"Effect of cutting parameters in the surface residual stresses generated by turning in AISI 4340 steel"

Machining processes induce a residual stress state in the machined part that, depending on its magnitude and sign, can be very detrimental to the service life of the machined components. 

This way, tensile residual stresses favour crack nucleation and propagation, leading to a reduction of fatigue life and corrosion and wear resistance. The final residual stress state in a piece depends on the material of the component and on the cutting parameters employed: cutting speed, cutting feed, depth of cut, kind of cutting tool (geometry, coating of the tool, etc), wear of the tool, lubrication, etc. Nevertheless, there is some lack of agreement in the literature regarding the specific tendency of residual stresses with each cutting parameter (feed and cutting speed) and there is not much literature about the effect of tool geometry and tool characteristics (coating). The present paper aims to put some light in this lack of agreement and complement the studies found in the literature.

In this work, it has been studied the effect on the final surface stress state in AISI 4340 steel of cutting speed, feed, tool nose radius, geometry of the tool chip breaker and coating of the cutting tool. For this study surface residual stresses have been measured, by means of X-ray diffraction, in AISI 4340 steel bars subjected to turning tests using different cutting speeds (between 200 and 300 m/min), different cutting feeds (between 0.075 and 0.200 mm/rev), and cutting tools with two nose radius (0.4 and 0.8 mm), two different surface states (one coated by CVD and the other without coating) and two different geometries of the chip breaker. In all cases surface tensile residual stresses have been measured, tending to be more or less tensile (and consequently more or less detrimental to the service life of the machined component) depending on the cutting conditions and the characteristics of the cutting tool.

In this work, not only the magnitude but also the orientation of the principal residual stresses has been determined. This is not commonly encountered in the literature, in spite of its significance: the direction of maximum residual stress is a critical direction because if it coincides with the direction of the stresses acting on the component as a result of the nominal load (load suffered by the component during its normal service), the service life of the part will be significantly affected, leading to possible premature failure of the component. That is the reason why knowing the orientation and not only the magnitude of principal residual stresses is of huge importance.