Nitride precipitation in duplex stainless steel weld metal

摘要

Duplex stainless steel base material was welded using gas tungsten arc welding with an Ar-10 percent 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 Cr_2N nitrides. Many of them had (100) directions and were parallel to the cleavage {100} plane in ferrite. An orientation relationship shown between Cr_2N precipitates and ferrite suggested that the axes of the Cr_2N precipitates were parallel to direction in ferrite and that they were more coherent along their long faces than at their tips. As a result, the tip of these Cr_2N 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 Cr_2N precipitates are located. Higher densities of Cr_2N precipitates were nucleated at solidification boundaries and at oxide inclusions in ferrite.