過熱脆化したNi-Cr-Mo-V鋼の延性粒界破壊とMnSの関係

抄録

Fracture appearance in an overheated Ni-Cr-Mo-V steel for low pressure turbine rotor was characterized as the occurence of intergranular fracture in ductile fracture range. The facet size of intergranular fracture surface corresponded to the austenite grain size of the steel heated at high temperature during heating before forging. The facet size was not changed by re-austenitizing treatment such as normalizing and quenching whose temperature was relatively low. The intergranular facet consisted of small equiaxed dimples and MnS particles located at the bottom of dimples. The number of MnS particles was approximately 50% of the number of dimples. Voids formed at interfaces between MnS particles and matrix, grew and coalesced, and consequently ductile intergranular fracture occured. The decreases in upper shelf energy and reduction of area by overheating were explained by the low strain requisite for void formation at the interface of MnS particle. Coarsening of MnS particles with slow cooling resulted in recovering toughness and ductility due to increasing the strain for void formation. Since most austenite grain boundaries before re-austenitizing were eliminated by re-austenitizing, most ductile cracks propagated in grain interior. Voids nucleated at MnS particles on prior austenite grain boundaries before re-austenitizing, grew and coalesced. Thus, facetted fracture surface was formed similar to ductile intergranular fracture surface. The susceptibility to temper embrittlement was not affected by overheating.