PSI - Issue 2_B

Shohei Asako et al. / Procedia Structural Integrity 2 (2016) 3668–3675 Asako et al/ Structural Integrity Procedia 00 (2016) 000–000

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specimens as shown in Table 2. On the other hand, comparing with effective grain size defined as the area surrounded by 15degree misorientation lines, fracture facet seems to be relatively large. This shows that the weakest link mechanism works in choosing the largest grain as the macroscopic brittle fracture initiation site of bainite matrix. The reason of high toughness of TMCP steel can be thought fine microstructure formation by TMCP and enhancement of critical fracture stress obtained as the result of microstructure refinement. As future work, in-situ observation for understanding of micromechanisms of brittle crack initiation will be carried out taking advantage of this configuration. Furthermore, suitable manufacturing process for enhancement of fracture toughness will be investigated. 4. Conclusions In order to clarify the mechanisms of brittle crack initiation in the microstructure manufactured by TMCP, several experiments including applying thermal processing reproduction apparatus and numerical analysis based on continuum mechanics, were carried out. Following results are obtained. 1) Specimen size is limited considering the restriction in the thermal processing machine so all used specimen is smaller than usual. As a result of SENB tests in -196 ° C whose thickness is only 3mm, brittle fracture was occurred before beginning of macroscopic plastic deformation. FEM analysis was carried out after formation of constitutive equation of the steel at -196 ° C by actual tensile test result. Critical fracture stress was concluded to be 2200~2400MPa. 2) According to observation of fracture surface, in most case, there seems to be the trace of MA at the trigger point. Fracture facet size in fracture toughness test is larger than the average value of effective grain size. This shows the possibility of crack retention at the length of one grain and existence of the weakest link mechanism in macroscopic fracture initiation. That seems natural from the viewpoint of crack length effect on the driving force of crack re-initiation based on the Griffith’s theory. 3) New specimen configuration was devised in order to observe the brittle fracture initiation procedure directly at surface, which is easy to observe. Microcracks formation was observed at the same position in several loading steps. As a result, microcrack is nucleated around MA. The mechanism of the microcrack formation can be estimated to be decohesion in the boundary between MA and matrix. Cracking within MA itself has never been observed. 4) By further experimental research, detailed process of brittle crack initiation will be investigated through the observation of the microstructure change by EBSD and so on. Reference Igawa, H., Ohshige, H., Tagami, T., 1980. Study on the Martensite-Austenite Constituent in Weld-Heat Affected Zone of High Strength Steel, Journal of the Japan Welding Society, 49, 467-472. J, Chen., Tang, S., Liu, G. Wang, 2013. Materials Science & Engineering A559, 241-249, 2013 J. H. Chen and R. Cao., “Micromechanism of Cleavage Fracture of Metals”, Elsevier Inc., 2015 Martin-Meizoso, A., Ocana-Arizcorreta, I., Gil-Sevillano, J., Fuentes-Perez, M., 1994. Modelling cleavage fracture of bainitic steels, Acta Metallurgica et Materialia 42, 2057-2068. Mimura, H., Iino, M., Haga, H., On the toughness and the micro structure in the low carbon steel subjected to the welding thermal cycles, Transactions of the Japan Welding Society 1(28-34), 1970-2004. Morito, S., Hayashi, A., Pham, H., Kawabata, T., 2016. Relathionship between the effective grain size of brittle crack propagation and microstructural size in low-carbon low-alloy bainitic steels, Tetsu-to-Hagane 102, 6 Nakajima, H., Araki, T., 1972. Experimental measurements of deformed crack tips in different yield-to-tensile ratio steels, The Iron and Steel, 58(14), 1993-2004. Thompson, A. W., Knott, J. F., 1993. Micromechanisms of brittle fracture, Metallurgical Transactions A 24, 523-534. He, X.J., al. 1984, Metal Science, 18(7), 367ò. Table 2 fracture facet size-area of each types X each Av. Equivalent circle diameter [μ m] 25.8 20.7 26.3 40.0 31.8