Low temperature aging behavior of type 308 stainless steel weld metal

The aging behavior of welded type 308 stainless steel was evaluated by mechanical property testing and microstructural examination. Aging was carried out at 475°C for up to 20,000 h. The initial material consisted of austenite with approximately 10% ferrite. Upon aging, the ferrite hardness increased up to 100%. This hardening was accompanied by a noticeable increase in the ductile—brittle transition temperature and a drop in the upper shelf energy, as measured by Charpy impact tests, and a degradation in fracture toughness, as determined by J-integral test. Tensile properties did not change significantly with aging. Microstructural analysis indicated that the ferrite decomposed spinodally into iron-rich α and chromium-enriched α′. In addition, abundant precipitation of nickel- and silicon-rich G-phase was found within the ferrite and M 23C 6 carbide formed along the austenite-ferrite interface. These effects are similar to the aging behavior of cast stainless steels. Occasionally, large G-phase or α precipitates were also found along the austenite-ferrite interface after aging more than 1000 h. After comparison of the mechanical property changes with the microstructural features, it was concluded that both spinodal decomposition as well as G-phase formation contribute to ferrite hardening. Spinodal decomposition results in embrittlement of the weld insofar as the ductile-brittle transition temperature is raised. G-phase formation and carbide precipitation are associated with a degradation in the ductile fracture properties, as shown by a drop in the upper shelf energy and a decrease in the fracture toughness. Le mécanisme de vieillissement de l'acier inoxydable type 308 soudéestétudiépar des essais mécaniques et par examen de la microstructure. Le vieillissement est effectuéà475°C pendant 20,000 h. La matériau initial est forméd'austénite avec environ 10% de ferrite. Aprés vieillissement, la duretéde la ferrite augmente jusqu'à100%. Ce durcissement est accompagnéd'une augmentation notable de la température de transition ductile—fragile et d'une chute de l'énergie du palier supérieur, mesurée graceàdes essais de résilience Charpy, et d'une dégradation de la tenacitéàrupture, déterminée par les tests d'intégrales J. Les propriétés en traction ne changent pas d'une manière significative au cours du vieillissement. L'analyse microstructurale indique que la ferrite se décompose spinodalement en α riche en fer et α′ enrichi en chrome. De plus une abondante préc