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25Cr2Mo1V钢螺栓在540℃长期使用(40000h以上)中变脆是由于含Mo的Fe_3Mo_3C(M_6C)缓慢析出,使铁素体中的Mo含量降低到0.3%Mo左右,不足以抑制P向晶界扩散并富集到8at-%P的程度,从而产生沿晶脆断. 已经变脆了的螺栓经680℃保温处理虽然可以消除P在晶界的富集及沿晶脆断,但是由于铁素体中Mo含量进一步降低到0.2%左右,螺栓在二次使用过程中的变脆倾向加剧.即使再次进行淬火回火处理,可能由于淬火并不能完全去除奥氏体中Mo的不均匀性,螺栓在二次使用时寿命较短.
25Cr2Mo1V steel bolt 540 ℃ long-term use (more than 40000h) brittle is due to the slow precipitation of Fe-containing Fe_3Mo_3C (M_6C), the ferrite Mo content is reduced to about 0.3% Mo is not enough to inhibit the P to the grain boundary diffusion And enriched to the extent of 8at-% P, resulting in brittle fracture along the crystal has been brittle bolts through the 680 ℃ heat treatment can eliminate the enrichment of P in grain boundaries along the grain and brittle fracture, but due to ferrite The content of Mo is further reduced to about 0.2%, the tendency of embrittlement of the bolts during secondary use is aggravated, and even if the quenching and tempering treatment is performed again, the inhomogeneity of Mo in austenite may not be completely removed due to quenching, Second life shorter life expectancy.