Effect of martensitizing temperature on creep strength of modified 9Cr steel

Abstract

A modified 9Cr-iMo steel was cooled to a temperature between the M s point and room temperature from the normalizing temperature and then directly heated to the tempering temperature. It was found that the time to rupture at 650 and 700°C of the steel heat-treated by the new heat treatment increased 2-3 times longer than that of the steel conventionally normalized and tempered. The microstructure of the improved steel was tempered martensite and the size of martensite blocks was larger than for the conventional heat treatment. The hardness of the improved steel was fully recovered after tempering. This strengthening is not caused by fresh martensite, but caused by the refining of M21C6 and V(C,N) particles in addition to the increase in the martesite block size, where M denotes metallic elements. ©2011 The Japan Society for Heat Treatment.