Quench Cracking

The quench-and-temper process induces enormous stresses, which can lead to part cracking during the process if good part design guidelines and proper heat treating practices are not followed. In the quenching action, the steel is subject to non-uniform cooling, thermal deformation, and a phased grain transformation that can literally pull the steel apart. Quench cracks can appear in any orientation in the part.

Quench cracks have a distinctive appearance. They run between the grains of the steel and to the unaided eye look somewhat like a river on a map. Since the crack usually appears during the quench, it is common to find a light gray scale within the quench crack that develops during the temper cycle. Conversely, a crack that existed before heat treating would have extensive decarburization and scale from the initial austenitizing cycle.

Sharp corners, notches, slots, holes, grooves, or changes of cross-section in the part are possible points for the cracking to initiate. Generous radii and gradual transitions are important in reducing the possibility of quench cracking. Holes should be packed before heat treating.

Quench cracking can appear to be associated with banding in the microstructure of the steel. Banding is a naturally occurring condition in any plate steel, and it arises as the ingot or strand cast slab cools from the outside in, which creates a slight variation in grain structure and chemistry through the cross-section; this variation is still present to some extent in the rolled plate product. Banding is more prevalent in thicker plate and in alloy plate; it may even be visible to the naked eye in some plate products, especially after machining, but it is not an indication of a defective product or that there is a problem with the microstructure. Quench cracking may appear to propagate along a banded area, but it is rare that the crack occurred because of this microstructure variation.