13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 2.2. Material The investigated material was the Nb-stabilized metastable austenitic stainless steel AISI 347 (1.4550, X6CrNiNb1810). The chemical composition is given in Table 1. The calculation of the austenite stability parameters (e.g. Md30 = 26 °C by Angel) results in a metastable state of the test material at ambient temperature. This calculation gives an approximate value, which means, that at a sufficient plastic deformation at ambient temperature an austenite-martensite-transformation can be expected. The initial microstructure (Figure 3) of the investigated steel has a mean grain size of 120 µm and a HV10 hardness of 140 HV. Also Nb-carbides were observed. The chemical composition of these carbides was analyzed via EDX-mapping. As expected the tensile strength, ultimate strain and reduction in area decrease with increasing temperature. At ambient temperature, at a plastic deformation of 66 %, 4.41 FE-% ´-martensite was formed. The ´-martensite formation starts at a total strain of 22 %. Magnetic measurements and micrographs after tensile tests at 300 °C indicated no ´-martensite formation. Table 1. Chemical composition in weight-% C Cr Ni Nb N Si Mn P S Al 0.040 17.600 10.640 0.620 0.007 0.410 1.830 0.020 0.007 0.016 Ti Sn Mo W Cu Co V Pb B Fe 0.020 0.008 0.290 0.030 0.060 0.010 0.070 < 0.008 < 0.005 bal. Figure 3. SEM-micrograph and EDX-analyses of AISI 347 in solution annealed state 3. Results and discussion 3.1. Evaluation of the fatigue behavior with deformation induced ´-martensite formation Figure 4 shows the development of the stress amplitude a and ferromagnetic martensite fraction (Figure 4a), the change in the mean value of time of flight tofmean (Figure 4b) and the change in the mean value of the electromagnetically activated ultrasonic amplitude peakmean (Figure 4c) versus the number of cycles N in fatigue tests at ambient temperature.
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