DIN--17022-5-2000 Heat Treatment of Ferrous Material

March2000

D EUTSCHE NORM NORM

Heat treatment of ferrous materials

{

Part 5: Surface hardening

17022-5

ICS 25.200 Wärmebehandlung von Eisenwerkstoffen – Verfahren der Wärmebehandlung – Teil 5: Randschichthärten In keeping with current practice in standards published by the International Organization for Standardization (ISO), a comma has been used throughout as the decimal marker.

Contents Page

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Sc op e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 No Norm rmat ativ ive e refe refere renc nces es . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Co n c e p t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Pr Prin inci cipl ple e of of met metho hod d ............................... .................................. 5 Ide Identi ntific ficati ation on of of heat heat treatme treatment nt condi conditio tion n . . . . . . .. . . . . . .. . . . . . .. . . . . . . .. . . . . . .. . . . . . .. . . 6 P ro c e d u re . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Pretreatment and preparation preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Austenitizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Subzero treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Tempering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Se Seco cond ndar ary y tre treat atme ment nt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 He Heat at tr trea eatm tmen entt med media ia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Cooling and quenching media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Subzero treatment media media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Ef Effe fects cts of sur surfa face ce har harde deni ning ng . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Effects on case structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Effec Effects ts on hardness hardness and effect effective ive case case depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Effects on shape and dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 De Defe fects cts in in heat heat treat treated ed prod produc ucts ts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 De Desi sign gnin ing g for for heat heat trea treatme tment nt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 St ra ra ig ig ht ht en enin g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Tes Testin ting g surf surface ace har harden dened ed pro produc ducts ts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 2 2 2 2 3 3 4 6 7 8 8 8 8 8 8 8 9 10 10 11 13 13

Foreword This standard has been prepared by Technical Committee Wärmebehandlungstechnik  of the Normen aus schu ss Werks  ausschu W erkstof tofftec ftec hnol ogi ogie e (Materials Technology Standards Committee).

1

Sco pe

This standard describes the surface hardening of products made of rolled steel, cast iron, or steel powder compacts.

Continued on pages 2 to 13.

Translation by DIN-Sprachendien DIN-Sprachendienst. st. In case of doubt, the German-language original should be consulted as the authoritative text.

© No part of this translation translation may be reproduced without without the prior permission permission of DIN Deutsches Institut Institut für Normung e. V., Berlin. Beuth Verlag GmbH, GmbH , 10772 Berlin, Germany, Germany, has the exclusive right of sale for German Standards (DIN-Normen) .

Ref. Re f. No No.. DI DIN N 17 1702 0222-5 5 : 20 2000 00-0 -03 3 Engl En glis ish h pri price ce gr grou oup p 11

Sale Sa les sN No.011 o.0111 1 03.01

P ag e 2 DIN 17022-5 : 2000-03

2

Normative references

This standard incorporates, by dated or undated reference, provisions from other publications. These normative references are cited at the appropriate places in the text, and the titles of the publications are listed below. For dated references, subsequent amendments to or revisions of any of these publications apply to this standard only when incorporated in it by amendment or revision. For undated references, the latest edition of the publication referred to applies. D IN 6 77 3

H ea t t rea tme nt o f f er ro us ma te ri al s – H ea t t rea te d p art s, r ep res en ta tio n a nd i nd ica tio ns o n drawings *) DIN 17014-3 Heat treatment of ferrous materials – Symbols for heat treatment processes D IN 17 02 2- 1 H ea t t rea tm en t o f f er ro us p ro duc ts – H ar de ni ng an d t em pe ri ng DIN 17022-2 Heat treatment of ferrous materials – Heat treatment methods – Hardening and tempering of tools DIN 17023 Heat treatment of f errous metals – F orms – Orders for heat treatment (WBA) DIN 50103-3 Rockwell hardness testing of metallic materials – Modified Rockwell scales Bm and Fm (for thin sheet steel) DIN 50190-2 Determination of the effective case depth of heat treated parts after surface hardening DIN 50192 Determination of depth of decarburization of steel D IN 5 06 01 M et all og rap hi c e xam in at io n – De te rmi na ti on of th e f er ri ti c o r a us te ni tic gr ai n s ize of ste el and ferrous materials D IN E N 5 71 -1 N on -d es tr uc ti ve te sti ng – P en et ran t t es ti ng – P ar t 1 : G en er al pri nc ip les D IN E N 1 00 52 V oca bu la ry of he at tr eat me nt te rms fo r f er ro us pro du cts D IN E N 1 26 26 Sa fe ty of ma ch in er y – L as er pr oce ss in g ma ch in es – S af et y r eq ui reme nts (ISO 11553 : 1996, modified) DIN EN ISO 6506-1 Metallic materials – Brinell hardness test – Part 1: Test method DIN EN ISO 6507-1 Metallic materials – Vickers hardness test – Part 1: Test method (ISO 6507-1 : 1997) DIN EN ISO 6508-1 Metallic materials – Rockwell hardness test (scales A, B, C, D, E, F, G, H, K, N, T) – Part1: Test method

