Figure 1. Cluster of cracks extending 0.004 inches (0.1 mm) from the surface of a 4145H shaft. Unetched (500x).

I always assumed you cannot get retained austenite during induction hardening (in this case 4145H steel) since you are only heating to the depth of case and can quickly (more completely) transform the austenite to martensite. Most problems I’ve seen with retained austenite happen with carburizing cycles.

We have experienced cracking (burning) during grinding this material after heat treatment (Fig. 1). The grinding process (wheel grade, coolant, feeds, speeds, etc.) has been closely reviewed along with the material chemistry, microstructure and induction heat-treating process. The parts are pre-hardened in the bar to HRC 32-36. The only thing I didn’t really like was the heat treater didn’t always get the parts tempered within two hours after quenching. But if this were a problem, I would expect it to lead to a more severe type of fracture.

I have noted that this problem occurs during the cold winter season. “If” there was any retained austenite, this could have transformed during shipping between our heat treater and our plant. If retained austenite could not form in the parts after induction hardening, obviously this could not happen. If there could be some fresh martensite forming, it could contribute to the grind-burning problem.

ANSWER (In collaboration with Professor Induction)[1]

Steels such as AISI 4145, 4345 and other high-carbon steels are quite prone to forming retained austenite and to the problem you’re describing.

The amount of retained austenite depends upon steel's chemical composition, hardening temperature, properties of quenchant (including temperature of quenchant) and homogeneity of the austenite. Here is a brief explanation how each of these factors affects the amount of retained austenite.
  • Steel's chemical composition affects Ms and, more importantly, Mf temperature. This is the reason why steels with high carbon content (e.g., AISI 1070 or 1080 or cast irons) will always have substantial amount of retained austenite practically regardless of quench severity because Mf is below an ambient temperature. Only cryogenic cooling can help to transform retained austenite into martensite and obtain fully martensitic structure for steels with carbon content of 0.43-0.48%C range (as it is with AISI 4145 steel).
  • Hardening temperatures – After reaching some high enough temperatures, an amount of retained austenite will start increasing.
  • If quenchant is not sufficient enough and its temperature is higher than expected, then amount of retained austenite will increase.
  • Homogeneity of austenite – Without sufficient hold/soak time at austenizing temperature (regardless of the fact that steel was heated above Ac3 critical temperature), areas with higher carbon concentration will act similar to high-carbon steels.
A cold core helps to increase cooling severity during quenching, but it all depends on the ratio of case depth area to cold core area. In some cases, the effect of the cold core can be significant, and in others it might not be significant.