Microstructural analysis of an embrittled 422 stainless steel stud bolt after approximately 30 years service in a fossil power plant

A microstructural analysis has been conducted of 12% Cr stainless steel (essentially Type 422) bolts that were found to be embrittled after service in a fossil-fired power plant for approximately 30 years at about 565–590°C. In the as-received condition, the Charpy V-notch absorption energy at 25°C was about 4 J. Fracture in the bolts and in the impact samples was intergranular, along the prior austenite grain boundaries. However, the fracture surfaces did have a fine, rough topology. The microstructure consisted of a rather uniform dispersion of fine particles in a ferrite matrix, with particles on the prior austenite grain boundaries. X-ray diffraction analysis of extracted particles, and TEM and STEM analysis of metallographic extraction replicas and thin foils, and of extraction replicas from the fracture surface, identified most of the particles as M₂₃C₆ carbides. In addition, Laves phase and a Ni-rich (e.g., 30 wt. % Ni) phase were found.

Several heat treatments were tried to improve the toughness. The highest energy absorption of 34 J was obtained by reaustenitizing for 2 h at 1,065°C, oil quenching, then tempering for 2 h at 750°C, followed by air cooling. The microstructure was similar in appearance to that of the as-received, embrittled material. However, no Laves or Ni-rich phase was found. There was markedly less intergranular and more transgranular fracture.

The cause of the embrittlement was not discerned, nor why the toughness was improved with the heat treatments. A brief discussion of possible causes is presented.