材料期刊网

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.

Bolts made of the 25Cr2Mo1V steel become gradually embrittled with increas-ing fraction of intercrystalline fracture after long exposure (more than 40000hours) at 540℃. This is found to be caused by the segregation of phosphorus upto 8 at% at austenitic grain boundaries accompanying the depletion of molybdenumin ferrite down to about 0.3% owing to the formation of a molybdenum-contain-ing carbide Fe_3Mo_3C (M_6C). Reheating of the embrittled bolts to 680℃ removes the segregation of phos-phorous at the grain boundaries and therefore also the intercrystalline embrittle-ment. However, the molybdenum content of ferrite is further lowered to about 0.2%and its scavenging effect for phosphorous is thus greatly lost, therefore the embrit-tling tendency of these bolts during reuse is enhanced. Even after requenchingand tempering, the service life of these bolts is still shortened, owing possibly tothe inhomogeniety of molybdenum in austenite not removed by the reaustenitizingtreatment.