13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- The material and the microstructure obtained by the thermo-mechanical forming process are described in the first section. Then the experimental set-up is described which has been developed for obtaining microstructure-dependent crack growth rates. Finally the experimental results are compared with findings of a FE simulation. 2. Material and graded microstructure The tested material is a low-alloy steel in tempered condition (German designation 51CrV4) with the chemical composition given in Table 1. It has a ferritic-perlitic microstructure and a yield strength of 520 MPa. Table 1. Chemical composition of 51CrV4 Element C Si Mn Cr V P S Weight % 0,47-0,55 ≤0,40 0,70-1,10 0,90-1,20 0,10-0,25 ≤0,035 ≤0,035 An integrated thermo-mechanical forming process, as shown in Fig. 1, was applied to a shaft resulting in a flange shaft with a graded microstructure [3]. First the shaft was heated up above austenitization temperature by induction heating and then put into the molding press to form the flange, which lead to a local martensitic transformation of the original ferritic-perlitic phase. Figure 1: Integrated thermo-mechanical forming process of the flange shaft [6] Due to the high natural strain during the forming process a fine grained microstructure is formed, which contains dispersed small defects like inclusions and carbides. The resulting phase distribution is given in the etched micrograph in Fig. 2. It is characterized by the ferritic-perlitic base cold-worked material (grey) in a in the core of the flange shaft and a martensitic phase (dark) at the outside of the flange. The phases are linked by a transition zone (white) of varying width and material properties. For an unambiguously phase identification micro-hardness measurements are used. As shown in Fig. 3, the martensite possesses hardness values above 650 HV (Pos. 1), the transition zone about 450 HV (Pos. 2) and the deformed base material has hardness values below 300 HV (Pos. 1).
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