Experimental characterization of torch brazed powder metallurgy borated austenitic stainless steel (SS304B4) to austenitic stainless steel (SS316LN) in simulated catalytic converter conditions

This study investigates the joining of SS304B4 and SS316LN stainless steels for automotive exhaust applications using torch brazing and diffusion bonding with interlayers (Cu, Ni, Ni–Cu–Ni, TiCuSil). Torch brazed joints were fabricated using Ag-based (SBA) and brass-based (CBA) fillers, designated as BAB, BAS, BBB, and BBS. Among these, the BAS joint (SS304B4-SBA-SS316LN) exhibited superior microstructural integrity with a refined interface and defect-free bonding. Texture analysis revealed dual-phase stability and uniform texture distribution in BAS, while BBS showed micro-voids and grain coarsening. Potentiodynamic polarization tests demonstrated a lower corrosion rate (8.68 mm/year) and higher breakdown potential (0.311 V) for BAS compared to BBS (16.01 mm/year, 0.221 V). BAS also showed lower mass loss (0.012 g) in hot salt corrosion over eight thermal cycles, and retained a smooth surface with fewer pits. Microhardness dropped at the interface (BAS: ∼145 HV), confirming filler softening. These findings establish BAS as a structurally and environmentally robust joint for high-temperature automotive service.