Effect of Si Content on the Microstructures and Toughness in Heat-Affected Zone of Offshore Engineering Steels with Mg Deoxidation

Herein, the effect of Si content on the microstructures and toughness in the heat-affected zone (HAZ) with the heat input of 100 kJ cm(-1) for Mg-deoxidized offshore engineering steels is presented. For the matrix, increasing Si content coarsens the lath bainites (LBs) in HAZ due to the higher A(r3) and lower A(r1). Thus, the high-angled grain boundary density (HAGBD) decreases from 0.88 to 0.53 mu m(-1), and the effective grain size increases from 10.6 to 18.3 mu m. M-A constituents are transformed from dotted and filmed ones to the stringer and blocky ones, with the area fraction increasing from 2.9% to 5.6%. Among them, the stringer M-A particles are likely to debond from the matrix, while the blocky M-A particles tend to initiate cracks because the interfacial energy of stringer particles is lower than that of blocky ones. Accordingly, increasing Si content from 0.0003 to 0.120 wt% results in both the increased crack initiation probability by more M-A constituents with the larger size and the lowered crack propagation barrier by the smaller HAGBD, thereby reducing the HAZ toughness at -40 degrees C from 202 to 58 J.