OPTIMIZING CONTROL OF PRECIPITATES IN T91 FERRITIC HEAT－RESISTAN STEEL

T91 steel is one representative of(9%—12%)Cr(mass fraction) ferritic heat-resistant steel,in which MX carbonitrides and M23C6 carbides are two major strengthened precipitates for long-term creep under high temperature.This work attempted to control the precipitation of MX carbonitrides and M23C6 carbides by applying new heat treatment procedures.With the assist of Thermal-Calc software calculation,two new heat treatment procedures have been designed for T91 steel based on its traditional normalized-tempered treatment,in which an isothermal.treatment at 850℃ was introduced between normalized treatment and tempered treatment.The mean size of M23C6 carbides decreased from 350 nm to 250 nm and the number density of M X carbonitrides increased due to the new heat treatment procedures.The calculated results of Thermal-Calc software showed that the nucleation rate of M23C6 carbides at 750℃ increased with the decrease of carbon content in the matrix,which might be the major reason why the mean size of M23C6 carbides decreased.