Microstructural analysis of an embrittled 422 stainless steel stud bolt after approximately 30 years service

A detailed microstructural analysis has been conducted on broken, embrittled, flange stud bolts. The bolts had been in service about 30 years in the temperature range 200-700°C in a fossil-fired power plant. The bolts were of a 12 % Cr, type 422 stainless steel. The bolts fractured primarily intergranuIarIy, along the prior austenite grain boundaries. Impact samples had a Charpy V-notch value (at 25°C of about 4 J), and fracture was intergranular. The microstructure consisted of a fine dispersion of particles in the ferrite matrix, and somewhat large, almost continuous grain boundary network of particles. Electron diffraction and scanning transmission electron microscopy established that most of the particles were M C carbides, but.also present were a Laves phase and an unidentified, high nickel (about 20 wt %) phase.

A series of heat treatments were used in order to improve the toughness. The best result of 34 J toughness was obtained by reaustenitizing at 1065°C for 2 hours, oil quenching, then tempering at 750°C for 2 hours, followed by air cooling. All heat treatments which improved the toughness also dissolved the Laves and the nickel-rich phase. The tougher material had markedly less intergranular fracture.

The exact cause of the embrittlement/ and the reason for the improvement in toughness upon heat treatment^ were not clearly established, A clear correlation between the presence of Laves and the nickel-rich phase and the embrittIement was not obtained.