THE INFLUENCE OF AUSTENITE GRAIN-SIZE AND ITS DISTRIBUTION ON CHIP DEFORMATION AND TOOL LIFE DURING MACHINING OF AISI 304L

In this article, the influence of austenite grain size and its distrib ution on chip deformation and tool life during machining of AISI 304L austenitic stainless steel bar is examined. Hot-forged bar and the que nched bars (at different quenching temperatures, 1050 degrees C, 1100 degrees C, 1150 degrees C, and 1200 degrees C) are machined at a high cutting speed. It was noted that the inhomogeneous distribution of gra in 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, co mpared to machining the quenched bars, due to the inhomogeneous distri bution of grain size. Moreover, the thickness of the secondary shear z one was reduced as the grain size increased. Interestingly, it was not iced that the chip work hardened during the machining process due to s train-induced twinning and epsilon martensite transformation. The stud ies of tool wear and failure revealed that a crack was initiated or th e flank face at the interface between the deposited workpiece and the tool substrate when machining the hot-forged bar. This crack was forme d due to either the thermal and mechanical fatigue or plastic deformat ion of the tool substrate. The fatigue crack propagated into the tool substrate through the decohesion of interface between carbides. The cr iterion of tool life when machining all of the quenched bars was norma l flank wear. Based on the studies of chip deformation and the mechani sms for tool wear and failure, the effects of austenite grain size and its distribution on tool life were explained.