Hardened steel heat treated steel is part four of a fire. Fe is a polycrystalline material, at low temperature for a body-centered cubic structure, the heating occurs to a certain temperature reorganization into a face-centered cubic lattice structure. For steel, the matrix structure corresponding to a low temperature of ferrite, is heated to a temperature above the eutectoid austenite. Hardened steel is fully austenitic (hypoeutectoid steel) or partial austenitizing (eutectoid steel), holding time, very cold in the quenching liquid (water, oil, etc.), the formation of Markov body, and can be considered a supersaturated martensite ferritic, martensitic so just after quenching thermodynamic point of view, belongs to an unstable state, but also because of the presence of fast-cooling tissue stress. So tempering is necessary, tempering the most basic reason is to eliminate the stress, if not promptly after quenching and tempering, cracks will appear, in this case, a piece of steel will become scrap.