3

Concepts

For the purposes of this standard, the heat treatment concepts defined in DIN EN 10052 shall apply.

4

Principle of method

The surface layer of a ferrous product is austenitized and then cooled at a suitable rate. Martensite is thus formed, increasing the hardness of the surface layer and enhancing strength and wear resistance. The area to be hardened is heated to a temperature above  Ac3 or  Ac m by means of either flame, induction, laser beam or electron beam hardening. For each material, the density of the heat flow rate of the heat source and the treatment time produce a specific thermal cycle during which the surface layer is austenitized to a certain depth at a high heating rate followed by a short soaking time as compared to other heat treatment methods. Because of the transformation behaviour of steel, higher heating rates require higher heating temperatures to obtain a sufficiently austenitic condition. The relationship between the heating rate and temperature can be derived from a time-temperature-transformation (TTT) diagram for continuous heating. Hardening actually occurs during the subsequent quenching of the product. Large areas can be hardened either by means of a suitable energy transfer or by moving the product itself. Between the hardened case and the non-hardened core lies a transition zone of several millimetres within which the depth of hardness gradually diminishes. The depth of this zone is influenced by the heating and quenching conditions. In many cases, surface hardening is followed by tempering.

5

Identification of heat treatment condition

The heat treatment condition shall be indicated on drawings as specified in DIN 6773. Instructions for performing surface hardening shall be formulated using either the ‘WBA’ form specified in DIN 17023 or in a ‘heat treatment plan (WBP)’. Symbols used to designate the heat treatment method shall be as specified in DIN 17014-3.

*) Currently at draft stage.

P ag e 3 DIN 17022-5 : 2000-03

6 6.1

Procedure Pretreatment and preparation

Products shall be pretreated and prepared to ensure a material condition suitable for surface hardening, particularly in terms of microstructure and residual stresses, and to obtain the required core strength in the final condition. Prior to laser hardening, it may be necessary to clean the product surface and pretreat it to improve absorption. Prior to electron beam hardening, the surface shall be cleaned and, if necessary, demagnetized. 6 .1 .1 P re tr ea tm ent 6.1.1.1 Stress relieving If residual stresses (e.g. due to cutting processes) might cause distortion of the product during treatment, it is recommended that stress relieving be carried out. Any resulting distortion can then be corrected by subsequent machining, although there shall be an allowance great enough to eliminate any unwanted changes to the surface layer (e.g. decarburization). The stress relief temperature shall be close to, but shall not exceed, the transformation temperature  Ac1 of the material being treated. In the case of quenched and tempered products, this temperature shall be lower than the tempering temperature in order to maintain strength, and soaking for more than thirty minutes during the heating phase will not be necessary. Heating and cooling shall be carried out slowly to prevent new residual stresses from building up. Cold-worked products should not be stress relieved, but rather normalized, if there is a risk that recrystallization would result in grain coarsening. 6.1.1.2 Normalizing Residual stresses in untreated products may also be relieved by normalizing, which at the same time alters the microstructure, thus preventing grain coarsening in critical areas. Normalizing parameters (normalizing temperature and duration, cooling) shall be taken from the steel manufacturer’s specifications or other documents. 6.1.1.3 Quenching and tempering It may be necessary to subject the product to quenching and tempering prior to treatment to obtain the desired strength and a homogenous material condition. See DIN 17022-1 and DIN 17022-2 regarding the procedure. To ensure that any changes to the surface layer (e.g. decarburization or oxidation) which occur during quenching and tempering do not adversely affect subsequent treatment, the product surface should be machined before further treatment. 6.1.1.4 Oxidizing Prior to laser hardening, it may be necessary to o xidize the surface to promote the absorption of the laser beam by the material. Normally, this is done by annealing the material in water vapour at a temperature between 450 °C and 550 °C. 6 .1. 2 P re pa ra ti on Machining or cutting residues (e.g. oxide layers, residues of cooling lubricants, cleaning agents or preservatives) can impede the surface hardening process, as can chips, burrs, rust, scale and nonferrous metals. The evaporation of residues during electron beam hardening can adversely affect the vacuum, while during laser hardening such residues can affect the transfer of energy to the surface layer. It is therefore necessary to carefully treat and thoroughly clean the products prior to hardening, depending on the degree of surface impurities and the required quality. The surface can be cleaned by washing, deburring, blasting or pickling. 6.1.2.1 Washing Normally, products are washed in hot water with suitable cleaning agents. To ensure that the surface is fully cleaned, it may be necessary to subject the surface to water-blast cleaning or ultrasound cleaning prior to washing. After washing, the products shall be thoroughly dried. 6.1.2.2 Deburring Burrs caused by machining can be removed by blasting, or chemical or thermal deburring. It should be noted that thermal deburring processes oxidize the product’s surface, while in chemical processes the material reacts with the electrolyte, so that in both cases treatment with electron beams or lasers can be impeded. When removing adherent chips, the product should be demagnetized. 6.1.2.3 Blasting Dry or wet blasting with suitable cleaning agents can be used to remove burrs, scale, rolling, forging or casting skin, colorants or flux residues.

