Unravelling the impact of Al-Ti content on the microstructure-toughness relationship in the coarse-grained heated-affected zone of super-high-strength pipeline steels

Abstract

This study aims to investigate the role of Al-Ti addition on particle modification in super-high-strength pipeline steels, and its following effect on microstructural evolution and impact toughness in the coarse-grained heated-affected zone (CGHAZ) is also explored. The steel plates with low and high Al-Ti content (0.009 wt-% Al + 0.006 wt-% Ti vs. 0.033 wt-% Al + 0.025 wt-% Ti), respectively, are subjected to gas metal arc welding with 6–7 kJ/cm heat input. The results reveal that the second-phase particles including micron-scale inclusion of Al-Mg oxide covered with CaS and nano-scale precipitate of titanium nitride are obtained in low Al-Ti steel, which are modified to the Al-Mg-Ti oxide core surrounded by CaS and TiN precipitate in high Al-Ti steel. The nano-scale TiN precipitate and micron-scale inclusion can restrict the austenite grain growth at high temperatures and induce acicular ferrite nucleation at medium temperatures, respectively, leading to the fine-grained mixed microstructure of bainite and acicular ferrite in CGHAZ of both steels. Because of the variant selection during the decomposition of austenite to bainite and acicular ferrite, those fine-grain structures with amounts of high-angle grain boundaries exhibit excellent CGHAZ toughness in both specimens. Compared with low Al-Ti steel, more precipitates induce smaller austenite grains, and the inclusions containing titanium increase the ability of inclusion to promote acicular ferrite nucleation in CGHAZ of high Al-Ti steel. Both of them resulted in smaller crystallographic grain and toughness improvement in CGHAZ of high Al-Ti steel than that in low Al-Ti steel.