Microstructures and mechanical properties of Fe-15% Cr-15% Ni austenitic stainless steels containing different levels of niobium additions submitted to various processing stages

In this work additions of 0.5, 1.0 and 2.0 wt.% of niobium were made to a Fe-15% Cr-15% Ni austenitic stainless steel. The microstructures and the mechanical properties of the as cast, hot forged, solution annealed and aged samples as well as the tape alloys (produced using the melt spinning process) were evaluated and compared. The microstructures of the steels were characterized and analyzed using several complementary techniques. Mechanical properties were measured using tensile tests and Vickers hardness. Results showed that niobium additions caused an enlargement of the solidification interval and led to Laves phase formation in the as cast samples. The Laves phase is hard and fragile and its formation between austenite dendrites caused loss in toughness and ductility. After hot forging, the microstructure of the samples was essentially constituted by recrystallized austenitic grains and a relative uniform dispersion of Laves phase particles in the alloys containing niobium. The samples were solution annealed at 1200 or at 1250 °C after hot forging. Annealing did not allow a complete dissolution of particles observed in the hot forged samples whereas the melt spinning process produced alloys free of Laves phase particles. Aging of niobium containing alloys in the temperature range between 600 and 800 °C caused significant precipitation hardening.