RESIDUAL STRESSES IN GRINDING HARDENED STEEL (SUPERABRASIVES, TEMPERATURE, CUBIC BORON-NITRIDE).

摘要

Cubic Boron Nitride (CBN) is a relatively new grinding abrasive. It is being used increasingly to grind steel parts especially those that will be subjected to bending fatigue loads. This is because it can produce highly desirable compressive surface residual stresses. Conventional abrasives such as Aluminum Oxide and Silicon Carbide, usually produce tensile surface residual stresses.;The mechanism that produces these stresses is of great interest. The present study attempts to add to the fundamental knowledge in this area. Hardened steel specimens were ground under a typical range of operating parameters, using Aluminum Oxide and CBN wheels. The residual stresses thus induced were measured using a modified etch/deflection technique. A computer program was developed to generate profiles of residual stresses as a function of depth below the ground surface. Scanning electron microscopy was used to study the ground surfaces.;The residual stresses in grinding are found to comprise two major contributions: mechanical and thermal. The abrasive particles produce extensive plastic deformation at the surface by mechanical action. The sub-surface layers, which are still elastic, restrain the plastic deformation yielding a compressive surface residual stress. This is independent of the type of abrasive used. The thermal properties of workpiece and the abrasive-bond system determine whether most of the heat generated in grinding will be extracted away towards the wheel or towards the workpiece. If there is thermal softening of the workpiece surface, differential cooling results in residual stresses. These will be tensile and compressive in the cases of heat extraction into and away from the workpiece, respectively. Since the net residual stresses are but the superpositions of the mechanical and thermal contributions, the latter should be minimized. In this regard, CBN is seen to have distinctly superior thermal properties compared to Aluminum Oxide.