. Spin hardening of gearing involves heating all of the teeth across the face simultaneously 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 SHOWN ARE NOT POSSIBLE FOR ALL SIZES AND DIAMETRAL PITCHES OF GEARING, AND ARE DEPENDENT UPON THE CAPACITY OF THE EQUIPMENT.

Fig 5---1 Variation in Hardening Patterns Obtainable on Gear Teeth by Flame Hardening ANSI/AGMA

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SPIN HARDENING

INDUCTION COIL OR FLAME HEAD

INDUCTION COIL OR FLAME HEAD

FLANK HARDENING INDUCTOR OR FLAME HEAD

INDUCTOR OR FLAME HEAD

FLANK AND ROOT HARDENING INDUCTOR OR FLAME HEAD

Fig 5---2 Variations in Hardening Patterns Obtainable on Gear Teeth by Induction Hardening ANSI/AGMA

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

used in place of more costly nitriding which cannot economically generate some of the deeper cases required. Contour 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 roots, but hardens teeth through the entire cross section, reducing core ductility of teeth and increasing distortion (see Fig 5---2).

Simple torch type flame heads are also used to manually harden teeth. Since there is no automatic control of this process, high operator skill is required. Induction hardening employs a wide variety of inductors ranging from coiled copper tubing to forms machined from solid copper combined with laminated materials to achieve the required induced electrical currents.

If high root hardness is not required, flame hardening is more available and more economical than induction hardening for herringbone and spiral bevel gearing. NOTE: AGMA quality level will be reduced approximately one level (from the green condition) after flame or induction hardening unless subsequent finishing is performed.

Coarser pitch teeth generally require inductors powered by medium frequency motor generator sets or solid state units. Finer pitch gearing generally utilizes encircling coils with power provided by high frequency vacuum tube units. Wide faced gearing is heated by scanning type equipment while more limited areas can be heated by stationary inductors. Parts are rotated when encircling coils are used.

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

5.2.3 Material. A wide variety of materials can be flame or induction hardened, including (cast and wrought) carbon and alloy steels, martensitic stainless steels, ductile, malleable and gray cast irons. Generally, steels with carbon content of approximately 0.35---0.55 percent are suitable for flame or induction hardening. Alloy steels of 0.5 percent carbon or higher are susceptible to cracking. The higher the alloy content with high carbon, the greater the tendency for cracking. Cast irons also have a high tendency for cracking. Selection of the material condition of the gearing can affect the magnitude and repeatability of flame and induction hardening. Hot rolled material exhibits more dimensional change and variation than hot rolled, cold drawn material because of densification from cold working. A quench and tempered material condition or preheat treatment, however, provides the best hardening response and most repeatable distortion.

5.2.2 Application. 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 and bending strength achieved by carburizing is not required. These processes are also

5.2.4 Prior Heat Treatment. For more consistent results, it is recommended that coarser pitched gears of leaner alloy steels receive a quench and temper pretreatment; for example, 4140 steel with teeth coarser than 3 DP. In both carbon and alloy steels, normalized or annealed structures can be hardened. These structures do, however, require longer heating cycles and a more severe quench which increase the chance of

Induction heating depth and pattern are controlled by frequency, power density, shape of the inductor, workpiece geometry and workpiece area being heated. Contour or profile hardened tooth patterns for 4---12 D.P. gearing can be obtained by dual frequency spin coil induction heating using both low (audio) frequency (AF) of 1---15 kHz and higher (radio) frequency (RF) of approximately 350---500 kHz. Initially low audio frequency is used to preheat the root area, followed by high radio frequency to develop the profile heated pattern, followed by quenching.

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cracking. The annealed structure is the least receptive to flame or induction hardening.

5.2.6 Process Considerations. Several areas must be considered when processing. Some of the more critical requirements are outlined below.

Successful induction hardening of either gray or ductile cast iron is dependent on the amount of carbon in the matrix. The combined carbon in pearlite will readily dissolve at the austenitizing temperature. Pearlite microstructures are desirable. Pearlite promoting alloy additions such as 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, this is usually not a problem with properly maintained equipment since electrical power characteristics, inductor movement and integral quench intensity can be readily controlled. Repeatabiltiy becomes more difficult with flame hardening. Equipment varies from hand held torches to tailor made machine tools with well controlled movement of burner heads. Equipment must be such that heating rates across the burner face are consistent from cycle to cycle. Gas pressure and mixing of heating gases must be uniform. Burner head location must be precise from cycle to cycle.

5.2.5 Hardening Patterns. There are two basic methods of flame or induction hardening gears, spin hardening and tooth to tooth hardening. See Figs 5---1 and 5---2 for variations of these processes and 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 of the equipment limits the pattern which can be attained. Root flame hardening by the tooth by tooth process is difficult and should be specified with care.

5.2.6.2 Heating with Flame or 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. Underheating results in less than specified hardness and case depth. Overheating can result in cracking. Flame hardening 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 face 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 equipment. 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 rotating in a coil, a spray quench usually follows behind the coil.

Flank or root and flank induction scan hardening (contour) can be applied to almost any tooth size with appropriate supporting equipment and kW capacity. However, for pitches of approximately 16 DP and finer, these methods are not recommended. Spin hardening in an induction coil is recommended. Spin hardening of finer pitches is 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 heating. Quench time and temperature are critical and in---spray quenching, pressure velocity and direction 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 can absorb heat from the heated zone.

The allowable durability and root strength rating for the different hardening patterns should be obtained from appropriate AGMA rating practices. These bending strength ratings are lower at the roots of teeth when only the tooth flanks are hardened.

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5.2.6.4 Tempering. Tempering is mandatory only when specified. However, for particular processes, judgment should be exercised before omitting tempering. It is good practice to temper after quenching to increase toughness and reduce residual stress and crack susceptibility. Tempering should be for 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 that AGMA decreases load ratings for gears which do not have hardened roots. AGMA gear rating standards should be consulted for appropriate stress numbers. 5.2.7.1 Heat Affected Zone. In flame hardening, the heat affected zone (HAZ) is a region that is heated to 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 1/8 inch up the flank from the critical root fillet or well below the root diameter.

5.2.6.5 Surface Hardness. Surface hardness is the hardness measured on the immediate surface and is primarily a function of the carbon content (see Fig 5---3). Hardness may be lower as a result of prior heat treatment, alloy content, depth of hardening, heating time, mass and quenching considerations.

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 quench 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 is normally defined as the distance below the surface at the 0.5 tooth height where hardness drops 10 HRC points below the surface hardness (see Fig 5---3). When a tooth is through hardened, effective case

60

MAXIMUM SURFACE HARDNESS

50 ∆ H= 10 EFFECTIVE CASE DEPTH HARDNESS

40

30 0.20

0.30

0.40 0.50 0.60 CARBON CONTENT --- PERCENT

0.80

0.70

Fig 5---3 Recommended Maximum Surface Hardness and Effective Case Depth Hardness Versus Percent Carbon for Flame and Induction Hardening

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(3) Results of magnetic particle inspection, if required.

5.2.7.2 Case Depth Evaluation (Hardness Pattern). Although it is not always practical, particularly on larger gearing, the only positive way to check case depth is by sectioning an actual part. For tooth by tooth hardening, a segment of a gear can be hardened and sectioned. Case depth should be determined on a normal tooth section, using an appropriate superficial or micro---hardness tester. When a gear cannot be sectioned, hardness pattern and depth can be checked by polishing end faces of teeth and nitric acid etching. Grit blasting is also occasionally used. Hardness can also be checked on end faces at flank and root areas.

5.3 Carburizing. Gas carburizing consists of heating and 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 (typically 0.70---1.10 percent carbon at the surface). Gear blanks to be carburized and hardened are generally preheated after the initial anneal by a subcritical anneal at 1100_F---1250_F (590---675_C), normalize, normalize and temper or quench and temper to specified hardness before carburize hardening. This is done for machinability, dimensional stability and possible grain refinement considerations. An intermediate stress relief before final machining before carburizing may be used to remove residual stress from 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, hardness may be lower at the ends, particularly at the root area. In this case, existence of a hardness pattern can be demonstrated by acid 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 at temperature to stabilize while maintaining the carbon potential, and direct quenched. Gearing may be atmosphere cooled after 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). Gearing may be subsequently given a refrigeration treatment to transform retained austenite and retempered.

5.2.8 Specifications. 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) Hardening pattern required. (4) Minimum surface hardness required. (Maximums may be specified for induction hardened 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 are to be used for determining the depth of hardening and the frequency of such inspection. (8) Tempering temperature, if required. (9) Magnetic 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, favorable 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 AGMA gear tooth ratings for contact stress, pitting resistance and root strength (bending). Carburized 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 obtained by subsequent hard gear finishing. Conventional hard gear finishing (skiving and grinding) results in some sacrifice of beneficial compressive stress at the surface and substantially increases costs.

5.2.9 Documentation. The heat treater should submit the following information: (1) Surface hardness range obtained and the number of pieces inspected.

Carburized gearing is used in enclosed gear units for general industrial use, high speed and aerospace precision gear units and also large open gearing for mill applications. Carburized gearing is also used for

(2) Depth of hardening obtained at each location specified when destructive tests are required, and the number of pieces inspected. ANSI/AGMA

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improved wear resistance. Specified finish operations after hardening depend upon accuracy and contact requirements for all applications.

carburized helical and spur gearing to 4 1/2 DP. The test bar should have 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 bar may be used for coarser pitch carburized gearing to 1.5 DP. The size of the bar for coarser than 1.5 DP gearing should be mutually agreed upon, and should approximate the inscribed diameter at mid height of the tooth cross section. The bar length should be 2---3 times the diameter. When specified, core hardness and core microstructure can be determined at the center of the round bar size shown in Table 5---1 according to diametral pitch.

