Effect of hardened surface layer obtained by frictional treatment on the contact endurance of the AISI 321 stainless steel under contact gigacycle fatigue tests

Previous research has shown that the contact endurance (resistance to the fatigue spalling) of the corrosion resistant AISI 321 steel is insufficient under cyclic impact loading. The application of surface hardening is effective for increasing the contact endurance of metal materials if the gradient hardened layer is formed. The aim of this work is to study the possibility of increasing the contact endurance of the AISI 321 steel by surface hardening. A hardened layer has been formed on the surface of the steel by means of the frictional treatment with a sliding indenter at the normal load of 294 N. Mechanical testing for gigacycle contact fatigue of the surfacehardened AISI 321 steel has been conducted in accordance with the pulsing impact "plane-to-plane" contact scheme with an ultrasonic loading frequency. Optical and scanning electron microscopy, energy-dispersive microanalysis, optical profilometry, X-ray diffraction analysis, microhardness measurement and instrumented microindentation are used as methods of investigation. It has been discovered that the surface frictional treatment forms a gradient hardened layer up to 500 mu m deep, which has a fairly homogeneous dispersed structure, an excellent quality surface with a quite low roughness value (Ra = 0.23 mu m), high microhardness (780 HV0.025), robust residual stresses (sigma(alpha)=-540 MPa), and higher resistance to elastic-plastic deformation according to the microindentation data. The hardened layer considerably increases the contact endurance of the steel under conditions of impact contact-fatigue loading. This manifests itself in a considerable decrease (from 20 to 1-2 mu m) in the depth of contact damages on the steel surface and the absence of damage localization, that is, a more uniform failure. However, localization of the contact damages can be facilitated by defects in the hardened layer (particularly pores), therefore, their number must be minimal.