Influences of up-milling and down-milling on surface integrity and fatigue strength of X160CrMoV12 steel
Article dans une revue avec comité de lecture
This paper aims to compare the influences of the two peripheral milling modes, up-milling and down-milling, on surface integrity and fatigue strength of X160CrMoV12 high-alloy steel. The experimental investigations showed an important difference between integrity of both milled surfaces. The down-milled surface is lowly work-hardened and well finished (lower roughness), but subjected to tensile residual stresses and severely damaged by folds of metal and short micro-cracks. The up-milled surface is highly work-hardened and subjected to compressive residual stresses, but poorly finished (higher roughness) and damaged by a density of micro-cavities due to carbide extraction. The results of 3-point bending fatigue tests revealed that the fatigue limit at 2 × 106 cycles of the up-milled state is largely higher of about 26% in comparison with the down-milled state. The effects of surface integrity induced by each milling mode on fatigue strength were evaluated using a HCF behaviour predictive approach based on Dang Van’s multiaxial criterion. The predictive results estimated that the pre-existing micro-cracks play a dominant role in the fatigue strength degradation of the down-milled surface while the other surface effects seem to be lower. On the contrary, the fatigue strength of the up-milled surface is less affected by the pre-existing micro-cavities. The detrimental roughness effect (stress concentration effect) is significantly reduced by the beneficial effects of superficial hardening and compressive residual stresses. So, this study revealed that up-milling is the more appropriate mode for a better surface integrity towards fatigue strength of X160CrMoV12 steel than the down-milling mode.
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