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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The acoustic emission (AE) parameters and intensity of the residual magnetic field were evaluated during tensile deformation. The magnetic field intensity by the eddy current method and the coercive force was measured at different stages during stops in loading. The percentage of the martensitic phase in stainless steel was evaluated by the eddy current technique. The following characteristics were evaluated: • intensity of acoustic emission signals Ṅ AE . • slope coefficient of the amplitude distributions of the AE signals ( b AE -value). • intensity of the residual magnetic field ( H r ) estimated by the method of magnetic memory of metals. • magnetic field intensity ( H EC ) was estimated by the eddy current method. • coercive force ( H C ). According to the measurement results, dependencies of the estimated characteristics on t he relative deformation ε* were plotted. The relative deformation ε* was defined as the ratio of the current strain to the strain at specimen failure. 2. Results and discussions 2.1. Assessment of the effect of the operation on the standard mechanical properties of the steels under study The standard mechanical properties of studied steels in the initial state and after the operation are presented in Table 2. As can be seen from the table, operation does not lead to noticeable degradation of the strength properties of the steels under study.

Table 2. Mechanical properties of studied steels

15Cr2MnMoV steel

σ YS , MPa

σ U , MPa

El, %

Initial state

890 865

1040 1030

15 17

After operation

12Cr18Ni10Ti steel

Initial state

234 236

593 610

71 60

After operation

The results of the fatigue tests of 15Cr2MnMoV steel show that operation did not lead to a decrease in fatigue strength (Fig. 1a). While hydrogen charging without loading caused a significant reduction in fatigue strength. This is due to the formation of delamination cracks due to hydrogen charging, which were observed at the fracture surfaces of the specimens. Operation resulted in a slight decrease in the fatigue strength of 12Cr18Ni10Ti stainless steel (Fig. 1b). Analysis of the fracture microrelief of fatigue specimens of the material after operation revealed numerous pores with a size of 1 to 5 microns, which were formed because of pitting corrosion during operation, and often served as origin of fatigue crack initiation. Accumulation of corrosive pores, apparently, was the reason for the decrease in the fatigue life of the stainless steel specimens after the operation. Impact bending instrumental tests of 15Cr2MnMoV steel specimens were carried out. It was established that the impact toughness of the specimens after operation practically did not change, but the fracture work in the temperature range from -40 to -60 °C was higher in the samples after operation in comparison with initial state (L.R. Botvina et al., 2021). The increase in the impact fracture energy of steel after operation is associated with the well-known effect of energy dissipation on defects that appeared during the service of the material in a hydrogen sulfide-containing environment. An analysis of the fracture surfaces of the specimens confirmed this conclusion since it revealed microcracks with a large opening.