Microstructure of a Heat-Resistant Austenitic Steel after Long-Term Use as Ultra-Supercritical Boiler Tube

Abstract

It is important to understand the mechanical properties of the material used at the operating temperature for stable operation of a thermal power plant, and necessary to analyze the nanometer scale microstructure that governs the properties. We have performed microstructure characterization of KA-SUS304J1HTB austenitic steel from ultra-supercritical boiler tubes after 130,718 hour use whose designed steam condition was 873 K / 24.5 MPa. EPMA, EBSD and STEM observation revealed the presence of Cr-rich σ phase mainly at grain boundaries. Neither Cu nor Nb(C, N) are present as coherent nanometer-sized precipitates; their average size were 70 nm and 60 nm, respectively. We also confirmed the presence of MX type Nb carbonitrides of nearly 5 μm in size and Cu precipitates of nearly 1 μm around σ phase. The decrease of coherent Cu and Nb(C, N) precipitates will reduce matrix strengthening and the formation of σ phase will cause grain boundary embrittlement.