| From a machining standpoint, materials with a hardness greater than 45 Rc are classified as “hard”. Components made of such materials, such as bearings and shafts, are usually finish machined by grinding. The advent of Polycrystalline Cubic Boron Nitride (PCBN) cutting tools has however opened up several new avenues in finish machining such as turning and milling which could potentially replace grinding on account of advantages such as greater process flexibility and environmental acceptability. The main disadvantage associated with hard turned components is the presence of undesirable surface artifacts known as white layers, which, depending on machining conditions, are sometimes associated with tensile states of residual stress and hence poor fatigue life. A model to predict white layer formation and hence surface integrity in hard machining would therefore be extremely useful.; There is evidence in literature suggesting that the mechanism of white layer formation is a function of the machining conditions used. Through a series of experiments using transmission electron microscopy, x-ray diffraction and nano-hardness testing, it was found that white layer formation in hardened AISI 52100 steel at low to moderate cutting speeds was dominated by mechanical grain refinement, and by phase transformation at high cutting speeds.; With this information, a model was developed to predict white layer formation and residual stresses in hardened AISI 52100 steel under thermally dominant conditions generated by using high machining speeds and tools with large amounts of flank wear. The model was a coupled thermo-mechanical finite element formulation utilizing a flow stress equation derived from orthogonal machining experiments. Predicted depths of white layer formed were validated satisfactorily with experiment. Significant differences in predicted residual stresses were found when white layer formation was not accounted for. |
