PSI - Issue 30

G.N. Sleptsov et al. / Procedia Structural Integrity 30 (2020) 154–161 Sleptsov G.N. et al. / Structural Integrity Procedia 00 (2020) 000–000

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Of all the machine methods included in GOST 26388-84 for testing welded joints for the formation of cold cracks, the Implant method is the most conventional and was deeply studied Mikhailov V.E. et al. (1999). Despite this, it is widely used in practice, since it has significant advantages: - the small size of the specimen-insert, which is a great advantage of the method, the specimen can be cut at various depths and orientations with respect to the surface of the steel sheets and the direction of their rolling; - the ability to arbitrarily set the final level and the law of increase in the applied load to the specimen; - the shape of the specimen-insert allows you to evaluate the most sensitive to the formation of cold cracks part of the heat affected zone of the welded joint, regardless of the technological strength of the weld metal. Russian GOST 26388-84 establishes for testing Implant cylindrical specimens with a diameter of 8 mm (6 mm can be used) with a screw notch, for the quantitative resistance of butt joints of steels up to 1000 MPa to the formation of longitudinal cracks in the HAZ across the rolling plane under conditions of an acute stress concentrator. The requirements are imposed on the equipment only in terms of smooth loading, average loading speed ≤ 10 MPa/s, load constancy within ± 5% during the entire loading period of 20 hours and they stop when the specimen breaks or a crack is visually detected. In contrast to this standard, at low temperatures to research the influence of the development and occurrence of cold cracks, on the specimen-insert according to the Implant method, they are welded at positive and at low ambient temperatures up to -48 ° С . Constant external loading in the installation is ensured by means of a lever and a calibrated elastic element of the installation, which in turn gives stable loading of the sample throughout the experiment. The load on the specimen-insert is set for at least 20 hours. To measure the displacements of the specimen-insert on the installation, was used a Vogel 241103-0.001 digital displacement meter, with the possibility of measurements up to 1 μ m. The registration of displacement data is carried out to assess the effect of plastic deformation using the developed program and is stored in the computer hard drive for further data processing. The accuracy of measurements of technological parameters of welding is provided by the AWR-224MD digital welding recorder, which also records and transfers the necessary data for storage and processing to a computer. The thermal cycle parameters were recorded on a TERKON universal precision instrument for measuring the signals of resistance thermocouples or thermocouples and their subsequent conversion to temperature values. Chromel-alumel type thermocouples are soldered into the sample using a discharge capacitor in special technological holes at a distance of not less than 1,0–1,5 mm from the bond line of the welded joint. Next, registration, processing are performed on specialized software and storage on the hard drive of a personal computer. When conducting the above tests, the occurrence of elastic vibrations is also recorded by the method of acoustic emission (AE). To register signals AE, was used the diagnostic complex Expert-2014, which is an acoustic emission diagnostic system that allows one to obtain both amplitude and energy characteristics of acoustic emission signals. The reliability of the obtained data was realized using location of AE signals on the specimens, using acoustic antennas from three or more AE sensors and software hardware (see Fig. 1). The registration of AE signals and the correct interpretation of the obtained data gives a complete picture of the kinetics of the development of fracture and deformation processes with a resolution of up to 1 μ m, reflected in the work Andreykiv A.E. (1989). Sleptsov G.N. (2016) experimentally established that the parameters of acoustic emission accurately register all stages of the development of delayed fracture. It is known that technological samples receive a qualitative or semi-quantitative estimation of the technological strength of the weld metal. In this case, the fact of the presence or absence of a crack, or the number and size of cracks, is used as an indicator. In this case, the fact of the presence or absence of a crack, or the number and size of cracks, is used as an indicator. Therefore, this type of test is used mainly for a comparative estimation of the weldability of steels, the selection of the welding mode and welding materials. Nevertheless, technological samples allow to get as close as possible to the thermo-deformation cycle of a real welded unit and to the selected welding technology. Consequently, such samples are necessarily present in the standards and normative documents for the control and acceptance of welding materials, welded joints and constructions. 2.2. The improved technological sample of Tekken method