Effect of nitrogen content and retained ferrite on the residual stress in austenitic stainless steel weldments

The aim of this study was to investigate the effect of added nitrogen to the argon shielding gas and retained ferrite on the residual stress in austenitic stainless steel weldment by using autogenous gas tungsten arc welding. The base metal of type 316L and 310 stainless steels were used in these experiments. During welding, the thermal cycles of different locations in the weldment were recorded. The residual stresses were determined by using the hole-drilling strain-gage method of ASTM standard E837. The depth-to-width ratio was observed with an optical microscope. A thermomechanical analyzer (TMA) was used to analyze the thermal contraction during the solidification process in investigated materials. A ferrite-scope was used to measure the ferrite number for each weldment. To study the variation of ferrite content from weld metal to heat-affected zone, an electron probe X-ray microanalyzer (EPMA) was employed to analyze the nitrogen content within the weld metal. The experimental results show that the cross-sectional area of weld metal increased with increased nitrogen content in the shielding gas. The 4% nitrogen addition has a minimum depth-to-width ratio value. The low heat input condition has higher retained ferrite content than the high heat input condition when the level of nitrogen in the shielding gas does not exceed 2%. During solidification, the thermal contraction can be moderated by the expansion of δ-ferrite. Therefore, the dual phase structure that δ-ferrite retains within the austenite matrix has lower residual stress in comparison with the single austenite phase matrix.