API 2B Q235B STEEL PIPE DN600

Why does microstructure become a "hotbed of crack source"

If chemical composition is the "innate gene" of steel, microstructure is its "acquired constitution", and improper microstructure will become a "hotbed" of hydrogen-induced cracking. High-grade steels such as X70 and X80 commonly used in LSAW steel pipes will form hardened structures such as martensite and bainite if the heat treatment process is deviated. The lattice distortion of these structures is serious, and there are a lot of internal stresses, and the solubility of hydrogen in them is high and the diffusion speed is slow, which is easy to form a "hydrogen trap".

After hydrogen atoms are trapped in these traps, they will accumulate to form high-pressure hydrogen molecules. When the local pressure reaches more than 300MPa, microcracks will occur. On the contrary, the uniform and fine ferrite-pearlite structure has better hydrogen resistance because of its regular lattice arrangement, small internal stress and difficult aggregation of hydrogen atoms. Due to insufficient tempering temperature in a steel pipe factory, a lot of bainite structures appeared in a batch of X80 steel pipes. As a result, the qualified rate plummeted from 98% to 62% in the hydrogen-induced cracking test, which fully confirmed the key role of microstructure. In addition, the grain size will also affect the risk-coarse grains will reduce the grain boundary area, increase the concentration of hydrogen atoms at the grain boundary, and increase the risk of cracking.