Influence of the Mg content on the austenite grain growth in heat-affected zone of offshore engineering steels considering TiN particle pinning and Mo solute drag effects

The submicrometre and nanometre particle characteristics, soluble element contents, and austenite grain growth behaviors in heat-affected zone of offshore engineering steels with 0.0002 (2Mg) and 0.0042 (42Mg) wt.% Mg during the simulated welding process were studied. With increasing the Mg content in steel from 0.0002 to 0.0042 wt.%, the submicron particles are decreased in number and size with their compositions evolved from TiN to TiN + MgO capped with Mo carbides, and the number density of small-sized nanoparticles increases and large-sized nanoparticles decreases. When the temperature is below 1250 °C, the grain growth rate of two steels is not much different due to the larger Mo solute drag effect in 2Mg and larger pinning force in 42Mg. When the temperature is 1250-1300 °C, the small-sized nanoparticles in 42Mg is more than that in 2Mg, resulting in the larger pinning force and smaller grain growth rate in 42Mg. When heated to 1300-1350 °C and soaked at 1350 °C for 300 s, since large quantities of particles smaller than the critical size (dcr) are dissolved, the grain growth rate in 2Mg is smaller than that in 42Mg due to the greater amount of the effective pinning particles and larger pinning force in 2Mg.