Hard turning of M50 steel with different microstructures in continuous and intermittent cutting

Different types of M50 steel (63 HRc) have been hard turned using cubic boron nitride (CBN) tools to experimentally investigate microstructual effects on both continuous and intermittent cutting. In continuous cutting, powder metallurgy (PM) M50 results in substantially lower tool wear and wear rate than conventional M50, presumably due to refined carbides that may delay delamination wear. In intermittent cutting, fine microstructures of PM steel also lead to reduced wear rate, however, not as drastically as in continuous cutting. In intermittent cutting, the bulk impact loading may be dominant in tool wear and attrition wear affected by carbide sizes becomes less significant. Nevertheless, fine carbides in PM steel seem to alleviate delamination wear on tool flank, which limits tool life at lower speed in intermittent cutting of conventional M50. Furthermore, low CBN content tools consistently outperform high CBN content tools in PM M50 intermittent cutting, contradictory to the results in intermittent cutting of conventional counterparts. This study provides guidelines on tool and work material selections in machining of hardened steel components. For continuous hard turning, PM steel may induce substantially lower tool wear, and thus, have much better machinability. In intermittent cutting, PM steel still offers reduced tool wear, however, less effectively.