Formation and characteristics of welding cracks
Under the joint action of welding stress and other brittleness factors, welding crack is the new interface caused by the destruction of the metal atom bonding force in the local area of the welding joint.
It is characterized by sharp notches and a large aspect ratio.
Classification of crack
1) Cracks can be divided into macro crack, micro crack and micro crack according to their size;
2) according to the production conditions and timing is divided into: cold crack and hot crack, reheat crack, lamellar tear and stress corrosion crack.
A. hot crack
A crack occurring near the AC3 line.Generally, it appears after welding, also known as crystallization crack.This kind of crack mainly occurs in grain boundary, the crack surface has oxidation color, loses the metallic luster.
B. cold crack
Refers to the cold after welding to the martensite transition temperature MS point below the crack, generally in a period of time after welding (a few hours, a few days or even longer), so it is also called delay crack.
C. Reheat crack
Cracks occur when the joint is cooled and reheated to 500 ~ 700℃.The reheat crack occurs in the coarse grain zone of the welding heat affected zone of the precipitation-strengthened materials (such as metals containing Cr, Mo, V, Ti and Nb) and generally develops from the fusion line to the coarse grain zone of the heat affected zone, showing the characteristics of intergranular cracking.
D. Lamellar tear
Stepwise cracks occurring in thick and large members with T or corner joints along the rolling direction of the plate.Lamellar tearing is mainly due to the steel in the rolling process, sulfide (MNS), silicate and other impurities caught in it, the formation of anisotropy.Under the use of welding stress or external confinement stress, metal along the rolling direction of debris cracking.
E. Stress corrosion crack
Cracks produced by the combined action of stress and corrosive media.Apart from residual stress or confining stress, stress corrosion crack is mainly related to the composition and shape of weld microstructure.
Hazard of crack
Crack is one of the most dangerous welding defects.
The crack is a kind of area defect, (the defect with three dimensional size is called volume type defect, such as: air hole, slag inclusion;Defects with two-dimensional dimensions are called areal defects, such as cracks and non-fusion. The appearance of these defects will significantly reduce the bearing area. What is more serious is that a sharp notch is formed at the end of the crack, and the stress is highly concentrated, which is easy to expand and lead to damage.
Factors affecting crystallization crack and prevention measures
Factors affecting crystallization crack:
A. Influence of alloying elements and impurity The increase of carbon and impurity elements such as sulfur and phosphorus will expand the sensitive temperature zone and increase the chance of crystallization crack.
B. Influence of cooling rate The increase of cooling rate will aggravate crystallization segregation and increase the crystallization temperature range, both of which will increase the chance of crystallization crack.
In the brittle temperature zone, the strength of the metal is extremely low, and the welding stress makes this part of the metal tensile, when the tensile stress reaches a certain degree, there will be a crystallization crack.
Prevention and control measures:
A. Reduce the content of harmful elements such as sulfur and phosphorus, and use materials with low carbon content for welding.
B. Add certain alloying elements to reduce columnar crystals and segregation.Such as molybdenum, vanadium, titanium, niobium and so on can refine the grain.
C. Adopt welding seam with shallow melting depth to improve heat dissipation conditions so that low melting point material floats on the surface of the welding seam instead of existing in it.
D. Reasonable selection of welding specifications, and the use of preheat and post-heat, reduce the cooling rate.
E. Adopt reasonable assembly sequence to reduce welding stress.
2021-04-01 
