In-situ Observation of Bainite Transformation in CGHAZ of 420 MPa Grade Offshore Engineering Steel with Different Mo Contents

抄録

The 420 MPa grade offshore engineering steels with the different Mo contents were fabricated through Ca deoxidation. The effect of Mo content on the microstructure and the impact toughness of the coarse-grained heat-affected zone (CGHAZ) after the welding heat input of 100 kJ/cm were investigated by welding simulation and high-temperature laser scanning confocal microscopy (HTLSCM). As the Mo content is increased from 0.08 to 0.16 wt%, the impact toughness at −40°C is increased from 15 to 160 J, and the fracture is changed from cleavage to ductile and quasi-cleavage. Increasing Mo content leads to the increase in lath bainite (LB) and the decrease in granular bainite (GB) and acicular ferrite (AF) in CGHAZ. Electron backscatter diffraction (EBSD) results show that the fraction of high angle grain boundaries (HAGBs) is increased from 45% to 58%, while the effective grain size (EGS) is decreased from 10.4 to 6.3 µm in the HAZ of the steels. The growth rates of bainite ferrite range from 39.7 and 134.5 µm/s for Mo8 steel, and 51.2 to 165.6 µm/s for Mo16 steel. In-situ observation shows that increasing Mo content decreases the austenite grain size from 114 to 100 µm. As for Mo8 steel, AFs nucleate from the inclusions firstly. Then the different bainites nucleate, grow, stop growth and widen as they impinge the other bainites and grain boundaries. The secondary bainites finally nucleate from the prior bainite and AF. As for Mo16 steel, the AF cannot nucleate, and the bainites have a similar nucleating and growth behavior as the Mo8 steel.