Role of relaxation processes in volume change during steel tempering

Carbon-free and low-carbon iron-nickel (18 and 10% Ni) alloys and medium-carbon steels alloyed with chromium, manganese and other elements were investigated for volume effects caused by tempering. The results were compared with the calculated values of the volume effect due to the precipitation of cementite and to the variations of the electric resistance, of the X-ray diffraction lines width and of the temperature dependence of the yield limit. It was found that the decrease in volume on the tempering of martensitic steel cannot be explained by the precipitation of carbon alone from the solid solution and the formation of the carbide phase. It is suggested that the decrease in volume is also caused by the healing of martensite structure defects, such as volume- stretched elastic areas of the crystalline lattice