P ag e 4 DIN 17022-5 : 2000-03 6.1.2.4 Pickling Pickling is suitable for removing rust, scale, or rolling, forging or casting skin. Care should be taken to fully remove all pickling residue, since this can begin to rust. Furthermore, too intensive pickling can leave pits in the surface layer. 6.1.2.5 Coating Prior to laser hardening, it may be necessary to supply the product with a coating that promotes laser beam absorption (e.g. using graphite powder). 6.1.2.6 Edge protection Prior to flame or induction hardening, it may be necessary to protect edges in the area to be hardened from overheating. This can be done by fitting suitable copper inserts into undercuts, flutes, slots, holes, etc.

6.2

Austenitizing

Surface hardening involves a localized heating of a product’s surface layer to austenitizing temperature for a certain length of time, with the heating process being performed once or several times, using one of several heat sources. The heating rate is determined by the energy supplied by the heat source and the heating time. The resulting temperature profile for the heated case is a function of the type and density of the flow rate of the heat source, the exposure time and the type of material being treated. The objective is to maintain a uniform temperature distribution within a localized heated area. Care should be taken to ensure that the maximum temperature within the case does not exceed the melting temperatures of the different phases 1 ) in the material . With surface hardening, the austenitizing temperature is reached within a much shorter time than with furnace heating, due to the relatively high density of heat flow rate (cf. table 1). For sufficient austenitizing, it is therefore necessary to heat to temperatures which are 50 °C to 100 °C higher than furnace temperatures, taking care that the temperatures in the external regions of the product are below the melting temperature of the material, to avoid unwanted fusion. Table 1: Density of heat flow rate of various heat sources

Density of heat flow rate, in W/cm2

Effective case depth, in mm

Laser beam

10 3 to 104

0,01 to 1

Electron beam

10 3 to 104

0,01 to 1

Induction: MF HF HF-impulse

10 3 to 104

Flame

10 3 to 6 . 10 3

Heat source

2 to 8 0,1 to 2 0,05 to 0,5 1,5 to 10

Plasma beam

10 4

–

Salt bath (convection)

20

–

 Air/ gas (convec tion )

0,5

–

The microstructural changes taking place during the heating process are described in a time-temperaturetransformation (TTT) diagram for continuous heating (see figure 1 for an example).

1 )

For example, 950 °C f or the phos phide e utectic mixture in c ast iron.

P ag e 5 DIN 17022-5 : 2000-03 Heating rate, in °C/s

   C    °   n    i  ,   e   r   u    t   a   r   e   p   m   e    T

Complete austenitization Partial austenitization

Ferrite + austenite + carbide

Ferrite + carbide

Time, in s Quenching/tempering parameters: 825 °C for 15 min in water, 600 °C for 60 min in air Figure 1:

Time-temperature-transformation (TTT) diagram for continuous heating of grade 42CrMo4 steel in the quenched and tempered c ondition

