Evaluation of Influencing Factors on Plasticity during Tempering Process

Abstract

During tempering process of steel materials, a plastic strain occurs along loading direction even if the applied stress is less than yield stress. The authors name this phenomenon “tempering plasticity”. In the temperature raising process during tempering, volume change occurs in each temperature range, and it is known that the cause is the decomposition of retained austenite and the precipitation of carbide. The influence of these factors on tempering plasticity has not been considered so far. In this paper the plastic strain was experimentally investigated by tempering of quenched specimen with tensile stress under various conditions such as “subzero treatment”, “change of tempering heating rate”, “the case of extremely low carbon material”. And the effects of decomposition of retained austenite and precipitation of carbide on tempering plasticity were quantitatively evaluated. As a result, decomposition of retained austenite has little influence on tempering plasticity, while precipitation of carbide has a large influence on tempering plasticity. Furthermore, assuming the similarity with the transformation plasticity phenomenon, the tempered plastic strain was formulated based on the tensile test during tempering, and the tempered plastic coefficient was obtained. This coefficient was proportional to the volumetric strain difference before and after tempering and inversely proportional to the lower yield stress of the parent phase and the new phase during tempering. This shows the similarity to the mechanism of transformation plasticity proposed by Greenwood-Johnson.