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

Jun MORII, Fumio SATO, Kentaro SUZUKI
2018 Journal of the Society of Materials Science Japan  
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
more » ... -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.
doi:10.2472/jsms.67.789 fatcat:b7fpxs62ybbf5jul2j6pmwfbgy