Finite element modelling of residual stresses in shot‐peened steam turbine blades

Abstract

The attachment region in steam turbine blades is critical to structural integrity, and hence, surface treatment by shot peening or roller burnishing is commonly used to induce compressive residual stresses in the fir‐tree region of the blade. In particular, the last stage blades on the low pressure rotors present the highest risk in terms of catastrophic failure due to their size and operating conditions. These blades have a mass of some 24 kg and operate in a wet steam environment at approximately 60 °C while rotating at 3000 rpm; potential failure modes include high and low cycle fatigue, stress corrosion cracking and corrosion fatigue. This paper reports the development of, and results obtained from, an elastic‐perfectly plastic finite element model of the residual stresses in the attachment region of a last stage blade and the influence on these compressive stresses of fatigue cycling. The model uses thermal quenching to simulate the residual stress profile obtained in the fir‐tree serrations after shot peening. Results were validated via extensive synchrotron and laboratory X‐ray diffraction measurements of the residual stress in shot‐peened specimens.

Numerical modelling of the fatigue crack shape evolution in a shot-peened steam turbine material

“…Shot peening is an effective surface treatment method used to improve the fatigue resistance of metallic components, particularly in regions with stress concentration features, such as the fir tree interface in steam turbine systems [1][2][3]. During the shot peening process, the surface of the workpiece is bombarded by a stream of small spherical shots at high velocities acting as tiny peening hammers, producing inhomogeneous plastic deformation near the surface.…”

“…Many researchers have systematically studied the residual stress relaxation behaviour in shot-peened specimens during fatigue loading both experimentally and numerically [2,6,7,11,15,16,[21][22][23]. It was found that the residual stress distribution generally reached a stabilised state after a logarithmic relaxation process during fatigue loading.…”

“…The same material as He et al [18,33] has been selected and the reported experimental data have been used as validation of the present modelling work. The current work is also relevant due to the relatively few investigations focusing on steam turbine materials subjected to shot peening compared with other material systems [1][2][3].…”

Effect of shot peening on metastable austenitic stainless steels

“…Among a great variety of surface treatments, shot peening is one of the most widely used techniques to increase the resistance of metal parts to fatigue in a large range of industries, such as automotive, aerospace and petrochemical [7,8,9]. It consists of blasting high velocity small beads on the metal component to generate surface hardening and compressive residual stresses which oppose to the nucleation of cracks and also exert a closure effect on those cracks already nucleated, avoiding their propagation [10,11]. Shot peening usually produces detrimental surface effects too, such as increasing roughness, nevertheless it has been shown that the average effect of both beneficial phenomena is more important than those induced by superficial detrimental effects [12,13].…”

Changes in the Surface Layer of Rolled Bearing Steel

“…The roentgenographic method also evaluates the crystal lattice deformation in the individual phases of the material under consideration [7,10]. The Philips D500 equipment that was used also allowed an assessment of the percentage representation of each metal phase.…”