PSI - Issue 52

Jiri Dvorak et al. / Procedia Structural Integrity 52 (2024) 259–266 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Hasegawa, (2015), Hald, (2017)). It was developed in the second half of the 1980s (Masumoto et al., (1986)). A further increase in creep strength was obtained by the addition of tungsten, which replaced Mo, and B. The main reason for the addition of W and B was to enhance solid solution strengthening of the matrix and to restrict precipitation coarsening. The inclusion of Nb and V in the chemical composition has effectively improved creep strength, as a result of the precipitation of slowly coarsened carbides NbC and fine intragranular VN precipitates. However, the microstructure of ferritic martensitic steels is thermodynamically unstable. A degradation of the creep properties is associated with coarsening of M 23 C 6 carbides, transformation of MX-type carbonitrides to Z-phase and precipitation of Laves-phase particles (Sawada et al.,(1999)). Therefore, great attention should be paid to the study of microstructural changes occurring during creep deformation. Creep behaviour, microstructure stability, and degradation of creep properties of the P92 steel are phenomena of major practical importance that often limit the performance of high-temperature components. This has motivated extensive experimental creep studies and theoretical modelling of the creep deformation mechanisms and microstructure evolution in P92 steel over the last three decades (e.g. Ennis et al., (1997), Hald, (1999), Sklenicka et al., (2003, 2011,2018), Seung et al., (2006), Magnusson and Sandström, (2007), Sakthivel et al. (2016) and Kral et al. (2017)). The possibility to timely monitor and asses the development and instantaneous state of damage is a fundamental requirement for the safe and long-term operation of high-temperature components. Currently, non-destructive methods are widely used for verification damage of power plant components. One of the modern defectoscopic methods, included in the group of non-destructive tests, is the acoustic emission (AE) technique. This method is based on the sensing of elastic waves, which arise as a result of dynamic processes occurring in the material when it is loaded by internal or external forces. Recently, the AE technique was considered a reliable method for continuously monitoring the progressive damage process of industry components in real time (Grosse et al., (2021)). The AE technique makes it possible to observe the accumulation of damage, the course of plastic deformation, the initiation and propagation of cracks (Nohal et al., (2019)). Acoustic emission enables the monitoring of defects with a sensitivity not detected by other methods. Advantage of AE is possibility long-term operational monitoring of pipeline condition and early warning before destruction. Therefore, the non-destructive acoustic emission (AE) method was applied to monitor the formation and accumulation of creep damage during creep tests of P92 steel to demonstrate the necessity for noninvasive examination rather than destructive evaluation to asses creep damage. 2. Experimental material and procedures The experimental material used in the present investigation was an advanced tungsten modified 9%Cr P92 steel. This material was attained from the pipe produced by Vallourec & Mannesmann Tubes, Deutschland. It is a pipe of nominal dimensions OD = 350 mm and a nominal wall thickness of 55 mm, which was delivered in the condition after operational heat treatment consisted of normalising at 1050°C/30 min /air followed by tempering at 765°C/60 min/air. The chemical composition of the studied steels is given in Table 1. Constant load creep tests in tension were carried out in protective argon atmosphere and in air using cylinder creep specimens having a gauge length of 50 mm and a diameter of 6 mm. Samples were machined in the longitudinal direction of the pipe. The creep tests, conducted under application of uniaxial tension with a constant load, were carried out at the temperature 600 °C in the stress range 180-250 MPa. The samples were heated up to 5h in the furnace before starting the test and then the temperature was maintained ±0.5°C during the test exposition. All tests were conducted to the fracture of the specimen. During the test, time, elongation, and creep rate were continuously monitored and recorded. Microstructural analysis was performed by Jeol 2100F transmission electron microscope (TEM) and optical microscope (OM). Fractographic examinations were performed on fractured specimens using scanning electron microscope using a Tescan Lyra 3 XMU FEG/SEMFIB/EBSD scanning electron microscope (SEM). The acoustic emission (AE) was continuously monitored during creep tests using a working prototype of the IPL-3 Tab. 1: Chemical composition of P92 steel pipe (in wt%). Fe C Cr W Mo 0.49 Mn 0.44 Mo 0.48 Si Ni V Nb N Al bal. 0.1 8.72 1.55 0.17 0.18 0.20 0.06 0.05 0.001