PSI - Issue 50

Svetlana Barannikova et al. / Procedia Structural Integrity 50 (2023) 33–39 S. Barannikova, M. Nadezhkin, P. Iskhakova / Structural Integrity Procedia 00 (2023) 000 – 000

35 3

vessel and was controlled by a chromel-alumel thermocouple brought in contact with the sample. The stress-strain diagrams of Fe-Ni-Cr alloy samples covered the ranges of elastic and plastic deformations and failure. The mechanical and acoustic properties of the steel are given in Table 1. According to the data, a decrease in temperature led to an increase in the speed of ultrasound. Simultaneously with the recording of loading curves, the speed of ultrasonic Rayleigh waves V and the attenuation coefficient α were measured by means of a separate and combined sensing unit that consisted of emitting and receiving piezoelectric converters based on CTS-19 piezoceramics with a resonant frequency of 5 MHz (Murav'ev et al. (1996), Barannikova et al (2016), Lunev et al. (2018)). Besides, lowering the temperature caused an increase in the strength and speed characteristics of ultrasound. For comparison, the damage parameter was analyzed using the ultrasonic wave propagation speed as follows: D V = 1 – V/V 0 , (1) where V and V 0 are the ultrasound speeds in deformed and initial states, respectively. Table 1. Mechanical and acoustic properties (tensile strength σ B , yield strength σ 0.2 , relative elongation to failure  , and speed of ultrasound in the nondeformed steel V 0 ) of Fe-18 wt.% Cr -10 wt.% Ni alloy T, K σ B , MPa σ 0.2 , MPa  V 0 , m/s 318 598±3 289±2 0.71±0. 02 2689±3 297 786±2 257±3 0.74±0. 02 2849±3 270 882±2 211±3 0.5±0. 01 3003±3 254 896±3 220±2 0. 48± 0.01 3033±3 227 954±2 289±3 0. 43± 0.02 3143±3 211 992±3 353±2 0. 38± 0.02 3277±3 180 1090±2 362±3 0. 35± 0.01 3379±3 As a result of plastic deformation, structural changes occur in the temperature range under consideration, which are accompanied by variations of the martensitic α '-phase formed via the γ - α ' -phase transformation (Talonen et al (2005)). Simultaneously with the measurements of the velocity of ultrasonic waves, a magneto-phase analysis of samples, consisting in determination of the volume fraction of ferrite, was carried out using a multifunctional MVP 2M eddy current device (ferritometer). 3. Results and discussions Synchronous recording of strain- stress diagrams and measurements of velocity V and attenuation α of Rayleigh acoustic waves make it possible to obtain dependences of the propagation velocity of ultrasound on the magnitude of the total deformation  and the effective stress σ . Since the test temperature exerts a significant effect on the plasticity and strength, the data are further presented in dimensionless coordinates  /  f (  f refers to deformation to failure) and σ/σ B ( σ B is the ultimate strength). The measured velocity V of ultrasonic Rayleigh waves, attenuation coefficient α and volume fraction of martensite f α′ in accordance with the stain-stress diagram σ(  ) of Fe-18% Cr -10 % Ni alloy at 211 K are shown in Fig. 1, where the relative deformation  /  f is indicated along the horizontal axis. In these dependences, four stages (referred to as I, II, III, and IV) of changes in acoustic parameters can be distinguished throughout the investigated temperature range. The duration of the stages varies with a decrease in the test temperature. The first stage I reflecting the change in acoustic parameters for stainless steel is probably related to the elastoplastic transition of the sample material. This is evidenced by an increase in the propagation velocity V and a decrease in the attenuation coefficient α of ultrasonic waves (Fig. 1). At the yield point (stage II), the velocity V reaches its maximum value and then remains almost constant against the decrease in α . At the hardening stage III, there is the maximum growth rate of the martensitic α' -phase, which is due to the deformation-induced γ - α' -phase transformation, and this process is accompanied by a drastic decrease in the velocity of ultrasound propagation V