Re: Tempering Question (Chris North) wrote in message
I need help in finding some information on tempering. The application is tempering Cr-Mo steels, ... and section sizes are
small enough that transformation to martensite is achieved fully. What I need is a way to predict final hardness based on tempering time and temperatures. The object is to minimize the time and still achieve acceptable hardness (as well as tensile and impact) properties. From school, I remember Larsen-Miller relationship P=T*(log(t)+C), but this was years ago and my old texts were lost over the years and all I can recall is the equation and not how to use it. Are there any better correlations out there? Maybe one based on chemistry? Any help would be appreciated. Thanks,
You're more than halfway there since you remembered there is an equation. If I recall correctly, Larson and Miller borrowed the tempering parameter from Hollomon and Jaffee.
The tempering parameter, P is defined by the equation P = T*(A + log(t)) where T is the absolute temperature in Kelvin, t is the time in hours, and A is a constant about twenty. Hardness (or yield or tensile strength) is supposed to depend on P. So, for the same steel a series of points at equal time and different temperatures could be used to develop a master curve.
G. Krauss's book STEELS: Heat Treatment and Processing Principles, ASM International, 1990 discusses the tempering parameter on pages 215-216.
Use of the tempering parameter is discussed in more detail in the older book Ferrous Metallurgical Design by J. H. Hollomon and L. D. Jaffe, Wiley & Sons, New York, 1947 They say that the constant A depends linearly on carbon content, with A = 21.1875 - 5.625*(%C) For short soak times you may need to get fancier and include the heatup. You have to integrate. I vaguely recall that S.L. Semiatin showed how to do this in another ASM book on induction heat treating. For modified 4130 steels with 1%Cr, 0.75%Mo and 0.025% Nb containing 0.15% to 0.37% carbon, and A varying with carbon content as previously shown, the yield strength in MPa depends on P as (see p. 364 of Hydrogen Effects in Metals, Met Soc. AIME, Warrendale, 1981):
YS(MPa) = 23168 - 2198*P + 53.519*P*P
Pittsburgh Pete
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