Embrittlement in the Weld Metal of High Purity 19Cr-2Mo Steel and Its Improvement by REM Addition

Study on Welding Metallurgy in High Purity Ferritic Stainless Steels (Report 3)

Abstract

A study has been performed to examine the influence of nitrogen and oxygen pick-up on the weld toughness of the commercial high purity 19Cr-2Mo steel using the mixed gas of argon and various percent of nitrogen, oxygen and air as the shielding gas. The effect of REM on the weld toughness was also investigated with the intention of improving it. The toughness of the weld metal was evaluated using subsize Charpy V-notch specimens. With addition of small amount of air to the shielding gas, the toughness of the weld metal drastically deteriorates due to the pick-up of nitrogen and oxygen. The increases of nitrogen and oxygen in the weld metal cause the precipitation of fine intragranular chromium nitrides and spherical oxide inclusions of more than l μm diameter, respectively, which induce the embrittlement of the weld metal. The weld toughness is found to be significantly improved by REM addition. Effective REM content ranges 100 ppm to 400 ppm when the amount of air in the shielding gas being less than 1%. The absorption rate of nitrogen and oxygen in the REM bearing weld metal seems to decrease due to the formation of the REM oxide film or the REM-absorbed layer on the surface of the molten pool. Consequently, the amounts of nitrogen and oxygen picked-up in the weld metal decrease, whichi result in improving the weld toughness.