Issue 41

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 41 (2017) 191-196; DOI: 10.3221/IGF-ESIS.41.26 192 In this study, four different industrially relevant forging conditions have been studied for two different pearlitic or ferritic- pearlitic steels. The effect of the processing route on the materials microstructure and mechanical behavior has been first evaluated. The 3D propagation behavior of fatigue cracks growing from shallow artificial defect ( i.e. growing mainly within the forged skin of the materials) has been obtained through in situ X-ray synchrotron tomography. The crack growth curves have been obtained for all materials and the effect of processing on crack growth behavior assessed. E XPERIMENTAL Materials our different forging condition of industrial relevance were investigated for two different steels. Cold forged cylindrical specimens produced with two different cross sectional area reduction (18 and 75 %) have been obtained for a 27MnCr5 ferritic-pearlitic steel [1]. A C70 pearlitic steel was also used to produce connecting rods through a four step hot forging process. Samples were extracted from the connecting rods directly in the as-forged condition or after a shot-blasted treatment applied in order to remove the scale [2]. Tab. 1 details the processes and the mechanical properties of the four different materials/conditions studied (hereafter simply called different materials with a greek numbering as shown in the table). It can be seen from this table that the cold forging process produces a lower grain size than hot forging and that the grain size decreases with the level of reduction. When shot blasting is used (material IV) a grain size gradient is observed. EBSD observations reveal a 10 µm thick layer of extremely fine grains (< 1 µm); below this region a layer of heavily deformed grains is observed evolving gradually towards more equiaxed grains with an average size of 25 µm at a distance of 150-200 µm below the surface, where the unaffected bulk material begins. A gradient of mechanical properties is concomitantly observed with hardness values ranging from 350 HV at the surface to 280 HV at 400 µm below the surface. Finally, in the bulk material, a residual stresses gradient (measured by X-ray diffraction) with compressive stress values ranging from - 500 MPa at the surface to 0 MPa at a distance of 400 µm below the surface was measured . More detailed information can be found in [2] Synchrotron tomography experiments In order to study the influence of the surface conditions induced by the finishing processes on the propagation of shallow fatigue cracks, small samples of 0.8 x 0.8 mm cross-sectional area were extracted at the surface of the forged components. The small cross section of the samples is the result of the large X-ray attenuation of Iron and the small voxel size required to detect accurately the small cracks [3]. Fig. 1 (a) and (b) show a schematic view of the fatigue samples extracted from the cold forged round bars (materials I and II) and from hot forged connecting rods (materials III and IV) by means of electro discharge machining. Symbol Material Process σ 0.2 (MPa) Grain size (µm) I 27MnCrFP Cold Forged (18%) 730 15 II 27MnCrP Cold Forged (75%) 1072 8 III C70P Hot Forged 800 25 IV C70P Hot Forged + shot blasting 800 1 to 25 Table 1 : List of the materials and forging processes investigated with the corresponding mechanical properties and grain sizes. In material IV the shot blasting process produces a grain size gradient (see the text for details). Residual stress measurements performed on the tomography samples showed values ranging from 0 to 20 MPa in samples of materials I, II and III. In the particular case of material IV, after spark machining the samples exhibited a curvature radius of 170 mm due to the presence of residual stress gradient. However, this curvature disappeared in the unloaded state after a few hundred fatigue cycles. Thus it can be concluded that the samples tested were free of residual stresses. F