Microstructure–property relations of Mo- and W-alloyed super duplex stainless weld metals

Abstract

The microstructure and resulting properties of manual metal arc super duplex weld metals deposited using three different filler metals and two heat inputs were investigated using various techniques. Analytical transmission electron microscopy, electron probe microanalysis, and scanning electron microscopy were used to study the microstructure and Charpy V notch impact energy and different corrosion test methods were used to characterise the weld metal properties. The fillers were of approximate composition (wt-%) Fe-25.5Cr-(3–4)Mo-9.5Ni-0.25N; one filler had additions of tungsten and one had additions of tungsten plus copper. The effect of composition and heat input on secondary austenite γ2 formation was evaluated experimentally and the results interpreted in terms of a theoretical model for diffusion controlled austenite growth. The equilibrium conditions were calculated using the Thermo-Calc database. The formation of γ2 in the as welded condition appears to be promoted by a low nitrogen and high copper content. Short time aging heat treatments in the temperature range 700–1050°C were performed to establish the tendency of intermetallic phase to form in the different weld metals. The kinetics for intermetallic formation, evaluated using metallographical techniques, was analysed using an Avrami type of expression. Tungsten favours the precipitation of χ phase which in turn serves as a nucleation site for σ phase, making the tungsten alloyed weld metals less resistant to short term thermal exposure such as higher heat inputs during multipass welding. Room temperature Charpy V notch impact energy tests were performed on as welded and aged specimens. The results could be correlated to the observed microstructural differences. Determination of the critical temperature for pitting was used to characterise the influence of alloying elements and microstructure on corrosion resistance. It was concluded that the γ2 fraction affects the corrosion resistance in the as welded and short term aged specimens but the effect is overridden by the subsequent precipitation of χ phase and the particularly detrimental σ phase on prolonged aging. The largest differences between the investigated fillers were in the as welded short term aged condition, i.e. in the technically most critical time-temperature range.