On the effect of σ-phase formation during metal dusting

Changes in microstructure and mechanical properties have been characterized in HK-40 steel subjected to high-temperature corrosion by metal dusting. Samples with several grades of corrosion were analyzed. σ-Phase precipitation and growth caused Cr and Si depletion in the surrounding metallic matrix next to it, which in turn reduced HK-40 metal dusting resistance. Average C content increased from 0.40 in the as-cast material, to 0.49 in samples with medium and high grades of metal dusting, due to C diffusion through the carbide layer present in the inner surface. In the samples, inner edge RB hardness reduced from 99 RB, in a sample with negligible metal dusting, to around 60 in one with severe metal dusting attack. This hardness reduction was mainly associated to a process of carbide precipitation and growth that took place as C diffused into the samples.