The effect of shot peening on notched low cycle fatigue

Abstract

The improvement in low cycle fatigue life created by shot peening ferritic heat resistant steel was investigated in components of varying geometries based on those found in conventional power station steam turbine blades. It was found that the shape of the component did not affect the efficacy of the shot peening process, which was found to be beneficial even under the high stress amplitude three point bend loads applied. Furthermore, by varying the shot peening process parameters and considering fatigue life it has been shown that the three surface effects of shot peening; roughening, strain hardening and the generation of a compressive residual stress field must be included in remnant life models as physically separate entities. The compressive residual stress field during plane bending low cycle fatigue has been experimentally determined using X-ray diffraction at varying life fractions and found to be retained in a direction parallel to that of loading and to only relax to 80% of its original magnitude in a direction orthogonal to loading. This result, which contributes to the retention of fatigue life improvement in low cycle fatigue conditions, has been discussed in light of the specific stress distribution applied to the components. The ultimate aim of the research is to apply these results in a life assessment methodology which can be used to justify a reduction in the length of scheduled plant overhauls. This will result in significant cost savings for the generating utility. KeywordsLow cycle fatigue, stress concentration, shot peening, residual stresses, ferritic heat resisting

3-D analysis of fatigue crack behaviour in a shot peened steam turbine blade material

“…A full description of the fatigue sample and experiment details have been provided in previous publications [31,[34][35][36] and the relevant details are reported here only briefly. The material in this study is a steam turbine blade material, FV448, a ferritic heat resistant steel with a tempered martensitic microstructure.…”

“…The material in this study is a steam turbine blade material, FV448, a ferritic heat resistant steel with a tempered martensitic microstructure. The composition of the steel evaluated in the present work is Fe-0.12 C-0.94 Mn-0.31 Si-0.74 Ni-11.0 Cr-0.58 Mo-0.31 V-0.34 Nb (wt %) [31]. This barstock material had been austenitised at 1150 °C, oil quenched, and then tempered at 650 ℃ [37].…”

“…For the baseline condition, the surface was polished down to 1μm, while for the T0 and T1 shot peened conditions, the surface roughness Ra is 3.6 and 1.25 μm, respectively. All these samples were subjected to similar strain range conditions (Δε 11 = 0.68 %), during the fatigue tests and the fatigue process was interrupted and the specimens removed from the fatigue test apparatus at 70~80 % of the fatigue life based on the S/N curves determined in a previous study [31]. The 3D crack morphologies resulting from testing polished and T0 shot peened samples were studied by serial mechanical sectioning, while a laboratory based X-ray CT imaging approach was employed for both the T0 and T1 shot peened cases.…”

“…Extensive efforts have been made to study the influence of shot peening induced factors in the near-surface (surface roughness, compressive residual stress regions as well as the plastic strain layer) in both high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes in different material systems (such as steel [31,32], Al alloys [3,33] etc.). However, most of these studies which have focused on the effect of shot peening on fatigue crack initiation and propagation behaviour or the effect of stress relaxation, were generally confined to surface observations which provide no information about the three-dimensional (3D) shape of these cracks.…”

“…The present study is part of a larger project detailed in [31,[34][35][36], focussed on understanding the effects of shot peening on fatigue crack initiation and propagation behaviour in steam turbine blade materials in the LCF regime. Both mechanical serial sectioning and X-ray tomography (using a lab source) were used to study in detail the 3D fatigue crack morphology under these shot peened conditions, and this paper focuses on these investigations.…”