With the development of steel, petrochemical, ship and electric power industries, the welded structures tend to develop in the direction of large-scale, large-capacity and high-parameters, and some are still working in low temperature, cryogenic, corrosive media and other environments.
Therefore, various low-alloy high-strength steels, medium- and high-alloy steels, super-strength steels, and various alloy materials are widely used. However, with the application of these steel grades and alloys, many new problems are brought about in welding production, among which the most common and most serious is welding cracks.
Cracks sometimes appear during welding, and sometimes during placement or operation, and are so-called delayed cracks. Because such cracks cannot be detected in manufacturing, the damage of such cracks is even 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:
No1. Hot crack
Hot cracks are produced at high temperatures during welding, so they are called hot cracks. According to the material of the welded metal, the shape, temperature range and main reason of the hot crack are also different. Therefore, the hot crack is divided into three categories: crystallization crack, liquefaction crack and polygonal crack.
1. Crystal cracks
In the late stage of crystallization, the liquid film formed by the low volume eutectic weakens the connection between grains, and cracks occur under the action of tensile stress.
It mainly occurs in the welds of carbon steel and low-alloy steel with more impurities (high content of sulfur, phosphorus, iron, carbon, and silicon) and the welds of single-phase austenitic steel, nickel-based alloys and some aluminum alloys middle. In individual cases, crystalline cracks can also occur in the heat-affected zone.
2. High temperature liquefaction crack
Under the action of the peak temperature of the welding thermal cycle, remelting occurs in the heat-affected zone and between the layers of the multi-layer welding, and cracks are generated under the action of stress.
It mainly occurs in high-strength steels containing chromium and nickel, austenitic steels, and some nickel-based alloys in the near seam zone or between multi-layer welds. When the content of sulfur, phosphorus and silicon carbon in the base metal and welding wire is high, the tendency of liquefaction cracking will increase significantly.
3. Multilateralized cracks
In the solidified crystallization front, under the action of high temperature and stress, the lattice defects move and aggregate to form a secondary boundary, which is in a low plastic state at high temperature, and cracks are generated under the action of stress.
Multilateralized cracks mostly occur in the welds of pure metals or single-phase austenitic alloys or in the near-weld region, and they belong to the type of hot cracks.
No2. Reheat cracks
For steels with thick-plate welded structure and some precipitation-strengthening alloying elements, the cracks that occur in the coarse-grained parts of the welding heat-affected zone during stress relief heat treatment or service at a certain temperature 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, pearlitic heat-resistant steels, austenitic stainless steels and some nickel-based alloys.
No4. Cold cracks
Cold cracks are a common type of cracks produced in welding, which are produced when the temperature is cooled 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 individual cases, such as when welding ultra-high-strength steels or certain titanium alloys, cold cracks also appear on the weld metal.
According to the different steel types and structures to be welded, there are also different types of cold cracks, which can be roughly divided into the following three categories:
1. Delayed crack
It is a common form of cold crack, the main feature is that it does not appear immediately after welding, but has a general incubation period, and is a crack with delayed characteristics generated under the combined action of hardened structure, hydrogen and restraint stress.
2. Quenching cracks
This kind of crack is basically not delayed, it is found immediately after welding, sometimes it occurs in the weld, sometimes it occurs in the heat affected zone.
Mainly there is a hardened structure, cracks generated under the action of welding stress.
3. Low plastic embrittlement crack
For some materials with low plasticity, when cold to low temperature, the strain caused by the shrinkage force exceeds the plastic reserve of the material itself or the cracks caused by the material becoming brittle. Because it is produced at a lower temperature, it is also another form of cold crack, but there is no delay phenomenon.
No4. Laminar tearing
In the manufacturing process of large oil production platforms and thick-walled pressure vessels, step cracks parallel to the rolling direction sometimes appear, so-called laminar tearing.
Mainly due to the existence of layered inclusions (along the rolling direction) inside the steel plate, and the stress perpendicular to the rolling direction generated during welding, resulting in a "stepped" layered shape in the heat-affected zone farther away from the fire. torn.
No5. Stress corrosion cracking
Some welded structures (such as vessels and pipes) are delayed cracks produced by the combined action of corrosive media and stress.
The factors affecting 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 occurs during service.