In-service degradation of gas trunk pipeline X52 steel

We study the mechanical and corrosion-mechanical properties and the behavior of hydrogen in X52 steel in the intact state and after operation for 30 yr. It is shown that the long-term operation of gas mains made of this steel is accompanied by the degradation of the metal in the bulk of the pipe wall. This is manifested, first of all, in the decrease in impact toughness, relative narrowing ψ, fracture toughness, and corrosion-cracking resistance. It is shown that the in-service embrittlement of the metal is characterized by the presence of two anomalies in its mechanical behavior, namely, its brittle fracture resistance decreases parallel with the drop of hardness and the decrease in ψ is accompanied by the increase in relative elongation. The characteristics of crack growth resistance are especially sensitive to the in-service degradation of the material. The detection of degradation is more efficient under the action of embrittling factors, such as the severe stress-strain states, low testing temperatures, and the presence of corrosive media. In general, the degradation is more pronounced for the bottom part of the operating pipes, which is explained by the negative effect of hydrogen penetrating into the metal from the transported media. A hypothesis is advanced concerning the microdamage of operating steel as the principal factor of degradation of pipes as a result of long-term operation. Note that the results of investigation of hydrogen permeability and the temperature dependences of the extraction of hydrogen from the metals in different states are in good agreement with the proposed hypothesis.