Microstructural and mechanical characterisation of dissimilar welded joints between SA553 9% Ni and SA537 CL1 steel plates with varying thicknesses

This study investigates how plate thickness and process parameters govern microstructure evolution and mechanical performance of dissimilar welds between SA553 (9%Ni) and SA537 CL1 steels. Joints with 6-, 8-, and 12-mm plates were produced using GTAW and SMAW. The base metals exhibit tempered martensite (SA553) and ferrite–pearlite (SA537); near the fusion line, local Ni dilution stabilises austenite that transforms to martensite, while the fusion zone shows dendritic austenite. Tensile and impact testing, including Charpy at −196 °C, quantify joint performance. Representative joints achieved yield strength ≈550 MPa and ultimate tensile strength ≈740 MPa (Type-1), and ≈420 MPa and ≈550 MPa (Type-2), compared with base metals of ≈350 MPa yield and ≈630 MPa UTS. Cryogenic Charpy energies for base metals were ≈117 J, and weld/HAZ values varied with notch location yet exceeded commonly cited minima for cryogenic service (≈27–41 J); among the weld metals, the E-NiCrMo-6 filler provided the highest low-temperature toughness. Fractography revealed mixed ductile–brittle modes linked to impurity distribution and joint heterogeneity. These results provide process–structure–property guidance for dissimilar SA553/SA537 joints operating at −196 °C, with practical implications for LNG tanks and other cryogenic structures.