Evaluating surface deformation and near surface strain hardening resulting from shot peening a tempered martensitic steel and application to low cycle fatigue K.A. Soady a,b,⇑ , B.G. Mellor a , G.D. West c , G. Harrison d , A. Morris b , P.A.S. Reed a a Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Highfield, Southampton SO17 1BJ, UK b E.ON New Build & Technology Ltd., Ratcliffe on Soar, Nottingham NG11 0EE, UK c Department of Materials, Loughborough University, Leicestershire LE11 3TU, UK d Materials Science Centre, University of Manchester, Oxford Road, Manchester M13 9PL, UK article info Article history: Received 18 December 2012 Received in revised form 20 March 2013 Accepted 22 March 2013 Available online 6 April 2013 Keywords: Plastic deformation Shot peening Low cycle fatigue X-ray diffraction Electron backscatter diffraction abstract The plastic deformation resulting from shot peening treatments applied to the ferritic heat resistant steel FV448 has been investigated. Two important effects have been quantified: surface roughness and strain hardening. 2D and 3D tactile and optical techniques for determining surface roughness amplitude param- eters have been investigated; it was found that whilst R a and S a were consistent, S z was generally higher than R z due to the increased probability of finding the worst case surface feature. Three different methods for evaluating the plastic strain profile have been evaluated with a view to establishing the variation in yield strength near the surface of a shot peened component. Microhardness, X-ray diffraction (XRD) line broadening and electron backscatter diffraction (EBSD) local misorientation techniques were applied to both uniaxially deformed calibration samples of known plastic strain and samples shot peened at inten- sities varying from 4A to 18A to establish the variation in plastic strain and hence the variation in yield strength. The results from the three methods were compared; XRD and EBSD profiles were found to be the most similar with microhardness profiles extending much deeper into the sample. Changes in the measured plastic strain profile after exposure to low cycle fatigue and the correlation of these changes with the cyclic stress–strain behaviour of the material are also discussed with a view to assessing the importance of the dislocation profile in component life assessment procedures. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Shot peening is a cold work process typically applied to compo- nents to improve fatigue resistance in critical areas. Stress concen- trating regions are bombarded with high velocity shot of a hard material. This results in plastic deformation at the surface of the component, characterised by a dimpled topography and near sur- face strain hardening, thus increasing the local yield strength of the material. Compressive residual stresses are also formed as a re- sult of the misfit strain between the plastically deformed surface and the elastically deformed sub-surface layers. Whilst this process has been applied for many years, current interest is focussed on including the effects of the shot peening process in component remnant life models rather than simply using the process to add extra conservatism to the model. One example application is to shot peened industrial low pres- sure steam turbine blade to disc interfaces. Non-destructive testing is typically carried out every 12 years and damage tolerant life assessment modelling is performed based on the worst case de- fects to underwrite service for the next period or to define a repair and replacement schedule. The development of life assessment methods with the potential to defer invasive inspections, improve the reliability of repair and replacement schedules and extend component life are of significant interest in the power generation industry where the loss during a typical outage of 8–12 weeks on a 500 MW unit is >£M. Inclusion of shot peening effects in remnant fatigue life model- ling requires detailed consideration of the inter-relationship be- tween surface roughness and strain hardening on initiation characteristics (surface roughness tends to accelerate initiation, whereas strain hardening tends to retard initiation) and residual and applied stresses (as influenced by near surface strain harden- ing) on crack propagation (compressive residual stresses tend to reduce damaging tensile applied mean stresses). In some cases, there may also be the additional complication of phase transforma- 0142-1123/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijfatigue.2013.03.019 ⇑ Corresponding author at: Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Highfield, Southampton SO17 1BJ, UK. Tel.: +44 (0) 2476 192010. E-mail address: K.A.Soady@soton.ac.uk (K.A. Soady). International Journal of Fatigue 54 (2013) 106–117 Contents lists available at SciVerse ScienceDirect International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue