Effects of Tempering Temperature on the Microstructure and Mechanical Properties of T92 Heat-Resistant Steel

Dandan Zhao,Huifang Xiao,Shenghua Zhang,Hai Zhang,Shilei Li,Yanli Wang,Xitao Wang

doi:10.3390/met9020194

Abstract

T92 heat-resistant steel is among the most promising candidate materials for structural components in the Generation IV (GEN-IV) reactors. The effects of tempering temperature on the microstructure and mechanical properties of the T92 steel were studied. The microstructural evolution of the T92 steel subjected to various temperatures of the tempering process were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and synchrotron radiation X-ray diffraction (SR-XRD). The mechanical properties of Vickers hardness, tensile test, and impact test were also investigated. The results showed that the grain size of the prior austenite does not significantly change during the tempering process, while the width of the martensite lath and the size of the carbide precipitates increased with increasing tempering temperature. The hardness and yield strength of the T92 steel decreased, and the plasticity and impact energy increased with increasing tempering temperature. Coarsening of the carbide precipitates during the tempering process was considered to be the dominant factor that reduced the yield strength in the T92 steel.

Highlights

T92 heat-resistant steel has excellent high temperature strength, high thermal conductivity, a low expansion rate, and good corrosion resistance, and it is widely used in the pipes for main steam and reheat steam in ultra-supercritical units [1]
Heat treatment had important effects on both the microstructure [5,10,11] and mechanical properties [5,11] of 9Cr F/M steels, and it has been reported that the key to improve the high temperature properties of T92 steel is to precipitate enough dispersed carbonitrides in the pre-treatment stage [12], which can remain stable during long-term high temperature service
It was reported that the grain size of the prior austenite was related to normalizing heat treatment [11], increasing with the increasing temperature during the normalizing heat treatment, while the normalizing temperature was unchanged in this work

Summary

Introduction

T92 heat-resistant steel has excellent high temperature strength, high thermal conductivity, a low expansion rate, and good corrosion resistance, and it is widely used in the pipes for main steam and reheat steam in ultra-supercritical units [1]. T92 heat-resistant steel has a microstructure of tempered lath martensite and precipitates, including M23 C6 carbides, MX V/N carbonitrides, and high density of dislocations in matrix [4,5,6], where all of them can contribute to the strength of the material. Heat treatment had important effects on both the microstructure [5,10,11] and mechanical properties [5,11] of 9Cr F/M steels, and it has been reported that the key to improve the high temperature properties of T92 steel is to precipitate enough dispersed carbonitrides in the pre-treatment stage [12], which can remain stable during long-term high temperature service. As the last process of heat treatment, the tempering process can significantly affect both the microstructure and the mechanical properties of the steel, including the recovery of martensitic lath, reduction of dislocation density, reduction of hardness and yield strength, etc. The understanding of the effects of the various microstructural evolution during tempering on the mechanical properties is important to both the optimum design of the materials and the industrial application

Results

Discussion

Conclusion