Issue34

P. Gallo et alii, Frattura ed Integrità Strutturale, 34 (2015) 180-189; DOI: 10.3221/IGF-ESIS.34.19 182 mode was observed to be transgranular. For such material, where transgranular crack initiation is faster in the specimen with a rough surface than in that with a smooth surface, the surface roughness has a strong influence on the creep-fatigue and pure fatigue life. The results were in agreement with a previous work by the same authors [12] considering only Cr- Mo-V steel. In that work they tested cylindrical specimens made of Cr-Mo-V steel at 550 °C. Two different modes of surface roughness were obtained by mechanically polishing specimens with emery paper 80 and 1200. They showed that a large fraction of the fatigue life is consumed in the crack initiation and that the effect of surface roughness on low-cycle fatigue life could be due to the reduction of the number of cycles for crack initiation. In the above background, the present work deals with high temperature fatigue tests of 40CrMoV13.9. At the best of authors’ knowledge, a complete set of data from un-notched and notched specimens at high temperature, as a complete characterization on the influence of the surface roughness, is not available in the literature for 40CrMoV13.9. With the aim to fill this lack, the present paper experimentally investigates the high temperature fatigue of notched components of the considered alloy at different temperatures up to 650°C. Subsequently, with the aim to define the influence of the rolls cooling channels roughness on the high temperature behaviour and the cracks initiation, uniaxial fatigue tests have been conducted on plate with central hole at the service temperature of 650°C, varying the surface finishes. A final synthesis of the present results together with previous data from multiaxial tests (at room temperature) on the same material [13] is carried out by means of the Strain Energy Density (SED) approach, as recently made for Cu-Be alloys [8] and a Titanium Grade 2 [9], at high temperature. E XPERIMENTAL DETAILS Material he material investigated in the present study is 40CrMoV13.9 steel, usually employed for hot-rolling of metals where the material is subjected to a combination of mechanical and thermal loading conditions. Preliminary static tensile tests on a standard specimen were carried out to evaluate the elastic and strength properties at 650°C: Young’s modulus E is equal to 135 GPa, σ Y is equal to 520 MPa and σ R to 610 MPa. The data-sheet reports the following mechanical properties at room temperature (25°C): elastic modulus E is equal to 206 GPa, tensile strength of about 1300 MPa and a yield strength of 1100 MPa with a percent elongation of 15%. The properties (at room temperature and 650°C) are also summarized in Tab. 1. The chemical composition of the material instead is given in Tab. 2. The material was first quenched at 920°C and subsequently tempered twice at 580°C and 590°C. A final stress relieving treatment at 570°C was carried out. The final microstructure was characterized by a high strength bainitic-martensitic structure and is shown in Fig. 1 at different magnification values. It is evident that the microstructure is homogenous along all the directions, also through the specimen thickness, due to the austenitizing and annealing processes. Temperature Young’s Modulus E (GPa) Ultimate tensile strength σ R (MPa) Yield stress σ Y (MPa) Elongation to fracture A (%) HRC Room Temp. 206 1355 1127 15.2 52 650°C 135 610 520 23.5 35 Table 1 : Mechanical properties of 40CrMoV13.9. Table 2 : Chemical composition wt.%, balance Fe. T C Mn Si S P Cr Ni Mo V Al W 0.38 0.5 0.27 0.006 0.003 3.05 0.24 1.04 0.24 0.013 0.005