Effect of undercooled austenite ausforming on the role of the M–A constituents in the CGHAZ toughness of the HSLA steels with bainite structure

The brittle martensite-austenite (M-A) constituents in the coarse-grained heat-affected zone (CGHAZ) of the bainite steel are known to have negative effect on toughness. To modify the microstructure of the M–A constituents and then improve the CGHAZ toughness, this work applied ausforming to the bainite steel during its CGHAZ simulating process when the temperature of the thermal cycle decreased to 600 °C. Morphology of the M–A constituents in the CGHAZ with and without the ausforming was observed by optical microscopy (OM) and scanning electron microscopy (SEM). The microstructure of the M–A constituents was characterized by transmission electron microscopy (TEM), the toughness of the specimens was assessed through instrumented Charpy impact test, and the crack behavior beneath the fracture was revealed by SEM. Results show that the ausforming contributes to the austenite formation within the M–A constituents, which helps to improve the CGHAZ toughness from 10.50 J to 33.70 J. This work proposed a nontraditional method that enables the microstructure modification of the M–A constituents and the toughness improvement in the CGHAZ.