Correlation between the microstructure and mechanical properties of dissimilar AISI 309 and 430 stainless steel joints welded via pulsed GTAW

The current study investigates the joining of dissimilar AISI 430 ferritic and AISI 309 austenitic stainless steels, a combination relevant for applications requiring a transition from corrosion-resistant to cost-effective sections in industries like thermal power and petrochemicals. The work quantitatively correlates the microstructure and mechanical properties of joints welded using pulsed-current Gas Tungsten Arc Welding (GTAW). Two filler metals, ERNiCrMo-3 (Inconel 625) and ER309L, were used under constant current and two pulsed current ratios (0.25 and 0.5). Microstructural analysis revealed that a 50% pulse ratio yielded the optimal refinement: Inconel 625 welds exhibited a threefold reduction in austenitic dendritic grain size (from 36 μm to 14 μm), while ER309L welds developed a fine mixture of acicular and lath ferrite. This microstructural refinement directly enhanced mechanical properties through the Hall-Petch mechanism, resulting in peak average weld metal hardness values of 250 HV and 398 HV for the Inconel and ER309L joints, respectively. Despite this weld metal strengthening, tensile tests consistently showed fracture in the heat-affected zone (HAZ) of the AISI 309 base metal (ultimate tensile strength ∼650 MPa), with all joints achieving a nearly identical tensile strength of 485 MPa. The key conclusion is that while pulsed GTAW significantly optimizes weld metal microstructure and hardness, the overall joint strength is ultimately governed by the mechanical limit of the weaker AISI 309 base metal component. • Optimal Pulsed-Current Parameter : A 50% pulse ratio in GTAW was identified as optimal, producing a threefold reduction in austenitic dendritic grain size (to 14 μm) in Inconel 625 welds and a superior fine acicular/lath ferrite mixture in ER309L welds. • Significant Weld Metal Hardness Enhancement : Microstructural refinement via the Hall-Petch mechanism resulted in peak hardness values of 250 HV (Inconel 625) and 398 HV (ER309L), a ∼60% increase for the latter. • Joint Strength Governed by Base Metal : Despite enhanced weld metal properties, all tensile specimens fractured in the HAZ of the AISI 309 base metal, establishing a consistent joint tensile strength limit of ∼485 MPa. • Quantitative Correlation Established : The study conclusively quantifies the link between pulsed-current parameters, microstructural evolution, and the final mechanical properties of the dissimilar joint.