A METALLOGRAPHIC EXAMINATION OF CRACK-PATH PROPAGATION IN EMBRITTLED 12-PERCENT CR STEEL

A 12% Cr 422 stainless steel high-temperature steam flange bolt was ex tremely embrittled after approximately 30 years of service at approxim ately 565-590 degrees C. The bolt microstructure is primarily Cr-rich M23C6 carbides in ferrite. Fracture was ''intergranular,'' along the p rior austenite grain boundaries, which are covered with a high density of primarily Cr-rich carbides. A detailed microstructural examination was made of the fracture surface profile of an impact sample and of c rack initiation and growth in a metallographically polished and etched surface deformed in bending. Much of the intergranular cracking occur red by separation at the interface of the grain boundary particles and the ferrite. Heat treatment by austenitizing and tempering to restore the toughness produced fewer and finer carbides on the prior austenit e boundaries. During aging, the density and size of these carbides inc reased, and this appears to be the main cause of the embrittlement. (C ) Elsevier Science inc., 1997.