Issue34

S. Henschel et alii, Frattura ed Integrità Strutturale, 34 (2015) 326-333; DOI: 10.3221/IGF-ESIS.34.35 327 the relative position of different voids, void sheets can be formed [6]. This process is characterized by strain concentration in a band between voids [7]. Hence, the macroscopic strain is relatively small. In addition to the inclusion characteristics size and volume fraction, the size distribution and, therefore, the distance distribution is an important microstructural parameter. According to [8], a uniform distribution of the formed voids leads to a minimum of strain to fracture. This result from numerical calculations is attributed to the spacing between the voids, which are relatively small in a uniform distribution. According to [8], the higher ductility of a non-uniform void distribution is explained by a suspended void coalescence. The void coalescence is suspended until sufficient void growth is achieved in the remaining material. On the other hand, a locally higher content of porosity leads to strain localization and, consequently, to fracture in this region [9]. In the transition range, an increase of loading rate results in a decrease of fracture toughness [10]. In the upper shelf regime, the opposite relation is found [10]. An increased loading rate promotes the void nucleation [11]. The void growth rate is reduced with increasing loading rate. This correlates with the increased yield strength and decreased work hardening rate at higher loading rates [11]. According to [12], strain localization is promoted in a material with inhomogeneities. The consequence of such strain localization during a dynamic fracture test is adiabatic heating in the plastic zone in front of a crack [13]. The aim of the present paper is the characterization of the damaging effect of an inhomogeneous distribution of non- metallic inclusions. To this end, crack initiation and growth was studied under conditions of quasi-static as well as dynamic loading. Clustering of non-metallic inclusions was investigated by means of fractography. Furthermore, the effect of non-metallic inclusion clusters on the formation of the crack path is discussed. M ATERIALS AND METHODS Investigated steel n this study, the quenched and tempered cast steel G42CrMo4 (DIN EN 10293, 1.7231) was tested. Non-metallic inclusions were intentionally added to the metallic melt. This was achieved by a "contaminating filter" in the casting gate coated with loose alumina particles. After contamination, the melt flowed through the actual metal melt filter, which was utilized to clean the melt. This cleaning filter was characterized by its functionalized surface. Information about the coating of the contaminating filter and the cleaning filter is shown in Tab. 1. Details of the filter manufacturing process can be found in [14]. The chemical composition of the casting plates is given in Tab. 2. Cast Contaminating filter Cleaning filter A Al 2 O 3 (0–200 µm) — B Al 2 O 3 (0–200 µm) Al 2 O 3 -SiO 2 (Mullite) C Al 2 O 3 (0–200 µm) Al 2 O 3 Table 1: Coating of the contaminating and cleaning filter. Substrate: Al 2 O 3 -C. Cast C Cr Mo Mn Ni Si Al S P Fe A 0.41 0.90 0.24 0.78 0.21 0.53 0.09 0.007 0.011 bal. B 0.43 0.92 0.25 0.79 0.22 0.54 0.09 0.007 0.011 bal. C 0.42 0.98 0.25 0.80 0.22 0.53 0.09 0.009 0.018 bal. Table 2: Chemical composition (in wt.%) of the casting plates determined by glow discharge optical emission spectroscopy. The machined samples were austenitized at 840 °C in a vacuum, followed by quenching in a stream of He. Tempering was performed at 560 °C in a N 2 atmosphere. Fig. 1 shows the microstructure of the material that consisted of tempered martensite. I