PSI - Issue 30

M.M. Kantor et al. / Procedia Structural Integrity 30 (2020) 45–52 M. М . Kantor et al. / Structural Integrity Procedia 00 (2020) 000–000

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low angle grain boundaries 2-15°) in regions adjacent to microcracks locations in IPF maps was observed, Fig. 4 (c). Evaluation of average GOS value, averaging the deviation between the orientation of each point in a given grain, was used for the assessment of local strain degree. 6,5° average GOS value in microstructure adjacent to the microcrack edges pointed at high degree of local plastic strain (2,2° in initial state). Microcracks in plastic zone under fracture surfaces containing splittings were predominantly residual ones, Fig. 4 (b). Almost straight microcracks propagation trajectories and their sharp tips were revealed. The same orientation of separated grain parts in IPF maps pointed at the transcrystalline character of microcracks formation, Fig.4 (d). Grain size and shape in microstructure of plastic zone under fracture surface containing splittings (9,8 µm) was almost identical to that, which was observed in initial state before fracture. However, the formation of developed substructure was a result of expended work of local plastic deformation, which is confirmed by rather high (5,3°) average GOS value. For the description of fracture process in DBT region we adopted the criterion of differences in local strain work expended at the initiation and propagation of ductile and cleavage cracks Shtremel (2014). We consider that the fracture process of low carbon microalloyed steels proceeds as follow. The residual microcracks occur in local embrittled regions of material on the early stage of fracture as a result of cleavage. Pre-strained regions with local ductility variations are fractured by the formation of splittings and by the ductile tearing during the main crack propagation, as studied by Ritchie et al. (1973), Kantor and Vorkachev (2018). The formation of splittings by the quasi-cleavage mechanism occurs in microstructure regions containing residual cleavage microcracks. It follows that splittings shields embrittled microstructure regions from overstress, thus preventing the formation of completely cleavage fracture surfaces, shown by Bramfitt and Marder (1977). More ductile ligaments between occurred microvoids are fractured by ductile tearing during with main crack propagation. Variations in ductility of plastic zone may be caused by the features of microstructure, Fig.5. The high temperature parent austenite microstructure is represented in Fig. 5 (a). Such microstructure contained pancaked elongated towards rolling direction grains. It was observed the presence of several populations of grains with thickness of 2-4 µm and 14-30 µm. The length of grains could reach 700 µm. It is considered that the formation of splittings could be a result of smooth almost straight parent austenite grain boundaries decohesion during the main crack propagation, as shown by Herø et al. (1977). Investigation of real microstructure by means of EBSD allowed to establish the formation of different ferritic structural constituents: proeutectoid ferrite, granular bainite, lath bainite, as pointed by Araki et al. (1990), Zajac et al. (2005), Fig. 5 (b). It was also observed the formation of A and M-A structural constituents.

Fig. 5. Microstructure of low carbon microalloyed steel: (a) parent austenite, etched by heated saturated aqueous solution of picric acid + natrium lauryl sulphate, OM; (b) real microstructure, IPF + BS, EBSD.

The results of quantitative grain size estimation represented in Fig. 6.