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 th e argon shielding gas and retained ferrite on the residual stress in austen itic stainless steel weldment by using autogenous gas tungsten are welding. The base metal of type 316L and 310 stainless steels were used in these ex periments. 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 solidificatio n process in investigated materials. A ferrite-scope was used to measure th e ferrite number for each weldment. To study the variation of ferrite conte nt from weld metal to heat-affected zone, an electron probe X-ray microanal yzer (EPMA) was employed to analyze the nitrogen content within the weld me tal. The experimental results show that the cross-sectional area of weld me tal increased with increased nitrogen content in the shielding gas. The 4% nitrogen addition has a minimum depth-to-width ratio value. The low heat in put 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 he moderated by the e xpansion of delta -ferrite. Therefore, the dual phase structure that delta -ferrite retains within the austenite matrix has lower residual stress in c omparison with the single austenite phase matrix. (C) 2001 Elsevier Science B.V. All rights reserved.