PSI - Issue 36

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 36 (2022) 306–312 Hryhoriy Nykyforchyn, Leonid Unigovskyi, Olha Zvirko et al. / Structural Integrity Procedia 0 (2021) 000 – 000

308

3

Table 1. Chemical composition of the VST3ps carbon steel in the as-received state and after operation. Elements (wt.%) C Si Mn Cr Ni Cu P S As-received steel 0.12 0.006 0.36 0.04 0.01 0.01 0.036 0.034 After operation 0.11 0.001 0.45 0.03 0.02 0.02 0.047 0.055

To evaluate mechanical properties of the steel, SSRT testing was carried out with strain rate  = 3×10 -4 s -1 using flat specimens with the working part thickness of 1.2 mm (Fig. 1), for which influence of deformation and fracture inside specimen on overall mechanical behaviour of the steel is minimized, as it was demonstrated by Yuan et al. (2012). The specimens were cut out from the pipe in the transversal direction to its axis, which enables taking into account the effect of rolling on anisotropy of mechanical properties of pipe steels, as shown by Marushchak et al. (2019).

Fig. 1. Scheme of a flat tensile specimen.

For both steel states, the following characteristics have been determined: strength (ultimate tensile strength and yield strength), plasticity (elongation and reduction in area) and the resistance to hydrogen assisted cracking by relative changes in plasticity characteristics as a result of preliminary hydrogenation of the specimens, evaluated by the parameter λ :

− H P P P

(1)

,

100%



=



where plasticity characteristics of steels are given for non-hydrogenated ( P ) and hydrogenated ( P H ) specimens. Specimens were preliminary hydrogenated in the H 2 SO 4 aqueous solution (рН 3.5) under a cathodic current density of 1 mА/сm 2 during 100 h. It was assumed that prolonged hydrogen charging and a small thickness of the specimens ensure the entire hydrogenation of their working parts and a more uniform hydrogen distribution. After completing hydrogenation, specimens were cleaned with acetone, dried and immediately loaded in air until fracture. Under such conditions, desorption of hydrogen from the metal during tensile testing was considered insignificant. The residual hydrogen content in steels was determined using hydrogen analyzer ELTRA H-500 at temperature 950 ºС. Specimens of 19.0×4.7×4.1 mm in size were polished, degreased with acetone, and then washed with ether. Residual ether was removed with a stream of hot air. Microfractographic investigations of the fracture surfaces of tensile specimens (both with and without preliminary hydrogenation) were performed using scanning electron microscope Carl Zeiss EVO-40XVP. Two zones at the macro fracture surface were distinguished: central part (perpendicular to the loading axis) and outlying one in the vicinity of the lateral surface (inclined to the loading axis). 3. Results and discussion Table 2 contains the results of tensile tests averaged from at least three tested specimens. Strength of the operated steel is lower (by 12 – 15%) comparing to the as-received one. Relatively low plasticity as for a given steel grade can