Structural stability of super duplex stainless weld metals and its dependence on tungsten and copper |
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Authors: | J. -O. Nilsson T. Huhtala P. Jonsson L. Karlsson A. Wilson |
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Affiliation: | 1. Physical Metallurgy, R&D Center, AB Sandvik Steel, S-811 81, Sandviken, Sweden 2. Central Laboratories, Esab AB, S-402 77, G?teborg, Sweden 3. Department of Engineering Materials, Lule? University of Technology, S-951 63, Lule?, Sweden 4. Tube Laboratory, R&D Center, AB Sandvik Steel, S-811 81, Sandviken, Sweden
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Abstract: | Three different superduplex stainless weld metals have been produced using manual metal arc 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 °C to 1110 °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 pct of intermetallic phase was 2 times faster for tungsten-rich weld metal. Microanalysis in combination with thermodynamic calculations showed that tungsten was accommodated in χ 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 χ phase compared with σ phase. The formation of secondary austenite (γ2) during welding was modeled using the thermodynamic computer program Thermo-Calc. Satisfactory agreement between theory and practice was obtained. Thermo-Calc was capable of predicting observed lower concentrations of chromium and nitrogen in γ2 compared with primary austenite. The volume fraction of γ2 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 γ2 in these. |
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