Corrosion behaviour of 304LN activated tungsten inert gas and flux-cored arc weld metals

Activated tungsten inert gas (A-TIG) and flux-cored arc (FCA) weld metals were prepared using 304LN stainless steel plate. The weld metals were thermally aged at 923, 973 and 1023?K for 100?h to study the decomposition of initial δ-ferrite in A-TIG (～10 ferrite number (FN)) and FCA (～5 FN) weld metals into secondary phases like M₂₃C₆ carbides, χ and σ. Ferrite number is the measurement of δ-ferrite based on the principle of magnetic property using ferritescope. Preliminary microstructural studies revealed the formation of carbides in FCA weld metals aged at 923?K for 100?h, which was correlated with higher carbon content (0.04?wt-%), and also ageing at higher temperature transformed δ-ferrite into χ/σ phases. However, A-TIG weld metals showed the transformation of δ-ferrite mainly into χ/σ phases. The δ-ferrite transformation kinetics was found to be sluggish in A-TIG weld metals compared to FCA weld metals. This difference was attributed to the difference in the carbon contents of A-TIG and FCA welds. Activated tungsten inert gas weld metals showed better uniform and pitting corrosion resistance compared to FCA weld metals in as-deposited and thermally aged conditions. Presence of higher amount of initial δ-ferrite content in A-TIG weld metal helped diffusion of minor alloying elements like sulphur and phosphorous into it, thereby reducing their microsegregation at the δ/γ interface boundaries and subsequent pitting corrosion attack. Thus, A-TIG welding process was found to be superior compared to FCA welding process.