PSI - Issue 28

V.V. Sudin et al. / Procedia Structural Integrity 28 (2020) 1637–1643 Sudin V.V./ Structural Integrity Procedia 00 (2019) 000–000

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located in the ductile to brittle transition temperatures region. At the same time, the occurrence of a brittle crack in the process of deformation significantly reduces the ability of the material to resist further crack growth, both due to an instant decrease of the cross-section, and due to the formation of a strong stress concentrator. The fracture energy measured in impact bending tests is usually divided into the components of elastic deformation, growth of the initial ductile crack, macroplastic deformation of the specimen, energy of a brittle crack formation and the energy of the ductile final fracture of the specimen Gerard et al. (1996). Among these energies, the most important and has the largest value are the energy of the ductile crack growth and macrodeformation energy, but the value of the energy of the ductile final fracture can also be significant. In some studies, it has been shown that the formation of the shear lips, which make up a significant fracture area, occurs after a brittle fracture Gerard et al. (1996). or after reaching the maximum force Tanguy et al. (2005). Thus, when a material is located in a critical range of cold fracture temperatures, its resistance to impact loads cannot be characterized only by the value of the impact strength and the proportion of the shear area of the fracture. The criterion of particular importance in this interval is the moment of formation of the brittle crack. Thus, if the failure begins with the formation of a brittle crack followed by a ductile final fracture, the resulting value of the impact strength and the percentage of the shear fracture may lead to an incorrect assessment of the level of reliability of the material. For example, the standard API 5L regulates the impact strength value of 50 J/cm 2 , which corresponds to the fracture energy of a standard specimen with a V-shaped notch of 40 J. At the same time, in the case when the impact bending tests conducts in the ductile to brittle transition temperatures range it is possible in the case of high energy of ductile final fracture, occurrence of situation in which a value of impact toughness higher than the standard, but the energy spent before the formation of a brittle crack is significantly lower than 40 J. The possibility of such an outcome may differ for a metal with different microstructures. In connection with the above, the purpose of this work was to establish the possibility of early formation of a brittle fracture during impact bending tests of metal with various microstructures, to establish the frequency of occurrence of this phenomenon and to develop methods for monitoring this type of fracture. 2. Materials and methods Standard Charpy V-notch specimens made according to GOST R ISO 148-1-2013 (with a 10×10 mm2 section, a 45° 2 mm depth notch) were cut out from the area of the welds made by six different methods, from weld butts metal (designations Weld method № or WM №), from base metal - steel 09G2S after quenching and tempering, or after normalization (designation 09G2S QT, 09G2S N), from heat affected zone (HAZ) (designation HAZ 09G2S QT, HAZ 09G2S N), and steel 17G1S after hot rolling. Impact bending tests were performed on an instrumented Roell Amsler RKP-450 pendulum impact tester (Zwick/Roell). The potential energy of the pendulum was 450 J and the speed of the pendulum was 5.24 m/s. The tests were carried out at temperatures from -60 to +20 °C. For each specimen a load-displacement curve was recorded. The metal of welds sometimes has defects in the form of gas pores and slag pores. Specimens that showed such defects during testing were excluded from further analysis. On the obtained load-displacement curves, the points of instant drop of the force corresponding to the beginning of growth of the brittle crack were found. By integrating a section of the load-displacement curve to the point of an instant drop in force, the energies corresponding to the work spent before the formation of a brittle fracture section were calculated. For tested specimens, images of fractures were obtained using a Canon-based macrophotographic station (EOS 6D + Macro Photo Lens MP-E 65mm). The areas of ductile and brittle fracture were highlighted on the images and calculated. For all specimens, the distance between the notch root and the top of the macroscopic cleavage region was measured using digital images. Also, for some specimens, the distance between the notch and the cleavage facets was measured by scanning electron microscopy with a CrossBeam 1540 EsB (Carl Zeiss). An example of a fracture image with a marked stable crack zone is shown in figure 1. Only those specimens that were in the transition ductile to brittle temperatures interval were selected for analysis. The criterion for finding in this interval was the value of the percentage of shear fracture of the specimen in the range from 5 to 95%.