Influence of Peak Austenitizing Temperature and Thermal Transformation Kinetics on the Degradation of High-Strength Low-Alloy Steel in Chloride Environments

One of the main challenges in marine engineering is the microscale evolution of microstructures and corrosion in alloys. The work focuses on whether thermal transformation kinetics, mechanical microanisotropy, and average macroscopic corrosion rates in A572 high-strength low-alloy steel are related. Three different austenitizing peak temperatures and cooling rates were applied to produce the three microstructural regimes. The samples were evaluated using optical microscopy, static laboratory testing (ASTM G31) in 3.5 % NaCl solution, and spatial Vickers microhardness testing. A correlation between electrochemical degradation and microstructural coarsening has been observed in experimental results. Carbonitride precipitates were not removed from the fine-grained matrix from a thermal-kinetic perspective, a requirement for the formation of the highly polished, interlocking morphology produced by low-thermal-input synthesis. This homogenized structural state exhibited excellent corrosion resistance, with a rate of 0.150 millimeters per year, and effectively minimized general mass loss. On the other hand, a thermodynamically supersaturated condition led to the formation of ordered bainitic packets and macroscopic softening, thereby accelerating the overall degradation rate. Extensive abnormal grain growth (Widmanstätten ferrite plates) was induced by the dissolution of precipitates under the extreme thermal cycle established for stabilization. This complex morphology led to a high anodic-to-cathodic area ratio and large interfacial strain gradients, resulting in a high corrosion rate of 0.402 mm/yr. In addition, spatial microhardness profiling has shown that high standard deviations of local hardness accurately reflect phase mismatches that cause galvanic susceptibility. Lastly, the current study reveals strong mechanical anisotropy effects in electrochemical degradation processes and indicates that local hardness variation is indicative of relative corrosion susceptibility.