1、IDC 621.785 : 669.1 4 : 539.54 DEUTSCHE NORMEN February 1976 Heat Treatment of Ferrous Metals Material Selection Steel Selection according to Hardenability DIN 17 021 Part 1 I Wrmebehandiung von Eisenwerkstoffen; Werkstoffauswahl; Stahlauswahl aufgrund der Hrtbarkeit Contents Poge 1 scope l 2 Concep
2、ts .1 2.1 Hardening .l 2.2 Tempering -1 2.4 Hardenability .2 2.4.1 Potential hardness increase .2 2.4.2 Hardness penetrability . .2 2.5 Hardened state . .2 2.5.1 Hardness increase .2 2.5.2 Hardness penetration .2 2.6 Hardness penetration depth . .2 2.7 Cooling characteristic . .2 2.8 Cooling rate .
3、.2 3 Basic considerations concerning hardenability .2 3.1 Potential hardness increase . .2 3.2 Influence of alloying elements . .2 3.3 Testing hardenability . .3 4 Factors influencing the choice of steel . .3 4.1 Dimensional change, distortion and risk of cracking . .3 2.3 Quenching and tempering .2
4、 1 scope This Standard provides guidance on the selection of steel grades for workpieces to be hardened or quenched and tempered. The information given applies mainly to steels for which, according to the Standards, testing of harden- ability by the end quench test (see DIN 50 191) is provid- ed, na
5、mely for the alloy quenched and tempered steels according to DIN 13 200, the alloy case hardening steels according to DIN 17 210; the aiioy steels for flame and induction hardening (surface hardening) according to DIN 17 212 and for other steels, as necessary, for which the hardenability response is
6、 substantiated with sufficient accuracy by the end quench test. These are primarly the alloy steels according to the comparable Euronorms and IS0 Standards which can be looked up, for example, in the German Standards indicated above. This Standard applies also to nitriding steels according to DIN 17
7、 211. This Standard does not apply for the purposes of steel selection on the basis of the prospective hardness pene- tration depth or case depth after surface layer hardening or case hardening or to steels with only very low hardness gain, .e. primarily carbon steels, or for the selection of steels
8、 for transformation in the bainite range. Page 4.2 Effect of cooling agents . .3 4.3 Effect of tempering during the quenching and tempering process . .3 4.3.1 General information .3 4.3.2 Embnttlement phenomena .4 5 Steed selection .4 5.1 Relationship between the cooling process in the end quench sp
9、ecimen and in workpieces . .4 5.1.2 Relationship between the cooling characteristic of cylindrical speci- mens and the end quench specimen .5 5.1.1 Principles .4 5.2 Relationship between hardness and other mechanical properties at ambient temperature. . .5 5.3 Examples for steel selection . .6 5.3.1
10、 Selection on the basis of cooling .6 5.3.2 Selection on the basis of tests under operating conditions .8 Expressly excluded are also tools for which the selection of steel grade is dominated by criteria other than harden- ability. Apart from the approach whereby steel selection is made on the basis
11、 of hardenability, it may be necessary to take account of a number of other properties, such as machin- ability, reformability, joinability, fatigue limit, carbunz- ing behaviour, mechanical strength, tempering behaviour. Another decisive factor may be the cost of procuring and storing: 2 Concepts T
12、he concepts in this section correspond with those in DIN 17 014 Part 1, March 1975 edition. 2J Harening: Austenitizing and cooling at such a rate as to bring about a considerable increase in hardness in more or less extensive regions of the cross-section of the workpiece through martensite formation
13、. 2.2 Tempering: Heating a hardened workpiece to a tem- perature between ambient temperature and Ac1 and hold- ing at this temperature, followed by appropriate cooling. Continued on pages 2 to 10 Explanations on page 11 DN 17 021 Teil 1 engl. Preisgr. 7 Vertr.-Nr. 0107 Page 2 DIN 17 021 Part 1 2.3 Q
14、uenching and tempering: Hardening and subsequent tempering in the upper feasible temperature range in or- der to achieve satisfactory toughness with a given tensile strength. 2.4 Harenability Concept comprising potential hardness increase and hardness penetrability, a common method of testing harden
15、ability is the end quench hardenability test (see DIN 50 191). 2.4.1 Potential hardness increase: Maximum hardness a t t a i n a b 1 e in a workpiece through hardeningunder optimum conditions. 2.4.2 Hardness penetrability: Maximum hardness pene- tration depth attainable in a workpiece by hardening u
16、nder optimum conditions. 2.5 Hardened state: Condition of increased hardness attained in a workpiece through hardening. 2.5.1 Hardness increase: Maximum hardness a t t a i n- e d in a workpiece after hardening (under the prevailing conditions). 2.5.2 Hardness penetration: Hardening in terms of the c
17、ross-sectional region of a workpiece a f f e c t e d by it and the resulting hardness characteristic. A measure of the hardness penetration is the hardness penetration depth. 2.6 Hardness penetration depth: Vertical distance from the surface of a hardened workpiece down to the point at which the har
