The Effect of Sigma Phase Formation on the Corrosion and Mechanical Properties of Nb-Stabilized Stainless Steel Cladding

Austenitic stainless steel surfacing is commonly used to protect ferritic pressure vessels from the corrosive environment in nuclear reactors and chemical process plants. The delta ferrite in a Nb-stabilized austenitic cladding decomposes during stress relief heat treatments (10 to 600 h at 670 degree C, or 1238 degree F), forming sigma phase and secondary austenite. Increases in sigma phase content enhance hydrogen embrittlement and reduce impact toughness. Slow strain-rate tests did not cause stress corrosion cracking in hot 0. 58N H//3BO//3 solution. In 1N HCl solution the cracks grew as the delta ferrite and sigma phases were selectively dissolved.