Abstract
Low pressure steam turbine (LPST) blades are made of martensitic stainless steel and Ti6Al4V alloy for the different ratings of steam turbines due to their high strength and toughness. These blade roots have a fir tree profile and experience severe stress concentrations all along their notched sections during turbine operation. The fatigue life of these blades can be increased by introducing compressive stresses either by shot peening or by laser peening. The present work deals with laser peening of these two materials to understand its effect on their fatigue properties, surface roughness and hardness. It is observed that laser peening has significantly enhanced fatigue life of Ti6Al4V alloy at 550 MPa stress as compared to the shot peened sample. The penetration depth of residual stress due to laser peening in the Ti6Al4V alloy was twice that due to shot peening. However, the fatigue life of steel was found to be similar for both the shot peened as well as laser peened samples. Similar response was observed from testing at lower stresses (400 MPa). Since the depth of penetration of compressive residual stresses for both the laser peened as well as the shot peened samples were similar for steels, it can be concluded that the fatigue life is a strong function of the penetration depth.
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