Issue34

P. Gallo et alii, Frattura ed Integrità Strutturale, 34 (2015) 180-189; DOI: 10.3221/IGF-ESIS.34.19 180 Focussed on Crack Paths High temperature fatigue tests and crack growth in 40CrMoV13.9 notched components P. Gallo, F. Berto Department of Management and Engineering, University of Padova, Stradella San Nicola 3, 36100, Vicenza, Italy pgallo@gest.unipd.it , berto@gest.unipd.it A BSTRACT . The present paper addresses experimentally the high temperature fatigue of 40CrMoV13.9 steel and the effect of surface roughness on fatigue strength and crack initiation. The 40CrMoV13.9 steel is widely used in different engineering high temperature applications among which hot- rolling of metals, where, in order to assure a constant temperature, the rolls are provided with cooling channels. These are the most stressed zone of the rolls where cracks systematically initiate. In order to completely characterize the high temperature behaviour of this steel, firstly uniaxial-tension load controlled fatigue tests have been conducted at different temperatures up to 650°C. Two geometries are considered: plain specimens and plates weakened by symmetric V-notches. Subsequently, with the aim to investigate the influence of the cooling channels roughness on the high temperature behaviour and the cracks initiation, uniaxial-tension load controlled fatigue tests have been conducted on plate with central hole at the service temperature of 650°C varying the surface roughness. After a brief review of the recent literature, the experimental procedure is described in detail and the new data from un-notched and notched specimens are summarized in terms of stress range, at the considered temperatures. Finally, fatigue data from un-notched and notched specimens are re-analysed by means of the mean value of the Strain Energy Density (SED) approach extended at high temperature. K EYWORDS . High temperature fatigue; Cracks growth; 40CrMoV13.9; Strain energy density; Surface roughness. I NTRODUCTION ot-rolling process is increasingly required for higher mechanical performances, fatigue strength and quality of laminated products. Different steels are usually employed in a large variety of applications to combine static and fatigue properties with an excellent wear resistance at high temperature and in corrosive environments. The 40CrMoV13.9 steel is widely used in different engineering high temperature applications among which hot-rolling of metals. Rolls for hot-rolling, despite different design needs, have some basic characteristics, such as the presence of cooling channels with the aim to assure a uniform and controlled temperature during the in-service operations. On the basis of numerous feedbacks, the cooling channels are the most stressed zone of the rolls and cracks initiate systematically from these areas. For these reasons, it becomes very interesting to investigate this kind of steel, with the aim to characterize its fatigue behaviour at high temperature, considering for example notch effects and/or the influence of H