With the development of iron and steel, petrochemical, ship and electric power industry, the welding structure tends to be large-scale, large capacity and high parameters. Some of them are still working in low temperature, cryogenic, corrosive media and other environments.
Therefore, all kinds of low alloy high strength steel, medium and high alloy steel, super strength steel, and all kinds of alloy materials are widely used. However, with the application of these steel grades and alloys, many new problems have been brought about in welding production, among which welding cracks are more common and more serious. Cracks sometimes appear in the welding process, sometimes in the process of placement or operation, and the so-called delayed cracks. Because this kind of crack can not be detected in manufacturing, so the harm of this kind of crack is more serious. There are many kinds of cracks in the welding process. According to the nature of cracks, the current research can be divided into the following five categories
1¡¢ Hot crack
Hot crack is produced at high temperature during welding, so it is called hot crack. According to the different materials of the welding metal, the shape, temperature zone and main causes of the hot cracks are also different. Therefore, the hot cracks can be divided into three categories: crystallization cracks, liquefaction cracks and multilateral cracks.
1. Crystal crack
At the later stage of crystallization, the liquid film formed by low volume eutectic weakens the connection between grains and cracks under tensile stress.
It is mainly produced in the welds of carbon steel and low alloy steel with more impurities (high sulfur, phosphorus, iron, carbon and silicon) and in the welds of single-phase austenitic steel, nickel base alloy and some aluminum alloys. In some cases, crystallization cracks can also occur in the heat affected zone.
2. High temperature liquefaction crack
Under the effect of peak temperature of welding thermal cycle, remelting occurs in the heat affected zone and between layers of multi-layer welding, and cracks are generated under the effect of stress.
It mainly occurs in high strength steel containing Cr and Ni, austenitic steel, and near seam zone of some nickel based alloys or between layers. When sulfur, phosphorus and silicon carbon in base metal and welding wire are high, the tendency of liquefying crack will increase significantly.
3. Multilateralization crack
In the solidification front, under the action of high temperature and stress, the lattice defects move and gather, forming a secondary boundary. It is in a low plastic state at high temperature, and cracks are generated under the action of stress. Multilateralization cracks usually occur in or near the weld of pure metal or single-phase austenitic alloy, which belongs to the type of hot cracks.
2¡¢ Reheat crack
For the steel with thick plate welded structure and containing some precipitation strengthening alloy elements, the cracks in the coarse-grained part of the welding heat affected zone are called reheat cracks during stress relief heat treatment or service at a certain temperature. Reheat cracks mostly occur in coarse grain of HAZ of low alloy high strength steel, pearlite heat resistant steel, austenitic stainless steel and some nickel base alloys.
3¡¢ Cold crack
Cold crack is a common kind of crack in welding, which is produced when the temperature is low after welding. Cold cracks mainly occur in the HAZ of low alloy steel, medium alloy steel, medium carbon steel and high carbon steel. In some cases, such as welding ultra-high strength steel or some titanium alloys, cold cracks also appear on the weld metal.
According to different types of steel and structures, cold cracks can be divided into three categories
1. Delayed crack
It is a common form of cold crack. Its main feature is that it does not appear immediately after welding, but has a general incubation period. Under the joint action of hardened structure, hydrogen and restraint stress, it produces a delayed crack.
2. Quenching crack
This kind of crack basically has no delay phenomenon. It is found immediately after welding, sometimes on the weld, sometimes in the heat affected zone. It is mainly composed of hardened structure and cracks under the action of welding stress.
3. Low plastic embrittlement crack
For some materials with low plasticity, when they are cold to low temperature, the strain caused by shrinkage force exceeds the plastic reserve of the material itself or the crack caused by material embrittlement. Because it is produced at a lower temperature, it belongs to another form of cold crack, but there is no delay phenomenon.
4¡¢ Lamellar tearing
In the manufacturing process of large oil production platform and thick wall pressure vessel, sometimes step cracks parallel to rolling direction appear, which is called lamellar tearing.
It is mainly due to the existence of layered inclusions (along the rolling direction) in the steel plate, and the stress perpendicular to the rolling direction during welding, which results in the "step" layered tearing at the place far away from the heat affected zone.
5¡¢ Stress corrosion cracking
Some welded structures (such as vessels and pipes) are subject to the combined action of corrosive medium and stress. The factors that affect the stress corrosion cracking include the material of the structure, the type of corrosive medium, the shape of the structure, the manufacturing and welding process, the welding material, and the degree of stress relief. Stress corrosion is produced in service.
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