Potential defects as a result of the stress (hardness stress) occurring during hardening if the steel has been heated too much, improperly cooled or insufficiently or too late tempered (fig. 1).
Due to quenching, heat stresses may occur during hardening of steel alloys as a result of uneven temperature distribution over the cross section. Additionally, transformation stress occurs due to structural transformation (increase in volume during martensite formation) that may lead in combination with heat stress to internal stress
This may lead to cracks during and after cooling. Capillary cracks are characterized by mostly dark fracture areas and often run in a straight, vertical line to the surface with internal crystalline branching.