Depth-Resolved Surface Integrity Evolution and Hydrodynamic Erosion Mechanisms in Abrasive Water Jet Machining of Dissimilar Stainless Steel–Carbon Steel Welds

Abrasive Water Jet Machining (AWJM) is increasingly used for post-weld surface modification of dissimilar metallic joints; however, the depth-dependent surface response of welded stainless–carbon steel joints remains insufficiently quantified. In this study, four dissimilar welded systems, TIG 316, TIG 309, ARC 316, and ARC 309, were systematically investigated to elucidate the combined influence of welding technology, filler composition, and jet parameters on surface integrity. Surface roughness was evaluated at multiple jet-penetration depths using amplitude (Ra, Rq, Rt, Rz) and statistical (Rsk, Rku) descriptors. The results reveal three distinct hydrodynamic erosion regimes governing texture evolution. Duplex welds (TIG 309 and ARC 309) exhibited highly stable erosion behavior, with Ra confined to 1.91–2.99 µm, low roughness gradients (ΔRa/Δdepth = 0.012–0.015 µm·mm−1), and near-Gaussian surface statistics (Rsk ≈ 0, Rku ≈ 3–4). In contrast, austenitic welds (TIG 316 and ARC 316) showed pronounced depth-dependent instability, with Ra increasing up to 4.54 µm and the normalized roughness ratio Rz/Ra reaching 5.69 in TIG 316 near the jet exit. Strong inter-parameter correlations in duplex welds (r ≥ 0.94) confirm uniform erosion kinetics, whereas weakened correlations in austenitic systems (r ≈ 0.70–0.83) reflect jet-energy dissipation. These findings establish a mechanistically grounded AWJM performance window for achieving Ra ≤ 3 µm in dissimilar welded steels.