1、UDC 621.7a5.61.7 : 669.1 : 621.9.02 r620.i DEUTSCHE NORM April 1989 Heat treatment of ferrous materials Heat treatment met hods Case hardening IDIN I 17022 Part 3 Wrmebehandlung von Eisenwerkstoffen; Verfahren der Wrmebehandlung; Einsatzhrten In keeping with current practice in standards published b
2、y the International Organization forstandardization (/SO), a comma has been used throughout as the decimal marker Contents Page 1 Scope and field of application 2 2 Terminology . 2 3 Principles of case hardening 2 4 Indication of case-hardened condition in documentation 2 5 Procedure . 2 5.1.1 Workp
3、iece preparation 2 5.1.2 Stress relieving . 2 5.1.3 Normalizing . 2 5.1.4 Hardening and tempering 2 5.1.5 Preparation for local carburizing or carbonitriding 2 5.1.6 Charging of workpieces . 2 5.1 Workpiece preparation and pretreatment 2 5.2 Heating to treatment temperature . 3 5.3 Carburizing., . 3
4、 5.3.1 Carburizing with constant carbon potential . 5 5.3.2 Carburizing with variable carbon potential 5 5.3.4 Carburizing in solid media (granulate) 6 5.3.3 Case depth . 5 5.3.5 Salt bath carburizing 6 5.3.6 Gas carburizing . 6 5.3.6.1 Gas carburizing with carbon potential control . 6 5.3.6.2 Gas c
5、arburizing without carbon potential control . 6 5.4 Carbonitriding 7 5.4.1 Salt bath carbonitriding 7 5.4.2 Gas carbonitriding 5.5.1 General . 5.5.2 Direct hardening 8 5.5.3 Single hardening . 8 5.5.4 Double quench hardening . 8 5.6 Sub-zero treatment . 5.7 Tempering 5.5 Quench hardening . 7 6 Heat
6、treatment media . 6.1 Heating media . 6.1.1 Liquid heating media . 6.1.2 Gaseous heating media 6.1.3 Vacuum . 6.2 Carburizing and carbonitriding media 6.2.1 Solid media (granulate) . 6.2.2 Salt baths . 6.2.3 Gases . 6.2.3.1 Gases for carburizing . 6.2.3.2 Gases for carbonitridina - 6.3 Cooling and q
7、uenching media . 6.3.1 Liquid quenching media . 6.3.2 Gaseous cooling media 14 6.3.3 Cooling media for sub-zero treatment 14 Page 12 12 12 12 12 12 12 12 12 12 14 14 14 7 Instructions on equipment used in heat treatment . 14 7.1 Heat treatment furnace . 14 7.2 Cooling and quenching equipment . 14 7.
8、3 Sub-zero treatment equipment 14 8 Defects in heat treated workpieces . 16 9 Heat treatment as a criterion in design . 18 10 Instructions on straightening 19 11 Inspection of heat treated workpieces . 19 Appendix A . 20 A.l Recommended surface carbon content for low retained austenite levels . 20 A
9、.2 Relationship between carbon potentiat and gas composition in the case of gas carburizing _ . _ ._ . _ _ ._. _ ._ 21 A.3 Local case hardening . 21 Standards and other documents referred to 22 Continued on pages 2 to 22 uth Verlag GmbH,Berlin, has the exclusive right of sale for German Standards (D
10、IN-Normen). DIN 17022 Part 3 Engl. Price group 13 10.90 Sales No. 0113 Page 2 DIN 17022 Part 3 1 Scope and field of application This standard describes case hardening procedures and gives instructions on the case hardening of steel work- pieces and tools. 2 Terminology The terminology associated wit
11、h heat treatment as used in this standard has been adopted from DIN 17014 Pari 1. 3 Principles of case hardening Cases hardening is intended to make the case (surface lay- er) of steel workpieces and tools substantially harderand to improve their mechanical properties. It consists of carburiz- ing o
12、r carbonitriding followed by quench hardening carried out either immediately after this treatment or after int- ercooling and reheating to a quench hardening tempera- ture suited to the particular application. This procedure enriches the case with carbon (carburizing) or carbon and nitrogen (carboni
13、triding) before quench hardening. As compared with carburizing, enrichment with nitrogen pro- duces a higher degree of hardenability by modifiying the transformation process in the case, and thus results in an improved retention of hardness after quench hardening. The steel may be tempered again or
14、treated at a tempera- ture below ambient and tempered after quench hardening, depending on the characteristics specified for the product or the requirements of subsequent machining (e.g. grind- ing). 4 Indication of case-hardened condition in documentation The case-hardened condition shall be indica
15、ted on draw- ings as specified in DIN 6773 Part 4. Where required, infor- mation relating to case hardening shall take the form of a set of heat treatment instructions asspecified in DIN 17 023 or of a heat treatment schedule, the designation given in DIN 17014 Part 3 being used to identify details