Figure 1 shows that as the heating rate increases, austenite formation and carbide dissolution take place at increasingly higher temperatures. The curves in the TTT diagram can be used to approximate the temperature above which a specific microstructure can be obtained at a given heating rate. The formation of austenite and dissolution of carbide are influenced by the type of alloying elements present and their quantities, as well as by the material condition prior to treatment. Although a complete dissolution of carbide is not generally desirable, enough carbide should be dissolved to ensure the carbon content of the austenite is sufficient to achieve the required hardness. In progressive methods the heat source or the product travels, allowing localized austenitizing with a varying microstructure. Patterns of hardened areas can be created by moving the product. A simple example of a spiral pattern is shown in figure 2. If a treated area is exposed a second time to the heat source, tempering occurs in the adjacent areas, making them subject to cracking. See DIN 6773 regarding the designation of such areas. Hardened

Not hardened Figure 2: Example of a spiral pattern of hardened areas on a shaft

P ag e 6 DIN 17022-5 : 2000-03 6.2.1 Induction hardening Here the heat required for austenitizing is generated by means of induction. Heating is accomplished by placing a product in the magnetic field generated by an alternating current passing through an inductor, usually watercooled. The rapidly alternating magnetic field induces current within the product and the induced currents then generate heat.  A conduct ive he atin g meth od can also be use d in whi ch the heated par t of the product s erve s as the indu ctor . The depth of heating produced by induction is inversely proportional to the frequency of the alternating current. Normally, this frequency is constant. The formation of the heated area is determined by the type and form of the inductor and its coupling, and thus by the distance between the inductor and the product surface. The heated case formed does not always absolutely conform to the shape of the product. The depth of heating is normally controlled by the alternating current power input, the inductive coupling, and the density of the electromagnetic field. To this end, a single-turn or multi-turn induction coil, or a magnetic inductor may be used. Normally, the inductor remains still and the product moves, for instance to cover large areas (‘progressive method’). A ‘spinning method’ in which the product is rotated is often used on symmetrical pieces for concentrated heating.  After the metal has been aus teni tized and the alter nati ng cu rrent turned off (or the pr oduct has been removed from the inductor), the product is quenched in a suitable medium. Where the depth of heating is not very g reat, the product can be ‘self-quenched’ by simply allowing the unheated core to draw off heat from the surface layer. 6.2.2 Flame hardening Flame hardening is a heating method in which the product surface is austenitized by heating with a torch, which is normally moving while the product remains still or is rotated. The depth of heating is determined by shape of the torch, the type of gas used to create the flame, and the flo w rate of the gas. The type of torch used determines the size of the heated area. The torch can be moved back and fo rth across the product to cover a greater area or to obtain a greater case depth by means of thermal conduction. Furthermore, a moving torch can help ensure that the surface temperature remains below melting temperature. Quenching is carried out after flame hardening in much the same manner as after induction hardening. 6.2.3 Laser hardening With this method, the heat source is a high-power laser beam. The laser is only partially absorbed by a very thin surface layer and the rest is reflected. The extent of absorption depends on the product material, the laser’s wavelength, the surface condition (roughness, degree of oxidation, cleanliness, etc.) and the product temperature. Absorption can be increased by adding coatings or using polarized radiation. The area covered by the beam can be influenced by manipulating the optical components or mirrors used to create the laser beam. The depth of heating is determined by the level of thermal conduction. By moving the laser beam and product in relation to each other, the area of treatment can be moved. Measures are to be taken to protect persons and property f rom direct and reflected radiation, as specified in DIN EN 12626. 6.2.4 Electron beam hardening With this method, the heat source is an electron beam formed by means of magnetic lenses and directed at the product’s surface. Both the electron beam and the product are in a vacuum. With their kinetic energy, the electrons heat the product to a depth of about 10 m m to 50 mm, with the actual depth of heating being determined by the level of thermal conduction. It should be noted that X-rays are emitted, depending on the accelerating voltage, and sputtering occurs at the surface. The electric charge of the product has to be dissipated via the product and its holder. The size of the treated area can be adjusted by changing the shape of the beam, or by guiding it or splitting it. The location of the area can be adjusted by moving the beam or the product.

6.3

Quenching

Quenching is performed using a medium that is suitable for the material’s hardness, and for the size and shape of the product. For smaller heating depths and where the relevant product dimension is about ten times the effective case depth, quenching may not be necessary because the bulk of the product acts as an adequate heat sink for ‘self-quenching’. For regular quenching, the product can either be dipped in the quenching medium, or nozzles can be used to spray the product with the medium.  As with heat ing, quen chin g produces differe nces between the core and case temperat ures , whic h in turn creates stresses that can lead to distortion or cracking. It may therefore be necessary to limit the quenching action.

