Chromium nitride precipitation behavior in weld heat-affected zone of high nitrogen stainless steel

Abstract

High nitrogen stainless steels (HNS) containing about 1 mass% (%) nitrogen without adding Mn were manufactured by pressurized electro-slag remelting method. The chromium nitride precipitation behaviors at the weld heat-affected zone (HAZ) of HNS with different compositions were investigated. We also evaluated the localized corrosion resistance of the welded joints. The nitride precipitates were identified as Cr2N containing a small amount of Mo and Fe by TEM/EDS analyses. Time-temperature-precipitation curves of some HNS were obtained. It was found that for 23Cr-4Ni-2Mo-1 N steel, aging for 2 s in the temperature range between 1 173 K and 1 373 K caused Cr2N precipitation and decreased the critical pitting corrosion temperature (CRT) in the 6% FeCI3 solution. Precipitation was delayed to 4s by decreasing the nitrogen content from 1 to 0.8% and was further slightly delayed to over 5s by adding the pre-deformation followed by the thermal treatment of 23Cr-4Ni-2Mo-0.8N steel. Precipitation at the HAZ was accelerated by a continuous thermal cycle test compared to the prediction based on the additivity rule, and that the existence of a ferritic phase affected acceleration was estimated. Joints welded by laser and minimum heat input conditioned plasma arc welding showed a 348 K CPT in a 6 % FeCI3 solution and no crevice corrosion occurred at 308 K in artificial seawater. The CPT dropped notably against holding time above 1 073 K at the HAZ. The CPT drop was slightly relieved by decreasing nitrogen, content in the base metal from 1 to 0.