Chemistry and microstructure of duplex stainless steel powders from recycled Z100 mixed with 316L steels

Recovered metallic waste can be used in additive manufacturing as a feedstock if the subsequent steps of the waste-to-product process are sufficiently mastered. In this study, impact of recycling of Z100 duplex steel mixed with 316L steel on the resulting powders microstructure and chemical composition was investigated. The utility of the original method of recycling stainless steels into a high-grade powder suitable for additive techniques has been demonstrated. By examining three gradations of powders, namely 20-50 mu m, 50-100 mu m and 125-250 mu m, differences in selected properties in relation to the average particle size are shown. The results suggest that with increasing the particle diameter, fine-crystalline gamma-austenite is favoured to precipitate at the boundaries and within the volume of the originally formed large delta-ferrite grains. It is reflected by a decrease of delta/gamma fraction ratio from 0.64 in the 20-50 mu m powders to 0.20 in the 125-250 mu m, respectively. Obtained results indicate non-diffusional, shear or semi-shear character of delta -> gamma + delta phase transformation. The resulting fine-crystalline austenite is characterised by a significant dislocation density, which induces dislocation strengthening effect, responsible for an increase in Vickers hardness from 145 HV and Young's modulus from 29 GPa in the 20-50 mu m group to 310 HV and 146 GPa in the 125-250 mu m fraction, respectively.