P ag e 7 DIN 17022-5 : 2000-03 Figure 3 shows an example of a continuous-cooling-transformation (CCT) diagram, which illustrates the phase transformations taking place in a grade 42CrMo4 steel during quenching at austenitizing temperature. The regions in which microstructural changes occur are shown as curves, whose position and shape are determined by the steel’s material composition and the austenitizing conditions. The expected microstructure of the case at ambient temperature and the relevant hardness can be approximated on the basis of the cooling curves in the diagram. In the case of surface hardening, a full transformation to martensite is desirable. This is only possible if the critical cooling rate, vKm, characteristic for each steel can be reached within the austenitized region. If the hardenability of the material is too low, the case is too deep, or the quenching effect is not sufficient, then other constituents (e.g. bainite, pearlite or ferrite) form in addition to martensite. Hypereutectoid steel can also contain undissolved or preeutectoid carbides, as well as retained austenite. The transformation of austenite into martensite begins once the cooling temperature goes below the Ms temperature, and is not complete until the Mf  temperature is reached, which can be below ambient temperature, depending on the composition of the material and the austenitizing conditions.  Aust enit zing temperat ure: 850 °C

Pearlite    C    °   n    i  ,   e   r   u    t   a   r   e   p   m   e    T

 Austenite

Ferrite

Pearlite

Bainite

Martensite

Bainite content (%) Retained austenite content (%)

Minutes Hours Time, in s

Days

Figure 3: Continuous-cooling-transformation (CCT) diagram for a grade 42CrMo4 steel

6.4

Subzero treatment

The amount of retained austenite at ambient temperature can be reduced by subzero treatment. This may be necessary if tempering would lower the hardness value, or there are special requirements regarding the dimensional stability of the product. However, subzero treatment increases the brittleness of the material, thus reducing tensile or fatigue strength. Because the retained austenite stabilizes immediately after quenching, subzero treatment should be carried out directly following the quenching process. Tempering at low temperatures can also lead to the stabilization of retained austenite. See subclause 8.2 for suitable subzero cooling media.

P ag e 8 DIN 17022-5 : 2000-03

6.5

Tempering

Tempering involves heating the product and soaking it at tempering temperature, then cooling it to ambient temperature. Either the entire product or the case only is heated. Tempering should immediately follow the hardening process, although the product should be allowed to cool to ambient temperature first. Normally, tempering is carried out between 180 °C and 220 °C, rarely above these temperatures. If tempering is performed in a furnace, the soaking time should be at least one hour.

7

Secondary treatment

Normally no secondary treatment is performed on surface hardened products aside from mechanical or chemical surface treatment.

8

Heat treatment media

8.1

Cooling and quenching media

Quenching can be carried out with or without a quenching medium. Quenching using a medium is necessary if self-quenching will not occur at the required critical cooling rate; this is normally the case for flame or induction hardening, while self-quenching is usually sufficient after laser or electron beam processes. 8 .1 .1 L iq uid m ed ia Common liquid quenching media include water with or without additives, and oil. It should be noted that polymer additives lower the cooling rate as compared to water without additives. The temperature of the quenching medium is maintained within a narrow range, with water normally being used at a temperature between 15 °C and 40 °C and oil normally being used either at ambient temperature or a temperature above 60 °C. 8.1.2 Gaseous media Still or forced air, and nitrogen may be used as gaseous quenching media. The quenching effect is dramatically lower in gaseous media than in liquids, although it can be increased by raising the pressure or the flow rate.

8.2

Subzero treatment media

In conventional freezers, the cooled air cools the products to about –60 °C. Special equipment can be used to lower the temperature to –140 °C. Temperatures below –60 °C may be reached by using dry ice, alcohol mixtures or liquefied gases (e.g. liquid nitrogen, which has a temperature of –196 °C).

9

Effects of surface hardening

9.1

Effects on case structure

Because the product surface is heated to a temperature well above  Ac3 and  Acm in current practice, the formation of austenite is to be expected, as shown in TTT diagrams for various steels. Throughout the heating process, the degree of austenite formation 2 ) decreas es with increasin g depth; the phase s formed are influenc ed by the heating and quenching conditions, and the product material. Figure 4 shows a schematic representation of the hardness profile and phases in a quenched and tempered and then surface hardened product. The microstructure of hardened or quenched and tempered materials can be divided into several zones. Starting at the core and moving towards the surface these are: a tempering zone, a mixed zone with martensite, bainite, pearlite, ferrite and carbide, and a martensitic zone. Decarburization can occur in the case, depending on the thermal cycle and material. Laser hardening can cause layers to form which promote absorption, leading to carburization.

