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Weld cold crack

Welded joints cooled to a lower temperature ( for steel, in the martensitic transformation start temperature Ms less ) generated by the weld cracking.

Commonly known as cold cracking, which is divided into delayed cracking, thermal stress cracking and lamellar tearing three kinds. Cold cracks sometimes appear immediately after welding, but sometimes after a few hours, days, or even longer to appear. These weld after a period of time before the emergence of crack called delayed cracking. Delayed cracking in the manufacturing process may not be found, and in the process of using it may cause extremely serious consequences. So it is more harmful than crack.

Judging from the cold crack forms have the following types: boundary cracks, under-bead cracks and root cracks. Boundary cracks from weld and base metal junction began to extend the base material. Under-bead cracks in the weld seam below the near area, not developed to the base metal surface. Cracks form originated in the root of the weld notch stress concentration at the heat affected zone, which extends into the base metal or welds.

Cold crack causes are analyzed as follows:
1, the role of hydrogen
In the welding temperature, some of the hydrogen atoms in the compound of the state of the precipitation of hydrogen resolution, large amount of hydrogen dissolved in the metal bath. As the bath temperature dropped, the solubility of hydrogen in the metal drastically reduced. However, quickly cooling of the weld pool, the hydrogen too late to escape and remains in the weld metal. Hydrogen in austenite and ferrite in the solubility and diffusion capacity are also significant differences.

2, quenching effect
Near the weld seam zone or cold cracks formed on the metal phase transition process and the mechanical properties of rapidly changing and complex stress state. Cold cracking occurs mainly in medium carbon steel, high carbon steel and high strength steel. The main features of this type of steel is easy to quenching, the austenite serious overheating , the grains grow significantly. Learn from metal shows that the austenite grain coarsening easier quenched into coarse martensite, the performance deterioration of metal near the weld area, especially the plastic drop fragility. At this time in the role of complex welding stress occurs under cold cracking.

Usually the carbon equivalent of the weld metal is lower than the base material, and thus the weld occurs at a higher temperature austenite decomposition, then near the weld area has not yet occurred austenite. Since the solubility of hydrogen in the weld metal sudden drop. As the temperature decreased, near the weld region of austenite transformation, the temperature has very low solubility of hydrogen lower and has very weak diffusion. So then with hydrogen gas state into the fine pores of the metal and cause a lot of pressure, so that the local metal with great stress, and thus the formation of cold cracks.





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