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 wires were employed to deposit duplex and fully ferritic weld metals having different nitrogen contents. Weld metal slow extension rate tensile (WM-SERT) testing was used to examine the hydrogen-induced cracking susceptibility and fractography of the weld metals. An increase in nitrogen content in fully ferritic stainless steel weld metal increased the density of precipitates and the hydrogen-induced cracking susceptibility. The facets on the quasi-cleavage fracture surfaces of broken WM-SERT test specimens were parallel to the {100} plane in ferrite. Scanning and transmission electron microscope observations revealed the crystallographic features and morphology of the precipitates. The precipitates were rod-like Cr2N nitrides. Many of them had [100] directions and were parallel to the cleavage {100} plane in ferrite. An orientation relationship shown between Cr2N precipitates and ferrite suggested that the axes of the Cr2N precipitates were parallel to [001] direction in ferrite and that they were more coherent along their long faces than at their tips. As a result, the tip of these Cr2N precipitates could act as sinks for hydrogen and may be preferential sites for initiation of hydrogen cracking; this could promote crack propagation on {001} cleavage planes in ferrite on which Cr2N precipitates are located. Higher densities of Cr2N precipitates were nucleated at solidification boundaries and at oxide inclusions in ferrite.