Effect of Mo content on nano-scaled particles, prior austenite grains and impact toughness of CGHAZ in offshore engineering steels

The effect of Mo on nano-scaled particles, prior austenite grains and impact toughness of coarse-grained heat-affected zone (CGHAZ) in offshore engineering steels with Ca deoxidation was studied. The heat-affected zone (HAZ) toughness of Mo16 steel is obviously higher than that of Mo8 steel at all the heat inputs of 50, 100, 150 and 200 kJ/cm, with HAZ toughness of both steels decreased with increasing the welding heat input. When the Mo content is increased from 0.08 to 0.16%, the size of nano-scaled particles in HAZ is decreased from 18 to 15 nm, and their number density is increased from 0.7 to 0.9 μm⁻². Thus, the Zener pinning force is increased, and the prior austenite grain size (PAGS) is decreased, leading to the improved HAZ toughness. Microstructural characterizations show that the nano-scaled particles in both steels are Ti(C, N) with the solute elements of Nb and Mo. The calculated critical particle size of TiN is 10.2 and 8.4 nm in Mo8 and Mo16 steels at 1350 °C, and the particles larger than the critical size are stable during the welding process. From the Zener pinning force calculation, Ti(C, N) particles play the more important role in the pinning effect on the prior austenite grain growth. Based on the regression analysis by the MATLAB results, the predicted values of PAGS at different heat inputs are well fitted with the experimental data.