Influence mechanisms of instantaneous overheating on fracture and precipitate distortion of high chromium nickel austenitic steel

doi:10.13251/j.issn.0254-6051.2021.05.041

Abstract

Abstract: Cracks were found in the sealing weld between the reheater tube (22Cr-15Ni steel) and the sleeve (18Cr-8Ni-Nb steel) during the test tube screen pressure test in a power plant. The cracking cause was studied through macro and micro methods, including macro-analysis of the sample tube, metallographic analysis of the crack, and SEM/EDS analysis of the fracture and precipitates. The results indicate that near the failure site there are obvious traces of mechanical stress input and weld stress concentration, meanwhile the welding heat input causes local microstructure overheating, and the abnormal growth of M₂₃C₆ globular precipitate causes microscopic tensile stress. Finally, the combination of the above reasons causes that the mechanical stress is input from the stress concentration to produce microcracks that propagate along the overheating-weakened grain boundary. Such cracks coalesce with the microcracks generated by the matrix precipitates and eventually propagate into a circumferential fracture. The wind with pressure erupts and leads to the distortion and fragmentation of the globular precipitates, finally pits on surface and internal dendritic crystal of M₂₃C₆ globular precipitates come to appear.

Key words: instantaneous overheating, crack, stress concentration, M₂₃C₆ phase, distortion and fracture

CLC Number:

TG115.1

Che Pengcheng, Shao Wenzhu, Cheng Yi, Wang Shuo, Xu Hang, Li Xianshuang, Tan Shuping. Influence mechanisms of instantaneous overheating on fracture and precipitate distortion of high chromium nickel austenitic steel[J]. Heat Treatment of Metals, 2021, 46(5): 235-241.

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