1、L, LI 2000ETM2 o, 1 The Calculation of Optimum Surface Carbon Content for Carburized Case Hardened Gears by: P.C. Clarke, David Brown Heatech Limited TECHNICAL PAPER Q e The Calculation of Optimum Surface Carbon Content for Carburized Case Hardened Gears Philip C. Clarke, David Brown Heatech Limited
2、 The statements and opinions contained herein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. Abstract The International Gearing Standard IS0 6336, in part 5, addresses the metallurgical requirements to achieve the
3、rated service performance. For high quality carburise case hardened gears the heat treater is required to achieve, in conjunction with other properties, a surface carbon level of Eutectoid Carbon + 0.20% to - 0.10%. This creates practical difficulties because, at present, there is no method to calcu
4、late eutectoid carbon from chemical analysis and the eutectoid carbon is not necessarily the best value to base the surface carbon requirement on. The UKrepresentatives on the international committee preparing the next issue of the standard agreed to attempt to solve this problem. As a result this p
5、aper defines the conditions to calculate an optimum carbon level to minimise the possibilities of producing undesirable metallurgical phases including retained austenite, cementite and bainite. A method is proposed to determine the optimum carbon level from regression equations which calculate CCT d
6、iagram bainite, ferrite, pearlite and cementite nose times plus martensite transformation temperatures from the alloy content and austenitising conditions. Copyright O 2000 American Gear Manufacturers Association 1500 King Street, Suite 201 Alexandria, Virginia, 22314 October, 2000 ISBN: 1-55589-763
7、-0 The Calculation of Optimum Surface Carbon Content Introduction for Carburise Case Hardened Gears Author : Philip C Clarke, David Brown Heatech Limited For high quality carburise case hardened gears achievement of close case carbon control is essen tial. Whilst tight carbon control is possible, vi
8、ews on what optimum carbon level to target can be wider than the tolerance. The IS0 standard IS0 6336 in part 5 makes an attempt to specify a target and the tolerance for the highest quality grade as Eutectoid Carbon Percentage plus 0.20%, minus 0.1 0%. This implies that either a method exists to ca
9、lculate Eutectoid Carbon Content from alloy content or the values have been determined for a wide range of steels and are widely available. Unfortunately neither exist. Also implicit is that the Eutectoid Carbon is the optimum. But no rationale is given. A simplistic interpretation is to use the Eut
10、ectoid Carbon Content from the Iron -Carbon Phase Diagram - see Fig 1. This value is 0.77% which seems reasonable at a first glance. However, experienced heat treaters realise that the higher alloy steels would develop excessive retained austenite if targeting 0.77% with the above tolerance. Fig 1 I
11、ron Carbon Phase Diagram I I 600 I I -1 I O %Carbon 1.0 2.0 In practice the optimum carbon for a grade of steel is determined empirically by experience and is chosen to minimise the risk of forming undesirable phases including retained austenite, carbides, bainite and pearlite. The conclusion is tha
12、t any calculation of optimum carbon content must reflect this requirement. The objective of this paper is to define a readily available methodology to calculate optimum carbon content from alloy content and austenitising temperature at the hardening stage. Continuous Cooling Transformation ( CCT ) D
13、iagrams To avoid undesirable transformation products we turn to the effect that carbon content, alloy content and austenitising conditions have on the formation of phases during cooling. CCT diagrams are one of the most effective ways of representing transformation behaviour. Over 1000 diagrams repr
14、esenting the whole range of carburising alloys, carbon levels and austenitising conditions are available in the public domain. Fig 2 is typical of an experimentally determined CCT diagram with hardnesses and microstructures. Temperature is the vertical linear axis and time is the logarithmic horizon
15、tal scale. Fig 2 CCT Diagram, Hardnesses and Microstructures I 1 Over 600 selected CCT diagrams, refs 1-7 have been translated into mathematical form, refs 8,9, by Multiple Linear Regression Analysis. These subsequently became one of the cornerstones of the STAMP and AC3 programs, refs lO,ll, which
16、have a mature pedigree in calculating CCT diagrams, microstructure and case hardness profiles from alloy content, carbon profile, austenitising conditions, part geometry and cooling media. The CCT equations create the ability to analyse the effects of carbon and alloy content on transformation produ
17、cts. Fig 3 is a calculated example of the effect of carbon on these transformations. Increasing the carbon content pushes the boundaries of the undesirable products; bainite, ferrite and pearlite to the right increasing hardenabiliy. Fig 3 Effect of Carbon on CCT Diagrams For 0.2% C, 0.5% C and 0.9%
18、 C soo . , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , I ,T”1 - , , , , , , , , , , , , ,I, , , 111, 800 - 5 300 200 100 O -100 P + 1 10 100 1000 10000 1OOOOO Time in Seconds The martensite transformation temperatures are lowered increasing the amount of retained austenit
19、e at ambient temperatures. Fig 4 is a calculated example of the effect of Nickel on the CCT diagram at a carbon level of 0.70%. Fig 4 Effect of Nickel on CCT Diagrams For 0% Ni, 1.5% Ni and 3.0% Ni at 0.70 Yo Carbon The effects of Nickel are similar but less pronounced compared to carbon. The key to
20、 defining the optimum carbon level is to examine how cesain features vary with carbon content. The features chosen by the method described later are those which exhibit the greatest sensitivity to carbon content and have a large influence on case hardenability. These are : Bainite Nose Time Pearlite
21、 Nose Time Cementite Nose Time Martensite Start Temperature The nose times are the lowest values on the start of transformation curves. For example the Bainite nose time for 0.20 Yo Carbon in Figure 3 is 7 seconds and the Pearlite nose time at 1.5% Nickel in Figure 3 is 5000 seconds. Multiple Linear
22、 Regression Equations The Bainite, Pearlite and Cementite nose times and the Martensite Start Temperature can be calculated form the following equations. Where : T = Austenitising Temperature, deg C t = Austenitising Soak Time, mins Bainite Nose Time (BTAU in s) (1) C 50.50 % Logio(BTAU) = -3.79 +8.