Carburizing technology is well established and the available equipment and controls make it a reliable process. Surface hardness, case depth, and core hardness can be specified to reasonably close tolerances, and the quality can be audited. Some gearing does not lend itself to carburize hardening because of distortion. Gearing which distorts and 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 after carburizing can be used to minimize distortion. Selected areas of gearing can be protected from carburizing (masked) to permit machining after hardening, or can be machined after carburizing and slow cooling before hardening.

Table 5---1 Test Bar Size for Core Hardness Determination

Gearing beyond 80 inch (2032 mm) diameter is difficult to carburize due to the limited number of available furnaces for processing. Maximum size of carburize gearing is currently in the 120 inch (3048 mm) diameter range. Most of this large gearing requires tooth finishing (skiving and/or grinding) after carburizing and hardening. 5.3.2 Materials. Material selection is an integral part of the design process. Selection should be made on the basis of material hardness and hardenability, chemistry, cleanliness, performance, and economical considerations. Performance criteria include, but are not limited to, the following: toughness, notch sensitivity, fatigue strength, bending strength, pitting wear resistance, and operational characteristics. Reference should be made to Table 4---1 for a list of typical carburizing materials and Appendix C for case hardenability considerations.

BAR SIZE

4 1/2 DP and finer

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

2 1/2 DP to less than 4 1/2 DP

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

1 1/2 DP to less than 2 1/2 DP

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

1 1/2 DP and coarser

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

Test discs or plates may also 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 equal to the inscribed diameter of the normal tooth thickness at mid face width.

5.3.3 Control With Test Bars. Test bars are used to show that the case properties and, when required, core properties meet specifications. Test bars should be of the same steel type as the gear(s), but not necessarily the same heat. Bars should accompany gearing through all heat treatments, including all post hardening treatments. Consideration should be given to evaluation of that portion of the case that is not removed during tooth finishing.

When disagreement exists as to the properties obtained on the test bar and the parts, an actual part may be sectioned for analysis. 5.3.3.1 Case Hardness. Case hardness should be measured with microhardness testers which produce small shallow impressions, in order that the hardness values obtained are representative of the surfaces or area being tested. Those testers which produce Diamond Pyramid or Knoop hardness numbers (500 gram load) are recommended. When measuring di-

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

DP

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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 the case depth relative to the depth of the impression made by the tester.

(0.13 mm) is used. Care should also be exercised in establishing the perpendicular to the mid tooth point when starting the traverse. Effective case depth at roots are typically 50---70 percent of mid tooth height case depths, and tips may be 150 percent of mid tooth height case depths. NOTE: See definition of case depth of carburized components, Section 3.

Low readings can be obtained when the indentor penetrates entirely or partially through the case. Microhardness tests for 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 surface. Care must be taken during grinding and polishing not to round the edge being inspected and 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 HRC 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 HRC point should be added to the 50 HRC effective case depth criterion (example, core hardness equals 47 HRC, effective case depth should be measured at 52 HRC). Case depth in these instances may also be measured on a test bar, if bar size has been previously correlated to the gear tooth section (refer to 5.3.3).

NOTE: Direct surface hardness readings (ASTM E18---79) or file checks at the tooth 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 indicate their presence. Microhardness inspection 0.002 to 0.004 inch (.05 to .10 mm) from the edge on a polished cross section of the tooth is more accurate. 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 surface stresses are lowered. Excessive tooth distortion and a loss of core ductility can also occur. Parts of this type should be carefully reviewed for case depth specifications and for use of lower hardenability steels such as 4620 and 8620.

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

5.3.3.4 Case Carbon 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 techniques have also been developed 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 through the case, depending on accuracy desired and depth of case. Grinding in steps through the case would be used with spectrographic techniques. Test specimens should be carburized with the parts. Care should be exercised to maintain surface integrity during cooling or in tempering for subsequent machining. Bar should be straightened to within 0.0015 inch (0.038 mm) (TIR) before machining. Test specimens must be clean and machined dry. Care must be taken to ensure that the turnings are

NOTES: See definition of core hardness, Section 3. Occasionally banding, which results from the steel melting practice, can cause variations in core hardness during testing with a microhardness tester. These variations should not fall below the minimum, when core hardness is specified. 5.3.3.3 Case Depth --- 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 microhardness traverse should be started 0.002 to 0.004 inch (.05 to .10 mm) below the surface and extend to at least 0.01 inch (.25 mm) beyond the depth at which 50 HRC is obtained. Usually an interval of 0.005 inch ANSI/AGMA

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free of any extraneous carbonaceous materials prior to analysis.

continuous atmosphere control is preferred, but other approved methods may be used.

5.3.3.5 Microstructure. The microstructure may be determined on a central normal section of the test bar or tooth, preferably mounted, after being properly polished and etched.

(3) Subzero Treatment (Retained Austenite Conversion Treatment). When the surface 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 characteristics, the heat treater should be given the following as a minimum:

NOTE: Caution 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) Case depth range (refer to Table 5---2). (3) Surface hardness range. When additional characteristics are required, the following additional items may be specified in whole or part:

(4) Carbide Control. When high surface carbon results in a heavy continuous carbide network in the outer portion of the case, parts should be reheated to typically 1650_F(900_C)in a lower carbon potential atmosphere, typically 0.60 percent carbon, to diffuse and break up the excess carbide. Carbide networks should be avoided whenever possible as they tend to reduce fatigue strength of the material. (5) Decarburization. Surface 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.

(1) Core hardness. Approximate minimum tooth core hardness, which can be obtained from some typical carburizing grades of steel and good agitated oil quenching, are shown in Table 5---3. (2) Core microstructure. (3) Case microstructure. (4) Surface carbon content. (5) Subzero treatment. (6) Areas to be free of carburizing by appropriate masking by copper plating or use of commercial stop---off compounds. 5.3.5 Carburizing Process Control. Precision carburizing requires close control of many factors including:

Gross 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 Control. Furnace equipment with temperature uniformity, close temperature control, and accuracy of temperature recording and control instruments. Controls should be checked and calibrated at 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 falls below approximately 0.60 percent.

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

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Table 5---2 Typical Effective Case Depth Specifications for Carburized Gearing Normal Diametral 1 Pitch

Normal Tooth 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 Bevel & Mitre 6 0.010 0.010 0.015 0.020 0.025 0.025 0.030 0.040 0.050 0.060 0.070 0.070 0.080 0.080 0.090 0.105 0.120 0.145 0.170 0.170

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

0.020 0.020 0.025 0.030 0.040 0.040 0.050 0.060 0.070 0.080 0.090 0.090 0.105 0.105 0.125 0.140 0.155 0.180 0.205 0.205

3, ,4 5

Worms with Ground 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 Gears with thin top lands may be subject to excessive case depth at the tips. Land width should be calculated before a case is specified. 3 Case at root is typically 50---70 percent of case at mid tooth. 4 The case depth for bevel and mitre gears is calculated from the thickness of the tooth’s small end. 5 For gearing requiring maximum performance, detailed studies must be made of the application, loading and manufacturing procedures to determine the required effective case depth. For further details refer to AGMA 2001---B88. 6 To convert above data to metric, multiply values given by 25.4 to determine mm equivalent. 7 Worm and ground---thread case depths allow for grinding. Un---ground worm gear cases may be decreased accordingly. For very heavily loaded coarse pitch ground thread worms, heavier case depth than shown in table may be required. 5.4 Carbonitriding. The purpose of this Section is to establish methods for specifying carbonitrided gearing. Information in 5.3 on carburizing will generally apply to carbonitriding, with noted exceptions. Typically carbonitriding is carried out at lower temperatures, 1550---1650_F (843---899_C), and for shorter times than gas carburizing. Shallower case depths are generally specified for carbonitriding than is usual for production carburizing. Its effect on steel is similar to liquid cyaniding and has replaced cyaniding because of cyanide disposal problems. ANSI/AGMA

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. Deep case depths require prohibitive time cycles. One of the

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spection of nitrided gearing. This section covers the selection and processing of materials, hardnesses obtainable, and definitions and inspection of depth of hardening.

advantages of carbonitriding is better case hardenability in lower alloy or plain carbon steels. The carbonitrided case has better wear and temper resistance than a straight carburized case. Carbonitriding 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 facts, along with lower alloy steels, result in the lower core hardness mentioned previously, thus reducing tooth growth and distortion. However, if higher core hardness and deeper case depths are required for bending resistance, carbonitriding may not be applicable.

Conventional gas nitride hardening of gearing, which has had a quench and temper pretreatment and is usually finish machined, involves heating and holding at a temperature between 950---1060_F (510---571_C) in a controlled cracked ammonia atmosphere (10 to 30 percent dissociation). Nitride hardening can also be achieved with the ion nitriding process. During nitriding, nitrogen atoms are absorbed into the surface to form hard iron and alloy nitrides. The practical limit on case depth is about 0.040 inch (1.0 mm) maximum, which requires a thorough stress analysis (for other than wear applications) of the effectiveness of the case for coarse pitch gearing.

Table 5---3 Approximate Minimum Core Hardness of Carburized Gear Teeth Grade

Hardness 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 approximately 1060_F(570_C) for short 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 0.001 inch (0.025 mm) or less, with a nitrogen compound layer to a depth of 0.015---0.020 inch (0.38---0.50 mm) which enhances fatigue strength.

1 Depending upon the Jominy curve of the particular material, maximum hardness will typically be 8---10 points higher than the minimums listed. Use 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 methods because 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 gears should not be specified if shock loading is present, due to inherent brittleness of the case.

