The influence of austenite grain size and its distribution on chip deformation and tool life during machining of AISI 304L

Abstract

In this article, the influence of austenite grain size and its distribution on chip deformation and tool life during machining of AISI 304L austenitic stainless steel bar is examined. Hot-forged bar and the quenched bars (at different quenching temperatures, 1050 °C, 1100 °C, 1150 °C, and 1200 °C) are machined at a high cutting speed. It was noted that the inhomogeneous distribution of grain size in the surface area, within a depth of 15 mm of the workpiece, resulted in tool edge breakage and lower tool life when machining the hot-forged bar compared with all of the quenched bars. In addition, a slight decrease in tool life was observed as the grain size increased in the quenched bars. The chip studies revealed that a higher segment height ratio of chip was gained when machining the hot-forged bar, compared to machining the quenched bars, due to the inhomogeneous distribution of grain size. Moreover, the thickness of the secondary shear zone was reduced as the grain size increased. Interestingly, it was noticed that the chip work hardened during the machining process due to strain-induced twinning and ɛ martensite transformation. The studies of tool wear and failure revealed that a crack was initiated on the flank face at the interface between the deposited workpiece and the tool substrate when machining the hot-forged bar. This crack was formed due to either the thermal and mechanical fatigue or plastic deformation of the tool substrate. The fatigue crack propagated into the tool substrate through the decohesion of interface between carbides. The criterion of tool life when machining all of the quenched bars was normal flank wear. Based on the studies of chip deformation and the mechanisms for tool wear and failure, the effects of austenite grain size and its distribution on tool life were explained.