Effect of Compositional Changes of Laves Phase Precipitate on Grain Boundary Embrittlement in Long-Term Annealed 9 Pct Cr Ferritic Steel

Abstract

In 9 to 12 pct chromium steels, the high-temperature mechanical properties are known to be strongly dependent on the formation and coarsening of Laves phase precipitates at boundaries. During high-temperature deformation, the Laves phase precipitate coarsening to over a critical size has been considered to trigger cavity formation at the precipitate-matrix interfaces. This coarsening, accompanied by the diffusion of W, Mo, and Cr, should change the mechanical properties and chemical composition of both Laves phase precipitates and the matrix. In this study, we aimed to clarify the effects of compositional changes of Laves phase precipitates on cavity formation during coarsening. The values of the Fe/Cr and W/Mo ratios in Laves phase precipitates were shown to induce different levels of strain energy in the vicinity of the Laves phase precipitate, consequently promoting the formation of cavities. Therefore, the compositional change of Laves phase precipitates was found to play a critical role in the grain boundary embrittlement of high Cr steel at elevated temperature.