Microstructural Gradient Architecture and Heterogeneous Strengthening Mechanism in Ultra-Narrow Gap MAG Welded 27SiMn Steel Joints

Ultra-narrow gap (UNG) MAG welding is increasingly utilized for thick-walled 27SiMn hydraulic cylinders; however, multi-pass thermal cycling induces pronounced through-thickness heterogeneity, leading to local mechanical mismatch near inter-pass interfaces and the fusion line. This study establishes a quantitative microstructure–property linkage for a 32 mm-thick UNG MAG welded 27SiMn joint to identify the underlying features governing its strengthening-toughening synergy. By partitioning the joint into six representative zones, zone-resolved EBSD was conducted to quantify local orientation gradients, geometrically necessary dislocation (GND) density, and grain-boundary characteristics, which were then correlated with full cross-sectional microhardness mapping and tensile testing. The results reveal a distinct "structure-orientation-dimension" gradient architecture from the weld metal to the heat-affected zone (HAZ). A significant crystallographic mismatch exists at the fusion line, where the HAZ exhibits severe strain concentration and elevated GND accumulation, while the weld zone demonstrates robust crystallographic ordering enriched with Σ3 twin boundaries. This heterogeneous interlocking of hard phases (acicular ferrite/lath martensite) and soft recovered zones (in-situ tempered sorbite) accommodates deformation through substantial hetero-deformation induced (HDI) back stress. Consequently, the joint achieves a reliable "strength over-matching" regime with an ultimate tensile strength of 678.6 MPa, characterized by tensile fracture exclusively in the base metal. Overall, the findings demonstrate that actively designing microstructural inhomogeneity, rather than eliminating it, provides a superior paradigm for optimizing heavy-duty thick-walled hydraulic components.