PSI - Issue 60

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

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increase in hardness beyond the average value of ~375 HV 0.5 . This can be attributed to the presence of fine laths at the laser hardened region that provide increase in hardness, which in turn improves the water droplet erosion resistance of the blade.

Fig. 3. Hardness profile of blade from suction side to pressure side

3.3. Blade performance testing and fractographic examination The laser hardening provided the blade with improved surface hardness at the leading edge towards suction side which also improved the water droplet erosion resistance. Subsequent to this development of hardening process, a complete performance test of the blade is imminent prior to its commissioning for service operations. Therefore, a few blades with laser hardened profile sections were tested in a test rig under service loads. Unfortunately, a few blades fractured in the profile region (Fig. 4(a)), whereas, a few blades developed cracks perpendicular to the axis of the blade (Fig. 4(b)). Upon stereomicroscopic examination of the fractured surfaces of these blades, the crack origin was observed at ~ 32 mm from the leading edge of the blade as seen in Fig. 4(c), which is at the interface of the laser hardened and the unaffected blade material. Fractographic examination using FESEM revealed the crack origin to be transgranular in nature (Fig. 5(a, b)) with the presence of a number of beach marks indicating crack propagation by fatigue. A higher magnification observation of the crack propagation region (Fig. 5(c, d)) reveals the presence of fine striations (~2 μ m) between beach marks regions suggesting that the mechanism of crack growth is high cycle fatigue. Beach marks appear due to major load variations during operation. Since, the blade origin was observed at the interface between laser hardened and unaffected blade material, a detailed evaluation of residual stress was conducted over the failed blade. The blade fractured at a location of ~ 240 mm from the blade tip. From this location, a grid of 5 mm x 5mm was marked upto a distance of ~35mm from the leading edge on to the suction side as presented in Fig. 6 (a). Residual stresses were measured at the intersection points of the grids. A contour plot of residual stresses is displayed in Fig. 6(b). It can be seen that closer to the fractured surface, tensile residual stresses of ~80 MPa were observed. Since, the blade considered was a fractured one and the tensile residual stresses seem to aid the fracture of the blade, residual stress development prior to service on laser hardened blades was investigated by measuring residual stresses using a 2.5 mm x 5 mm grid upto a 3.4. Residual stress mapping