Electron Microscopic Examination of a Service Embrittled 21/4Cr-lMo Steel

概要

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The microstructure of a 21/4Cr-1M_0 steel embrittled by service in a boiler header operated at 813 K for 88,000 hours was examined with the aid of transmission and scanning electron microscopy. Comparison with, a virgin fleet of the same grade revealed the following three major microstructural changes due to long service which could be associated with the embrittlement of the steel: Considerable coarsening and precipitation of carbides during long Venice, particularly within the bainitic grain which showed low hardness and low carbide density, and along prior austenite grain boundaries which were covered with thick elongated films of M_6C and M_<23>C_6 type carbides. Microcracks were observed to propagate along the interfaces between. the grain boundary carbides and the matrix in specimens after impact tests, suggesting weakening of cohesion at these interfaces. Grain boundary segregation of phosphorus. Although a considerable scatter existed in the gram boundary concentration of phosphorus as measured by Auger Electron Analysis, phosphorus segregation was confirmed. Formation of voids during service. Isolated voids of mean size 1.3 μm were observed along grain boundaries. On average, there was one void per 2300 μm^2.
社団法人溶接学会の論文
1986-10-01