A novel powder-metallurgical eco-friendly recycling process for tool steel grinding sludge

This work comprehensively investigates the reuse of AISI D2 tool steel grinding sludge as secondary raw material into a powder metallurgical process route. The tool steel grinding sludge initially contains about 15 mass% of silicon carbide and aluminium oxide abrasive particles as well as residual cooling lubricant emulsion consisting of water and oil leading to an increased carbon content of 2.56 mass%. A three-stage decontamination procedure of the grinding sludge is performed, including drying, removing non-metallic abrasive particles via magnetic separation, and recovering the cooling lubricant oil by supercritical carbon dioxide extraction. Subsequently, the prepared grinding metal chips are densified by a novel energy efficient short-time sintering technique called electric discharge sintering. The carbon content of the sludge could be reduced to 1.71 mass%. However, it was found that remaining contaminations lead to a change in the alloying composition of the recycled material, which influences the tempering behavior. Increased carbon and silicon levels promoted a high retained austenite fraction after quenching of more than 67 vol%. By adopting the heat treatment, the hardness of the recycled samples could be adjusted to a level similar to that of the cast reference material in quenched and tempered condition. Reducing the retained austenite content to 2.3% by tempering led to a hardness of roughly 700 HV1. However, the recycled material showed less than 40% of the compressive strength of the cast reference ma-terial. On the one hand, the lower compressive strength can be attributed to the inadequate removal of oxidic abrasive particles during the recycling process and their poor integration into the metal matrix of the compacted specimens. In addition, oxidized layers on the chip surfaces counteract metallurgical bonding between the chips during electric discharge sintering.