Morphology and crystallography of microstructures in Mg-deoxidized offshore engineering steels after simulated welding thermal cycles

Abstract

The morphology and crystallography of microstructures in heat-affected zone of offshore engineering steels with 0.0002 and 0.0042 wt% Mg were studied. With increasing Mg content from 0.0002 to 0.0042 wt%, the prior austenite grain (PAG) size increases from 92 to 163 μm, which decreases the PAG boundary density and increases the bainite transformation temperature. So the bainite nucleation sites and driving force reduce, affecting bainites with morphology and crystallography. For morphology, the sympathetic nucleation reduces and widening growth rate of bainitic laths increases, coarsening microstructures. For crystallography, the variant selection is intensified, coarsening the unevenly distributed Bain zones with specific variants and reducing the high-angle grain boundary (HAGB) density from 1.4 to 0.6 μm^–1. The coarsened microstructure with lower crack resistance increases the ductile-brittle transition temperature, contributing to the ductile fracture transforming to cleavage fracture and the HAZ toughness at –40 °C decreasing from 201 to 58 J.