Fatigue properties of a low-alloy steel with a nano-structured surface layer obtained by severe mechanical treatments S. M. Hassani-Gangaraj 1,a , A. Moridi 1,b and M. Guagliano 1,c 1 Politecnico di Milano, Dipartimento di Meccanica, Via La Masa, 1, 20156 Milano, Italy a seyyed.hassani@mail.polimi.it, b atieh.moridi@mail.polimi.it, c mario.guagliano@polimi.it Keywords: Severe deep rolling, Severe shot peening, Nano-structured surface, Fatigue, Residual Stress. Abstract. Recent development in mechanical technologies and processes have shown that by performing traditional mechanical treatments with unusual and severe parameters it is possible to obtain metal surfaces characterized by grain size with dimension in the order of 50-100 nm. This confers peculiar and superior properties to the surface layer of material. Since the surface is the usual point of fatigue crack initiation it is expected that the parts treated this way show a better fatigue behavior with respect to the coarse grain materials, even if treated with conventional mechanical treatments. This work explores any opportunities to obtain nano-structured surface layers by means of two popular mechanical treatments, shot peening and deep rolling. To this end particularly severe processing parameters are applied on a low alloy steel fatigue test specimens. The treated surface is characterized by means of optical Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) analysis of residual stress and roughness measurements. In the end a series of fatigue tests on smooth specimens severely treated, conventionally treated and not treated were executed. The results show the potential benefits of severe mechanical treatments and were interpreted in the light of peculiar effects of these novel treatments on the characteristics of the treated surfaces. Introduction Outstanding physical and mechanical properties of nano-structured materials have recently attracted tremendous attention of both scientific and technological parties [1]. One potential benefit of nano-structured materials could be fatigue behavior improvement. Since most often fatigue cracks initiate from the surface and propagate to the interior, a component with a nano-structured surface layer and coarse-grained interior is expected to have highly improved fatigue properties. This justifies the attention to the processes able to obtain nano-structured surface layers of materials, leaving the inner unchanged. Among these processes, the so-called severe plastic deformation methods are achieving an increasing success due to their relative simplicity and applicability for different class of materials. The basis of severe plastic deformation methods is to increase free energy of the poly-crystals and much more defects and interfaces (grain boundaries) in non-equilibria processes and eventually to fragment the grains up to 100 nm or less [2]. In this paper a low-alloy steel was treated with two popular mechanical treatments, shot peening and deep rolling, applied with particularly severe parameters. The treated surface was characterized by means of optical Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) analysis of residual stress and roughness measurements. In the end a series of fatigue tests on smooth specimens both severely treated, conventionally treated and not treated were executed. Material and Experiment The material object of this study was low-alloy steel ESKYLOS6959. Mechanical properties evaluated through tension test are the following: yield stress 878 MPa, UTS 1010 MPa and percent elongation at break 17.7 %. The geometry of rotating bending fatigue test specimens is presented in Fig.1. Key Engineering Materials Vols. 577-578 (2014) pp 469-472 © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/KEM.577-578.469 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 18.115.1.218-30/07/13,16:14:59)