5.4.2 Materials. Typically carbon and low alloy steels such as 1018, 1022, 1117, 4022, 4118 and 8620 steels are used for carbonitriding. 5.4.3 Specification and Inspection. Case depth, microstructure, hardness, etc. for carbonitrided parts can all be specified and evaluated as prescribed in the section for carburized gearing. Case depth is specified and measured as effective or total, depending upon application. Cases shallower than 0.010 inch (0.25 mm) are generally specified as total 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, are required in order to form stable nitrides at the nitriding temperature. Typical steels suitable for nitriding are 4140, 4150, 4340, the Nitralloy grades, and 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. 5.5.3 Pre--- treatments. Parts to be nitrided must be quenched and tempered to produce the essential-

5.5 Nitriding. The purpose of this section is to provide information, means of specifying, and inANSI/AGMA

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ly tempered martensitic microstructure required for case diffusion. Microstructure must be free of primary ferrite, such as is produced by annealing and 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 after quenching. In order to minimize distortion of certain gearing designs, intermediate stress relieving after rough machining at 25---50_F (14---28_C) below the tempering temperature may also be required prior to finish machining to relieve machining stresses before nitriding.

part by dimensional analyses both prior to and after nitriding. 5.5.4 Nitriding Process Procedures. Variables in the nitriding process are the combined effects of surface condition, degree of ammonia dissociation, temperature, and time of nitriding. Nitrogen adsorption in the steel surface is affected by oxide and surface contamination. In order to guarantee nitrogen adsorption it may be necessary to remove surface oxidation by chemical or mechanical means. The nitriding process affects the rate of nitrogen adsorption and the thickness of the resultant brittle white layer on the surface. A two stage nitriding process (two temperatures with increased percent of ammonia dissociation at the second higher temperature) generally reduces the thickness of the white layer to 0.0005---0.001 inch (0.013---0.026 mm) maximum. The white layer thickness is also dependent upon the analysis of steel.

In alloys such as series 4140 and 4340 steels, nitrided hardness is lessened appreciably by decreased core hardness prior to nitriding. This must be considered when selecting tempering or stress relieving temperatures.

The ion nitride process uses ionized nitrogen gas 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 very critical, the newer ion nitriding process should be considered. Nitriding can be accomplished at lower temperatures with ion nitriding than those used for conventional gas nitriding.

5.5.5 Specific Characteristics of Nitrided Gearing. Nitriding does not lend itself to every gear application. The nitride process is restricted by and specified by case depth, surface hardness, core hardness and material selection constraints.

Nitriding over decarburized steel causes a brittle case which may spall under 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 hard nitrides as discussed in 5.5.2.

Where it is desired to 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 proprietary paints specifically designed for this purpose.

5.5.5.2 Core Hardness. Core 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 hardness

Nitrided parts will distort in a consistent manner when all manufacturing phases and the nitriding process are held constant. The amount and direction of growth or movement should be determined for each

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most specifications only specify a minimum case depth requirement.

obtained on typical nitrided steels are as follows: Steel Type

Minimum Surface Hardness, HRC

4140 4150 4340 Nitralloy 135

Case depth should be determined using a microhardness tester. At least three hardness tests should be made beyond the depth at which core hardness 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 Surface Hardness. Surface hardness is limited by the concentration of hard nitride forming elements in the alloy and the core hardness of the gear. Lower core hardness 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 the results 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 the following specified:

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

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

Material grade Preheat treatment (see 5.5.5.2) Minimum surface hardness Minimum total case depth Maximum thickness of white layer, if required (6) Areas to be protected from nitriding by masking, if required (7) Nitriding temperature (8) Metallurgical test coupons

Table 5---4 Approximate Minimum Surface Hardness --- Nitrided Steels Steel Type 4140 4150 4340

Minimum Surface Hardness R15N HRC! 85 48 85 48 84 46

Nitralloy (contains Al)

90

60

2 1/2 percent Chrome (EN 40B & 40C and 31CrMoV9)@

89

58

5.6 Other Heat Treatments. Gearing may also be heat treated by other means, including laser heat treating and electron beam heat treating. Both laser and electron beam surface hardening of gears are selective in nature and are generally 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 quantity 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 of radiation) or electron (kinetic energy of electrons) beam, while the underlying mass provides the heat sink to quench harden the surface. Use of electron beam heat treating for gear

1 Converted to HRC 2 British and German analyses, respectively NOTE: Data infers a 269HB minimum core hardness. 5.5.5.4 Case Depth. The specified case depth 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, ANSI/AGMA

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por bubbles and restrict the flow of quenchant should be avoided.

teeth is restricted, however, to full gear tooth contours, and is better suited for flat than curved surfaces. This is true because the stream of 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). Dual laser beam optics have been developed, however, for flank and root contour surface hardening of gear teeth.

There are a variety of quenchants to choose from such as: oil, polymer, molten salt, water, brine and gases. Each variety is available with a wide range of quench characteristics. Table 5---5 associates some material grades and their normally used quenchants. Agitation is externally produced movement of the quenchant past the part. The degree and uniformity of agitation 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 Metals Handbook, 9th Edition, Volume 4 on Heat Treating for additional information on laser and electron beam heat treating, as well as other modifications of heat treatments applied to gearing.

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

5.7 Quenching. 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 and 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 treatment is inevitable and varies with the hardening process. The part design and manufacturing process must consider movement during heat treatment. 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 quench variables to obtain the required properties in the gear. The quench needs to be fast enough to avoid secondary transformation products, but slow enough to reduce distortion and avoid cracking. The material hardenability will determine how severe the quench has to be for a particular part geometry.

5.8.1 Causes. Dimensional changes of gearing resulting from heat treatment occur principally when steel is quenched. These changes occur in both quenched and tempered and surface hardened gears. Distortion is due to mechanical and thermal stresses and phase transformation. Process variables and design considerations have a significant effect upon the amount of 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 than processes that require liquid quenching.

Quench cracks usually originate at sharp corners or substantial section size changes. However, even with perfectly uniform sections, parts can easily crack if made of high---carbon, high---hardenability steels and the quench is too severe. Delayed quench cracks can occur hours or days after quenching, especially if improperly tempered or stress relieved. It is good practice to immediately temper after quenching if quench crack problems are a concern.

5.8.2 Quenching and Tempering. Quenched and tempered gearing changes size and distorts due to mechanical and thermal stresses and microstructural transformations. Quenching the structure to martensite prior to tempering results in steel growing 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 factors which control the quench rate are: part geometry, type of quenchant, degree of agitation and quench temperature. The geometry will affect how quickly and uniformly the quenchant will circulate around the part. Pockets which trap va-

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Table 5---5 Commonly Used Quenchants for Ferrous Gear Materials Material Grade

Quenchant

Remarks

1020

Water or Brine

Carburized and quenched with good quench agitation.

4118 4620 8620 8822 4320

Oil

Carburized and quenched 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

Carburized and quenched 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

Water, Oil or Polymer

Type of quenchant depends upon chemistry and section size. Large sections normally require water or low concentration polymer. Smaller sections can be processed in well agitated oil.

1141 1541

Oil or Polymer

Good response in well agitated conventional oil or polymer. Induction or flame hardened parts normally quenched in polymer.

4140 4142 4145

Oil or Polymer

Same as above; however, thin sections or sharp corners can represent a crack hazard. Hot oil should be considered in these cases. With proper equipment, air quench can be used for flame hardened parts.

9310

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

Oil or Polymer

Gray or Ductile Iron

Oil, Polymer or Air

If conventional oil is used, parts are often removed warm and tempered promptly after quench. Crack sensitivity applies also 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. Quench media depends upon alloy content. High alloy irons can be air quenched to moderate hardness levels. Unalloyed or low alloy irons require oil or polymer. In this section parts and flame or induction hardened surfaces can be crack sensitive.

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

5.8.3.1 Carburized Gearing. Distortion of carburized gearing makes it one of the least repeatable of surface hardened processes. Lack of repeatability is due to the greater number of variables which affect distortion. Close control is, therefore, required. Distortion results from microstructural transformation, and residual stress (from thermal shock, uneven cooling, etc.) considerations. Transformation in the case results in growth which sets up residual surface compressive stress. This stress is balanced by corresponding residual tensile stress beneath the case. Principal variables affecting the amount of growth, distortion, and residual stress include: (1) Geometry.

Normally, rough gear blanks (forging, barstock, or casting) have sufficient stock provided so distortion can be accommodated 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 and 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 quenching. (4) Carbon potential of the carburizing atmosphere. (5) Carburizing temperature and temperature prior to quenching. (6) Time between quench and temper for richer alloys. (7) Quenchant type, temperature and amount of agitation.

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

(8) Resultant metallurgical characteristics of the case, such as carbon content, case depth, amount of retained austenite, carbides, etc. NOTE: Direct quenching generally results in less distortion than slow cooled, reheated and quenched gears, 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 to 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 and induction hardening results essentially in only distortion 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 is heated and quenched as with carburizing.

ANSI/AGMA

Stock removal by grinding after carburize hardening should be limited to approximately 0.007 inch (0.18 mm) per tooth surface or 20 percent of the case depth, whichever is less. Exception may be made for coarser pitch gearing 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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General design considerations of carburized gearing related to distortion include the following (refer to Fig 5---4):

Distortion of carburized gearing also exhibits the following typical characteristics (refer to Fig 5---5): (1) Reduction in tooth helix angle (“helix unwind”), which often requires an increased helix angle to be machined into the element prior to carburizing (more prevalent in pinions). Teeth on larger diameter, smaller face width gears may exhibit “helix wind---up” after hardening.

(1) Larger teeth (lower DP) distort more. (2) Rim thickness should 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 face width for precision gears. Near solid “pancake” gear blanks, designed with moderate recess on both sides of the web section, distort less. The recess is provided to enable clean---up grinding of the rim and hub end faces after 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 are often crown cut prior to hardening to compensate for reverse crown or are chamfered at the ends of teeth. Teeth may also be both crown cut and chamfered.

(4) Holes in the web section close to the rim, to reduce the weight or provide holes for lifting, may cause collapsing of the rim section over the holes.

