Influence of Long Term Ageing and Superimposed Creep Stress on the Microstructure of 1 mass%Cr -0.5 Mass%Mo Steel

Abstract

In order to understand the effects of high temperature ageing and that of superimposed creep stress on the microstructural variations in a 1 mass%Cr-0.5 mass%Mo steel, the shoulder and gage portions of the specimens, subjected to stress rupture tests at 520 and 560°C were studied by transmission electron microscopy. The optical microstructure of the steel consists of 30 percent ferrite and the remainder bainite. The ferrite is free from any precipitates and ferrite laths with a high dislocation density are frequently observed adjacent to bainite. The bainitic carbide is cementite in a plate-like morphology. Ageing at 520°C upto 10151 h with or without the superimposed creep stress do not bring about any noticeable change in either ferrite or bainite. While ageing at 560°C for 1235 h has not brought about any significant change in microstructure, the superimposed creep stress has led to the precipitation of fine M2C platelets in ferrite and some recovery of ferrite matrix. The exposure at 560°C, for 34667 h with or without creep stress, has led to the precipitation of M2C and M6C in ferrite. The superimposed creep stress has, however, shortened the length of M2C, but increased its profuseness. The long term exposure at 560°C with or without creep stress also led to significant recovery of ferrite and increased spheroidization of cementitie. © 1992, The Japan Institute of Metals. All rights reserved.