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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(18 bit/15 MHz) data acquisition apparatus developed by DAKEL company, with a sampling frequency of 15 MHz and a total gain of 130 dB, allowing the capture of emission events that were previously beyond the detection limit. AE measurements were achieved using MDK-42AS42 sensor. This sensor was magnetically attached via an ultrasonic gel to the ends of the tension system. During the creep tests, an AE signal was detected and subsequently converted into a basic parameter (CRE, RMS and others). Selected emission events were finally evaluated using neutron networks. The data were recording three times a day for 15 minutes.

3. Results and discussion 3.1 . Microstructure

Fig. 1 shows the microstructure of P92 steel in as-received state in the normalized and tempered condition. The microstructure of as-received state contains prior austenite grains with laths boundaries and subgrains in their interiors. Fig. 2 shows the microstructure after tensile creep testing at 600°C and 180 MPa. In the microstructure can be found M 23 C 6 carbides which are predominantly located along prior austenite grain boundaries and subgrain/ lath boundaries. Inside the lath boundaries and subgrains can be seen fine MX precipitates.

Fig. 1. OEM of P92 steel in its as-received state.

Fig. 2. TEM micrograph shows P92 steel after creep exposition at 600°C and 180 MPa.

3.2. Creep properties Fig. 3a shows the series of standard creep curves for different stress conditions of P92 steel tested at 600 °C. Most samples were tested in a protective argon atmosphere only one sample was tested in air for comparison of creep results. Creep deformation is characterised by short instantaneous strain followed by well-defined primary, secondary and tertiary stages. The results show that the applied stress does not have a more pronounced effect on the final elongation of the samples. In general, oxidation of the sample can lead to the formation and exfoliation of the oxide layer, which has the effect on reducing the cross-section of the sample and thus to faster degradation of the test material. When studying the influence of the ambient atmosphere (argon vs. air) on the creep characteristic, no effect was demonstrated of air atmosphere on creep resistance. The results obtained at the same stress of 180 MPa are more or less the same within the variance typical for creep behaviour. One of the reasons is the fact that the high content of Cr prevents oxidation of the sample and creep exposition time is too short for more noticeably oxidize the sample. From standard creep curves it is a difficult determine particular stadia of creep exposition. A better overview of individual creep states can be provided by the transformation of standard creep curves into the form of instantaneous strain rate ̇ at time t (Figs. 3(b) or instantaneous strain rate ̇ to creep strain ε (Fig. 3(c). Creep curves thus better pronounce transient and accelerating of particular creep stages. The beneficial effect of the replotted curves is also a reliable determination of the value of the minimum creep rate ̇ , important parameter for determination of creep mechanisms.