Reliable establishment of the technical condition of metal structures, including gas pipelines is based on three fundamentals: definition of the level of defectiveness, establishment of the stress-strain state, and the degree of degradation of steels mechanical characteristics. If the first two fundamentals shoot the moon in recent years, this cannot be said about the degree (magnitude) of degradation of the mechanical characteristics. To date, the main phenomena were established, which cause damage and crack formation in steel trunk pipelines under the influence of operational factors. Most of these phenomena are associated with hydrogenation factors, the role of which depends on the source of hydrogen entering the metal, and the level of actual stresses.
The main hydrogenation factors are considered: stress corrosion cracking, hydrogen corrosion, delamination, hydrogen brittleness causing damage growth in steel and cracking in the structures, and in some cases their destruction. On the example of low-alloy steel grade 17GS, it is shown that the development of stress corrosion cracking leads to a change in the mechanism of crack propagation: a significant increase in the share of intergranular failure in the fracture, primarily in the rolling plane. As a result, the anisotropy coefficient of metal impact strength increases and depends not only on the carbon (perlite) and sulfur (sulfide) contents in steel, but also on the degree of weakening of the ferrite grain boundaries.
Using the example of an oil pipe with a diameter of 530 mm, the effect of hydrogen corrosion on the fracture mode of impact specimens is studied. It is established that the presence of hydrogen in the metal leads to the appearance of KCV — B bond, where B is the fraction of the viscous component in the fracture of two linear sections. Section I is characterized by low-energy fracture (4.4–15 J/cm2), where a shear fracture mechanism is acting along the rolling planes. In section II, a ductile cleavage mechanism is acting with high work (32–246 J/cm2) of crack propagation. It is shown that determining the degree of intergranular embrittlement of steel makes it possible to more reliably establish the technical condition of the structure and its residual life.
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