Optimizing control of precipitates in T91 ferritic heat-resistan steel

Abstract

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°C 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 MX 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°C increased with the decrease of carbon content in the matrix, which might be the major reason why the mean size of M23C6 carbides decreased. © Copyright.