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

768

4

a

b

550

Initial state After operation Hydrogen saturation, 1 week

Initial state After operation

900

500

800

450

400 σ max , MPa

700 σ max , MPa

600

350

10 5

10 6

10 7

10 4

10 5

10 6

10 7

N

N

Fig. 1. Fatigue curves of (a) 15Cr2MnMoV and (b) 12Cr18Ni10Ti steel specimens in initial state, after operation and hydrogen charging (15Cr2MnMoV steel).

2.2. Study of the fracture process of 15Cr2MnMoV steel specimens after the operation As shown in Section 2.1, the operation did not result in significant degradation of the mechanical properties of 15Cr2MnMoV steel. However, the AE characteristics decreased sharply compared to the same characteristics for the initial state, which indicates the exhaustion of the plastic properties of the material. Thus, a comparison of Figures 2a, b shows that the intensity and total number of AE signals after operation decreased by almost an order of magnitude both at the initial and final stages of fracture. Strain, at which there is a sharp drop in the b AE -value, has also decreased, indicating the appearance of high-amplitude signals after operation. The period of acoustic emission gap before the final failure, noted in (Botvina and Tyutin, 2019), on the contrary, increased. Four stages of changes in the AE characteristics were distinguished during tensile deformation of specimens in the initial state (Fig. 2a) and after operation (Fig. 2b), following the considerations described by L R Botvina et al. (2021). The initial stage I of elastic deformation up to the yield point is accompanied by high acoustic activity and high b AE values, especially at testing the material in the initial state. Transition to stages II, III leads to a decrease in AE activity. The boundary between stages II and III passes through a point where the slope of the strain curve changes. In the transition region from stage II and within stage III, the slope coefficient of the strain dependence of the total number of acoustic emission signals also changes, indicating the beginning of strain localization before fracture. When reaching the ultimate strength, at the end of stage III, a knee point appears on the deformation dependence of the total number of acoustic emission signals, after which the values of this parameter remain constant or increase slightly until final failure. Thus, at stage IV, there is a sharp decrease in the b AE -value and in AE activity, similar to the acoustic quiescence that Botvina et al. (2020) and Tyutin et al., (2020) observed earlier before the fracture of carbon steel specimens. 2.3. Test results of 15Cr2MnMoV steel specimens after hydrogen charging The influence of structural degradation due to hydrogen charging without and under load on the mechanical properties and acoustic emission parameters was estimated. Table 3 shows the test results, and Fig. 2c-e shows the dependences of acoustic emission parameters obtained during tensile tests of 15Cr2GMF steel specimens after hydrogen charging. The structure of 15Cr2MnMoV steel is bainite with carbide clusters along the boundaries of the original austenitic grain (L.R. Botvina et al., 2021). Hydrogen charging under load produces delamination cracks, which develop both along the boundaries and along the body of the austenitic grain (Fig. 3). As can be seen from Table 3, charging without load does not cause a decrease in strength properties, however, the elongation and fracture work A f , defined as the area under the loading curve, decrease. In specimens that were charged under load, there is a significant decrease in yield strength, elongation, and fracture work.