440c heat treat 2

[mwagers]

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).

 

[AJohnson]

The linear coefficient of thermal expansion for 440C steel is approximately 11 x 10^-6 /C or about 6 x 10^-6 /F.

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

 

[mwagers]

Oops, I was unclear. I have a production manager who insists that 440C will undergo PERMANENT shrinkage when heat treated (specifically to Rockwell 58-62C)

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

 

[AJohnson]

A volume change makes sense now that I think about it.

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

 

[mwagers]

Andy,

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

 

My experience is that a test is worth 10,000 "expert" opinions. Further I have learned during my 37 years experience not to rely on hocus pocus calculations to predict materials behavoir. A good source of information for HT of 440C is "Heat Treater's Guide by ASM Intl. There are many variables which effect the shrinkage/expansion of 440C after heat treatment 1) austenitizing temperature which effects retained austenite 2)cold stablizing and whether single or double temper 3)initial level of primary carbides in the base material and percentage of primary carbides which decomposes during austenitizing 4) the rate of cooling from austenitizing temperature. If application is critical do testing. Bob