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

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