PSI - Issue 36

Ihor Dmytrakh et al. / Procedia Structural Integrity 36 (2022) 298–305 Ihor Dmytrakh et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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the metal, as well as the local concentration in the zones of high mechanical stresses, are the critical parameters that define the strength and fracture resistance of materials in the assigned testing conditions: Somerday et al. (2014). As it was demonstrated by Murakami et al. (2010), depending on the hydrogen concentration, we can observe the processes of different nature, such as the facilitation of plasticity at a very low concentration of diffusible hydrogen; hydrogen trapping at a higher concentration that leads to hydrogen embrittlement due to the limitation of the yield of material. Very high hydrogen concentration induces additional stresses in the material and also promotes such phenomena as blistering and so on. Consequently, the consideration of hydrogen concentration is mandatory for the correct evaluation of hydrogen effects on materials behaviour under the different operating conditions. The presented work removes this disadvantage because the tests were carried out under the known values of the volume concentration of hydrogen in the specimens.

Nomenclature σ

tensile stress in MPa yield stress in MPa fracture stress in MPa

 Y  f C H N H

hydrogen concentration in wppm

number cycles of hydrogen charging-discharging

total square of defects in μm 2

S

As the background of presented study, we consider our recent work (Dmytrakh et al. (2015)), where the ambiguous effects of the hydrogen concentration in specimens on the value of the plasticity limit was found. It was shown that there is some characteristic value of diffusible hydrogen concentration С H in the bulk of the ferrite pearlitic pipeline steel at which the mechanism of hydrogen influence changes, namely: near or below this value the decreasing of the yield stress takes place and above – the plasticity limit increases. For this steel such value equal to С H  0.23 wppm. Below, we studied this specific case ( С H  0.23 wppm) for clarifying and a possible explanation of its nature, based on the chain ‘hydrogen concentration – structure – mechanical behaviour’. 2. Material and methods The pipeline steel with nominal chemical composition (in weight %): C = 0.17 – 0.24; Si = 0.17 – 0.37; Mn=0.35 – 0.65; S < 0.04; remainder Fe was the object of present study. This material consists of the grains of ferrite-pearlite, typical for all steel of this class. The standard cylindrical tensile specimens with a diameter of 5 mm were manufactured from the real pipe. We have conducted the special tests of these specimens, which were preliminary hydrogen charged to the low concentration about С H  0.23 wppm and then fully hydrogen discharged before testing. For each series of tests, this procedure was repeated to obtain the specimens with the different number of cycles of hydrogen charging discharging N H , namely: 0, 2, and 5. The hydrogen charging of the specimens was made by the electrochemical method under cathodic polarisation at some constant potential E cath = const. With the aim to simulate the hydrogen entry under real operating conditions of the buried pipeline, the procedure described by Capelle et al. (2008) and Capelle et al. (2010) has been applied. The special deoxygenated, near-neutral pH NS4 solution, which is the model of underground water, was chosen as the electrolyte for the hydrogen charging of steel. The chemical composition of the NS4 solution is given in Table 1. Taking into account the situation of the freely corroding system that exists for the real pipeline, the potential of polarisation E cath was slightly more negative than the free corrosion potential E corr for a given steel, i.e.: E corr = – 600 mV (SCE) and E cath = – 800 mV (SCE). Under these conditions, the cathodic current density (hydrogen charging current density) is very low and its values vary within range i cath = 27…110 μ m/cm 2 , depending on the time of exposure. Here, we would like note that under such conditions it is not possible that the electrochemical charging can damage the specimen surface.