Effect of Nitrogen on Sigma Transformation in Duplex Stainless Steel Weld Metal at High Temperatures

A type 329J4L duplex stainless steel was GTA(gas tungstenarc)-welded without filler material in argon-nitrogen mixed gas atmosphere. Weld metals with different nitrogen contents were annealed a 1900-1300K. Time-Temperature-Precipitation (TTP) diagrams for sigma initiation and change in sigma content at 1200K during annealing were measured with the base and weld metals. In weld metals, nose temperature was around 1200K and initiation time of sigma was about 10^2.5s, regardless of nitrogen content. Nitrogen didn't affect the nose temperature and initiation time of sigma. On the other hand, the increase in nitrogen content decreased sigma content over annealing time of 10⁵s in the weld metals. The effect of sigma content on fracture toughness of weld metal was examined using Small Punch (SP) test which can evaluate fracture toughness of small area as SP energy. A small amount of sigma in the weld metal led to a marked decrease in the SP energy. Crystallographic characteristics of sigma initiation were examined using Orientation Imaging Microscopy (OIM).