PSI - Issue 13

Atsuhisa Kitade et al. / Procedia Structural Integrity 13 (2018) 1845–1854 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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and planning new facilities. In achieving this final goal, this paper focuses on the following two items. First, we tried to make a formula to obtain the critical stress value necessary for the DWTT model from microstructural information such as MA size, distribution, grain size distribution. For this purpose, it is quite essential to clarify the microscopic mechanism of brittle fracture. In this thesis, after carrying out the Double notch test, we conducted a delicate polishing and directly observe the origin of the brittle fracture occurrence, so that an important suggestion on the micro mechanism is obtained. This is shown in Chapter 2. Second, we will develop a numerical calculation model which can simulate DWTT test faithfully. In this paper, by experimentally obtained critical stress, a model was developed for the future work which integrates and connect the micromechanism and prediction of DWTT properties. This is shown in Chapter 3. 2. Experimental observation using polishing to elucidate the mechanism of brittle fracture The chemical composition of the steel used in this experiment is shown in Table 1. In this experiment, we decided to change the rolling temperature and rolling reduction as shown in Table 2 and consider the influence of each. Table 1 Chemical composition of the steel used C Si Mn P S Ni Cr Mo Ti Nb % % % ppm ppm % % % % % 0.065 0.225 1.55 80 20 0.20 0.40 0.30 0.0135 0.065 2.1. Material used

Table 2 Condition of rolling temperature and rolling reduction

Mark

Rolling temperature

Rolling reduction

700℃ 700℃ 850℃ 850℃

700-50 700-75 850-50 850-75

50 ％ 75 ％ 50 ％ 75 ％

2.2. Experiment procedure

In order to enhance the knowledge of the relationship between the brittle fracture triggering behavior and the crystal grain size and its orientation which have not been sufficiently clarified, especially in case of TMCP steel, double notch tests were conducted to facilitate discrimination of brittle fracture origin. As shown in Fig.2, the double notch test has two deep sharp notches at the two longitudinal direction of the specimen, and by three-point bending, the fracture driving force is generated equivalently to the two notch bottoms. Since brittle fracture is a probabilistic event in the microstructure level, if fracture occurs at one of the notch bottoms, it rapidly propagates and reduces the loading capacity of the whole specimen. That is, the other notch is preserved unloaded state just before crack occurrence. As shown in Fig.3, one notch is terminated without breaking. After the test, remaining unloaded notch part of the specimen is fractured by low stress fatigue crack propagation and the trigger of brittle fracture is observed by SEM. Fig. 4 shows a schematic diagram of this experiment. Fig.5 shows a SEM observation of a fatigue fracture surface of “850 - 50” . Clear river patterns that exhibits trigger point of brittle fracture at the place surrounded by black circle lines. In this time, we chose the trigger point of C out of the three starting points shown in Fig.5 for the sectional observation by delicate polishing.