2 )

The degre e of austenite form atio n is given by the degr ee of carb ide disso luti on, the uniformit y of the austenite and the austenitic grain size (cf. DIN 17022-1).

P ag e 9 DIN 17022-5 : 2000-03

Fully and uniformly austenitized and quenched with

Hardness profile

V  ö V km

Fully austenitized but quenched with V  < V km

  s   s   e   n    d   r   a    H

Partially austenitized Quenched and tempered original microstructure

Primarily martensite

Martensite, bainite, pearlite

Martensite + bainite + pearlite + ferrite

Original microstructure with tempering at higher temperatures

Distance from surface Figure 4: Example of hardness profile and microstructure in a quenched and tempered steel (schematic)

9.2

Effects on hardness and effective case depth

The hardness of the case formed by surface hardening is a function of the type, amount and distribution of martensite, bainite, pearlite, ferrite and carbide. Figure 5 shows a schematic hardness profile for two steels with different carbon concentrations.

   V    H   s   s   e   n    d   r   a    H

Distance from surface, in mm Figure 5: Hardness profiles for surface hardened steels with different carbon concentrations in the non-tempered original condition (schematic)

Page 10 DIN 17022-5 : 2000-03

9.3

Effects on shape and dimensions

Localized heating and quenching can cause extensive dimensional changes and can produce residual stresses in different areas of the product due to thermal expansion and phase transformations. Since this can lead to distortion, products should be fixed in place or prestressed. Furthermore, products which had residual stresses before hardening and which were not sufficiently tempered or stress relieved will become distorted during the hardening process.

10

Defects in heat treated products

Defects in heat treated products are rarely due to a single cause. In addition to the heat treatment process itself, possible causes include the material and shape of the product, the machining process and service conditions. Table 2 lists some of the most common defects which occur in practice and which can be attributed to the surface hardening process, assuming the products have been delivered in good condition without any defects. Table 2: Defects and their possible causes

Defect

Cause

Refer to subclause

Heat treatment error

1 Surface hardness too 1.1 Insufficiently a) Austenitizing temperature low transformed pearlite, too low ferrite, or insufficiently b) Austenitizing time too short dissolved carbides

6.2

1.2 Insufficient amount of martensite in case... 1.2.1 due to formation of bainite, pearlite or ferrite

1.2.2 due to retained austenite

1.3 Martensite too soft, possibly localized

1.4 Too much retained a us te ni te or to o f ew carbides1 ) diss olved

2 Surface hardness too Martensite in case too great hard

3 Effective case depth too small

3.1 Austenitization does not cover entire area

a) Austenitizing temperature too low b) Austenitizing time too short c) Insufficient quenching (through medium or selfquenching) d) Too much oxidation of edges e) Case is decarburized

6.2 6.3, 8.1 8.1 8.1

a) Austenitizing temperature too high (overheating) b) Austenitizing time too long c) Insufficient, improperly timed, or no subzero treatment d) Insufficient, improperly timed, or no tempering e) C ar bur iz at io n d ue to u se o f coatings

6.2

a) Tempering temperature too high b) Tempering time too long c) Overlapping treatment of already hardened areas

6.5

a) No tempering or tempering d on e a t wr on g ti me b) Temp erin g tempera ture too low c) Tempering time too short

6.2 6.4 6.5 9 .1

6.5 6.2 6.5 6.5 6.5

a) No tempering b) Tempering temperature too low c) N ot te mpe red en oug h ti me s

6.5 6.5

a) Austenitizing temperature too low b ) A us te nit izi ng ti me to o s ho rt

6.2

(continued) For 1 ), s ee page 11.

