FATIGUE FAILURE ANALYSIS OF LOW-PRESSURE STEAMTURBINE BLADE

Abstract

Steam turbine blades are the most vital and critical components in the power plants for electricity generation as they convert heat energy into mechanical work. A single failure of a blade can cause the complete shutdown of a plant. Low pressure (LP) blades are exposed to critical working conditions of large centrifugal forces, and dynamic forces due to changing steam loads along with corrosive effects that contribute to the susceptibility of LP blades to fatigue failure. Thus, in particular, L-0 stage blades are most susceptible to fatigue and fracture type of failures. Therefore, the reliability and performance of LP blades depend on preventing or understanding such failures. This paper provides a comprehensive review of fatigue failure behavior of low-pressure steam turbine blades using both an experimental investigation and an integrated computational and residual stress analysis. A failure analysis of a martensitic stainless-steel alloy X10CrNiMoV 12-2-2, L-0 stage low-pressure steam turbine blade was conducted in this research. A transverse crack existed at the leading edge of the blade that propagated toward the trailing edge of the blade. Detailed examinations of the morphology and origin of the crack were performed using a combination of mechanical testing, fractographic examination utilizing scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), and visual inspections. The microstructural examination showed a disturbed martensitic structure which is typical of a heat-treated turbine-grade steel and SEM fractography showed typical fatigue striations and intergranular cracking. The EDS results confirm the presence of corrosion promoting products such as chlorine, silicon and oxygen and Sio2 particles on the fracture surface of the blade which results in corrosion fatigue a predominant failure mode. A residual stress analysis was conducted to gain a further insight into the internal stress conditions that contribute to the initiation and propagation of cracks, using a µ-X360 FULL 2D portable X ray residual stress analyzer that relies on the cos v α method of analysis. The findings showed that tensile residual stresses on the blade surface exist, which are the reason for stress concentrators and enhance fatigue crack development during cyclic loading. The residual stresses that were generated during the manufacturing and service in operation were discovered to have a major impact on the fatigue performance and life of the blade. The stress distribution at steady operational loading was assessed by means of the static structural analysis to determine the potential regions that are critical and could fail during operation. Dynamic analysis was also used to calculate the natural frequencies and critical speeds of the blades by Campbell diagrams, to avoid resonance at start-up and shut-down periods. The simulations of fatigue crack propagation were possible with the help of a hybrid computational method combining ANSYS and FRANC 3D. The rubber box method in FRANC 3D was used to create a sub model with a template radius of 0.5 mm and an initial edge crack of 2 mm and then the evolution of stress intensity factors KI was analyzed through a series of load cycles. The driving force that controlled the crack propagation was assessed using the stress intensity factor KI and the simulated fatigue life was about 38,414 cycles. When the loading continued, KI reached a maximum of approximately 3640 MPa√mm at the 86th step, which was greater than the fracture toughness of the material and signified the beginning of an unstable crack propagation. The findings emphasize the importance of advanced simulation tools in predicting the fatigue behaviour of LPST blade in order to minimize the catastrophic consequences associated with failure. In addition to providing an integrated understanding of fatigue failure in low-pressure steam turbine blades via a correlation of metallurgical studies, residual stress distributions, and computational fracture mechanics; the study also shows that both tensile residual stress and vibrational resonance contribute to the failure of these blades; as well as corrosion fatigue.