Decomposition of Thermal and Mechanical Effects on Microstructure and Hardness of Hard Turned Surfaces

Hard turning involves the mechanical deformation of materials, coupled with the heat generation from friction and dissipation of deformation energy. The mechanical and thermal effects on microstructure and hardness of hard turned surfaces are decomposed by a proposed method using X-Ray diffraction line profile analysis and hardness test. It is revealed that the structure could be represented by the diffraction profile and further by a parameter, Gaussian Curve Parameter (GCP), from fitting the diffraction line peak by a Gaussian curve. The thermal effect on structure can therefore be identified from the change in diffraction profiles. For heat treated parts, identical structures or profiles yield the same hardness. However, hard turned specimens with the same peak line profile often have higher measured hardness than the heat treated ones. Thus, the residual mechanical hardening effect, from strain and strain rate hardening, can be separated and reflected by the amount of hardness difference. The mechanical effects beneath the machined surfaces vary in depth, but little effect on the very surface is found. Larger tool wear and depth of cut usually give rise to deeper mechanical affected and thermal affected layers.