he strength for greater ductility through reduction of the carbon supersaturation initially present and replacing it with more stable structures. Additionally, the retained austenite associated with martensite in steels containing more than about 0.7 wt% C can be decomposed during the tempering process. In carbon steels containing small percentages of the common alloying elements, one distinguishes the following stages during tempering. In steels alloyed with chromium, molybdenum, vanadium, or tungsten, formation of alloy carbides occurs in the temperature range 500 to 700 oC. During stage 1, the hardness increases slightly while during stage 2, 3, and 4 the hardness decrease. Cooling Media and Quench IntensityThe depth of hardness at a given workpiece dimension is determined by the chemical composition of the steel, the austenite grain size as established during the austenitizing treatment, and the cooling rate. The steel is normally chosen on the basis of hardenability. The choice of cooling medium, on the other hand, is less exact and crude rules are normally applied (unalloyed steel is quenched in water, alloy steels in oil, and high-alloy steels in air). Molten salt is often used for bainitic hardening of medium-carbon steels and martempering of carburized parts. Judicious selection of cooling medium is critical for obtaining optimum mechanical properties, avoiding quench cracks, minimizing distortion and improving reproducibility in hardening. Of most interest are the liquid quenching media, and they also show the most complicated cooling process. Thermal Stressed during and Residual Stresses after Heat TreatmentHeat treatment of steel, especially martensitic hardening, is usually accompanied by the evolution of large residual stresses, that exist without any external load on the part considered. Residual stresses can be divided into three categories. A macroresidual stress is the average of the residual st...
