Residual stresses are also produced because of the volume change.The surface of the quenched steel cools rapidly and transforms to martensite. When the austenite in the center later transforms, the hard surface is placed in
tension, while the center is compressed. If the residual stresses exceed the yield strength,
quench cracks form at the surface.
However; if we first cool to just above the Ms (
Martensite Start) and hold until the
temperature equalizes in the steel, subsequent
quenching permits all of the steel to transform to martensite at about the same time. This heat treatment is called
marquenching or
martempering.