16、of the treat- ment applied. 5 Procedure 5.1 Workpiece preparation and pretreatment Preparation or pretreatment of workpieces is intended to prevent their final condition being adversely affected by in- ternal stresses (risk of distortion) or by the surface condi- tion,as well as to ensure that heat
17、treatment is not interrup- ted by workpiece failure. Protective surface treatment may also be applied to limit the extent of carburization. 5.1.1 Workpiece preparation Depending on the degree of surface impurities and the quality required, it may be necessary to prepare the work- pieces before carbu
18、rizing or carbonitriding by means washing, drying, pickling, abrasive blasting, deburring, chip removal or other suitable processes so a) that carburizing or carbonitriding is not inhibited, for example, by sulfurous cooling lubricant residues, rust, scale, rolling, forging or casting skins. or othe
19、r such unwanted coatings; b) that salt baths are not polluted by burrs, chips, rust, scale, rolling, forging or casting skins; c) as to prevent splash from salt baths due to the sudden evaporation of water or other liquids. Bolts or screws used to close threaded or unthreaded holes shall be removed
20、before heat treatment,or before cleaning. 5.1.2 Stress relieving If internal stresses in the workpiece unduly influence its susceptibility to distortion during case hardening, stress relieving will be required, the resulting changes in size and shape being taken into account by providing an adequate
21、 machining allowance. The temperature for stress relieving shall be close to trans- formation temperature,Acl, but should not exceed it.Soak- ing after heating is then not required.The heating and cool- ing process shall be controlled so that noadditionai or new internal stresses are produced. For c
22、old formed workpieces, normalizing shall be given preference to stress relieving if the latter is likelyto result in grain coarsening due to recrys- tallization. 5.1.3 Normalizing As stated above, internal stresses in the workpiece blank may be reduced by normalizing which,at the same time,can reduc
23、e differences in the microstructure and prevent grain coarsening at critical points of workpieces of complex shape. The temperatures required may be found in the relevant technical delivery conditions for steel or in documentation provided by the steel supplier. 5.1.4 Hardening and tempering Another
24、 method of internal stress reduction is to heat the workpiece to austenitizing temperature which also in- creases the homogeneity of the material. Subsequent cooling of the workpiece blank in the same way as in quench hardening will reduce the likelychanges in size and shape as a result of quench ha
25、rdening of the workpiece after machining, and permit the extent and direction of the changes to be estimated. Following this, the workpiece shall be tempered so as to allow further machining and to compensate for any changes in size and shape. This treatment of the blank,which includes machining oft
26、he workpiece prior to hardening, has proved its value particu- lary where exacting demands are to be met in respect of dimensional stability. 5.1.5 Preparation for local carburizing or carbonitriding Where specific parts of a workpiece are to be carburized or carbonitrided,one of the following measu
27、res may be taken: application of a coating affording protection against carburizing or carbonitriding (e.9. a paste or an electro- plated copper coating), such coating, however, not being effective in the case of salt bath carburizing or carbonitriding; fitting of a protective component (e.g. sleeve
28、 on shaft ends); its protective effect is. however, limited or negli- gible in the case of salt bath treatment; parts ofthe workpiece may be packed in asolid medium which inhibits carburization, this being only effective where a granulate is used for carburizing. After quench hardening, the surface
29、hardness in the non- carburized or non-carbonitrided zones corresponds to the carbon content of the steel in its initial condition. The width of the transition zone between carburized and the non-carburized zones is a function of the method of sur- face protection used and is narrowest where coating
30、s are applied. 5.1.6 Charging of workpieces When charging the workpieces, care shall be taken to ensure that their arrangement is such that a) in the case of carbonitriding. all parts of the surface to be treated are fully exposed to the treatment medium, and 1) See appendix A for further measures.