(3) Eccentricity (radial run---out) of gears and their bores is dependent upon how they are fixtured in the furnace.

(5) High length/diameter ratio pinions distort more. Journals may be required to be masked 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 across the face (tapered teeth), bore taper and “hour---glassing” of the gear bore can occur due to non---uniform growth of teeth across the face and non---uniform shrinking of the bores.

(6) Cantilever pinions, with teeth on the end of the shaft, and “blind ended” teeth on pinions, where the adjacent diameter is larger than the root diameter, present problems from both distortion and finishing standpoints.

(5) Bowing of the integral shaft pinions. Integral shaft pinions should, whenever possible, be hung or fixtured in the vertical position (axes vertical) to minimize bowing.

CANTILEVER PINION

BLIND ENDED TEETH

HIGH L/D RATIO CONCENTRIC BLANKS

Fig 5---4 General Design Guidelines for Blanks for Carburized Gearing

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STRAIGHT HELICAL UNWIND

TAPER

HOURGLASSING

BOWING

END GROWTH

(REVERSE CROWN)

ECCENTRICITY

Fig 5---5 Typical Distortion Characteristics of Carburized Gearing Gears may be fixtured vertically through the bores or web holes on a support rod (axes horizontal), or fixtured horizontally (individually or stacked) to minimize distortion, depending on size and face width. Larger ring gears are positioned horizontally with sufficient stock for clean---up of the teeth. Bores and web sections can be masked to prevent carburizing, and enable subsequent machining.

(2) Increased growth of the teeth (greater than for carburized gearing) because the entire tooth cross section may be hardened in finer pitch gearing. (3) Crowning or reverse crowning of the teeth across the face dependent upon the heat pattern. Crowning is more desirable from a tooth loading standpoint. (4) Taper of teeth due to varied heat pattern and case depth across the face. Distortion of the teeth from spin induction hardening is often considered more repeatable than with spin flame hardening, because of fewer human error factors involved during machine and inductor set--ups with induction hardening. Spin flame hardening involves more manual set---up factors, 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.

Thin section gears, such as bevel ring gears, may be press quenched to minimize distortion. 5.8.3.2 Flame and Induction Hardened Gearing. Flame and induction hardened gearing generally distort less than carburized gearing because only the teeth are heated and subsequently quenched. Contour induction hardening of tooth profiles produce less distortion and growth than spin hardening methods.

CAUTION: Deep spin hardening of gear teeth may cause excessive tooth growth and may affect bore size. 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 treatment, which results in distortion, is done before machining and nitriding. Parts are also not heated above the transformation temperature or previous tempering temperature of the steel during nitriding, and are not quenched, as occurs during carburizing, flame or induction hardening. Therefore, nitrided gear teeth are not generally required to be

During both spin flame and spin induction hardening, the entire tooth cross section is often hardened to the specified depth below the roots of teeth. For high bending strength applications, it is not desirable to have the hardening pattern terminate in the roots of the teeth because of residual tensile stress considerations. Distortion increases as a greater cross---section of a tooth is hardened. Spin flame and spin induction hardening generally produce the following distortion characteristics: (1) Helical unwinding of the gear teeth, as with carburized pinions. ANSI/AGMA

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ground or lapped after hardening to meet dimensional tolerance requirements. Bearing diameters of shaft extensions are often ground after nitriding with only minimum stock provided. Surfaces can also be masked for subsequent machining.

5.9.2 Process Control. 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 repeatability. 5.9.2.1 Intensity Control. Intensity refers to the kinetic energy with which the peening media strikes the part. This energy 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 as the part will be peened. The strip is held flat on 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 inches with a gauge and is called the arc height (see Fig 5---6). There are three classifications 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. Strips are SAE 1070 cold rolled spring steel, hardened and tempered to 40---50 HRC. Flatness tolerance is + --- 0.0015 inch + ( --- 0.04mm). Figure 5---6 also shows the dimensions 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. Whenever a processing procedure is developed for a new part, an intensity curve must be developed which establishes the time required to reach peening saturation 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 than a 10 percent increase in arc height.

When close tolerances are required, gearing can be rough machined and stress relieved at 50_F(28_C) below the prior tempering temperature to relieve rough machining residual stress prior to finish machining and nitriding. During nitriding, outer surfaces grow approximately 0.0005---0.001 inch (0.013---0.025 mm). Bores 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 of 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. Because the process increases bending fatigue strength, it may be used either to salvage or upgrade a gear design. Contact fatigue strength may also 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 are cleaning operations. 5.9.1 Equipment. Machinery used for shot peening should be automatic 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 capable of consistently reproducing the shot peening intensity and coverage required.

5.9.2.2 Shot Control. Shot size and shape must be carefully controlled during the shot peening process, to minimize the number of fragmented particles caused by fracturing of the shot. These fragmented particles can cause surface damage. Also, as a result of lower mass, fragmented shot particles will lengthen the time to reach a specified peening intensity. Periodic inspection of the shot is required to control shot size and shape within specification limits. When these limits are reached, the shot should be classified and separated to restore size and shape integrity as shown in MIL---S---13165B. 5.9.2.3 Coverage Control. Coverage refers to the percentage of indentation that occurs on the surface of the part. One hundred percent coverage is de-

Regardless of the type of equipment used, the gear must be rotated 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 toothform. This type of equipment is generally used for high performance gearing, although centrifugal wheel equipment is often used for very high volume production. ANSI/AGMA

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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 the latter process, full coverage has been achieved when no traces of the dye remain when viewed under ultraviolet light. A minimum of 100 percent coverage is required on any shot peened part.

quired to obtain multiples of 100 percent coverage is that multiple times the time to reach 100 percent coverage (200 percent, 300 percent, etc.). 5.9.3 Design Consideration. The following sections describe items that the designer should include in a shot peening specification. 5.9.3.1 Governing Process Specification. A commonly referenced shot peening specification is MIL---S---13165B which identifies materials, equipment requirements, procedures, and quality control requirements for effective shot peening. The SAE Manual on Shot Peening, SAE---J808a---SAE HS84, may also be used.

Coverage must be related to the 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 STRIP

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

A STRIP

PEENING NOZZLE

C STRIP

0.745 to 0.750 in (18.9 to 19.0 mm)

ALMEN STRIPS

SHOT STREAM 4 to 6 in (102 to 152 mm)

MEASURING DIAL

10--- 32 SCREWS ALMEN TEST STRIP

HARDENED BALL SUPPORTS

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 PEENING TEST (a)

STRIP REMOVED, RESIDUAL STRESSES INDUCE ARCHING (b)

STRIP MOUNTED FOR HEIGHT MEASUREMENT (c)

Fig 5---6 Shot Peening Intensity Control 5.9.3.2 Shot Size and 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 ferANSI/AGMA

rous materials is accomplished with cast steel shot. Cast steel shot is available in two hardness ranges: 45---55 HRC, and 55---62 HRC. When peening gears higher in hardness than 50 HRC, the harder shot

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should be specified to achieve higher magnitudes of compressive stress (refer to Fig 5---7).

5.9.3.5 Masking. At times, it is desirable to mask finished machined areas of the part from shot impingement. Typical masked areas 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 Drawing Example. A typical example of drawing or blueprint specification for shot peening would be as follows:

5.9.3.3 Intensity. The intensity governs the depth of the compressive layer and must be specified as the arc height on the A, C, or N strip (see 5.9.2.1). The range of arc height is generally 0.004 inch (0.10 mm) wide, but it can be specified to a closer tolerance for more repeatable results. Figure 5---8 illustrates the depth of the compressive layer on steel at 31 and 52 HRC 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). Other areas optional. Use 55---62 HRC shot, 100 percent minimum coverage.

5.9.3.4 Coverage. In most cases, 100 percent coverage is adequate. In some instances, it may be desirable to specify multiples of 100 percent in an attempt 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 SHOT ---200

---1500

---250 0

0.004

0.008

0.012

0.016

DEPTH IN INCHES

Fig 5---7 Residual Stress by Peening 1045 Steel at 62 HRC with 330 Shot

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1.0

.040 HRC 31 .035

.75

.030

.025

.50

.020 HRC 52 .015

.25

.010

.005

0

0

.002 .005

0

.004 .010

.015

.006 .020

.008 .025 A

.010C

0

INTENSITY

Fig 5---8 Depth of Compressive Stress Versus Almen Intensity for Steel Table 5---6 gives shot size and intensity for various diametral pitches.

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

Table 5---6 Typical Shot Size and Intensity for 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) Generally all machining of areas to be peened are complete prior to shot peening. It is possible to restore surface finish in peened areas (and retain beneficial effects) by lapping, honing, or polishing, if material removal is limited to 10 percent of the depth of 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---ray diffraction. Currently this must be measured on a cut sample in a laboratory X---ray diffraction 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 other specifications more desirable.

(5) When there are significant machining marks in the tooth roots, it is desirable to achieve an intensity sufficient to produce a depth of compressive stress to negate the stress riser effect of the machining mark. However, shot diameter 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 performed before shot peening. ANSI/AGMA

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5.10 Residual Stress Effects. Residual stresses play an important role in the manufacture and performance of gears. Residual stresses created by machining 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 favorable or unfavorable) are induced mechanically, thermally, by phase transformation, or by modification of surface chemistry (such as by nitriding). Each of these, singularly and in combination (such as by carburizing), can affect the degree of in---process distortion and the residual stress state present in the finished parts. The following sections briefly discuss the causes of each type of induced residual stress.

induced. Thermal, phase transformation and modification of surface chemistry stresses result from heat 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 also be considered a phase transformation stress. Quenching, particularly fast quenching to form martensite, generates both thermal and phase transformation 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 small diameter bars is a tensile stress at the surface and a compressive stress at the center. This stress pattern results from the surface of a bar cooling faster than the center. The phase transformation to martensite creates volume expansion producing tensile stress at the surface. This in turn creates a compressive stress at the center.

