Effects of Ferrite Content on the Tensile Strength and Impact Toughness of 2.25Cr-1Mo-0.25V Steels

Abstract

In order to evaluate the effects of a pro-eutectoid ferrite content on the tensile strength and impact toughness of 2.25Cr-1Mo-0.25V steels, several samples with a different area fraction of ferrite in the range from 0 to 80% were prepared by a control of cooling rate from an austenitization heat treatment temperature of 930oC. The steels were then tempered at 690oC, followed by a heat treatment at 705oC as a simulation of postweld heat treatments. The results of microstructural observation indicated that the ferrite was uniformly distributed in the bainitic matrix. The results of tensile tests revealed that the tensile strength at room temperature and 450oC was slightly reduced with the ferrite content of up to 20%, but it was considerably reduced with the ferrite content of 80%. On the other hand, the results of impact tests at -60, -80 and -100oC indicated that the impact toughness was rapidly degraded with increasing ferrite content. These results attributed to the segregation of carbides in the bainitic matrix as a result of the formation of ferrite phase with low solubility of carbon. It is concluded that the ferrite content in the 2.25Cr-1Mo-0.25V steels could exert a sensitive inference on the impact toughness rather than on tensile properties.