Phase Transformation and Its Effect on Flank Wear in Machining Steels

In machining, the main wear mechanism on the flank surface of a tool is commonly believed to be abrasive wear [1,9]. Accordingly, work materials with a higher concentration of hard inclusions are expected to develop higher flank wear rates. However, the previous turning experiment [9] with plain carbon steels containing varying amounts of cementite inclusions did not exhibit the expected flank wear behavior. Other imperative phenomenon must be occurring at the tool-work material interface during machining, which diminishes the abrasive action of the cementite inclusions. To investigate this behavior, a series of turning tests with AISI 1045, 1070, and 4340 steels have been conducted; and the newly generated surface layers are examined for phase identification using Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM). At high cutting speeds (>200 m/min), flank wear is diminished as the cementite phase at the newly formed surface dissociates and diffuses into the matrix of the austenitic phase. Because the heated austenite phase is cooled extremely rapidly, martensitic and in some case even retained austenitic phases are formed. This is the evidence of phase transformation, which explains the flank wear data observed in [9]. In addition, phase transformation explains the scatter in flank wear data in the literature because the onset of phase transformation depends on the exact composition of the work materials as well as interfacial conditions such as temperature and pressure. This paper reports the experimental evidence of phase transformation and its consequence on flank wear in machining annealed steels.