Effect of solution annealing on structure and properties of high Mo superaustenitic stainless steel castings

In the present study, the effect of solutionising temperatures of 1100, 1150, 1200 and 1250 degrees C on the tensile strength, impact toughness and pitting corrosion of 6.5, 7.5 and 8.5 wt-% Mo cast superaustenitic stainless steels (SASS) is investigated. These alloys were produced by a conventional induction melting furnace, and subsequently, they were annealed at the above mentioned temperatures to obtain a completely austenitic structure. Thermo-Calc software was used to study the stability of secondary phases for SASS annealed at these temperatures. The increasing Mo content raises the temperature of stability of the sigma phase. Solution annealing enhances yielding toughness, as demonstrated by the increase in impact energy, and obtains the expected hardness. In addition, the pitting resistance increases with the increase in solution annealing temperatures. Quite contrary to the expectations, higher Mo containing alloys showed a tendency to pitting compared to the lower Mo containing alloy for the same set of treatment conditions. This behaviour is related to the precipitation of intermetallic phases of the heat treated alloys.