With the development of steel, petrochemical, ship and electric power industries, welding structures tend to develop in the direction of large size, large capacity and high parameters, and some still work in low temperature, deep cold, corrosive media and other environments.

Therefore, a variety of low alloy high strength steel, medium, high alloy steel, super steel, as well as a variety of alloy materials are increasingly widely used.

However, with the application of these kinds of steel and alloy, many new problems are brought to welding production, among which welding cracks are more common and ten serious.

Cracks sometimes appear in the welding process, also sometimes appear in the process of placement or operation, and the so-called delayed cracks.  The damage is all the more serious because the cracks cannot be detected during manufacturing.  There are many kinds of cracks in the welding process. According to the nature of the cracks, it can be divided into the following five categories:

Hot crack 

Hot crack is produced at high temperature during welding, so it is called hot crack.    The morphology, temperature zone and main causes of thermal cracks are also different according to the different materials of the welded metal, so the thermal cracks are divided into three types: crystallization cracks, liquefaction cracks and polylateral cracks.

1. Crystal crack  

At the later stage of crystallization, the liquid film formed by low volume eutectic weakens the bond between grains and cracks occur under tensile stress.  

It is mainly produced in the welds of carbon steel and low alloy steel with more impurities (containing sulfur, phosphorus, iron, carbon and silicon on the high side) and single-phase austenitic steel, nickel base alloy and some aluminum alloys.  In rare cases, crystal cracks can also occur in the heat affected zone.

2. High temperature liquefaction crack  

Under the action of peak temperature of welding thermal cycle, remelting occurs between the heat affected zone and the layers of multilayer welding, resulting in cracks under the action of stress.  

It mainly occurs in high strength steel containing chromium and nickel, austenitic steel, and some nickel-based alloys near the seam area or between multi-layer welding.  When the sulfur, phosphorus and silicon in base metal and welding wire are too high, the tendency of liquefaction crack will be significantly increased.

In the solidified crystal front, lattice defects move and gather under the action of high temperature and stress, forming a secondary boundary. It is in a low plastic state at high temperature, and cracks are generated under the action of stress.  Polylateral cracks mostly occur in or near the weld zone of pure metal or single-phase austenite alloy, which is a type of hot cracks.

Second, reheat crack  

In the process of stress-relieving heat treatment or service at a certain temperature, the cracks in the coarse-grained part of the weld heat affected zone (HAZ) are called reheat cracks.  Reheat cracks mostly occur in coarse-grained parts of welding heat affected zone of low alloy high strength steel, pearlite heat resistant steel, austenitic stainless steel and some nickel-based alloys.