Improvement of the blasting induced effects on medical 316 LVM stainless steel by short-term thermal treatments

Blasting of 316 LVM steel was performed by using alumina and zirconia particles that yield rough (up to Ra ~ 8 μm) and nearly smooth (up to Ra ~ 1 μm) surfaces, respectively. Besides roughening, the severe plastic deformation imposed during blasting yields subtle sub-surface microstructural changes such as grain size refinement, α′-martensite formation, and work hardening. In this work we investigate the thermal treatment at 700 °C of the blasted steel, aimed to reverse the strain induced α′-martensite to austenite, with special emphasis in the correlation of the microstructural changes with the fatigue resistance. Blasting with rounded zirconia particles increases the fatigue resistance whereas the opposite effect is observed using alumina ones, which are more abrasive and cause a huge amount of embedded particles acting as stress raisers. It is shown that thermal treatment at 700 °C is very effective in the reversion of martensite, even after 2 min of annealing. Interestingly, the short thermal treatment of the alumina blasted steel increases the fatigue strength to values close to non-blasted material (400 MPa). Prolonged exposures, however, contribute to a full relaxation of the beneficial compressive residual stresses, being the net effect a decrease in the fatigue limit (340 MPa).