PSI - Issue 60

A.H.V. Pavan et al. / Procedia Structural Integrity 60 (2024) 277–285 A.H.V.Pavan/ StructuralIntegrity Procedia 00 (2024) 000 – 000

284

8

Similar spike in residual stresses post laser hardening in 17-4 PH steel was reported by Wang et. al. (2011) at the interface of laser hardened and unaffected material. 4.3. Influence of shot peening on residual stress post laser hardening treatment In order to mitigate the tensile residual stresses formed at the interface of the laser hardened and the unaffected base material, a novel approach of shot peening the blade profile was followed subsequent to laser hardening treatment. The whole blade profile region was shot peened with >100% coverage. Fig. 7 presents the residual stress contours obtained post shot peening treatment. It can be observed that the whole surface is under compressive residual stresses. Wang et. al. (2011) reported that shot peening: (i) reduced surface residual stresses from tensile to compressive state, and (ii) improved fatigue life, in 17-4 PH steel. Subsequent to this treatment, the laser hardened and shot peened blades were tested in a test rig under service loads. It was observed that none of the blades failed nor they developed any cracks at the interface of the laser hardened and unaffected blade material region. Thus, the process has been developed and these blades have been put in service. During service exposure of the processed blades, failures did not occur further.

Fig. 7. Residual stress maps on the blade after shot peening the blade profile subsequent to laser hardening

5. Conclusion Laser hardening treatment was conducted on X5CrNiCuNb16-4 steel for improving the surface hardness of the turbine blade material at the leading edge for enhancing water droplet erosion resistance. The hardness improved substantially upto ~2.75 mm from the surface of the blade material treated. However, this led to formation of tensile residual stresses at the interface of laser hardened and unaffected blade material which are detrimental during service operation of the blade. With a view to mitigate this deleterious effect of laser hardening, shot peening over the entire blade profile was conducted, post which only compressive residual stresses were present. This treatment further homogenized residual stress distribution across the profile of the blade to a compressive state, thus, preventing premature cracks in the profiles of the blades.