Does heat treating steel change its Thermal Expansion Coefficient?

Given two identical samples of heat treatable steel in both composition and geometry, if one of the pieces is annealed or hardened would there be a measurable difference in their thermal expansion coefficient?

• Just so we are all in agreement on what we are answering, the question is if we have three otherwise identical pieces of, say, 1040 steel, we anneal one, harden another and leave the third alone, Will the coefficient of thermal expansion be different in the three samples? That is if, after annealing or hardening, we machine each to as close to the same dimensions as we can get and measure their dimensions at different temperatures, will they react differently. – user1683793 Jan 1 '18 at 5:26
• @user1683793 That is exactly the question. – Joe Kissling Jan 1 '18 at 5:27

In my experience, tables/data listing the thermal expansion coefficients of different materials do not make any differentiation between hardened/annealed etc. states of any given material. The degree of accuracy to which the TEC can be measured is limited by the tolerances to which each material is made up of, and this variability will likely be larger than any change from heat treatment.

One related thing to note, is that when you heat treat one of your samples, it will likely change size as a result of the treatment. See https://vacaero.com/information-resources/the-heat-treat-doctor/1316-dimensional-changes-after-heat-treatment.html for more details

• I’ve never seen a distinction between heat treat states of various materials either, nor was it ever mentioned in any class I took. I want to make sure I’m not missing something or the classes were all idealized situations. – Joe Kissling Dec 31 '17 at 20:30

I don't see how they can have the same value. Hardening or annealing affects the crystal structure of material and in the case of steel, how much of the carbon (and other hardening elements) gets absorbed into the iron. Both of these will affect how they react to changes in temperature. Is the coefficient of thermal expansion higher or lower? That part, I can not tell you. I have searched and searched on the web looking for examples where the hardening is listed with the coefficient of thermal expansion and I came up empty. I expect that the variability due to differences in steel chemistry has more effect than that of hardening. (Keep in mind that, say, 1040 steel can have between 0.370 and 0.440 % carbon and between 0.60 and 0.90 % manganese.)

• Intuitively I agree with you, various heat treats influence the crystal structure, thereby altering the material properties of the material. That should change the expansion coefficient. The thing is I have not seen any documented evidence of this effect. I’m not concerned with how it changes, since any dissimilar expansion can be problematic. – Joe Kissling Jan 1 '18 at 6:13
• This is the same conclusion I came to. If the effect of crystal structure is smaller than the chemical variation, then it will be next to impossible to measure, and not very useful if you were able to find out. – Jonathan R Swift Jan 1 '18 at 12:07
• If you have samples cut from the same bar, they will be chemically identical. The question was not if it can be measured separately from the chemical variation but if it exists. Useful? What's that? – user1683793 Jan 2 '18 at 16:19

In a career as a metallurgist , I never saw a distinction noted for different thermal expansion rates. However in a hardened condition steel will be expanded slightly. And for some high alloys that may be ferritic or austenitic ( depending on heat-treatment) at ambient temperatures, there will be a coefficient difference.