Utilization of Waste Steel Scrap in Powder Metallurgy

Abstract

Most production processes of steel tools like machining and forging generate large amount of steel scrap in the form of chips. From the economic, strategic and environmental point of view, these steel scrap should be reused and recycled. Powder metallurgy provides a very suitable and economical way for the utilization of waste steel scraps. In the present investigation, an attempt has been made to prepare steel powder from the waste steel scrap by high energy planetary milling and finally the consolidation of the powder has been carried out to study the feasibility of a useful product. Both ultralow carbon and low carbon steel chips were milled separately in a dualdrive planetary mill for 5 hours using stainless steel balls with ball to powder weight ratio of 12:1. Toluene was added to avoid high temperature oxidation during milling. Samples were collected for the intervals of 0.5, 1.5, 3, 4 and 5 hours for analysis. Samples were characterized by XRD, SEM and TEM. A 1 wt. % nano yttria dispersed and yttria free ultralow and low carbon steel powders were cold compacted under a hydraulic press at the pressure of 874 MPa. PVA was used as binder during compaction. The compact samples were sintered at 1100 °C for 1 h in a tubular furnace under Ar atmosphere. The sintered samples were polished and then density, hardness values were measured and microstructures were revealed in optical and SEM. Fig. 1 shows XRD spectra which present the presence of ferrite as the main constituent in ultralow carbon steel after 5 hours of milling. Phase analysis was also reconfirmed from SAED ring pattern diffraction obtained from TEM. Fig. 2 shows the SEM micrographs which exhibit gradual refinement of powder during milling and an average particle size of 5 μm were obtained in both cases.