Characteristics and influence of nugget microstructures of spot welds in ultra-high-strength steel on cracking

This study investigated the cross-tension strength of 1470 MPa-grade ultra-high-strength steel spot welds, focusing on spatial correlations between phosphorus (P) segregation and martensitic structures. Mechanical and metallurgical evaluation in three steels varying P contents reveals that cross-tension strength is governed by brittle crack propagation resistance at the nugget edge. Columnar prior austenite grains exhibited significant crystallographic anisotropy, with specific packet groups occupying up to 85% by area. In low-P steel, martensitic hierarchical structures impede crack propagation. However, increasing P promotes cleavage fracture within grains with specific packet groups. In high-P steel, severe P-segregation at grain boundaries overrides the crystallographic barrier effect, inducing intergranular fracture and reducing cross-tension strength. These findings highlight the importance of microstructural control for improving ultra-high-strength steel spot weld toughness.