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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Fig. 6. (a) Grid prepared for rresidual stress measurements conducted over failed blade on suction side at leading edge, (b) Residual stress contours over the failed blade, and (c) Residual stress contours on the blade after laser hardening without any service history. 4. Discussion 4.1. Causes for the failure of turbine blades in their profile section Failure of thermal power plant turbine blades in their profile sections is caused by several factors. Kim (1998) suggested that the presence of defects or discontinuities over the blade profile during dynamic stress concentrations arising as a result of natural frequency excitation modes at several locations are to be examined and removed, else they would cause failures at the locations of these excitation modes. Similarly, a study by Kubaik et. al. (2007) inferred that if a repair operation of the shrouds in the blades is attempted without proper assessment of the natural frequency, the blades are at a risk of resonance leading to their failure. Sometimes, presence of discontinuity of the blades with the lacing wire either due to loosening or failure in older generation thermal power plant sets, could lead to fretting between blade material and lacing wire. This causes crack initiation and aggravates crack growth due to resonance, thereby, causing excessive vibrations leading to blade failure which is in agreement with Mukhopadhyay et. al. (2001). Another possibility for the occurrence of failure in blade profile as per Das et. al. (2003) is when pits/ grooves exist on the edges of blades and the water chemistry contains Cl- ions which initiate a crack due to crevice corrosion. Fatigue mechanism further propagates this crack leading to failure. In case of high frequency induction hardening of the blades, due to non-optimization of hardening parameters, the localized hardened material increases both hardness and brittleness, thus, leading to increase in the blade cracking susceptibility during service exposure according to Wang et. al. (2007). This can be controlled by provide post-hardening tempering heat treatment to reduce the brittleness referring to the same study. In summary, there are three main factors which cause failures: (i) Operational causes, (ii) Environment, and (iii) Processing parameters. 4.2. Influence of laser hardening on residual stress formation In case of laser hardening, it can be seen that there are tensile residual stresses that occur at the interface, i.e., ~29 mm and ~33 mm from the leading edge (Fig. 6(c)). These tensile residual stresses seem to occur exactly at the location where crack origin was present over the fractured surface as seen from Fig. 4(c). Therefore, it can be concluded that the combined with the operational stresses and the tensile residual stresses caused the premature failure of the blade material in the profile region. This cause for this failure is related to processing parameters.