5.10.1 Mechanically Induced Residual Stresses. There are two types of mechanically induced residual stresses, machining stresses and finishing operation stresses. Machining stresses are created by the cutting of the gear shape and can be either beneficial or detrimental. Parts given a final heat treatment after finish machining may have the gross residual 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. Grinding after final heat treatment must be performed very carefully since it can create residual tensile stresses in the surface of the gear which can adversely affect performance. Lapping, honing or careful 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 typically performed on finished gears to improve the pitting and surface bending fatigue resistance.

The second and opposite 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 transformation to martensite, setting up residual tensile stress at 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 quench. When the sum of these 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 the center and 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 Surface Chemistry. This type of residual stress must also be considered in conjunction with thermal residual stress because modification of surface chemistry requires heating, and heating can introduce thermal stresses, which must be taken into account. Carburizing, the most common type of surface chemistry modification, will serve as a good example of these types of residual stresses. In quenched 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 favorable effect on the residual stresses in the finished gear. Under extreme grinding conditions, however, CBN grinding may also induce surface tempering residual tensile stresses. Other hard gear finishing methods (e.g. skiving) will need to be individually evaluated as to effect on residual stress levels. 5.10.2 Metallurgically Induced Residual Stress. The other types of residual stress, although quite different, can all be categorized as being metallurgically ANSI/AGMA

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temperature than the case, and as discussed in the previous section, the austenite to martensite transformation creates a volume expansion. Therefore, as the part 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. Compressive stresses in the case help reduce surface pitting caused by tooth contact stress above and below the pitchline. They help counteract tensile stresses caused by bending in the root.

ASTM A370, are normally surface hardness tests made using: (1) Rockwell (2) Brinell (3) Rebound Tests (Equotip & 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 after final heat treatment.

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

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

Refer to Appendix D on Service Life Considerations.

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

Spectrographic Analysis X---Ray Analysis Atomic Absorption Wet Chemistry

When eight hardness tests are specified, four tests shall be on the cope side, (two over risers approximately 180 degrees apart, two between risers also approximately 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. Hardness may be specified but cannot be used to identify grade. Bronze material grades are normally qualified using chemical analysis 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. Large segmented gears shall be hardness inspected on the cope and drag rim edge of each segment per agreement between the customer and supplier.

Brass material grades are identified by chemical analysis. NOTE: Source certification is commonly accepted for analysis certification. 6.2 Incoming Material Hardness Tests. Material hardness tests, often specified in accordance with ANSI/AGMA

2 4 8 16

When two hardness tests are specified, one shall be on the cope side, preferably over a riser; the other on the drag side, approximately 180_ away.

6.1 Incoming Material Quality Control. Material grade is certified by chemical test. Generally this is a 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 (Rim Face)

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6.2.2 Forged Pinions and Gears. Forged pinions and gears include cylindrical shapes, disc shapes and rings.

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

6.2.2.1 Cylindrical Shaped Forgings. (1) A minimum of four hardness tests shall be taken on the major (tooth) diameter of forgings up to fifteen inches. Two readings, 180 degrees apart, shall be taken at the center of the length of the major diameter (center of tooth section at mid face). One reading shall be taken approximately 1 inch (25 mm) from each end of the major diameter, 180 degrees apart.

(4) When a total of eight hardness tests are specified, they shall be made 90 degrees apart on each rim edge. 6.3 Incoming Material Mechanical Tests. Mechanical property test bars, for tensile 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 diameter. Three readings, 120 degrees apart, shall be taken at the center of the length of the major diameter (center of the tooth section at mid face). One reading shall be taken approximately 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 Disc Shape Forging.

Test bar stock, approximately 1.5 ¢ 5 ¢ 6.0 inch (38 ¢ 127 ¢ 152 mm) long, are normally attached to the drag (bottom) rim edge of the 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 radius on forgings over 18.0 inches (457 mm) in diameter.

Test bar stock should remain attached to or accompany the rough stock until all thermal treatment is completed.

6.2.3 Forged Rings (Reference ASTM A290). Recommended number of hardness tests is as follows: Diameter of Ring, in (mm) Up 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, and also in ASTM A290, A291 and A148.

Number of Tests Recommended 2 (180_

apart)

4 (180_

apart)

6.4 Heat Treat Process Control. The many variables involved in the heat treatment of gear materials makes process control complex. Process variables include: time, temperature, rate of heating and cooling, heating media, cooling media, types of controls, base material composition, condition of process equipment, evaluation techniques, and part geometry.

6 (120_ apart) 8

(90_

apart)

Heat treat processes change the microstructure and mechanical properties of the gear material. Any dimensional change, such as distortion or part growth, and any cosmetic change, such as coloration or surface texture, are 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 as follows:

(1) When a total of two hardness tests are specified, they shall be made 180 degrees apart, one on the ring edge and the other on the opposite ring edge. (2) When a total of four hardness tests are specified, they all shall be made 180 degrees apart on ANSI/AGMA

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6.4.1 Temperature. Temperature selection and control is an important parameter in the heat treatment of gear materials. In carburizing and nitriding, the rate of diffusion into steel is dependent on temperature. The carbon concentration in the furnace atmosphere is also temperature dependent. Specific temperature ranges are required to harden the various grades of steel. Hardness and mechanical properties of a material grade are 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 atmosphere. The water vapor concentration is measured using a dew cell or dew pointer. The water vapor concentration is expressed as the atmosphere dew point measured in degrees fahrenheit.

6.4.1.1 Temperature Uniformity. Since the properties obtained in gear materials 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 desired.

(2) Carbon Dioxide Concentration. 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 gas analyzer and expressed as a percentage.

6.4.1.2 Thermal History. It is advisable to make a time temperature plot of the heat treat processes as 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 and carbon monoxide level. The oxygen concentration is measured with an oxygen probe positioned in the furnace heat chamber.

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

6.4.5 Quench Control. Control of the quenching operation involves monitoring the variables which affect the rate and uniformity of part cooling. This includes inspecting the condition, cleanliness and concentration (if applicable) of the quenchant; the proper operation of any device used for agitation; and ensuring that the quenchant stays at the proper temperature (refer to 5.7).

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

There are several methods available to monitor and quantify the cooling rate of the quenching 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 hardenability is accounted for (refer to 5.7 on quenching).

6.4.3 Rate. The rates of heating and cooling are important considerations. For example, if an induction hardened part is heated too slowly, the core material will get too hot and lose its mechanical properties. If a steel gear is cooled too quickly, 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 atmosphere is critical to carburizing and the protection of surfaces from carbon pickup or depletion during the hardening process. There are three ANSI/AGMA

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

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 other hardness measurements, the part was satisfactorily quenched. If the part hardness is low, this is an indication of decarburization, inadequate quenching, excessive retained austenite, undissolved carbides, too high tempering temperature, inadequate case depth, or low surface carbon.

6.5.1 Hardness. Hardness is the most common characteristic used to measure results of the heat treat process. There are numerous types of hardness testing devices which can be used, but each type has its own application 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 to Rockwell C 50. 6.5.1.6 Retained Austenite Examination. If the surface hardness of a carburized part is low, it may be due to the presence of retained austenite in the carburized case. Retained austenite can be transformed to martensite by freezing the carburized part. If the surface hardness improves after freezing, there was retained austenite in the carburized case which is an indicator 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 is a good indicator of the heat treat process. Many factors determine the as quenched hardness such as decarburization and retained austenite. High as quenched hardness is the result of good heat treatment. Low as quenched hardness usually results from one or more of many factors such as deteriorating quenchant, malfunctioning quench agitators, 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 that 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 hardness measurement near the original location. If both measurements are the same, there is no decarburization. If the hardness increases, there is possible decarburization. To determine the depth of decarburization, a test coupon or part that was run with the load should be sectioned, mounted, polished and etched. It should be noted, however, that in most cases decarburization is not permissible. 6.5.1.3 Post Temper Hardness Examination. Tempering parts 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 to monitor furnace soak time and uniformity. If the part hardness is greater in a heavy section compared to 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 pieces in a furnace load, this is a good indication of a processing problem. ANSI/AGMA

6.5.2 Microstructure. The composition of the various phases in the microstructure of a gear will tell a lot about the heat treat process. It is recommended that a trained metallographer or metallurgist perform the microstructure analysis. 6.5.2.1 Tempered Martensite. 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 adequate. 6.5.2.2 Bainite. If a gear has been improperly quenched, the microstructure might be interspersed with bainite, which is characterized by a feathery appearance if severely under quenched, or a darker acicular pattern for marginal quenching. 6.5.2.3 Retained Austenite. 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 characterized by blocky white regions in a matrix of martensite and retained austenite. 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. Continuous intergranular 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 surface finish, generally 64 microinches (5 microns), or: (2) If the size of the hardness impression on the test surface is permitted, or: (3) Mass of the test surface will support the test load.

6.5.3 Test Coupons. Test coupons of representative 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 obtaining the specified hardness at these locations. When hardness testers are not available for accurate measurement at roots of teeth, destructive sectioning and testing may be required.

6.6 Metallurgical, Mechanical and Non--- Destructive Tests and Inspections. Tests and 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 Fatigue Testing. Fatigue (life) testing of the final product is the proof of the suitability of the design for 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 the unknown workpiece with a hardened ball between the two test surfaces. Comparison is made of the ball 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 this testing while maintaining validity of the test data. This can be done by running the test at some overload ratio and evaluating the damage with time for the test conditions. Damage can be compared with that for the product design conditions. This comparison must be made for both the beam strength and the surface durability of the teeth. Miner’s Rule is a widely accepted method of making these comparisons.

