Reducing conservatism in life assessment approaches: industrial steam turbine blade to disc interfaces and the shot peening process

Abstract

Damage tolerant life assessment and the resulting component repair and replacement scheduling is crucial to the continued operation of UK power plant. Typically a major outage on a conventional power station takes 8 -12 weeks; there is a drive to minimise the scope of remedial works undertaken allowing the length of the outage to be reduced. Recent trends in life assessment methods are towards approaches that can be used to justify deferring invasive inspections and reducing the scope of the maintenance works. This thesis reports the development of a life assessment protocol for industrial steam turbine blade to disc interfaces which explicitly accounts for the residual stresses and strain hardening resulting from shot peening with a view to improving life cycle efficiency and reducing operator cost. A modelling approach has been developed which allows for the effect of shot peening to be explicitly accounted for in the damage tolerant life assessment process. The approach requires the input of experimentally determined residual stress, surface roughness and strain hardening profiles. Whilst the methods for determining residual stresses are well standardised, the optimum means of determining both the surface roughness characteristics (two and three dimensional) and the strain hardening profile (microhardness, X-ray diffraction and electron backscatter diffraction) were investigated as a first step towards standardising these measurements: It was found that a three dimensional analysis of the surface topography is most likely to capture the worst case defect. X-ray diffraction and electron backscatter diffraction measurements of cumulative plastic strain were shown to be most similar, with microhardness measurements appearing to overestimate the strain hardened region. Furthermore, the retained benefit of the shot peening process in terms of total life even under very high strain range LCF conditions has been clarified in terms of the applied stress distribution, specific material cyclic characteristics and residual stress relaxation. Under high strain range three point bend fatigue loading, the residual stresses and strain hardening in notched samples of the tempered martensitic steel under investigation did not change significantly. This allows the shot peening process to be specified for notched components under high strain range cyclic bend load, such as low pressure turbine disc blade interfaces, with an improved understanding of the resulting benefits