Deterioration mechanism of impact toughness in the coarse-grained heat-affected zone of Ca-treated shipbuilding steel plate with high-Cr content

The deterioration mechanism of low-temperature toughness in the coarse-grained heat-affected zone (CGHAZ) of shipbuilding steel plates with Ca-treatment, induced by the addition of Cr, was systematically investigated. Through a combination of experimental research and theoretical calculations, the effects of Cr content (ranging from 0.019 to 0.17 wt%) on microstructural evolution , M 3 C carbide formation, fracture morphology , and low-temperature impact toughness in the CGHAZ were evaluated. The mechanisms responsible for the decline in impact toughness in 170Cr (ω Cr =0.17 wt%) steel have been clarified. Thermodynamic calculations and in situ observations reveal that increasing Cr content reduces the C diffusion coefficient and the austenite-to-ferrite (γ→α) phase transformation temperature. Consequently, there is a decrease in the content of intragranular acicular ferrite (IAF) and an increase in the formation of brittle microstructures within the CGHAZ. It is effectively interpreted by first-principle calculations that the formation energy of M 3 C carbides is reduced when Fe atoms in Fe 3 C are replaced by Cr. The formation energy of Fe 3 C (−0.0487 eV/atom) is greater than that of (Fe, Cr) 3 C (−0.1048 or −0.1111 eV/atom), indicating that the M 3 C type carbides are easier to form in 170Cr steel. In 170Cr steel, strip M 3 C carbides are prone to forming at sub-grain boundaries, weakening the bonding force between sub-grains and making these boundaries more susceptible to crack propagation . Besides, the reduction in the variety of bainite variants decreases the content of high-angle grain boundaries (HAGBs) in bainites, further reducing the crack propagation resistance of 170Cr steel. Furthermore, the increasing C supersaturation in the ferrite of 170Cr steel reduces the plasticity of the CGHAZ, thereby reducing crack initiation energy. In conclusion, the addition of Cr results in a decrease in the low-temperature impact toughness of CGHAZ. As the Cr content increases from 0.019 wt% to 0.17 wt%, the low-temperature impact toughness at −40°C declines from 226 J to 121 J. ● The increasing Cr content enhances the selectivity of bainite variants while increasing the content of brittle microstructures in the CGHAZ. This microstructural evolution reduces high-angle grain boundaries in bainite , thereby facilitating crack propagation . ● Elevated Cr content lowers the formation energy of M3C carbides through substitution of Fe atoms in Fe3C. This chemical modification facilitates carbide precipitation at grain boundaries, thereby weakening the bonding forces between sub-grains. ● An increase in Cr content leads to an enhanced carbon supersaturation in the steel matrix, which further reduces the energy required for crack initiation.