Fracture mechanisms of CGHAZ for Mg and Ca-Mg deoxidized HSLA steels after HHIW based on microstructures, crack behaviors and mechanical properties

The fracture mechanisms of coarse-grained heat-affected zone (CGHAZ) for Mg and Mg-Ca deoxidized high-strength low-alloy (HSLA) steels after high heat input welding (HHIW) were investigated based on the microstructures, crack behaviors and mechanical properties. Compared to Mg-Ca steel, the proportion of intergranular acicular ferrites (IAFs) and polygonal ferrites (PFs) in Mg steel increases from 59.97% to 90.16%. The high-angle grain boundaries (HAGBs) and geometrically necessary dislocations density increase from 55.5% and 4.30 × 10¹⁴ m^-2 to 70.4% and 5.48 × 10¹⁴ m^-2, respectively, while effective grain size decreases from 9.46 to 8.12 μm. The area fraction of radial zone in Mg steel decreases from 80.8% to 37.7% and cleavage plane is smaller with more curved and finer tearing ridges. The inclusions distributed at the center of cleavage planes and along river lines can serve as crack initiation sites. The zigzag pattern of primary crack propagation path has width of 476 μm and the length of secondary cracks remains below 10 μm. These cracks are deflected or arrested by IAFs, PFs and HAGBs, and tend to propagate along 110 plane family. These factors contribute to superior overall mechanical properties of Mg steel, especially increasing low-temperature impact toughness from 23 to 175 J.