440c heat treat
440c heat treat
(OP)
Does 440C typically undergo any shrinkage during heat treat. If so what expansion coefficient should be used (or suggestions as to where to find this).
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RE: 440c heat treat
So a piece of 440C steel that is raised to the austenitizing temperature and then cooled, will expand (a little) as it is heated and then shrink back to original size once it is cooled. When the metal is tempered the same expansion and contraction cycle will be repeated.
Remember not to temper your 440C steel in the 475-550 C range or you will end up with a case of martensitic temper embrittlement and a very brittle piece of metal.
Andy
RE: 440c heat treat
The only reason I can think of is that the martensite is changing to ferrite and cementite, which is causing an EXTREMELY small amount of shrinkage. I havent had a chance to look at my MAT-SCI book on this yet.
Thanks for the insight
Matt
RE: 440c heat treat
So a little digging revealed the following:
assume a 1.0 % carbon steel:
lattice paramter of austentite is a=.3599
volume of austenite unit cell
v=a*(a/sqrt2)*(a/sqrt2)=.0233nm^3
martensite lattice parameters are
a=.2853
c=.2982
volume of martensite unit cell
V=c*a*a=.0243nm^3
the change in volume is .0243-.0233=0.0010nm^3
NOTE: This is an INCREASE from austenite to martensite
relative change in volume is 4 percent
relative change in length (assuming equal changes in x,y,and z directions) is 1.3 percent
So there is an increase in volume from austenite to martensite upon the initial austenitizing heat treat and air quench when forming the martensite.
The tempering heat treatment decomposes some of the retained austenite (FCC) into ferrite (BCC), which is smaller in volume. The resulting volume decrease would depend on the percentage of transformed austenite. I think it is safe to say that the length change would be less than the 1.3% achieved in the conversion of austenite to martensite.
Andy
RE: 440c heat treat
WOW. Thanks a lot. Im not sure how long it would have taken me to arrive at that conclusion. I think it might take a few hours to set in, but I can definitely follow you. I really appreciate the help.
Matt
RE: 440c heat treat