A metallographic examination of crack-path propagation in embrittled 12% Cr steel

A 12% Cr 422 stainless steel high-temperature steam flange bolt was extremely embrittled after approximately 30 years of service at approximately 565–590°C. The bolt microstructure is primarily Cr-rich M23C6 carbides in ferrite. Fracture was “intergranular,” along the prior 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 crack initiation and growth in a metallographically polished and etched surface deformed in bending. Much of the intergranular cracking occurred 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 austenite boundaries. During aging, the density and size of these carbides increased, and this appears to be the main cause of the embrittlement.