PSI - Issue 65

S.A. Barannikova et al. / Procedia Structural Integrity 65 (2024) 11–16 S.A. Barannikova, A.M. Nikonova/ Structural Integrity Procedia 00 (2024) 000–000

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Rayleigh acoustic waves V R allows us to obtain the dependences of the velocity on the value of the total strain V R (ε ) and the effective stress V R (σ) (Fig. 1a). As shown in (Barannikova, 2016; Lunev, 2018) the propagation velocity of ultrasound changes significantly after tensile of metals and alloys, and the dependences V R (ε ) and V R (σ ) are quite complex. Thus, the V R (ε) curve of AISI 420 steel in the initial state after heat treatment (quenching + tempering), shown in Fig. 1a, has an S-shaped form (three-stage character), which indicates a difference of the relationship between the velocity of ultrasound and strain in the corresponding intervals of plastic deformation. a b

Fig. 1. (a) Dependences of the ultrasound propagation velocity on the total deformation V R ( ε ) and stress V R ( σ ); (b) dependences of the strength degradation coefficients (1) and hydrogen embrittlement (2) on the acoustic properties degradation coefficient.

The analysis of mechanical properties demonstrates a strong effect of hydrogen on the yield strength, limit strength and relative elongation before breaking in comparison with the initial state without hydrogen. At a lower hydrogen content in the material, plasticity decreased first, while maintaining strength indicators. When the hydrogen content becomes higher than a certain value, the tensile strength also began to decrease in the process of hydrogen embrittlement. The value of the critical hydrogen concentration, at which 80% of the original mechanical properties are preserved, was estimated based on the data on the change in mechanical characteristics. For the dependences of strength and ductility degradation, two different values of the critical hydrogen concentration can be obtained. At the current density and cathode potential specified in the experiment, these values are close cr H C ≈ 5 ppm. The results of analysis of the stress-strain curves σ ( ε ) showed that beyond the yield point, the stage of linear work hardening (II) is recorded both in the initial state after heat treatment and for all samples pre-saturated with hydrogen. The stages of parabolic work hardening (III) and pre- failure are present. However, the duration of the plastic flow stages decreases with increasing hydrogen charging time. For samples saturated with hydrogen for 17 and 24 hours, the stages of parabolic work hardening and pre-failure were completely absent. With increasing hydrogen concentration, the type of destruction changes from ductile to brittle. The nature of the change in the velocity of Rayleigh waves with an increase in the total strain in all studied states are qualitatively similar. At the stages of linear work hardening, the constancy of the ultrasound propagation velocity was noted (Fig. 1a), which can be effectively used to control the stability of this stage of the deformation process. A decrease in the ultrasound velocity in the studied material is observed immediately after the stage of linear work hardening and up to failure. Based on experimental data, the following coefficients were estimated:  the hydrogen embrittlement index (HEI) K ε = (δ H − δ 0 ) / δ 0 , where δ 0 and δ H are the values of the total elongation of the samples with hydrogen and without one,  the strength degradation coefficient K σ= (σ H − σ 0 ) / σ 0 , where σ H and σ 0 are the ultimate strength values of the samples with hydrogen and without one,