It is desired that surface hardened gearing be hardness inspected, non---destructively, so as 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. Conventional Rockwell test machines can be used to hardness inspect surface hardened gearing when size of the gearing permits and where a visible impression is permitted. Hardened files, including those tempered to lower hardness than 60---64 HRC, can also be used to approximate hardness by the scratch test (Reference SAE J---864). Inspection of the hardness on the flanks of surface hardened coarse gearing with non---destructive portable hardness testers can be improved when the instrument can be fixed for perpendicularity to the test surface. Hardness measurement in the roots of teeth may not

When damage value accumulated on the test equals the damage 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 band 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 units to run at ten times the threshold life to validate the product design. This would constitute a Miner’s Rule damage of ten. 6.6.2 Hardness Testing on the Gear Product. Through hardened finish machine gearing can be conventionally hardness tested by standard and porANSI/AGMA

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be reliable due to accessibility in the radius of curvature and surface roughness. For improved accuracy and where permitted, through hardened steel and cast iron gearing should be hardness inspected directly in Brinell (not converted). Hardness of surface hardened gearing should be directly measured in Rockwell (C or A scale) or converted to Rockwell with suitable portable instruments.

be used in some instances. Caution should be exercised if the heavier load C scale is used. 6.6.4 Magnetic Particle Inspection. Magnetic particle inspection is a non---destructive testing method for locating surface and near surface discontinuities in ferromagnetic material. When a magnetic field is introduced into the part, discontinuities laying approximately transverse to the magnetic field will cause a leakage field. Finely divided ferromagnetic particles, dry or in an oil base or water base suspension, are applied over the surface of the material under test. These particles will gather and hold at the leakage field making the discontinuities visible to the naked eye.

Portable instruments vary in accuracy and reliability. Users, therefore, should take precautions to insure accurate calibration and test results. Hardness testing equipment manufacturers should be contacted and literature searched for additional information on principles of hardness inspection, available test equipment and their capabilities. Statistical process control is a useful tool to be used with hardness 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 basically 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 and by alternating currents differ in many characteristics. The main difference, which is of prime importance in magnetic particle testing, is that fields produced by direct current generally penetrate the entire cross section of the part, whereas the fields produced by alternating current are confined to the metal at or near the surface of the part under test. From this, it is evident that when deep penetration of field into the part is required, direct current must be used as the source of magnetizing force. By far, the most satisfactory source of D.C. is the rectification of alternating current. Both single phase and three phase A.C. are furnished commercially. By the use of rectifiers, reversing A.C. is rectified and the delivered direct current is entirely the equivalent of straight D.C. for magnetic particle testing purposes.

6.6.3 Surface Temper Inspection. Surface temper inspection is used to detect and classify localized overheating on ground surfaces by use of a chemical etch method. Details of the process are covered in AGMA 230.01, Surface Temper Inspection Process. Inspection criteria includes a class designation for critical and non---critical areas. To evaluate the severity of surface temper, grinding burns are classified by intensity of color from light gray to brown to black. Severe burning or re---hardening is indicated by patches of white in the darkened areas. Cracking may also be present. Re---hardening or cracking are cause for rejection. Tables I and II in AGMA 230.01 cover temper classes ranging from Class A (Light temper) to Class D (Heavy temper). Class C (Moderate 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 down to 115, 230, or 460 volts. This is accomplished by means of transformers to the low voltages required. At these low voltages, magnetizing currents up to several thousand amperes are often used. The trend in Europe is to use A.C. current for magnetic particle testing because the intent of their testing is location of surface discontinuities only. Subsurface discontinuities are best detected by radiography or ultrasonic non---destructive test methods. A.C. currents tends to give better particle mobility, and

Case depth shall be determined on a normal tooth section. Hardness testers which produce small shallow impressions should be used in order that the hardness values obtained will be representative of the surface area being tested. Microhardness testers which produce Diamond Pyramid or Knoop Hardness number are recommended, although other testers such as Rockwell superficial A or 15 N scales can ANSI/AGMA

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

(7) For prod magnetization with direct current, a minimum of 60 amperes per inch of prod spacing will produce a minimum magnetizing force of 20 oersteds at the midpoint of the prod line for plate 3/4 inch thick or less. A safer figure to use, however, is 200 amps per inch, unless this current strength produces an interfering surface power pattern. Prod spacing for practical inspection purposes is limited to about eight (8) inches maximum, except in special cases.

There are two essential components of magnetic particle testing, each of equal importance for reliable results. The first is the proper magnetization of the part to be tested, with proper field strength in the appropriate direction for the detection of defects. The second is the use of the proper magnetic particles type to secure the best possible defect indications under prevailing conditions. 6.6.4.1 General Principles. Some general principles and rules on magnetizing means, field strength, current distribution and strength requirements are listed below (refer to Figs 6---1 and 6---2).

(8) All parts should be demagnetized after magnetic particle inspection.

FIELD

(1) Fields should be at 90 degrees to the direction of defects. This may require magnetizing in two directions.

HEAD BATH

(2) Fields generated by electric currents are at 90 degrees to the direction of current flow. (3) When magnetizing with electric currents, pass the current in a direction parallel to the direction of expected discontinuities.

CURRENT DISCONTINUITY

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

HEAD SHOT CIRCULAR MAGNETIZATION LOCATES DISCONTINUITIES OCCURRING 45 --- 90 DEGREES TO THE DIRECTION OF THE FIELD.

(5) Circular magnetization specifications generally require from 100 to 1000 amps per inch of part diameter. Amperage requirements should be incorporated into the magnetic particle procedure.

INSPECT FOR PARTICLE INDICATIONS SHOWING LONGITUDINAL DISCONTINUITIES --- MARK DISCONTINUTIES.

Fig 6---1 Circular (Head Shot) Magnetic Particle Inspection

(6) For coil magnetization, a widely used formula for amperage calculations is: NI = 45 000 L/ D

6.6.4.2 Magnetic Particles. The particles used are finely divided ferromagnetic material. Properties vary over a wide range for different applications including magnetic properties, size, shape, density, mobility and visibility or contrast. Varying requirements for varying conditions of test and varying properties of suitable materials have led to the development of a large number of different types of available materials. The choice of which one to use is an important one, since the appearance of the particle

(Eq 6.1)

where NI = ampere turns required, L/D = length to 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 or not a pattern is formed. FIELD

methods is in the range of 60 to 40 microns. Particles larger than this tend to settle out of suspension rapidly. In general, wet method materials exhibit a greater sensitivity than dry powders. Fluorescent particles have the greatest contrast of the wet method materials. Although fluorescent wet particles have the greatest sensitivity and contrast, they can provide a confusing background on surfaces with a finish greater than 250 RMS.

CURRENT THROUGH COIL

BATH

6.6.4.3 Documented Procedures. Written procedures for magnetic particle testing should as a minimum include: (1) Which ASTM, ASNT or agency specifications the procedure meets. (2) Qualifications--(a) Indicate that the operators are qualified and tested to ASNT---TC---1A Level II, MIL--STD---271F, etc. (b) Indicate type of equipment used for inspection, A.C. and D.C. full wave rectified, etc. (c) Indicate type of particles used for inspection, fluorescent or black visible, wet or dry particle. For the wet method, particle concentration should also be indicated. (3) General--(a) State when inspection is to be done; after heat treat, finish machining, etc. (b) State what the surface will be; for example, 250 RMS, black forge, etc.

DISCONTINUITY

COIL SHOT LONGITUDINAL MAGNETIZATION LOCATES TRAVERSE DISCONTINUITIES.

INSPECT FOR PARTICLE INDICATIONS SHOWING TRANSVERSE DISCONTINUITIES. NOTE: EFFECTIVE LENGTH MAGNETIZED BY COIL SHOT IS A FEW INCHES ON EITHER SIDE OF COIL. MAXIMUM LENGTH OF ARTICLE COVERED BY ONE SHOT IS 18 INCHES (46 CM). ON LONG ARTICLES, REPEAT SHOTS AND BATHS DOWN THE LENGTH OF ARTICLE. PLACE ARTICLES CLOSE TO THE COIL BODY.

Fig 6---2 Coil Shot Magnetic Particle Inspection (1) Dry Powders. It is evident that size plays an important part in the behavior of magnetic particles. A large, heavy particle is not likely to be arrested and held by a weak field when 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 mass is very small. Extremely fine particles may also adhere to the surface where there are no discontinuities, especially if it is rough, and form confusing backgrounds. Most dry ferromagnetic powders used for detecting discontinuities are careful mixtures of particles of all sizes. The smaller ones add sensitivity and mobility, while the larger ones not only aid in locating large defects, but by a sweeping action, counteract the tendency of fine powders to leave a dusty background. Thus, by including the entire size range, a balanced powder with sensitivity over most of the range of sizes of discontinuities is produced.

(c) State amps per inch of diameter for circular magnetization and the formula used for calculation of longitudinal magnetization. (d) State what method will be used for determining field magitude; such as pie gage, etc. (e) State demagnetization, if required, and level of demagnetization required. (4) Standard of Acceptance (a) Indicate maximum size and density of indications permitted. (b)Indicate reporting procedures if needed. For further information on magnetic particle testing, refer to: Principles of Magnetic Particle Testing, C.E. Betz Metals Handbook Volume II Eighth Edition Nondestructive Inspection and Quality Control Nondestructive Testing Handbook, Edited by Robert C. McMasters for the Society for Nondestructive Testing

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

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6.6.5 Ultrasonic Inspection. Ultrasonic 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 transformed into voltage and monitored on an oscilloscope screen.

