Insight into stress corrosion cracking in heat-affected zone of Nb micro-alloyed high-strength low-alloy steel in SO₂-polluted seawater

In this work, the effects of Nb micro-alloying on the stress corrosion cracking (SCC) behaviors of the heat-affected zone (HAZ) of high-strength low-alloy (HSLA) steel exposed to SO₂-polluted seawater were investigated using electrochemical measurements, static load U-bend immersion tests and slow strain rate tensile experiments combined with fractographic observations. Results indicated that Nb existed in solute status in the coarse-grained HAZ (CGHAZ) while simultaneously in the form of solute status and NbC precipitate in the fine-grained HAZ (FGHAZ) and inter-critical HAZ (ICHAZ). With reduced prior austenite grain sizes, Nb micro-alloying refined the lath bainite in the CGHAZ as well as induced the formation of acicular ferrite in the FGHAZ and granular bainite in the ICHAZ. Moreover, the three types of HAZ microstructures had a high SCC susceptibility in SO₂-polluted seawater in the increasing order of ICHAZ < FGHAZ < CGHAZ under the combined mechanism of anodic dissolution and hydrogen embrittlement. Nevertheless, attributing to the variations of microstructural constituents and the presence of NbC precipitates, Nb micro-alloying was capable to enhance the SCC resistance of HAZ microstructures by inhibiting the initiation and propagation of SCC cracks, with the SCC susceptibility decreased by ~ 5%, ~ 28% and 24% for CGHAZ, FGHAZ and ICHAZ, respectively.