Quench cracks are referred to as cracks that occur during tempering of steel that has been tempered in a furnace, due to quenching, heating, or changes in microstructure. Common hardening and tempering steels such as high-speed steel and SKD11 die steel also undergo quenching after high-temperature tempering. This steel experiences a primary deformation of martensite particles during secondary quenching and a secondary deformation of martensite particles (retained austenite particles transforming into martensite particles) during tempering, which can lead to cracking. To prevent temper cracks, it is necessary to cool slowly starting from the tempering temperature, and to avoid prematurely removing the heat treatment from the quenching and tempering process.

The primary cause of tempering distortion is the residual hardness or tissue changes produced by the tempering furnace during tempering, which includes contraction due to the elimination of tensile stresses and expansion due to the elimination of compressive stresses. This encompasses the partial contraction of the precipitated E carbides during the early stages of tempering, significant contraction during the sintering process of cementite, expansion of the residual austenite iron into loose iron in the sparks, and expansion of the residual austenite iron into ductile iron. These factors lead to distortion in the workpiece after tempering.

Preventive measures include: (1) stress-relieving heat treatment; (2) using hot bath or air quenching to reduce residual stress; (3) machining correction; and (4) allowance for deformation.