Machining pre-sintered compacts of a sinter-hardenable powder metallurgy steel: A novel methodology for selecting sintering temperatures for the optimum machinability

Abstract

Machining of compacts sintered at a temperature below the final sintering temperature, i.e., pre-sintered compacts, is a promising avenue that deals with the issues in machining sinter-hardened compacts of a powder metallurgy (PM) steel, as well as their green counterparts, i.e., green machining. The present study analyses this avenue by performing a turning operation on the compacts prepared from a Rio-Tinto-produced FLC-4608 (Metal Powder Industries Federation denomination, standard 35) sinter-hardenable steel powder-premix pressed to nearly 7.00 g/cm3 density sintered at 600 °C, 800 °C, 1000 °C, and 1150 °C (the temperature in the conventional sinter-hardening cycle), using a cutting tool comprised of Seco-make DCGW 07 02 04 – AL insert and SDNCN 12 12 M 07 holder. It was found that at a fixed cutting velocity and feed rate, an increase in the sintering temperature affects the machinability as follows: on one hand, it increases Fc (cutting force component in the direction of cutting motion) and enhances difficulties in chip formation and handling; but on the other, it improves the surface finish of the machined surface and reduces the size of exit-edge-breakout (damage at the edge from where the cutting tool leaves the workpiece during machining). Given these opposing trends, a novel methodology was proposed to identify the range of appropriate sintering temperatures that enables the optimum machinability of pre-sintered compacts over both green machining and the machining of sinter-hardened counterparts. In this methodology, a newly defined machinability criterion, the ratio of machining outcome to machining effort, was graphically represented with the machining effort. Its significance in design for manufacturing (DFM) could be referred to from the current report. Nevertheless, based on the methodology, the 750 °C to 850 °C was identified as the range of appropriate sintering temperatures for the subject material-tool pair. Overall, this study is potentially helpful to the domain industries for improving the manufacturing process flow with due consideration of machining pre-sintered compacts.