NITRIDE PRECIPITATION IN DUPLEX STAINLESS-STEEL WELD METAL

Duplex stainless steel base material was welded using gas tungsten are welding with an Ar-10%H-2 shielding gas and laboratory-made filler wi res were employed to deposit duplex and fully ferritic weld metals hav ing different nitrogen contents. Weld metal slow extension rate tensil e (WM-SERT) testing was used to examine the hydrogen-induced cracking susceptibility and fractography of the weld metals. An increase in nit rogen content in fully ferritic stainless steel weld metal increased t he density of precipitates and the hydrogen-induced cracking susceptib ility. The facets on the quasi-cleavage fracture surfaces of broken WM -SERT test specimens were parallel to the {100} plane in ferrite. Scan ning and transmission electron microscope observations revealed the cr ystallographic features and morphology of the precipitates. The precip itates were rod-like Cr2N nitrides. Many of them had [100] directions and were parallel to the cleavage {100} plane in ferrite. An orientati on relationship shown between Cr2N precipitates and ferrite suggested that the axes of the Cr2N precipitates were parallel to [001] directio n in ferrite and that they were more coherent along their long faces t han at their tips. As a result, the tip of these Cr2N precipitates cou ld act as sinks for hydrogen and may be preferential sites for initiat ion of hydrogen cracking; this could promote crack propagation on {001 } cleavage planes in ferrite on which Cr2N precipitates are located. H igher densities of Cr2N precipitates were nucleated at solidification boundaries and at oxide inclusions in ferrite.