The cause of the retained austenite can be attributed to the following conditions:
- Quenching from too high of an austenitizing temperature
- Quenching into too slow of a quench medium (slack quench)
- Excessive surface carbon in a carburized steel
Fig. 3. The interior components of a Rockwell hardness-testing unit
There are two remedies for the decomposition of a retained-austenite condition. The first is to cryogenically treat the steel, followed by tempering when the steel is back at room temperature. The second method is high-temperature tempering.
Another condition can be that of “spotty hardness,” which is hard and soft spots on the surface of the steel. This could be caused by the following:
- Vapor pockets during the quench
- Soot, if the furnace atmosphere is not effectively controlled
- Surface-chemistry changes if vacuum heat-treated
In the case of surface-chemistry changes, this could be caused by running the process at too low a vacuum level in relation to the austenitizing temperature. Outgassing of the surface elements could be occurring that would cause the change in surface chemistry.
Fig. 4. Possible causes for low hardness responses to heat treatment
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