Shot peening is a process where small balls are shot at (impacting on)a target, e.g. gear teeth, so that indents are formed on the targetsurface. On gears the purpose of the peening is to increase the fatiguestrength. The indents are formed because the surface is plasticallydeformed which in turn cause three main property changes; residualcompressive stresses are created close below the surface,microstructural changes to the steel material and surface topographychanges.The shot peening process in itself is controlled with parameters suchas: Intensity, Coverage, Media hardness and shape. Simulating shotpeening requires information about the impact velocity. A method todetermine the velocity during the shot peening is presented. A materialmodel that includes plastic deformation and retained austenitetransformation is developed and can predict the experimental resultswith adequate accuracy for the basic model. Parameter variations showthat increased accuracy can be obtained by including a distribution inmedia shape. This would, however, increase the complexity andsimulation time considerably. Using spherical media was judged to bea good compromise. Residual compressive stresses and work hardeningare predicted well. Surface roughness predictions on ground partswould need to include the ground surface on the simulation target. 

Shot peening eller Kulpening/kulbombning på svenska, är en process där små kulor skjuts mot ett objekt så att intryckningar bildas på ytan. På kugghjul används kulpening för att höja utmattningsstyrkan. Intryckningarna bildas för att ytan deformeras plastiskt vilket i sin tur orsakar tre huvudsakliga förändringar av egenskaperna; tryckrestspänningar i ett område nära ytan, mikrostrukturförändringar hos stålet och topografiska ändringar på ytan.

Kulpening kontrolleras med intensitet, täckning, kulornas hårdhet och form. Vid simulering av kulpening måste kulornas islagshastighet vara känd och en metod för att bestämma denna från den praktiska kulpeningen har utvecklats. En materialmodell som tar hänsyn till både plasticitet och restaustenitomvandling har utvecklats och kan prediktera de experimentella resultaten med tillräcklig noggrannhet för den enklaste modellen. Parametervariationer visar att ökad prediktionsförmåga kan fås om en fördelning av kulornas form används. Detta skulle dock öka komplexiteten avsevärt och innebära längre simuleringstider. Sfäriska kulor rekommenderas som en bra kompromiss för generella slutsatser. Restspänningar och hårdhetsförändringar predikteras bra. För ytfinhetsutvärderingar på slipade komponenter, måste en slipad struktur användas på den simulerade ytan.

A material model for predicting fatigue related changes due to shot peening is developed. The model includes plastic behaviour at large strains and transformation of retained austenite. The model is simplified to limit the amount of material parameters needed and an iterative approach is used to fit the parameters to experimental data in the form of uniaxial tests on through hardened specimens, single impacts of carbide balls on polished case hardened test plates and on shot peened, ground test plates. With good accuracy, reasonable computer resources and basic experimental techniques to fit the model parameters, the model can predict; residual stresses, retained austenite transformation and work hardening in form of Vickers hardness increase. The model is also adequate for parameter studies as exemplified by changing media hardness, coverage or media shape variations. For surface roughness predictions, the initial surface roughness of the target must be included and future work is needed there.

Shot peening is a manufacturing process commonly used to increase fatigue life in components for the automotive and aircraft industry. In this paper the effect of shot peening is described for a case hardened gear steel. For gears there are three main factors from shot peening that influence fatigue life: residual stresses, microstructure and surface roughness. The paper describes an experimental series where these parameters were measured for common industrial shot peening settings. The aim was to show how several different measurement techniques and results correspond to each other for certain shot peening parameters. The aim was also to gather experimental results that can be used for verification of shot peening simulations. To simplify measurements and decrease variation, flat steel plates were used as targets. Residual stress, full width at half maximum (FWHM), retained austenite, surface roughness/topology, hardness and Barkhausen noise were measured and related to microstructural changes. The mean indentation diameter was measured for individual shots at low coverage of each intensity which was used to determine the average velocity of the media. The mean diameter and hardness of the shot peening media was also determined.

Shot peening is a manufacturing method that makes indentations on a components surface by impacting it with small steel balls (media). In a production environment the shot peening process is usually controlled by defining media size, coverage and Almen intensity. For simulating shot peening, the media velocity is needed and therefore it must be measured or correlated to the Almen intensity. This paper details a method to record indentations on a test plate and then compare them to single shot indentations with measured velocities. For small media sizes the results are in reasonable agreement with other published results but at larger sizes there is a larger spread in published results. It is therefore recommended that the media velocity is either measured directly or that the indents are analysed as presented in this paper.