23、68*C -5.35*C2-1 .70*Mn*C +1.56*Mn +0.79*Cr +0.92Mo +0.41 *Ni +0.32*Mo*Ni +0.0058*T +0.0002 1 *T*lOgl o(t) -100 , , . . . , . , , , , , , . . , . , , , , , , , , , , I 10 IO0 1000 tomo 110000 Tim. in S.sond. 2 (2) 0.50 I C 50.80 % Loglo(BTAU) = a + d*(C-0.8)2 Where : a equals the lower of : logl o( f
24、2) -g2 /4O or loglo( ti) + 0.50 and d = (loglo( ti) - a)/0.09 t, = BTAU at C = 0.50% from Equ (1) t2 = BTAU at C = 0.85% from Equ (4) g2 = is the gradient of the curve BTAU vs C at 0.85% from Equ(4) (7) C 2 0.80 % Loglo( PTAU) = -3.96 +0.95/C +0.73*Mn +0.54*Cr +3.33*Mo +0.65*Ni +0.00340*T (3) 0.80 C
25、 0.85 % Loglo(BTAU) = a + b*(C - 0.8) Where b = 400*(loglO( t2) - a) and a, t2 are as defined previously in (2) (4) C 10.85 % Log,(BTAU) = -7.30 + (1.69 -0.36*Ni)/C +0.57*Mn +0.57*Cr +1.81 *Mo +0.93*Ni +0.0065*T Pearlite Nose Time (PTAU in s) (5) C 10.60 % LOglO(PTAU) = -3.45 +2.77*C + 2.67*MO*C - 0
26、.75*Ni*C -3.00*C2 +1.26*Mn +1.52*Cr +4.54*Mo +0.98*Ni - 0.30*Cr2 -1.45*Mo*Cr +0.00233*T (6) 0.60 C e 0.80 % Loglo(PTAU) = Loglo( ti) + 5*(C - O.6)*L0g10( tdt, ) Cementite Nose Time (CTAU in s) Loglo(CTAU) = -1.24 +(-6.76 -0.1 1 *Mn -0.1 VCr +1.69*MO -0.06*Ni+O.O0602*T)/C +3.42*C2 +0.00047*T*Logl O(t
27、)/C Martensite Start Temperature (MS in OC) (8) C I 0.50 % (Andrews Formula, ref 12) MS = 51 2 - 453*C - 71.5*Mn*C - 67.6*Cr*C + 217*C2 + 15*Cr - 9.5*Mo - 16.9*Ni (9) 0.50 1,0.2, IF(LOG1o ( No.85 /No.5)-2,0,0.2*(LOGio (N0.85 IN0.5 ) + 2 113 ) Where : = lowest nose time at 0.50% N0.85 = lowest nose t
28、ime at 0.85% 5 Carbon equivalent, CM, to 25% Retained Austenite at an Ambient of 30 OC : CM = 1.1 - SQRT(0.36*(117 - MS1.1)/ (MS1.1 - MS0.5 ) Where : MSo.5 = 339.75 - 35.75*Mn - 18.8*Cr - 9.5*Mo - 16.9*Ni And MSI,l = 436 + 40*Cr - 5*Mo - 7*Ni -0.339*T - 0.023*( Mn + Ni*Cr )*T Calculation of Final Op
29、timum Carbon c = ( C” + (C, - 0.1) ) / 2 Results These formulae have been used to form the basis of an excel spreadsheet. Examples for selected steels are tabulated in Appendix 1. Points to emerge from appendix 1 include R For direct quenching the retained austenite criteria tends to dominate the fi
30、nal optimum carbon. R For reheat quenching the nose time equations tends to dominate. IC The highest Nickel steels have the lowest optimum carbons. Conclusions 9 A definitive method to calculate optimum carbon levels for carburise case hardened gears has been described. 9 The optimum carbon level mi
31、nimises the risk of forming undesirable transformation products including retained austenite, carbide, bainite and pearlite. 9 The method uses Multiple Linear Regression Equations derived from over 600 published CCT diagrams to calculate key points on the CCT diagrams. , 9 The accuracy of the coeffi
32、cient , a = - 1 .l x 10. in the Koistinen and Marburger equation : Where : Vy = o/o Retained Austenite MS = Martensite Start Temperature Tq = Ambient Temperature, needs to be re-evaluated because it was based on light microscopy measurements of retained austenite and more accurate methods of measuri
33、ng retained austenite by X-Ray diffraction and Electron microscopy are available and have demonstrated than light microscopy can give seriously misleading resu Its References 1 F. Wever, A. Rose et al “Atlas zur Warmebehandlung der Stahle”, Verlag Stahleisen, Part 1 1954-58 and Part 2 1973. 2 Atlas
34、of Isothermal and Cooling Transformation Diagrams, ASM publication 1977. 3 W. W. Cias Phase Transformation Kinetics and Hardenability of Medium Carbon Alloy Steels, Climax Molybdenum Company publication . 4 M.Atkins Atlas of Continuous Cooling Transformation Diagrams for Engineering Steels, British
35、Steel Corporation publication 1977. 5 J. Woolmer and R. A. Mottram The Mechanical and Physical Properties of the British Standard EN Steels, BISRA publication, VOIS 1-3, 1969. 6 G. Delbart, A. Constant and A. Clerc Courbes de Transformation des Aciers de Fabrication Francaise”, IRSID publication 196
36、0. 7 A. Schrader and A. Rose De Ferri Metallographia, Verlag Stahleisen 1966. 6 8 9 10 11 12 P. C. Clarke and D. W. Ingham The Effect of Variations of Chemical Composition on the CCT Characteristics of Through Hardening and Carburising Alloy Steels, David Brown Report H/R/24X 1979. P. C. Clarke The
37、Effect of Austenitising Heat Treatment, Silicon Content and Other Factors on the Continuous Cooling Transformation Characteristics of Wrought Gear Steels, David Brown Report H/R/24Y 1979. D. W. Ingham and P. C. Clarke Carburise Case Hardening: Computer Predictions of Structure and Hardness Distribut
38、ion, Heat Treatment of Metals, Vol 1 O NO 4, p 91-98 1983. D. W. McCurdy Software Simulation of Atmosphere Carburising and Hardening, ASM Proceedings of Carburising Processing and Performance 1989. K. W. Andrews Empirical Formulae for the Calculation of some Transformation Temperatures JISI Part 7,
39、721-727 1965. D. P. Koistinen and R. E. Marburger A General Equation describing the extent of the Austenite - Martensite Transformation in pure iron - carbon alloys and plain carbon steels, Acta Metallurgia, Vol 7 p 59-60 1959. 7 I Grade Pearlite 0.5% C 0.85%C 91 7 201,210 308,704 321,558 276,565 2,
40、554 1,338 46,420 39,135 86,492 17,254 91,698 1 01,056 152,126 89,359 736 179 18,577 9,733 36,669 3,775 IC Bainite Cementite % Carbon for 0.5% C 0.85% C at 0.85%C 25% Ret Aust 71 4 1,733 1 .IO 16,176 142,786 91,002 O. 72 29,007 121,691 240,623 O. 83 37 1,049 285,216 0.87 779 14,564 319,331 0.81 187 5
41、,217 618,630 O. 85 10,561 12,181 2,762 0.82 19,412 9,924 10,681 1.10 17 13 13,463 1.10 435 591 15,689 1 .o2 95 140 37,969 1.10 11 16MnCr5 253 264 295 281 289 253 264 295 281 289 655M 13 17CrNiM06 8620 4320 8822 655M 13 17CrNiMo6 8620 4320 8822 65 111 115 98 111 110 155 154 138 151 Appendix 1 Results
42、 of Sample Calculations Base Composition Aust Mn Cr Mo Ni Temi, 1 .o0 1 .o0 0.00 0.00 820 O. 45 O. 82 0.11 3.20 930 O. 48 1.62 0.29 1.52 930 o. 74 0.47 o. 19 0.47 930 O. 52 O. 47 0.24 1.73 930 O. 84 0.47 O. 34 0.51 930 0.45 0.82 0.1 1 3.20 820 0.48 1.62 0.29 1.52 820 0.74 0.47 0.1 9 0.47 820 0.52 0.47 0.24 1.73 820 0.84 0.47 0.34 0.51 820 Supplementary Calculations Optimum Carbon 0.70 O. 62 o. 73 o. 77 O. 71 o. 75 0.72 0.76 0.78 0.79 0.79 Grade 16MnCr5 655M13 17CrNjMo6 8620 4320 8822 655M13 17CrNiMo6 8620 4320 8822 8
