PSI - Issue 33

M.R. Tyutin et al. / Procedia Structural Integrity 33 (2021) 765–772 M.R. Tyutin , L.R. Botvina, A.V. Ioffe/ Structural Integrity Procedia 00 (2019) 000–000

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confirmed by a burst of high-amplitude AE activity before final failure, which was not detected in specimens in initial state and after operation (Fig. 2a, b). Deformation dependences of the b AE -value under all considered conditions (Fig. 2) reveal periodic changes consistent with the stage-by-stage changes in other acoustic characteristics: a decrease in the parameter at or near the boundary of stages I-II, some increase in the middle of stage III (Fig. 2a) and a sharp decrease at stage IV before failure. Analysis of the obtained results shows that at least three acoustic parameters - the accumulated number of signals, acoustic activity, and the b AE -value, can serve as diagnostic criteria characterizing the change in the state of the steel under study. The informativeness of these parameters, recorded during tension, has been noted by many researchers (Botvina et al., 2015; Nazarchuk et al., 2017; Ono, 2008). a b

Fig. 3. Crack formation in 15Cr2MnMoV steel specimens due to hydrogen charging in H 2 S- containing media at load of 0.3∙σ Y .

Table 3. Mechanical properties of 15Cr2MnMoV steel after hydrogen charging (HC) in NACE solution HC stress σ Y , MPa σ US , MPa El, % А f , J Σ N AE b AE Initial state 890 1040 15,0 413,0 2574 1,06 0 σ Y 891 1062 12,3 271,7 421 0,67 0 ,1 σ Y 856 1011 11,5 267,0 1260 0,95 0 ,2 σ Y 873 1062 9,5 215,2 909 0,68

2.4. Effect of operation and hydrogen charging on the magnetic characteristics of 15Cr2MnMoV steel In addition to the acoustic emission characteristics, the physical properties of the 15Cr2MnMoV steel were evaluated after hydrogen charging. Measured magnetic characteristics of steel after hydrogen charging exhibit a certain change at the boundary of stages I-II and III-IV, i.e., at the beginning of the plastic flow and when reaching the ultimate strength. The sensitivity of these characteristics is different, as well as the form of their deformation dependences. It was established that the change in load, at which the hydrogen charging was carried out, has little effect on the magnetic characteristics, the increase of which is observed only in the final stage of fracture, after reaching the ultimate strength. The similarity of the stages of change in the magnetic field intensity, mechanical properties, as well as acoustic emission parameters is noteworthy. It should be noted that at testing specimens after the hydrogen charging, stages are expressed to a lesser extent and the value of the residual magnetic field intensity Н r is lower in comparison with testing initial specimens. The magnetic characteristics change most intensively at two stages - at the initial stage I and at the final stage IV, after the beginning of the deformation localization process. It can be concluded that these magnetic characteristics depend to a greater extent on the material changes that occur during the transition from elastic to plastic deformation, as well as during the development of damage in the final fracture stage.