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,

----------------------------------------------------------

Chris:

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 +

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,

----------------------------------------------------------

Chris:

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*

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

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***P

Pittsburgh Pete

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