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Information × Registration Number 0216U006890, 0116U006348 , R & D reports Title Determining the causes of tension zone bends damages of the 7-th unit of TPP DTEK Burshtyn main steam pipeline based on the fractography features examination of the fracture surfaces and estimation of the technical state of operated metal popup.stage_title Head Nykyforchyn Hryhoriy, Registration Date 29-07-2016 Organization Karpenko Physico-Mechanical Institute of the National Academy of Science of Ukraine popup.description2 Although compliance with requirements of the regulations of the chemical composition, hardness and mechanical properties at the tension transverse specimens from the 12H1MF steel (from all areas of the bends and the entire cross section pipe) analyzed bends, they nevertheless destroyed to form axial cracks in their tension zone (TZB). The heat transfer agent leaked outwards through a crack in one of the bends. Thus, under identical operating conditions in the other bend was found only shallow (its maximum depth does not exceed 8 mm) axial crack. An impact toughness of the 12H1MF steel from the TZB of both bends were lower in two or three times than the regulated value. This demonstrates the ultra-low resistance to brittle fracture of this class of steels, which achieve the critical state of degradation. Detailed analysis of the damages existing in the diameter section of pipes has not given grounds to conclude that the damages outside the zone of formation of macro fracture in the TZB correspond to supercritical state of metal. While near the macro-crack the metal had unsatisfactory state (long chain along the grain boundaries, which coalesce into the microcracks) and corresponded to 6 points of damage. Thus, the analyzed elements had long ago replaced. Fractography proved that the destruction of the two bends derived from the outer surface of the pipe by the intergranular mechanism due to creep and hydrogen embrittlement. The boundaries of the intergrain fragments were contrasting decorating by secondary cracks. Intensively oxidization of the crack surfaces indicated that oxides formed during longtime high-temperature operation of the tube. During the cooling of the steam pipeline the oxides between crack surfaces promotion the cracks growth by the creep mechanism due to their additional loading caused by a mismatch of volumes of steel and its oxides. Based on these studies have shown that damages of the analyzed bends of steam pipelines caused by the supercritical structural degradation of the TZB metal, which creates prerequisites for accelerating the creep process in this area before macro-cracks initiation. In other metal bends of steam pipelines, which operated at commensurate time of operation and moved about the same number of shout-downs, could be expect the relevant changes in the structure. Their damage assessed as sufficient, so that the overloading of the bends in its TZB formed intergranular microcracks, its coalescence will entail the destruction of steam pipelines. To prevent such extraordinary situations in the future to TPP staff observance of the standard regulatory requirements for temperature and force, and vibration mode of operation of steam lines is addition recommended: 1. To use the sandblasting (better wet) treatment for cleaning the outer surface of pipes, which provides compressive stress (200 MPa) in the surface layer of the pipe and thus allow to move in the time the replacement of critical degraded bends of the main steam pipeline. 2. To use the lower operating parameters of steam pipelines up to the gradual replacement of critical degraded elements. 3. To reduce in the three time the survey frequency of other bends steam pipelines the operation duration of which exceeded the economic life. 4. To limit the shutdown numbers of the HPP blocks, which intensify the degradation of steel at the stage of structural changes, the formation of scattered damage and macro-cracks. 5. Incrementally replace of the bends with a service life more than 270000 h, in which the metal TZB has reached the critical state of degradation, or close to it. To control the state of the metal is proposed non-destructive testing methods based on electrochemical approaches developed in the Karpenko Physico-Mechanical Institute of the NAS of Ukraine.5481 Product Description popup.authors Бабій Леонтій Омелянович Кречковська Галина Василівна Курнат Іван Миколайович Рабик Богдан Іванович Студент Олександра Зиновіївна Цирульник Олександр Тимофійович popup.nrat_date 2020-04-02 Close