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Toughening coarse grained heat affected zone of high strength offshore engineering steel by enhancing the completeness of austenite-bainite transformation
•Toughness of CGHAZ is very sensitive to the holding time at peak temperature.•Slightly prolonging the holding time can toughen CGHAZ significantly.•Enhancement of austenite-bainite transformation can refine the final microstructure.•Control of block inter-spacing can inhibit brittle crack and enhance ductile zone.
A novel and feasible method of enhancing the low temperature (−40 °C) impact toughness of coarse grained heat affected zone (CGHAZ) is put forward. By slightly increasing the holding time at peak temperature, the completeness of austenite-bainite transformation was enhanced and the toughness of CGHAZ was improved significantly. The product bainite was refined with increased holding time and more crystallographic variants from different Bain groups, namely, more Bain group boundaries were formed, which proved to be effective in preventing crack propagation. Moreover, when the inter-spacing of these boundaries is controlled below the critical Griffith crack length by self-accommodation during transformation, micro-cracks can be arrested by these high angle boundaries, thereby suppressing brittle fracture initiation and increasing fracture properties.