With the development of industries such as steel, petrochemicals, ships, and power, the welding structure has tended to be large, large capacity, and high parameters. Some work in low temperature, deep cold, and corrosive media.
Therefore, various low-alloy high-strength steels, medium and high-alloy steels, super-strength steels, and various alloy materials are increasingly used. However, with the application of these steel types and alloys, many new problems have been brought about in welding production, among which the more common and ten serious ones are welding cracks. Cracks sometimes appear in the welding process, and sometimes appear during placement or operation, and so-called delayed cracking. Because such cracks cannot be detected during manufacturing, the damage of such cracks is more serious. There are many kinds of cracks generated in the welding process. According to the current research, according to the nature of the cracks, they can be roughly divided into the following five categories:
1. Hot crack
Hot cracks are generated at high temperatures during welding, so they are called hot cracks. Depending on the material of the metal being welded, the shape, temperature range and main cause of the generated thermal cracks are also different. Therefore, thermal cracks are divided into three categories: crystalline cracks, liquefied cracks and multilateralized cracks.
1. Crystal crack
In the later stage of crystallization, the liquid film formed by the low-capacity eutectic weakened the connection between the grains and cracked under the action of tensile stress.
Mainly produced in welds of carbon steel and low alloy steel with high impurities (containing sulfur, phosphorus, iron, carbon, silicon is too high) and single phase austenitic steel, nickel-based alloys and some aluminum alloys in. In some cases, crystal cracks can also be generated in the heat affected zone.
2. High temperature liquefaction crack
Under the action of the peak temperature of the welding thermal cycle, remelting occurs between the heat affected zone and the layers of the multi-layer welding, and cracks are generated under the stress.
It mainly occurs in the near-seam area or multi-layer welding layer of high-strength steel containing chromium-nickel, austenitic steel, and some nickel-based alloys. When the sulfur, phosphorus, and silicon carbon in the base metal and the welding wire are high, the tendency of liquefaction cracks will increase significantly.
3. Multilateral cracking
The solidified crystalline front, under the action of high temperature and stress, lattice defects move and aggregate to form a secondary boundary, which is in a low plastic state at high temperature and cracks generated under the action of stress. Multilateral cracks mostly occur in the welds of pure metals or single-phase austenitic alloys or near the seam area. It is a type of thermal crack.
2. Reheat crack
Thick plate welded structure, and steel containing certain precipitation strengthening alloy elements, during the process of stress relief heat treatment or service at a certain temperature, the cracks that occur in the coarse-grained parts of the welding heat-affected zone are called reheat cracks. Reheat cracks mostly occur in the coarse-grained parts of the welding heat-affected zone of low-alloy high-strength steels, pearlite heat-resistant steels, austenitic stainless steels, and some nickel-based alloys.
Three, cold crack
Cold cracking is a relatively common type of cracking that occurs during welding. It is generated after cooling to a lower temperature after welding. Cold cracks mainly occur in the welding heat affected zone of low alloy steel, medium alloy steel, medium carbon and high carbon steel. In some cases, such as when welding ultra-high strength steel or certain titanium alloys, cold cracks also appear on the weld metal.
Depending on the type and structure of the steel being welded, there are different types of cold cracks, which can be roughly divided into the following three categories:
1. Delayed cracking
It is a common form in cold cracks. The main feature is that it does not appear immediately after welding, but has a delay characteristic of cracks produced by the common incubation period under the combined action of hardened structure, hydrogen and restraint stress.
2. Quenching crack
This kind of crack basically has no delay phenomenon, it is found immediately after welding, sometimes occurs on the weld, sometimes appears in the heat affected zone. Mainly has a hardened structure, cracks generated under the action of welding stress.
3. Low plastic embrittlement cracks
Some materials with low plasticity, when cooled to low temperature, the strain caused by the contraction force exceeds the plastic reserves of the material itself or the cracks caused by the material becoming brittle. Because it is generated at a lower temperature, it is also another form of cold cracking, but there is no delay.
Four, layered tear
In the manufacturing process of large oil production platforms and thick-walled pressure vessels, sometimes stepped cracks parallel to the rolling direction appear, so-called lamellar tears.
Mainly due to the presence of layered inclusions (in the rolling direction) inside the steel plate, the stress generated during welding perpendicular to the rolling direction results in a "step" type lamination in the heat affected zone slightly away from the fire Torn.
5. Stress corrosion cracking
Some welded structures (such as containers and pipelines) have delayed cracks caused by the combination of corrosive media and stress. The factors that affect stress corrosion cracking include the material of the structure, the type of corrosion medium, the shape of the structure, the manufacturing and welding process, the welding material, and the degree of stress relief. Stress corrosion is generated during service.
Analysis of types and basic characteristics of welding cracks
