13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Rotating Bending Fatigue Tests of PH-42 steel Plasma Nitrided José Divo Bressan1,*, Daniel Kohls2 1 Department of Mechanical Engineering, Center for Technological Sciences, University of Santa Catarina State, Campus Universitário, 89.223-100 Joinville, Santa Catarina, Brazil. 2 Faculty of EEFF, Institute of GGHH, City Post Code, Country * Corresponding author: dem2jdb@joinville.udesc.br Abstract: The aim of this study was to evaluate the influence of plasma nitriding process on the fatigue limit of PH-42 Supra steel provided by Schmolz + Bickenbach of Brazil. The paper presents the experimental results of rotating bending fatigue tests for specimens as received and treated by plasma nitriding process with a nitrided layer of 0.3mm. This material is employed in manufacturing polymer injection molds and inserts, and offers high hardness and good machinability. The fatigue tests were carried out by a rotating bending fatigue machine which allowed to plot and compare the Wöhler’s curve for both types of steel specimens. Moreover, the hardness of the nitrided layer and substrate, the hardness of the non-nitrided specimens, the ultimate strength from simple tensile tests were determined experimentally. The fatigue testing results were used to obtain the material empirical Basquin’s fatigue life equations. Photographs by SEM of the fractured surface were obtained and provided a visual aspect analysis of nitrided and non-nitrided specimens in tensile and fatigue testing. The fatigue limit stress increased from 440 MPa to 710 MPa for non nitrided and plasma nitrided PH-42 steel respectively. In addition, the fatigue limit behavior and fracture mechanisms, depending on the application of advanced plasma nitrided layer, are also discussed. Keywords: PH-42 steel, rotating bending test, plasma nitriding, fatigue limit. 1. Introduction Rupture in a structural or mechanical component frequently initiate at metal surface under two loading conditions, owing to metal fatigue process. Firstly, it is related to high loading cycles of component, leading to resultant stresses greater than the material fatigue limit. Under this type of high loading, rupture occur at shorter number of cycles due to the inception of surface micro-cracks which are originated from shear bands in preferential sliding atomic planes and directions in the material. Secondly, there are situations of fracture occurring in components due to surface crack nucleation which are originated from surface defects such as grooves, scratches, pores, holes or metallic inclusions, even for low stresses. However, fatigue cracks can also be originated from non-metallic inclusions and defects situated in sub-layers just below the component surface [1]. This kind of rupture has been referred in literature to explain the behavior of high strength steels. In surface treated steels, fatigue limit is expected to increase proportionally to hardness increase. However, it is worth to recall that for steels with hardness above 400 Vickers there is a reduction of fatigue limit which is related to non-metallic inclusions. This can be observed in Fig. 1, showing the fatigue limit of various steel grades versus hardness [2]: fatigue limit increases up to a maximum and then falls with increasing hardness. Moreover, there are other issues related to fatigue resistance of steels such as the presence of profile of residual stresses near the surface site of crack nucleation due to machining and shot peening processes [3].
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