Enhancement of fatigue strength of SAE 9245 steel by shot peening S. Tekeli * Materials Division, Metallurgy Education Department, Faculty of Technical Education, Gazi University, 06500, Besevler-Ankara, Turkey Received 20 September 2001; received in revised form 26 March 2002; accepted 28 March 2002 Abstract In this study, the effect of shot peening on fatique strength of SAE 9245 steel, which is used in industry for spring production, was investigated. Shot peening process was carried out to create residual compressive stresses caused by deformation hardening at the surface and to remove decarburized layer of the surface resulted in heat treatment. The fatique specimens were heated up to 850 jC and held at that temperature for 20 min to transform microstructure completely to austenite and hardened by water quenching. The specimens were then tempered at 500 jC to eliminate residual stresses caused by the quenching. A group of fatique specimens, only as heat-treated and other specimens shot peened by using an air high-pressure type of equipment with CS 230 shot, was tested on Wo ¨hler fatique test machine. The fatique results for unpeened and peened conditions were compared. It was seen that shot peening improved the fatique life by about 30%. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Shot peening; Fatique strength; Residual compressive stresses 1. Introduction Shot peening can be defined as the process of work hardening the surface of components by means of propelled stream of spherical shot [1–3]. This process has now been used for many materials, including high-strength steels, aluminum alloys, titanium alloys and other engineering alloys [3]. It has been recently demonstrated that [4] shot peening is also an effective way of improving fatigue strength in P/M parts. Fatigue strength of P/M parts is usually improved by hot forging. Smith [5] showed that a mean com- pressive stress has a marked beneficial effect on the tensile fatigue life of notched specimens. An exter- nally applied mean compressive stress rarely exits in practice. However, the use of surface treatments resulting in residual compressive surface stresses ensures that a part placed in service under alternating tensile stressing would in fact be working with com- pressive stress. This condition is achieved by a num- ber of treatments, of which shot peening is one [5]. A residual compressive stress on the surface has the effect of increasing the intrinsic fatigue strength of the surface and therefore would be beneficial in reducing the probability of fatigue damage. By shot peening, the surface layer of a material is caused to yield plastically under the impact of shot. Residual compressive stresses result from the inability of the plastically 0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0167-577X(02)00838-8 * Tel.: +90-312-4399760; fax: +90-312-2120059. E-mail address: stekeli@tef.gazi.edu.tr (S. Tekeli). www.elsevier.com/locate/matlet December 2002 Materials Letters 57 (2002) 604 – 608