18、dness corresponds to an appropriately defined *) limiting value. 2.7 Cooling characteristic Specific temperature distribution in a workpiece during cooling down as a function of time. No te : In the nawower sense, the cooling churacter- istic gives a family of cooling curves for uarious points on a
19、workpiece. 2.8 Coolingrate Time-related temperature decrease for a given point or a given range on a cooling curve. No t e : The cooling characteristic depends on the heat transfer coefficient, which in turn is determined by the material and the coolant, and on the coefficient of ther- mal conductiu
20、ity churacteristic of the material. For the steels indicated in Section 1, there are only minor differ- ences in the coefficient of thermal conductiuity. 3 Basic considerations concerning hardenability 3.1 Potential hardness increase For the purpose of this Standard, the potential hardness increase,
21、 .e. the maximum hardness attainable, depends on the portion of the carbon content of the steel which is dissolved in the austenite. The size of the dissolved portion is determined by the austenitizing conditions. The relationship existing between the carbon portion dissolved in the austenite and th
22、e hardness attainable after quenching as a function of the martensite structure is presented in Fig. 1 and Fig. 2 for alloy and carbon steels. The hardness increase obtained in the workpiece depends not only on the potential hardness increase but also on the cooling conditions. I I I I l I HRC 60 t
23、50 8 c LO 2 I 30 2o,l 0.2 0.3 0,4 % by wt. 0.6 Carbon content - Figure 1. Relationship between as quenched-hardness, carbon content and martensite structure of hardened alloy and carbon steels (according to Hodge/Orehoski, Trans. AIME, 167,627,1946) %I HRC 0.6 60 OS 0,4 50 0.3 40 303 40 50 60 70 80
24、90%100 70 f C 0.2 I 3y wt. Martensite structure - Figure 2. Relationship between as quenched-hardness, carbon content and martensite structure of hardened alloy and carbon steels (according to Hodge/Orehoski, Trans. AIME, 167,627,1946) 3.2 Influence of alloying elements Hardness penetration is deter
25、mined mainly by the kind and quantity of the alloying elements dissolved in the austenite, also by the dissolved carbon and the cooling characteristic. Increasing proportions of alloying elements intensify the hardness penetration effect. The most effec- tive alloying elements are manganese, chromiu
26、m and mo- lybdenum. From certain minimum cross-section onwards it is neces- sary, if an approximately uniform distribution of hard- ness over the cross-section is to be obtained when harden- ing under identical cooling conditions, to use alloy steels instead of carbon steels. No t e : If, for exampl
27、e, hardness penetration right through to the core is required, the use of carbon steels oughtonly be considered up to diameter of about 20 mm. *) For determination of hardness penetration depth, see DIN 50 190 Part 2. DIN 17 021 Part 1 Page 3 The larger the cross-section of a component and the more
28、uniform the desired hardness distribution over the cross- section is, the larger must be the proportion of suitable alloying elements to be brought into solution in the aus- tenite. Assuming the same requirements regarding hard- ness penetration, the carbon content of alloy steels can be smaller tha
29、n that of carbon steels; however, this results in a smaller hardness increase. 3.3 Testing hardenability As a rule the hardenability is determined by the end quench test according to DIN 50 191 and represented in the form of end-distance hardness curves. For this test, a specimen which has undergone
30、 austenitizing is placed in a suitable fixture and is quenched only on its bottom end face by a jet of water under constant conditions. The cooling rate decreases with increasing distance from the quenched end face. It is characterized by the cooling period between 800 and 500 “C (see Fig. 3). 40 80
31、 120 160 s200 Figure 3. Cooling period from 800 to 5OO0C- Relationship between cooling period and end distance of end quenched specimens of trans- formable steels (according to A. Rose, Atlas zur Wrmebehandlung der Sthle (Steel Heat Treatment Atlas), Vol. 1 and H. Brandis/H. Preisendanz, Das Abkhlve
32、rhalten in Stirnab- schreckproben (Cooling Behaviour in End Quenched Specimens), Bnder, Bleche, Rohre, Oct. 1963) Hardness measurements made along the cylindrical sur- face of the specimen genedly reveal decreasing hardness values. Their distribution characterizes the hardenability . For steels suit
33、able for testing by the end quench method, it is possible in this way to establish so-called hardenability scatter bands corresponding to the scatter of the heat; for an example see Fig. 4. 60 HRC 50 M 40 Iu 2 30 C I 2o0 10 20 30 LO 50mm60 Distance from quenched - end face Figure 4. Hardenability sc