6.2

6 .5

6 .2

Page 11 DIN 17022-5 : 2000-03 Table 2 (concluded)

Defect

Cause 3.2 Too little martensite

4 E ff ect iv e c as e d ept h too great 5 Too much distortion

6 C racking

1 )

11

Quenching rate too slow

6.3, 8.1

T em pe rature too high wh en a) Au st en it iz in g t em pe rat ur e preheating to austenitizing too high temperature b) Austenitizing time too long

6 .2

Thermal and transformation stresses too great or unevenly distributed

6.3, 9.3

a) Too quickly or unevenly heated and austenitized b) Product not properly arranged c) Area to be treated not suitably designed

Thermal and a) T oo quickly or unevenly transformation stresses too heated and austenitized g reat (localized brittleness) b) Too quickly or unevenly quenched c) No tempering

7 Distortion of corners and edges or warping

Refer to subclause

Heat treatment error

Unintended fusion

6.2

9.3 6.2 6.2 6.3 6.5

d ) T emp er in g te mp er atu re t oo low e) Tempering time too short f) Overlapping treatment of already hardened areas

6 .5

a) Temperature too high b) Treatment time too long c) Power of heat source too high d) Edges overheated

6.2 6.2 6.2

6.5 6.2

6.1, 2.6

Appl ies only to stee ls h avin g under gone se condary hard enin g.

Designing for heat treatment

The product shape and size are major factors influencing the hardness profile and stresses created during the hardening process, as well as the resulting distortion. By selecting a suitable design the likelihood of distortion and the risk of cracking can be minimized, and often the life o f the product can be increased. Abrupt changes in cross section can have different effects on the hardness profile, depending on the method used. The following design principles should therefore be taken into account. – A suitable mass distribution can be obtained by avoiding designs with abrupt changes in cross section (cf. figure 6). – Instead of abrupt changes in cross section, give preference to rounded or bevelled transitions (cf. figure7). – Where the case extends to an edge of the product, include a chamfer (cf. figure 8). – Symmetrical designs should be used wherever possible (cf. figure 9).

Unsuitable

Suitable

Figure 6: Examples of suitable and unsuitable mass distribution

Page 12 DIN 17022-5 : 2000-03

Unsuitable

Suitable

Figure 7: Examples of suitable and unsuitable changes in cross section

Unsuitable

Suitable

Figure 8: Examples of suitable and unsuitable edge designs

Unsuitable

Suitable

Figure 9: Examples of asymmetrical and symmetrical designs

If it is not possible to design the product so that it is suitable for surface hardening, the product should receive its final form after hardening, as shown in figure 10.

Detail X

Detail X Removed by machining after hardening

Figure 10: Giving the product its final form after hardening (example)

Page 13 DIN 17022-5 : 2000-03

12

Straightening

When straightening hardened products, it should be noted that the case has practically no deformability and can therefore break even when only slightly deformed. For this reason, straightening of such products should be avoided. Slight distortions can be removed by bending the product using a straightening press, machine or bench, or by subjecting it to selective heating. When thermally straightening localized areas, care should be taken that the case is not tempered by the heating, since this will reduce hardness. It should be noted that the residual stresses induced by straightening may create renewed distortion. Straightening should be performed before tempering, since then the risk of cracking, the formation of residual stresses, and of deformation is lower.

13

Testing surface hardened products

When the effective case depth is determined, the product is destroyed. If this is not permitted, a reference specimen, preferably of the same material condition, size and shape as the product, should be heat treated along with the products, or an extra number of products are to be treated. Table 3 lists methods for testing the effectiveness of the heat treatment procedure. If a batch of several products has been treated, sampling should be carried out following statistical principles. The product user shall decide whether test results are suitable for determining the performance characteristics of the product. Table 3: Testing surface hardened products

Property/characteristic tested

Test method

1 Hardness

As in DIN 50103-3, DIN EN ISO 6506-1, DIN EN ISO 6507-1 and DIN EN ISO 6508-1

2 E ffective case depth

As in DIN 50190-2

3 Extent of unintended fusion

Visual examination of cleaned products, without any further pretreatment

4 Soft spots

a) Hardness testing b) Visual or macroscopic examination of etched (preferably polished 1 )) or blas ted surface

5 Cracking

a) Visual examination of cleaned products b) Micrographic examination (macro- or microscopic) c) Penetration testing as in DIN EN 571-1 d) Ultrasound testing e) Eddy current testing f) Magnetic flaw detection

6 Microstructure: 6.1 Form, number and structure of constituents (martensite, bainite, pearlite, ferrite, retained austenite and carbides) 6.2 Grain size and form 6.3 Decarburization or carburization of surface layer 1 )

Micrographic examination

As in DIN 50601 As in DIN 50192

Poli shing shal l be carr ied out so that the sof t spots in t he s urface laye r ar e no t re move d.