Fracture analysis of 20Cr1Mo1VTiB steel fastening bolts used for intermediate pressure main stop valve

doi:10.13251/j.issn.0254-6051.2023.11.048

Abstract

Abstract: Two 20Cr1Mo1VTiB high temperature fastening bolts were found to be broken during the disassembly and maintenance of the intermediate pressure main stop valve used for a steam turbine unit. The reasons for the premature failure of bolts were analyzed by means of macro and micro morphology observation, chemical composition analysis, impact fracture analysis as well as mechanical property test, and the fracture failure mechanism was also clarified. The results show that the fracture behavior of the two 20Cr1Mo1VTiB steel high temperature fastening bolts is mainly belong to brittle fracture triggered at the bottom of the first thread, which is mainly related to the long-term high temperature operation condition. The carbide particles in the matrix of the 20Cr1Mo1VTiB high temperature fastening bolt are gradually Ostwald ripening with the combined action of long-term high temperature and alternating cyclic stress, resulting in a decrease of toughness. Besides, creep cavities are also formed at the nut end of the first thread bottom with high stress concentration, leading to premature fracture failure. In addition, the linearly distributed Ti(N, C) inclusions with average size of 5 μm further lead to the reduction of impact property of the No.1 bolt, causing the fracture of No.1 bolt prior to bolt No.2. Then the overall stress condition of the intermediate pressure main stop valve is changed, which leads to brittle fracture of the No.2 bolt with a small impact property margin.

Key words: intermediate pressure main stop valve, 20Cr1Mo1VTiB steel, fastening bolt, fracture analysis, inclusion, carbide

CLC Number:

TG146

Zheng Jianjun, Fan Ziming, Cheng Xu, Zhang Yanfei, Shi Quanqiang. Fracture analysis of 20Cr1Mo1VTiB steel fastening bolts used for intermediate pressure main stop valve[J]. Heat Treatment of Metals, 2023, 48(11): 293-299.

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