Fatigue Crack Growth Rate Studies on Stainless Steel Welds

Abstract

The low carbon, nitrogen enhanced SS 304 L(N) stainless steels are one of the most potential candidates for the structural members in chemical industries and powerplants operating at hostile environments of temperature and corrosion. In service, the structural members fabricated using welding process, when subjected to a combination of mechanical load and elevated temperature can fail by fatigue. The Welding of Austenitic stainless steels using Tungsten Inert gas (TIG) is often limited by the depth of weld penetration, which can be achieved during a single pass. This necessitates for the use of multiple passes resulting in weld distortion and generation of residual stress. The Use of an electronegative flux (Activating flux) during the TIG welding (A-TIG) is known to enhance the weld penetration, thereby reducing the number of passes. The present study evaluates the fatigue crack growth in stainless steel weldment (304L(N) welds) joined using conventional Multipass TIG welding and Activated flux TIG welding at 673K. Compact Tension (C(T)) specimens having a width of 50.8 mm and a thickness of 4 mm were extracted from the location of heat-affected zone (HAZ) and weld metal (WM) for A-TIG and MP-TIG configurations. From the micro-structural evaluation of A-TIG welds, it is noted that high heat input in a single pass has favored the formation of coarse equiaxed grains along the weld center. The use of multiple passes at reduced heat input has resulted in the formation of finer grains, with the orientation of grains changing along each weld pass interface. This finer randomly oriented grains has resulted in increasing crack path resistance through the MP-TIG welds compared to A-TIG welds. Thus from a view point of fatigue crack growth, due to the presence of fine grains, conventional Multi-pass weld is superior compared to A-TIG, but in cases where there is a creep or creep-fatigue combination, the A-TIG weld may prove to be useful.