Scanning sensitivity and indication limitations are often determined using test blocks by establishing a distance---amplitude reference line on the oscilloscope screen as illustrated in Fig 6---4. As an example, sensitivity may be adjusted to establish the specified indication height [2 1/2 inch (63 mm)] from the flat bottom hole (FBH) in the 4 inch (102 mm) block, and at the same sensitivity, the indication from the same size FBH in the 12 inch (305 mm) block is noted on the oscilloscope screen. A straight line is drawn between the two points. Any indication noted must not exceed the determined distance---amplitude reference line. Also, indications are often specified not to exceed a certain magnitude and 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 coupling the ultrasonic transducer to the heat treated work piece. Untreated coarse grained structures do not lend themselves to ultrasonic testing. Surfaces to be scanned, such as the outside diameter and ends or end faces of cylindrical or disc shaped rough stock are generally machined to 125---250 micro---inch maximum surface roughness. 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 surfaces to be inspected. The second method uses water as the couplant, with the transducer and work piece submerged in a tank.

Reference can be made to the equipment manufacturer’s literature, or to the American Society for Metals (ASM) Metals Handbook, Volume 11 on “Non---Destructive Testing” (SNDT), for additional information. Important considerations include appropriate transducer frequency, operator requirements and qualification, application limitations, work piece requirements (grain size), instrument calibration, test block requirements, test specifications and interpretation of test results.

With the most common technique of ultrasonic inspection, namely, the pulse echo technique, the transducer both emits sound waves and receives the returning signals from the back surface and possible defects. The returning signals are subsequently monitored on an oscilloscope screen as shown in Fig 6---3. The indication to the left of the oscilloscope screen in Fig 6---3 is caused by the sound wave entering the steel and is called “initial pulse” or “contact interference.” The indication 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 distance 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 that each marker shown on the sweep line represents a standard distance or depth. Depth of the defect from the transducer contact point on the scanning surface can, therefore, be determined.

The American Society for Testing Materials and AGMA specifications which follow may be used for ultrasonic inspection of wrought and cast gearing. Forgings and bar stock: (1) AGMA 6033---A88, Section 10. (2) ASTM A388, Ultrasonic Examination of Heavy Steel Forgings. Castings: (1) AGMA 6033---A88, Section 11. (2) ASTM A609, Steel Castings, Carbon and Low Alloy, Ultrasonic Examination Thereof. 6.7 Microstructure. The major function of the material selection and heat treating process is to achieve the desired microstructure at the critical locations so that the part will have the desired contact and bending strength capacity. Hardened steel gearing microstructure should be tempered martensite at the entire tooth surface. The microstructure will vary around the gear tooth flank and throughout the tooth cross section. The tooth mass will have a significant effect on the resulting microstructure and hardness throughout the tooth section. The heat treatment variables will

Before testing, the instrument must be calibrated according to the test specification. Scanning sensitivity is often established as either the sensitivity to just obtain a specified back reflection height, or at the sensitivity to obtain an indication of specified height from a flat bottom hole drilled into test blocks. ANSI/AGMA

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significantly effect the microstructure achieved. Gear tooth quality control must include microstruc-

ture considerations as well as hardness control.

TRANSDUCER SUITABLE COUPLANT ON SURFACE

X Y

DEFECT

BACK REFLECTING SURFACE

INITIAL PULSE

BACK REFLECTION

Y

3 in (76 mm)

X DEFECT

MARKERS

Fig 6---3 Ultrasonic Inspection with Oscilloscope Screen

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INDICATION FROM FBH IN 4 in (102 mm) BLOCK

INDICATION D ---A REFERENCE LINE FROM FBH IN 12 in (306 2 1/2” mm) BLOCK (63 mm)

3 in (76 mm)

11 in (279 mm)

TEST BLOCKS: 12 AND 4 in (306 AND 102 mm) TEST BLOCKS CONTAINING SAME SIZE FLAT BOTTOM HOLE DRILLED TO A DEPTH OF 1 in

Fig 6---4 Distance --- Amplitude Reference Line for Ultrasonic Inspection

austenite. Some research has shown that microcracks are produced by subzero treating.

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.

In carburized and hardened steel gears, carbide forms and distribution are an area of microstructure concern. Continuous network carbide is generally considered to be unacceptable microstructure. Discontinuous carbide network is generally allowed within limits.

Microstructure evaluation 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. Data and opinions vary as to the allowable limits for retained austenite. Subzero treatment is specified for some applications to reduce retained

ANSI/AGMA

Bainite, pearlite, and ferrite are undesirable at the gear tooth surface of surface hardened gearing. These structures will exist in core microstructures of coarse tooth gearing.

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(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. Generally, smaller section test bars and sections show improved mechanical properties. (2) Castings--(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 standard integral or separate cast test coupons, and its effect related to improved solidification mechanism (reduced micro---segregation and micro---unsoundness) and increased response to heat treating, causes mechanical property variance compared to larger cast sections. (b) Location of the test coupon. Test coupon may be better located during heat treatment, causing increased response to heat treating and improved mechanical properties.

6.8 Mechanical Property Test Bar Considerations. Test coupons are specified by company and industry standards for evaluating mechanical properties of wrought and cast steel and non---ferrous materials used for gearing. NOTE: It should be realized, however, that mechanical properties obtained from test coupons for wrought and cast steel, cast iron and non---ferrous alloys are not equivalent to the actual properties of gearing from which the test coupons were obtained or associated. Smaller section test coupons are typically specified for economic considerations and instrument testing limitations. 6.8.1 Reasons for Mechanical Property Variance. The reasons for mechanical properties obtained from test coupons not being equivalent to those of gearing include the following considerations: (1) Wrought Forgings and Bar 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 directions are 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 manufacturers 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.

6.8.2 Mechanical Properties Affected. Mechanical properties obtained from test coupons, especially tensile ductility (percent elongation and reduction of area measured after tensile testing), impact strength and fatigue strength, are generally higher for test coupons than for actual forged or cast 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 range. 6.8.3 Interpretation. Mechanical properties obtained from test coupons should be considered as an indication of the quality of gear materials, but should not be interpreted as representing the precise mechanical properties of gearing for the reasons cited in 6.8.1 and 6.8.2. Specified mechanical properties for test coupons should be minimum properties, not typical properties. Designers should incorporate appropriate factors of safety based on experience for design of gearing to accommodate variance between measured and actual properties of gearing. In addition to test coupons providing indications as to the metallurgical quality of gear materials, test coupons provide a comparison of steel quality between different orders and can often help identify problems in steel making and heat treating.

Location or depth of the test coupon from the forged section (e.g. from the outside diameter, mid--section or from the center) and its effect with respect to the degree of mechanical working and segregation, causes variance in mechanical properties. Segregation is increased and degree of mechanical working is reduced towards the center of hot worked or wrought sections. (b) Mass effect. Small section of the test bar being tested, and the smaller section of the gearing from which the test coupon may have been obtained

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Bibliography ASTM A148---83,

Specifications for Steel Castings for High Strength Structural Purposes

ASTM A291---82,

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

ASTM A356---83,

Specification for Steel Castings, Carbon and Low Alloy, Heavy---Walled, for Steam Turbines

ASTM E125---63 (1980), Reference Photographs for Magnetic Particle Indications on Ferrous Castings ASTM E186---80,

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

ASTM E280---81,

Standard Reference Radiographs for Heavy 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) in Thickness

ASTM E609---83,

Ultrasonic Examination of Carbon and Low Alloy Steel Castings

ASTM E709---80,

Magnetic Particle Examination

MIL---H---6875G (Feb 86), Process for Heat Treatment of Steel

Reference Addresses American Society for Metals Metals Park, OH 44073 (216) 338---5151 Metals Handbooks Heat Treaters Guide Metals Reference Book

American Iron and Steel Institute 1000 16th Street, NW Washington, D.C. 20036 (202) 452---7100 AISI Steel Products Manuals Naval Publications and Forms Center 5801 Tabor Avenue Philadelphia, PA 19120 (215) 697---3321 Military Standards

American Society for Testing and Materials 1916 Race Street Philadelphia, PA 19103 (215) 299---5400 ASTM Standards

Metal Powder Industries Federation 105 College Road East\Princeton, NJ 080540 (609) 542---7700 MPIF Standard 35

Society of Automotive Engineers, Inc. 400 Commonwealth Drive Warrendale, PA 15096 (412) 776---4841 SAE Handbook AMS Standards

ANSI/AGMA

Other: Gray and Ductile Iron Castings Handbook Cast Steel Handbook Modern Plastics Encyclopedia

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Appendix A Plastic Gear Materials [This Appendix is provided for informational purposes only and should not be construed as part of AGMA Standard 2004---B89, Gear Materials and Heat Treatment Manual.]

A1. Purpose. The purpose of this Appendix is to provide information on plastic materials which have been used for gearing. For physical properties, refer to appropriate product standards.

moplastic material are used, with the latter 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 (polytetrafluorethylene) and graphite. Phenolics are generally used in applications requiring stability, and when higher temperatures are encountered. A5.2 Polyimide (T/S). Polyimide is usually 40---65 percent fiber glass reinforced and has good strength retention when used at high operating temperatures. A5.3 Nylon(T/P --- indicates thermoplastic). Nylon is a family of thermoplastic polymers. The most widely used of any molded gearing material is nylon 6/6, but nylon 6 and nylon 12 are also used. Some nylons absorb moisture which may cause dimensional instability. Nylon may be compounded with various types and amounts of glass reinforcing materials, mineral fillers, and such lubricants as PTFE and MoS2 (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 MoS2, as well as one version with fibrous PTFE. A5.5 Polycarbonate (T/P). Polycarbonate is generally used with the addition of glass fiber and/or PTFE lubricant and is a fine, low shrinkage material for producing consistently accurate molded gears.

A2. Tolerances. Under certain operating conditions, the tolerances for plastic gears may be less critical than for metal gears for smooth and quiet performance. Ordinarily, however, the same care in manufacturing, testing, measuring, and quality 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. A3. Operating Characteristics. Generally, plastic gearing materials are noted for low coefficient of friction, high efficiency performance, and quiet operation. Many plastic gearing materials have 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. Load Carrying Capacity. The maximum load 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). Polyurethane is generally noted for its flexibility and, therefore, has the ability to absorb shock and deaden sound. A5.8 SAN (Styreneacrylonitrile) (T/P). SAN is a stable, low shrinkage material and is used in some lightly loaded gear applications.

