Using the magnetic anisotropy method to determine the boundaries of stress-corrosion failure and prevent hydrogen damage to pipe steels

Using the STRESSVISION scanner and the magnetic anisotropy method (MAM), stresses remaining in a thin (up to 0.5 mm) surface layer of plates made of 08ps, 20 and 17GS pipe steels after their electrolytic hydrogenation in a stressed state in a 5 % H2SO4 solution with the addition of 1.5 g/l CS(NH2)2, as in the process simulating the incubation stage of stress corrosion of main gas pipelines were determined. It was found that the effect of hydrogen on the plates of all analyzed steels has the same effect as their stretching under three-point bending and uniaxial tension, i.e. leads to an increase in the MAM signal. This allowed to conclude that hydrogenation is accompanied by the appearance of tensile stresses in the metal, the value of which (up to 50 MPa) is close to that described in the literature (from 40 to 70 MPa). According to metallographic studies, the cause of the above stresses was changes in the structure of the surface layer of steels. Based on the results of magnetic anisotropic measurements and metallographic analysis carried out on a fragment of a pipe measuring 63×5 mm made of 35G2F steel with defects after operation characteristic of stress corrosion, it was established that the structure of the metal in areas of the outer surface of the fragment characterized by reduced values of residual compressive stresses is distinguished by an increased proportion of the light component - ferrite, as well as a decrease in microhardness (from ~ 350 to ~ 320 HV), which corresponds to the process of decarburization of ferrite-pearlite steel during stress corrosion. A conclusion was made on the possibility of using MAM to record stresses arising in the surface layers of pipe steels as a result of hydrogenation, and, as a consequence, to develop a conclusion on the changes that occurred in the structure of the pipe material as a result of stress corrosion.