Shot-peening of steam turbine blades: Residual stresses and their modification by fatigue cycling

Abstract

Power generation in thermal stations typically relies on large steam turbines. The corrosion resistant steel blades used in the last stage of a typical low pressure rotor set are approximately 1m long and experience high centrifugal loading during service. They operate in a wet steam environment, at approximately 120 °C while rotating at 3000 rpm, and failure modes are typically either stress corrosion cracking or corrosion fatigue. The blades are retained by a fir tree root which is typically shot-peened to generate compressive residual stresses that resist crack initiation. Finite element (FE) modelling has indicated that, in the absence of shot-peening, stresses above yield are induced at the fir tree root during operation. To date, no systematic information has been obtained on the level of residual stresses induced in the fir tree by shot-peening and their relaxation, nor are there any guidelines as to the magnitude of residual stresses necessary to ensure integrity of the turbine over a life span of at least twenty years. At least one of these blades has failed in recent years causing a catastrophic failure and severe damage to the turbinegenerator set.This paper will report results from a comprehensive program of residual stress measurements at the shot-peened fir tree roots of service blades, and in specimens that simulate the root, using diffraction data from laboratory and synchrotron X-ray radiation (SXRD). Shot-peening coverage between 75% and 200% was used and stresses were measured up to 5 mm into the blades/specimens. Measurements were made in the as-peened condition and after applying cyclic stresses representative of overspeed proof testing and service operation. The results will be used to calibrate FE modelling of residual stresses and as input into fatigue life prediction.