Cold crack is a serious flaw which may arise in the process of line pipe welding. Cold cracking generally appears during the cooling process, near the martensitic transformation start temperature Ms point or below the temperature range, and more occurs below 100. Cold cracks can appear immediately after welding, or often emerges over time, and thus tend to have delayed cold cracking characteristics.
Mass of practical and theoretical production studies have shown that the tendency of hardened steel, hydrogen content and stress distribution in welded pipe joints and welded joints are the three main factors that cold cracking appearing when welding pipeline. So, in essence, welding cold cracks induced embrittlement.
Because HAZ has a larger hardening tendency and gap effect, so cold cracks occurs mainly in the pipeline of the heat affected zone. Meanwhile, the cold cracks may also occur in the weld. Cold cracks are often generated at grain boundaries, with a large number of expansion along the grain, but sometimes also be extended along the grain boundaries.
Pipeline pipe molding usually low hydrogen weld double submerged arc welding method(lsaw steel pipe and sawh steel pipe), meanwhile the welding heat input is relative great, the cooling speed after welding is lower than spot ring welding. And the heat affected zone is not easy to appear organize with high hardness, so cold crack infrequently appears.
Pipeline spot welding mostly uses thin ring cellulose SMAW, easily lead to penetrating of large amount of hydrogen, meanwhile low heat input, cooling faster, easier to produce high hardness, low toughness low temperature transformation products, thereby increasing the cold crack sensitivity.
