Role of Mo and W during sensitization of superaustenitic stainless steel - Crystallography and composition of precipitates

This study concerns the crystallographic identification and compositions of precipitates formed in superaustenitic stainless steel. Three experimental alloys, all containing 24 wt pct Cr, 22 wt pct Ni, and 0.5 wt pct N but with varying amounts of Mo and W, were investigated after sensitization heat treatment (aging) at 900 °C. The contents of Mo and W in the three alloys were 7 wt pct Mo, (6 wt pct Mo+2 wt pct W) and (5 wt pct Mo+5 wt pct W), respectively. While σ and χ were the main secondary phases found in the W-free alloy, replacement of Mo by W was found to promote the formation of Laves-related phases with high Mo+W content. The complex crystallographic nature of Laves-related precipitates was exemplified through the formation of intergrowing C14 Laves, μ, and C phases, all with closely related crystal structures. There was no difference in chemical composition between the three phases. Prolonged aging resulted in intragranular precipitation of different intermetallic phases, as well as formation of nitrogen bearing phases, π and Cr2N, adjacent to previously formed intermetallic precipitates. The content of Mo+W was found to decrease with increasing aging time for all secondary phases.