API 2B Q235B STEEL PIPE DN700

"Congenital deficiency" is hard to avoid? How can internal defects accelerate cracking?

Even if the chemical composition and microstructure are qualified, the "congenital defects" in steel may become the "starting point" of hydrogen-induced cracking. In the process of steel smelting and rolling, if the process is not properly controlled, it is easy to produce internal defects such as porosity, porosity and inclusion. These defects are equivalent to "holes" in steel, and hydrogen atoms will preferentially diffuse and gather in these areas, because the stress concentration effect at the defects will reduce the escape energy of hydrogen.

Take the loose defects in steel as an example, the tiny pores connected inside will form "hydrogen accumulation channels", which will make the hydrogen concentration reach the critical value in a short time. The accident investigation of an oil and gas pipeline shows that the source of the crack is an inclusion defect with a diameter of only 0.1 mm inside the steel pipe. After hydrogen atoms gather here, it takes only four months to expand the tiny defect into a crack that runs through the pipe wall. It is worth noting that these internal defects are often hidden and difficult to find by routine inspection, but they will continue to "amplify" the harm under the action of hydrogen and become "untimely bombs" for the safe operation of steel pipes.