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

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A8.3 Burrs. Feather edge burrs, if not eliminated by back up discs or subsequent removal by other means, will impair inspection of gearing and possibly contribute to noise during operation.

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

A9. Laminated Phenolics Plastics.

A5.10 Polymer Elastomer (T/P). Polymer elastomer is a newcomer to the gearing field, and has excellent sound deadening qualities and resistance to flex fatigue, impact, and creep, among other advantageous characteristics.

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 fabric, glass fabric, or mat. These materials 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. Part Combinations. Several plastic gears can be molded together as a gear cluster. Combinations of gears, pulleys, sprockets, and cams can also be produced as a single part.

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

A7. Gear Blanks. Many of these plastic materials, notably unfilled nylon and acetal, are available in standard extruded shapes, such as rounds, squares, and rectangles of various sizes from which gears can be machined. Gears can be molded at 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 tooth surface is superior to the machined 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 tooth cavity of the mold. Gear cutting is done on standard machines and with standard tools. The following considerations will assist in obtaining higher quality machined parts.

Asbestos---phenolic grades have excellent thermal and dimensional stability. The glass fabric base grades have good heat resistance and very high tensile and impact strength. A9.2 Performance Characteristics. Phenolics are used for fine pitch gears due to economy, high resiliency, and high wear resistance. Lower density than metals often provides higher strength to weight ratios. It should be noted that all 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 as a gear but may require finish machining to meet most commercial quality requirements.

A8.1 Inspection. 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 is almost mandatory to avoid errors in measurements.

A10. Plastic Gearing References. AGMA 141.01, Plastics Gearing --- Molded, Machined and Other Methods.

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

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Appendix B Approximate Maximum Controlling Section Size Considerations for Through Hardened Gearing [This Appendix is provided for informational purposes only and should not be construed as part of AGMA Standard 2004---B89, Gear Materials and Heat Treatment Manual.] B1. Purpose. This Appendix presents approximate maximum controlling section size considerations 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 for gearing, and recommended maximum controlling section sizes for several low alloy steels from AGMA 6033---A88, Marine Propulsion Gear Units, Part 1, Materials. B2. Definition. The controlling section of a part is defined 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 required. The maximum controlling section size for steel is based princi-

pally on hardenability, specified hardness, depth of desired hardness, quenching and tempering temperature considerations. Reference should be made to 4.6 of the Standard for hardenability considerations. B3. Illustrations. Figure B---1 illustrates controlling sections for quenched gear configurations whose teeth are machined after heat treatment. NOTE: Evaluation of the controlling section size for the selection of an appropriate type of steel and/or specified hardness need not include consideration of standard rough stock machining allowances. Other special stock allowances such as those used to minimize distortion during heat treatment must be considered.

Table B---1 Approximate Maximum Recommended Controlling Section Size* Specified Brinell Hardness 223---262 248---293 262---311 285---311 302---352 321---363 341---388 w 363---415 w

**

Alloy Controlling Section Size, in (mm) AISI 4140

AISI 4340

To 8.0(203) included To 5.5(140) included To 4.5(115) included To 4.0(102) included To 3.0 (76) included Not recommended Not recommended Not recommended

No restriction ] No restriction No restriction To 25.0 (640) included To 15.0 (380) included To 12.0 (305) included To 8.0 (203) included To 3.75 (95) included

4350 Type [ No restriction ] No restriction No restriction No restriction No restriction No restriction No restriction To 23.0 (585) incl.

NOTES: * Maximum controlling section sizes higher than those above can be recommended when substantiated by test data (heat treat practice). Maximum recommended controlling section sizes for nitrided gearing are less than those above for the same hardness range because of higher tempering temperature required for nitriding gearing (refer to 5.5). Maximum recommended sizes for flame or induction hardening gearing would be same as above, dependent upon specified core hardness. [ 4350 Type Steel is generally considered equivalent to AISI 4340 for chemical analysis, except that carbon is 0.48---0.55 percent. ] “No restriction” indicates maximum controlling section size is not anticipated to provide any restrictions for conventional size gearing w 900_F(482_C) minimum temper may be required to meet these hardness specifications. ** Higher specified hardnesses (e.g. 375---415 HB, 388---321 HB and 401---444 HB) are used for special gearing, but costs should be evaluated due to reduced machinability.

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B4. Recommendations. Table B---1 provides approximate recommended maximum controlling section sizes for oil 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/hardenability data. Maximum controlling section sizes for rounds greater than 8.0 inch (205 mm) O.D. generally require in---house heat treat experiments of larger 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 obtained with the same type steel (hardenability) is considerably lower, however, and higher hardenability steel may be required. In---house normalized and tempered/hardness testing experiments are required.

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 “Chart Predicting Approximate Cross Section Hardness of Quenched Round Bars from Jominy Test Results” published in Practical Data for Metallurgists by Timkin Steel Co.,

TEETH

TEETH 2 inch (50)

--- --- --- --- --- --1.5 inch (38)

8 inch (203)

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

10 inch (254)

6 inch (152)

Controlling Section: 8 in (203 mm) Diameter

Controlling Section: 2 in (50 mm) Face width

TEETH

TEETH

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

36 inch (914)

--- --- --- --- --- --- --- --12 inch (304)

Controlling Section: 2 in (50 mm) Wall Thickness (If the bore diameter is less than 20% of the length of the bore, then the outside diameter)

32 inch (813) 36 inch (914)

Controlling Section: 2 in (50 mm) Rim Thickness

Fig B---1 Illustrations of Controlling Section Size

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Appendix C Case Hardenability of Carburizing Steels [This Appendix is provided for informational purposes only and should not be construed as part of AGMA Standard 2004---B89, Gear Materials and Heat Treatment Manual.] 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 to the location of gear teeth which governs the rate of heat removed during quench hardening. C3. Selection of Steel. To ensure that the steel under consideration has sufficient case hardenability to be capable of satisfactorily hardening the case in the roots of teeth, Fig C---1 should be used. Figure C---1 is based on hardenability and controlling section size considerations. Steels are presented in order of hardenability on the ordinate of Fig C---1. Steels not shown on Fig C---1, therefore, can be evaluated by comparing hardenability to those steels presented to determine the approximate maximum recommended controlling section size (as indicated by the solid line in Fig C---1).

C1. Purpose. This Appendix assists in the selection of a grade of carburizing steel to insure that the carburized case has sufficient hardenability to be capable of hardening roots of teeth to meet specified surface hardness requirements. The method used is based on steel hardenability considerations and standard hardening procedures used for carburized gearing. It may be used in conjunction with design and other considerations to select the appropriate grade of steel. 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 describes examples of how the controlling section size is determined for through hardened gearing when the teeth are cut after heat treating. The same examples

0

Approximate Controlling Section Size, mm 400 600 800 1000

200

AISI 9310 AISI 4820

1200

1400

ADEQUATE CASE HARDENABILITY

AISI 4320 CASE MAY OR MAY NOT HARDEN

AISI 8822 AISI 8620

NO CASE HARDENABILITY Source: The Influence of Microstructure on the of Case ---Carburized Components by Geoffrey Parrish, ASM Text (1980) AISI 4118 0

5

10

15

20 25 30 35 40 45 Approximate Controlling Section Size, inch

50

55

60

Fig C---1 Effect of Controlling Section on the Case Hardenability of Carburizing Grades of Steel ANSI/AGMA

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Appendix D Service Life Considerations [This Appendix is provided for informational purposes only and should not be construed as part of AGMA Standard 2004---B89, Gear Materials and Heat Treatment Manual.] D1. Purpose. Gears are generally removed from service due to wear, pitting, plastic flow, or breakage. If the service life is less than expected, an in---depth investigation should be initiated. This Appendix deals briefly with the causes of gear failures and the types of failures encountered.

tion criticals in the system causing vibration, inadequate grounding, etc. D2.7 Material Causes. Although materials rarely are the principal cause of failure, they can contribute to failure if material selection results in less than the required combination of properties compatible with the design and application. Improper selection of material can result in inadequate hardness (surface or subsurface) and toughness, or improper microstructure after heat treatment. Wrought materials such as hot rolled bars can have serious banding, which is alloy and carbon segregation in banded form. Banding 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, flakes, and bursts from insufficient forging temperature.

D2. Causes of Lower than Expected Life. When shorter than expected life is obtained, a number of factors should be reviewed. These factors are gear design, manufacture, heat treatment, assembly and installation, maintenance, service conditions and material 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. Manufacturing 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 Heat Treatment. Heat 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 fracture 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. Types of Gear Failures. Types of gear failure are pictured in AGMA 110, Nomenclature of Gear Tooth Failure Modes.

D2.4 Assembly and Installation. Improper assembly and installation are major contributors to premature failures and manifest themselves in excessive loading, wear, and misalignment.

D3.1 Wear. The most common wear failure modes are adhesion, abrasive scoring, corrosion, and flaking. These usually occur at or above 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 inadequate rigidity, faulty gaskets, seals, and bearings; and corrosion.

D3.2 Pitting. Pitting modes are initial pitting, destructive pitting, and spalling, and 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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D3.3 Plastic Flow. Plastic flow modes are rolling, peening, rippling, and ridging. Bending plastic flow occurs when the load exceeds the yield strength of the material.

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D3.4 Breakage. The majority of breakage failures (90 percent) are due to low and high cycle fatigue. Brittle failures may occur in low temperature service, in heat affected zones of welds or in notch sensitive materials. Overload failures result from misapplication, misalignment, and impact loading.

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