PSI - Issue 16

Borys Paton et al. / Procedia Structural Integrity 16 (2019) 176–183 Borys Paton et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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location, monitoring large industrial facilities with a small number of transducers, including facilities with a complex geometry and in difficult-of- access sections (underground, coated by insulation). Let’s consider how AE method can be applied and what results can be obtained at continuous monitoring of high-temperature components of thermal power plants. It is known that fracture of materials of power plants operating under high-temperature creep conditions is characterized first by initiation and propagation of microcracks on grain boundaries, then their gradual transformation into pores with subsequent pore coalescence and formation of the main crack, leading to material fracture (Lebedev et al. (1995), Nedoseka (2008)). Fig. 1 shows the graph of pore accumulation in service in 12Kh1MF steel quite extensively applied in pipeline systems of thermal plants.

Fig. 1. Damageability by pores at creep (a) and creep of 12Kh1MF steel (b), (Beresina at al. (1991)).

One can see from the graph that the closer the operating time to critical value ( t cr ), the larger number of pores forms in the metal, which coalescence to eventually form the main crack. Change of working temperature within the limits only slightly affects the mechanism of pore formation. At the same time, the value of change of 12Kh1MF steel relative elongation depends on temperature, and the higher the working temperature, the higher is the relative elongation. Material testing with AE method application showed that pore formation and coalescence during material fracture at high temperatures occurs just as discreetly, as at normal temperatures. Fig. 2 shows a graph of testing 15Kh1M1F steel at 560 ° C temperature. In the graph: P is the tensile load on the sample (blue curve); a is the amplitude of AE signals arising at material fracture (green) and Rt is the AE parameter “rise time”, characterizing the “rigidity” of the fracture process (pink). The graph demonstrates clearly pronounced discrete nature of the process of material fracture at this temperature. Investigation of material fracture at tension in the temperature range of 350 to 915 ° C showed the same results. Therefore, the procedure of AE testing can be applied also at evaluation of the state of materials operating at high temperatures (Nedoseka (2007)), (Lobanov et al. (2009), (Nedoseka et al. (2011)), Paton et al. (2012)). The graph shows that with load increase material fracture intensity and its rigidity change in a broad range. This is indicative of clearly pronounced discrete nature of fracture and of the influence on it of continuously changing material strengthening. The above considerations are fully applicable to materials of pipes and, in particular, steam pipelines, operating at high temperatures, for instance, in enterprises of thermal energy sector. Recent research showed the possibility of effective application of AE method at high temperatures, in particular its industrial application in operating pipelines of thermal power plants. Continuous monitoring of structures and equipment in service , acquisition of continuous flow of information about their state should be the initial stage in the sequence of ensuring their reliability. Taking into account positive results of preliminary studies, already gained experience of industrial application and available testing procedure (Paton et al. (2012), Beveridge et al. (2009)), a decision was taken to set up a system of continuous AE monitoring of hot industrial steam overheating pipelines at Kyiv TPP-6.