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There are many kinds of cracks about welded steel pipe in the welding process. According to the nature of the cracks, they can be divided into the following five categories
Hot crack
Hot crack is produced at high temperature in welding process, so it is called hot crack. Different welding materials have different shape and temperature zone of hot crack.
1.Crystal crack
The liquid film formed by low solution eutectic weakens the connection between grains and causes cracking, which mainly exists in the weld of carbon steel and low alloy steel with more impurities. In some cases, crystallization cracks can also occur in the heat affected zone.
2.High temperature liquefaction crack
Under the action of thermal cycle, remelting occurs between HAZ welding and multi-layer welding, resulting in cracks, which mainly exist in high strength steel containing chromium and nickel, austenitic steel and near seam zone or multi-layer welding layer of some nickel based alloys.
3.Multilateral crack
Under the action of high temperature, the secondary boundary of lattice defects is formed, and cracks are produced under the action of stress, which mostly occur in pure metal or single-phase austenite alloy or near crack zone.
Reheat crack
Reheat cracks occur in the coarse-grained parts of HAZ welding of steels containing some precipitation strengthening alloy elements after stress relief heat treatment or used at a certain temperature. They mainly occur in the coarse-grained parts of HAZ welding such as low-alloy high strength steel and austenitic stainless steel.
Cold crack
Cold crack is a kind of common crack in welding, which is produced at a lower temperature after welding. It mainly occurs in the heat affected zone of low, medium alloy steel, medium carbon steel and high carbon steel. In some cases, such as welding ultra-high strength steel or some titanium alloy, cold crack will also appear on the weld metal.
Lamellar tearing
In the manufacturing process of thick wall pressure vessel, the step cracks parallel to the rolling direction appear, the layered inclusions exist in the steel plate, the stress perpendicular to the rolling direction is generated during welding, and the "step" lamellar tearing occurs.
Stress corrosion cracking
Under the joint action of corrosive medium and stress, some welding structures will produce delayed cracks. The material and shape of the structure, the type of corrosive medium, manufacturing and welding process, welding materials, and the degree of stress relief are the factors that affect the stress corrosion cracks. Stress corrosion is produced in the service process.
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