Hardness stress

For heat treatment, it may be required for a workpiece to undergo quenching from hardening temperature to a lower temperature; this may cause great stresses within the material.

In this context, the volume increase during martensite formation and uneven shrinkage strain are two factors that play a decisive role. Hardness cracks may be the consequence or excessive hardness stresses.

Additional references:
Martensite point
TTT diagram