Effect of post weld heat treatment on the microstructure and mechanical properties of a gas-tungsten-arc-welded 304 stainless steel

In this work, the microstructural evolution and mechanical properties of 304 austenitic stainless steel welds produced by GTAW process were examined after post weld heat treatment (PWHT). Thermal treatment was carried out at different temperatures ranging from 450 to 1050 °C. The optical microscope and scanning electron microscope were used to study the microstructure and fracture surface of the welded joints. X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) techniques were used to investigate the possible formation of intermetallic phases as σ or chromium carbide. Based on the results, no σ phase or chromium carbide precipitation was found. Also, the mechanical behavior of the joint was evaluated by tensile and microhardness tests. The microstructure of the 304 stainless steel was found to consist of equiaxed austenite grains (γ) of various sizes limited by grain boundaries and twin crystals with small amount of delta ferrite (δ) in grain boundaries. The microstructure of the weld metal was austenite with dendritic structure containing skeletal delta-ferrite. The skeletal type delta-ferrite changed from columnar + equiaxed to cellular type by progression during heat treatment. The base metal (BM) and heat affected zone (HAZ) grain size decreased with increasing heat treatment temperature. Therefore, the mechanical testing results were appropriately correlated with the microstructure analysis. Tensile testing analysis showed that the highest tensile strength value was recorded for the as-welded specimen. On the other hand, the PWHT temperature has less effect on the tensile strengths but improves the elongation of the weldments. The micro-hardness for all the weld zones WZs exhibited lower values than that of the base metal BM and the heat affected zone HAZ. The latter presents similar micro-hardness values for all samples and slightly higher than that of the BM except for the weld heat treated at 850 °C, which attained high micro-hardness values. The weldment solution heat treated at 1050 °C shows the best and stable micro-hardness profile similar to that of the as-weld specimen but with higher values in the weld zone.