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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Introduction Long-term operation of structures in various fields of industry leads to degradation of materials due to the effect of stress, corrosive environment and other factors that cause a change in the structure, mechanical and physical properties of the material. The effects associated with mechanical degradation under loading are difficult to separate from structural degradation due to changes in macro- and microstructure. But when exposed to a corrosive environment, or when using a structurally unstable material that undergoes a phase transformation during loading, the effects of structural changes prevail, which allows us to speak of structural degradation of the material and its effect on the properties of the material. For a more detailed study of the influence of various factors on the fracture process, it is important to investigate the fracture stages of structural materials. Baskov (2011) in his work has paid much attention to this problem using acoustic emission monitoring data. Many studies are devoted to the problem of structural degradation using destructive methods and fracture characteristic assessment (Krechkovska et al., 2018; Nykyforchyn et al., 2010; Pluvinage, 2021; Qin et al., 2016), or nondestructive testing methods with the assessment of physical properties and acoustic emission registration (Calabrese et al., 2020; Zvirko and Tsyrulnyk, 2021). In the present study, we used both approaches. Standard tensile mechanical properties, fatigue and impact characteristics were determined and physical characteristics - acoustic emission and magnetic parameters - were evaluated. This allowed us to relate the stages of material fracture to the stages of change in physical characteristics.

Nomenclature A f

tensile fracture energy

slope coefficient of the amplitude distributions of the acoustic emission (AE) signals

b AE

Σ N AE cumulated number of AE events Ṅ AE intensity of AE signals H r

intensity of the resulting self-magnetic field

magnetic field intensity estimated by the eddy current method

H EC

coercive force

H C

1. Materials and methods Two steel grades widely used in industry were selected as materials for the study – bainitic 15Cr2MnMoV steel and austenitic corrosion-resistant 12Cr18Ni10Ti steel. The study was carried out under tension of dog-bone shaped flat specimens with reduced section sizes of 40×10×5 mm (15Cr2MnMoV steel) and 80×20×6 mm (12Cr18Ni10Ti steel). Table 1 presents the chemical composition and standard mechanical properties of the investigated steels. Bainitic 15Cr2MnMoV is used as a material for oil sucker rods. We investigated this steel in three states: initial, after operation in a hydrogen sulfide-containing environment for 150 days in the Baituganskiy oilfield of the Orenburg region of Russia, and after hydrogen charging, which was carried out following the ASTM G39 standard in NACE TM0177 solution A for 1 week without load and at a load of 0.1 and 0.2 of the yield strengths under 4-point bending conditions. Hydrogen charging was carried out to enhance the structural degradation effect. Austenitic 12Cr18Ni10Ti steel specimens were studied in two states: in the initial state, a 6 mm sheet, and after operation as a material of vacuum chamber that was taken out of service after six years due to a vacuum leak.

Table 1. Chemical composition (%, mass.) and standard mechanical properties of studied steels. Steel C Si Mn Ni Cu Cr Mo Ti El., %

σ YS , MPa

σ U , MPa

Bainitic (15Cr2MnMoV) Stainless (12Cr18Ni10Ti)

0.16 0.07

0.28

1.12

0.30 9.52

0.075 0.358

2.12

0.19

-

15 71

890 234

1040

0.278 0.909

18

0.246 0.35

593