The effect of adding MgO nanoparticles to ship plate steel at welding heat-affected zones (HAZs) was experimentally studied. The results showed that the added nanoparticles obviously optimised the inclusions; the average size of the inclusions was significantly reduced, and the distribution was relatively uniform. The toughness of the base metal and HAZ increased by 20–60 J, apart from a slight decrease in the strength of the fine-grained heat-affected zone and a nearly unchanged strength in the coarse-grained heat-affected zone, and approximately ∼2 μm (Mg–Al–Ti)O-containing inclusions induced acicular ferrite effectively, thereby simultaneously enhancing the strength and toughness of the inter-critical heat-affected zone. In conclusion, these performance improvements in both the base metal and HAZ could be attributed to grain refinement and inclusion optimisation.
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