Microstructural Evaluation and Tensile Properties for GTAW Weldments of Stainless Steel 304 Seam Pipes

This study examines the microstructural characteristics and tensile properties of autogenous orbital gas tungsten arc (GTA) circumferential butt welds produced on commercially rolled 304 stainless steel seam pipes (outer diameter 38.1 mm, wall thickness 2.0 mm) for high-purity fluid distribution systems. A three-segment current profile was employed using an AMI 8-4000 orbital system, with peak currents of 70, 67, and 65 A for the penetration, remelting, and downslope (crater-fill) segments, respectively, under high-purity Ar (99.999%) shielding with back purging. Electron backscatter diffraction (EBSD) analysis, including image quality (IQ), inverse pole figure (IPF), and kernel average misorientation (KAM) mapping, showed that the weld metal consists of epitaxially grown columnar austenite grains strongly oriented along the solidification direction, whereas the heat-affected zone (HAZ) exhibits finer equiaxed grains with an increased Σ3 twin boundary fraction and elevated low-angle boundary fraction, indicative of partial recrystallization. Only sparse, discontinuous δ-ferrite stringers were detected in the fusion zone, and no non-metallic inclusions were observed on fracture surfaces, supporting the weld metal’s suitability for semiconductor-grade cleanliness. Vickers microhardness profiles revealed modest hardness differences (typically within 10–20 HV) between the weld metal, HAZ, and base metal, with no pronounced HAZ softening. Cross-weld tensile tests conducted in accordance with ASTM E8/E8M-22 yielded yield strengths above 200 MPa, ultimate tensile strengths of 650–680 MPa, and total elongations approaching 40%, comparable to the as-received pipe. Scanning electron fractography confirmed fully ductile failure via microvoid coalescence without evidence of cleavage, intergranular decohesion, or weld-defect-induced embrittlement. Collectively, these results demonstrate that the three-segment autogenous orbital GTAW procedure produces structurally sound, particle-clean joints suitable for 304 stainless steel seam pipes used in high-purity industrial piping.