Studies towards development of laser peening technology for martensitic stainless steel and titanium alloys for steam turbine applications

Fatigue is a major life limiting factor affecting service life of low pressure steam turbine blades which, depending on turbine rating, are generally made of martensitic stainless steel or Ti6Al4V alloy. The results of the present study has demonstrated that with respect to conventional shot peening, laser peening brought more than two times longer mean fatigue life of Ti6Al4V alloy. Enhanced fatigue performance of laser peened specimens was attributed to the deeper peened layer and smoother finish. In the case of DIN X20Cr13 stainless steel, laser peening did not register significant improvement in fatigue resistance over shot peening. At maximum test stress of 550 MPa, DIN X20Cr13 stainless steel specimens, shot peened and laser peened specimens exhibited comparable fatigue lives. However, at lower magnitude of maximum test stress of 400 MPa, laser peened specimens displayed about 30% longer fatigue lives than their shot peened counterparts. Similar fatigue lives of shot peened and laser peened specimens of DIN X20Cr13 stainless steel specimens is attributed to comparable magnitude of surface residual stress and case depth produced by the two peening treatments.