Cryogenic Toughness of Austenitic Stainless Steels After Aging

In the present work, a study of the effect of precipitation on the fracture toughness at cryogenic temperatures was carried out in two austenitic stainless steels, nitrogen-containing steel and 316-type steel, after isothermal aging. Both steels were solution treated, cold-water quenched, and then aged at temperatures of 600, 700, 800 and 900 °C for times between 10 and 1000 min. The precipitation of these steels was characterized with a scanning electron microscope, and precipitates were analyzed by X-ray diffraction analysis of extracted precipitates, after electrolytic dissolution of austenitic matrix. The fracture toughness of steels was evaluated by the Charpy V-notch impact testing at − 196 °C, and fracture surfaces were observed in a scanning electron microscope. The results showed an intergranular precipitation of carbides M₂₃C₆ for both aged steels. However, the kinetics and percentage of intergranular precipitates were higher in the N-containing steel than that in the 316-type steel. The decrease in Charpy impact energy with aging time was higher in the N-containing steel and associated with its higher percentage of intergranular precipitation. That is, the N-containing steel is more susceptible to embrittlement due to isothermal aging than the 316-type steel. The fracture mode of the aged 316-type steel was transgranular ductile. In contrast, that of the N-containing steel changed from transgranular ductile to intergranular brittle as the aging process promoted more abundant intergranular precipitation.