Structural stability of super duplex stainless weld metals and its dependence on tungsten and copper

Three different superduplex stainless weld metals have been produced using manual metal are welding under identical welding conditions. The concentration of the alloying elements tungsten and copper corresponded to the concentrations in commercial superduplex stainless steels (SDSS). Aging experiments in the temperature range 700 degrees C to 1110 degrees C showed that the formation of intermetallic phase was enhanced in tungsten-rich weld metal and also dissolved at higher temperatures compared with tungsten-poor and tungsten-free weld metals. It could be inferred from time-temperature-transformation (TTT) and continuous-cooling-transformation (CCT) diagrams produced in the present investigation that the critical cooling rate to avoid 1 wt pet of intermetallic phase was 2 times faster for tungsten-rich weld metal. Microanalysis in combination with thermodynamic calculations showed that tungsten was accommodated in chi phase, thereby decreasing the free energy. Experimental evidence supports the view that the formation of intermetallic phase is enhanced in tungsten-rich weld metal, owing to easier nucleation of nonequilibrium chi phase compared with sigma phase, The formation of secondary austenite (gamma(2)) during welding was modeled using the thermodynamic computer program Thermo-Gale. Satisfactory agreement between theory and practice was obtained. Thermo-Gale was capable of predicting observed lower concentrations of chromium and nitrogen in gamma(2) compared with primary austenite. The volume fraction of gamma(2) was found to be significantly higher in tungsten-rich and tungsten + copper containing weld metal. The results could be explained by a higher driving force for precipitation of gamma(2) in these.