31、DIN 17022 Part 3 Page 3 5.3 Carburizing In carburizing, which is to be effected at temperatures be- tween 880C and 1050“C, preferentially between 900C and 950“C, the workpieces are fully exposed to the carbu- rizing medium,from which carbon is released and deposit- ed on the workpiece surface. If th
32、e carbon activity of the medium is greater than that of the stee1,carbon will diffuse into the workpiece surface, this causing an increase in car- bon content. In practice,the carbon activityof the carburizing medium is generally characterized by the carbon content, known as the carbon potential, th
33、is being defined as the maximum carbon content,expressed as a percentage by mass,which a pure iron foil will absorb over its entire cross section. The enrichment of the workpiece surface zone with carbon is represented by the carburization diagram (ci. figure 5), which is mainly a function of the fo
34、llowing parameters: a) the carbon potential; b) the carbon transition index, p; c) the rate of carbon diffusion in the steel; d) the carburizing temperature: e) the carburizing time. The carbon content in the surface zone (surface carbon content, for short) is largely a function of the carbon potent
35、ial.lf the carbon activityof the carburizing medium is less than the saturation limit, Cs3), the carbon potential is the maximum that can be achieved in the surface zone; if it is greater, the carbon potential cannot be defined and the surface carbon content can reach a level equal to CS. The rate a
36、t which the surface carbon content approaches the carbon potential or the saturation value depends on the level of the carbon transition index,B,and the rate of carbon diffusion in the steel. If the carbon transition rate is signifi- cantly higher than the diffusion rate, a short carburizing time wi
37、ll be sufficient. b) in the case of quenching, all zones to be hardened are exposed equally and, at the same flow rate, to the quenching medium. Accordingly, bulk material shall not be heaped too high or packed too densely,intermediate grids being used to avoid this where necessary, workpieces shall
38、 be arranged sepa- rately or kept moving in the treatment medium, or forced movement of the medium is to be provided. Face-on-face contact between workpieces shall be avoid- ed, linear contact may have adverse effects, whereas the effect of point contact is generally negligible. During quenching, wo
39、rkpieces closed on one side or pot- shaped workpieces shall be arranged with their open end up to allow water bubbles to escape.*) Workpieces shall be packed so that they are deformed nei- ther by their self-weight nor by other components lying on them. 5.2 Heating to treatment temperature The tempe
40、rature in the surface zone and the core of work- pieces of simple geometry and of a more or less uniform cross section.when they are brought to carburizing, carbo- nitriding or quench hardening temperature, is shown as a function of time in a graph in figure 1. Workpieces of non- uniform cross secti
41、on yield different heating curves forsur- face zone and core of each cross section. The sum of heating and soaking time gives the holding time for a workpiece in a furnace. When workpieces are heated,differences in temperature at the surface and at the core arise, the differences being a function of
42、 the rate of heating and the thermal conductivity of the material. The differences and the microstructural transformation to which they give rise at different points in time produce internal stresses that, in turn, may lead to dis- tortion of the workpieces. Hence, large workpieces orwork- pieces wi
43、th large differences in their cross section,particu- larly when made of alloy steel, shall be heated slowly or in stages (cf. figure 2). Figure 3 provides guidance on the heating time for circular, square or rectangular workpiece cross sections when heat- ed in a salt bath,figure 4 providing informa
44、tion for heating in air-circulating furnaces and chamber furnaces. Carburizing, carbonitriding, quench hardening temperature Core Time Holding time - Soaking time carburizing or carbonitriding time Figure 1. Typical thermal cycle fora workpiece undergoing carburizing, carbonitriding or quench harden
45、ing 2) The resulting loss of quenching medium will have to be accepted. 3, The saturation limit is the carbon content corresponding to the point at which a line drawn from the carburizing temperature intersects with SE curve associated with the particular content of alloying elements. Carburizing, c
46、arbonitriding, quench hardening temperature L 2 F 3 Y a E Figure 2. Typical thermal cycle foraworkpiece undergoing carburizing, carbonitriding or quench hardening with a single preheating stage Page 4 DIN 17022 Part 3 20 : o) .E c 10 cn c a .- c ,“5 - 3 LL 0 10 20 30 40 50 60 70 80 90 100 mm 120 Dia
47、meter or thickness of square or rectangular cross sections- Figure 3. Full heating time for salt bath heating (curves originating from tests on cylindrical test pieces) O 20 40 60 80 100 120 140 160 180mm200 Diameter or thickness of square or rectangular cross sections - Figure 4. Full heating time
48、in air-circulating and chamber furnaces (curves originating from tests on cylindrical test pieces) 1 Surface O carbon C u w Distance from surface, in mm Figure 5. Typical carburization diagram DIN 17 022 Part 3 Seite 5 Carbon transition rate higher than diffusion rate, but less than In some cases,ca
49、rburizing is required to achieve a specific surface carbon content. Recommended maximum values which will give low retained austenite levels after quench hardening are given in table A.l forthe most common case hardening steels. To allow for the influence of the alloying elements on the carbon activity in the steel, the carbon potential for carbu- rizing alloy steel shall differ from that for unalloyed steel. The difference in carbon potential may be determined with adequate accuracy using the following equation i: cb lg - = - 0,055.O/0 Si + 0,013 .Oh Mn + 0,040
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