Effect of nitrogen on the microstructure and stress corrosion cracking of stainless steel weld metals

The nitrogen content of conventionally-produced 18Cr-8Ni stainless steel weld metals can vary considerably. Variations in base and filler metal nitrogen contents and nitrogen pickup from the atmosphere during welding can result in weld metal nitrogen contents ranging from below 0.04 to above 0.3 wt-%. Unfortunately, aside from its influence on ferrite content, the importance of nitrogen content on 18Cr-8Ni weld metal properties has been for the most part undetermined. Results of the present investigation have shown that nitrogen content in a range from approximately 0.04 to 0.25 wt-% influences both the mode of weld metal solidification and weld metal stress-corrosion cracking properties. At low nitrogen levels (below about 0.1 wt-%) weld metal solidification initiates as primary ferrite, wi th the final regions solidifying as an austenite-ferrite eutectic at the primary-ferrite cellular-dendrite boundaries. During subsequent cooling of the weldment, nearly all the ferrite is transformed to austenite by a di f fusionless transformation, and ferrite is retained to room temperature at the cores of originally-solidified, primaryferrite dendrites. Increased nitrogen contents promote the primary solidification of austenite, wi th the final regions solidifying as an austenite-ferrite eutectic at primary-austenite cellular-dendrite and grain boundaries. As the weldment cools, some of this eutectic ferrite is transformed to austenite and a room-temperature microstructure exhibiting a discontinuous ferrite morphology results. At high nitrogen levels (greater than about 0.2 wt-%) an entirely austenitic room-temperature microstructure is produced. The absence of ferrite at austenite grain boundaries during weldment cooling allows considerable austenite grain growth. Constant-extension rate tensile tests performed on Ferrite Numbers 6 (0.44 wt-% N), 3 (0.10 wt-% N), 1 (0.14 wt-% N), and 0 (0.24 wt-% N) for 18Cr-8Ni weld metals in boil ing MgCL solutions found that the stress-corrosion cracking resistance decreases wi th increases in nitrogen content and decreases in ferrite content. It was also determined that the stress-corrosion cracking morphologies are closely associated with weld metal microstructural characteristics. The susceptibilities of weld metals which solidify as primary ferrite (Ferrite Number 6) are similar to those of wholly-austenitic Type 304 base metal. Stress-corrosion crack propagation in these weld metals occurs interfacially at austenite-ferrite boundaries in the weld metal substructure. Ferrite Number 1 and 3 weld metals, which solidify as primary austenite, exhibit Paper to be presented at the AWS 60th Annual Meeting in Detroit, Michigan, during April 2-6, 1979 W. A. BAESLACK, III is Materials joining Engineer/Metallurgist, joining Technology Group, Air Force Materials Laboratory, Wright Patterson Air Force Base, Ohio; VV. F. SAVAGE is Professor and Director of Welding Research; and D. I. DUQUETTE is Professor, Rensselaer Polytechnic Institute, Troy, New York. similar susceptibilities to stress-corrosion cracking which are somewhat greater than that observed with the Ferrite Number 6 weld metals. Crack propagation in these low-ferrite weld metals occurs by a mixed transgranular-intergranular mode. Wholly-austenitic weld metals are particularly susceptible to stress-corrosion cracking by a predominantly intergranular mode. The large grain size appears to promote rapid stress-corrosion crack propagation.