PSI - Issue 30
L.V. Moskvitina et al. / Procedia Structural Integrity 30 (2020) 100–104 Author name / Structural Integrity Procedia 00 (2019) 000 – 000
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Fig. 1. Deformation of structures in the incubatory period: (a) a plastic zone under the cavity, optical microscopy; (b) structure of deformations in the plastic zone under the cavity, TEM; (c) a shift of grain boundaries, TEM; (d) a cavity surface, TEM. Extended along deformed ferrite grains the pearlite grains split up and gradually crumble to form large pores at rotational processes. Turns of grains resulted in opening of grain-border microcracks , which determined an intragranular type of the cavity surface. The deepening of cavities has caused to formation of fatigue cracks. 3.2. Structures of deformation and destruction in the fatigue zone The mouth cracks of the valve opening directed perpendicularly to the welded seam were stopped in the thermal affected zone. The two cracks got further development parallel to the fusion line and symmetrized to the opening. The smoothed sites related to the fatigue areas have the lengths 2,5 and 2,78 mm, these parameters amounting 0,05% of all circumference of the rim. With accelerated development of the crack, there are sharp boundaries in the fatigue zone. The surface of the fatigue zones of the basic crack like a fracture structure of the radial cracks is also divided into an initial rough zone and a smoother zone. The presence of the rough area testifies that the cavities were the initial nidus of the fatigue cracks parallel the weld. In the rough area along with the intragranular facets there are flat facets, characterized as fissions of subgranular boundaries. In the cyclic deformed area, preceding the development of fatigue crack the processes of deformation are intragranular. Such transition from the grain-border processes of the incubation stage to the intragranular ones in the fatigue stage causes the further growth of the crack in the closed system. The electronic-microscopic research of the microstructure of the smooth fatigue zone has shown that at the beginning of the fatigue crack in the body of deformed grains there are more relief and bound sliding strips on which the metal extrusion is observed (Fig. 2 а ). The fracture on such structures consists of an alternation of quazifacets and branching fatigue grooves (Fig. 2b). The Figure 2a evidently shows that in slightly deformed ferrite grains dispersed particles of the second phases are revealed, while the other grains remain relatively clean. In addition, there are more large excretions of the second phases on the grain boundaries. The release of these phases is apparently caused by deformation aging, which could be conditioned by preliminary ageing under the influence of welding heat.
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