Ageing of forged superaustenitic stainless steel: Precipitate phases and mechanical properties

Cast superaustenitic stainless steel (SASS), of composition 19 wt% Cr, 20 wt% Ni, 7.5 wt% Mo and 0.37 wt% N, is hot forged at 1200 °C. The forging was then solutionized at 1250 °C and aged for 1 h and 10 h at different temperatures in the range of 500–1000 °C. Effect of these treatments on (i) hardness and (ii) fracture toughness based on impact energy is reported. Chi is formed from low temperatures up to 800 °C, and sigma at temperatures above 900 °C. A model is proposed to illustrate the dissolution of chi and sigma in austenite at elevated temperatures. Compared with chi, sigma contains more Mo. Toughness decreased with increasing amounts of chi and sigma precipitates. However, in the temperature range of 850–950 °C, low toughness was observed for relatively a short ageing time, although virtually the volume of sigma phase is very low. This is attributed to the presence of incoherent sigma in austenite matrix. Fractographs of the impact-tested samples indicated an increased tendency for brittle fracture with increasing ageing temperatures (increase in sigma content). Thermodynamic calculations substantiated (i) EDS results of composition of secondary phases present in the aged SASS and (ii) the proposed model. From these studies it is concluded that sigma is enriched in Mo at 900 °C and can be as detrimental to toughness of SASS as chi or carbides and nitrides.