34、atter band for 34 CrMo 4 steel according to DIN 17 200 4 Factors influencing choice of steel 4.1 Dimensional change, distortion and risk of cracking For definitions of dimensional change and distortion see DIN 17 014 Part 1. Points which also need to be given consideration when deciding which type o
35、f steel should be used for a given component on the basis of hardness are dimensional change, distortion and the risk of cracking. Distortion and risk of cracking are influenced by the different volume changes taking place throughout the cross-section during hardening. Tests are necessary for obtain
36、ing optimum correlation of component shape, dimensional change, dis- tortion, risk of cracking and steel composition. The internai stresses, varying in their distribution and magnitude as a result of hardening, are the criterion for the magnitude of distortion and the risk of cracking. The more rapi
37、dly cooling from hardening temperature takes place and the more complex the shape of the work- piece, the more adverse are the effects of the stresses likely to be. Slower cooling can bring about reduced stresses. If the hardenability of the steel is then insufficient for attaining the properties re
38、quired, it will be necessary to select a steel with greater hardenability. 4.2 Effect of cooling agent Assuming the same steel grades and dimensions, the heat extraction during hardening is determined by the cooling agent used. its properties and temperature plus any move- ment of the cooling agent
39、and/or the item to be cooled will influence the cooling. in individual cases correspond- ing variations may also occur in the dimensional changes, distortion and risk of cracking. The cooling agents differ in their cooling capacity in the temperature ranges which are important for hardening. Of the
40、cooling agents commonly used, air provides the slowest cooling and water the fastest. Between these is cooling in oil or fused salt, depending on the physical properties (e.g. viscosity, specific heat, thermal conduc- tivity). 4.3 Effect of tempering during the quenching and tempering process 4.3.1
41、General information The effect of tempering on modification of properties, increase of toughness, elongation and percentage reduc- tion of area at break, as well as the decrease of hardness, tensile strength and yield point, depends on the temper- ing temperature and tempering time. The two factors
42、are interchangeable within certain limits. When it is required to temper steels to the same hardness or strength it is normally found that alloy steels need higher tempering temperatures than carbon steels. Guidance will be found in the tempering diagrams (see, for example, DIN 17 200). By adapting
43、the heat-treatment to the particular applica- tion it is possible to impart to workpieces mechanical strength values differing from the data in the Standard. On the basis of the hardness stipulated after tempering, and with allowance made for the drop in hardness values, compared with the as quenche
44、d-condition, brought about by tempering, Fig. 5 indicates the as quenched-hardness needed prior to tempering. Page 4 DIN 17 021 Part 1 Tempered 60 HRC 55 t 50 f L5 2 z ln f LO 2 35 o) 30 2- 2515 20 25 30 35 LOHRC45 As tempered-hardness - Figure 5. Relationship between hardness before and after tempe
45、ring when heat treating steels to DIN 17 200 (according to unpublished investiga- tions by H. Brandis) 4.3.2 Embrittlement phenomena It should be noted that various steels may undergo em- brittlement during tempering if certain temperature ranges cannot be avoided. Such embrittlement is a factor to
46、be specially considered when impact loading of,components is involved and its existence is proved preferentially by the notched bar impact bending test. In this connection a distinction is made between a tem- perature range approximately around 300 OC (termed “300 degree embrittlement“) and a range
47、between 350 and 55OOC (termed “temper embrittlement“). 80 i c a .- 5 U L LO d U C a O LT 20 O To avoid the 300 Okmbrittlement, tempering in the range 250 to 350C should be avoided as far as possible. Temper embrittlement mainly affects steels alloyed with Mn, Cr, MnCr, CrV and CrNi when such steels
48、are cooled slowly after tempering above 600 OC or when tempering is carrled out between 350 and 550OC. Embrittlement can be reduced by low phosphorus content, using steels alloyed with molybdenum up to about 0.6 % by weight or by cooling rapidly after tempering above 600 OC. 5 Steelselection 5.1 Rel
49、ationship between the cooling process in the end quench specimen and in workpieces 5.1.1 Principles When a workpiece is quenched from the austenitizing tem- perature, the cooling characteristic established in the various areas of the piece depends on its shape and dimen- sions and also on the action of the quenchant. The end quench specimen also exhibits similar cooling characteristics. It is, therefore, possible to assign certain points or areas of a component with adequate accuracy to specific points on the cylindrical surface of the end quench specimen having the same rate
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