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, M3C carbide formation, fracture morphology, and lowtemperature impact toughness in the CGHAZ were evaluated. The mechanisms responsible for the decline in impact toughness in 170Cr (wCr=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 (gamma-*alpha) 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 M3C carbides is reduced when Fe atoms in Fe3C are replaced by Cr. The formation energy of Fe3C (-0.0487 eV/atom) is greater than that of (Fe, Cr)3C (-0.1048 or -0.1111 eV/atom), indicating that the M3C type carbides are easier to form in 170Cr steel. In 170Cr steel, strip M3C 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 degrees C declines from 226 J to 121 J.