Shrinkage and Induction Hardening

There are a few equations that will try to explain the volume changes in Heat treatment

%V = -4.64 +2.21C (annealed structure to austenizing) and

= 4.64 - 0.53C (austenizing to hardening ie martensite transformation)

as per ASM (Tool materials by JR davis)

There is likely to be a nett increase in volume (and he have physically seen it too) but your entry denotes it is a decrease in volume. That is only possible if you have very low martensite transformation.

Just forgot to add (in case ) C=%Carbon content and it is not temperature.

These equations are the expansion in volume due to HT : expansion due to conversion of austenite to martensite and reduction in volume due to austenisation.

There are a few other volume change too which are accompanied by volume change.

The Stress relieving operation is not associated with micro change and hence volume change. These though do affect the geometry.

BTW: at our place all the Induction hardening, carburising etc are proceeded by final grinding/ honing as the case may be (ID or OD)

Could it be stress relieving that is changing the diameter?

What temperatures are these processes working at and for how long?

Thinking of the processes involved from fabrication at the mill to final induction hardening, it would seem more likely that the piece would expand if it were stress relieved due to the removal of the outer layer and the relaxation of the previously in-tension inner core. Or is the mill process originally putting the inner core of the bar in compression?

So I am not at all sure that my answer is helpful in any way than to set someone else thinking or to be able to discard this idea.

If it is causing a mechanical fitting problem the part should be machined to allow for this problem.

Obviously the hardening process alters the grain structure, when being heated a considerable contraction occurs at the critical range, and when cooling there is a corresponding instantaneous expansion. It depends on where you you stop and start these processes as to how much of the steel is converted from one form to another which will cause a dimension change.

There are some materials that undergo shrinkage during the anneal or the age hardening procedure. The BeCu alloy 20C or Alloy 25 for example has a .2% linear shrinkage during the age hardening cycle. Items would often be partially machined leaving some finish on the critical dimension areas, then drilled, tapped as needed (while still in the annealed state) and then age hardened and lastly machined to the required critical dimensions in the hardened state.

Check with your material supplier concerning properties. He should be able to advise you as to how much to leave on for the final machining. Then do all the close tolerance dimensions after all annealing and aging has been performed.