Causes and preventive measures of various welding cracks in welded pipes

Causes and preventive measures of various welding cracks in welded pipes

Welding crack is a kind of crack produced by the damage of the bonding force of the metal atoms in the local area of the welding joint under the joint action of welding stress and other brittle factors. According to the size of the crack, it can be divided into macro crack, micro crack and micro crack. According to the generation conditions and time mechanism, it can be divided into cold crack and hot crack, reheat crack, lamellar tear and stress corrosion crack.

Crack is one of the most harmful welding defects. It is a kind of area defect. Its appearance will significantly reduce the bearing area. What's more serious is the sharp notch formed at the end of the crack. The stress is highly concentrated, and it is easy to expand and cause damage. The harm of crack is very great, especially cold crack.

The hot crack occurs at the end of weld metal solidification, and the sensitive temperature zone is around the high temperature zone near the solidus. The common hot crack is crystal crack. The reason is that during the solidification process of weld metal, crystal segregation makes the eutectic with low melting point produced by impurities accumulate in the grain boundary, forming the so-called "liquid film". In the specific sensitive temperature zone (also known as brittle temperature zone), the crystal segregation makes the eutectic with low melting point accumulate in the grain boundary, Its strength is very small, and because of the solidification shrinkage of the weld, it is subjected to tensile stress, and then cracks to form cracks. The common situation of crystal cracks is along the length of the weld center, which is longitudinal cracks, sometimes occurs between two columnar crystals in the weld, which is transverse cracks.

Factors affecting crystal cracks and preventive measures

The factors influencing the crystal crack are as follows

Under the influence of alloying elements and impurities, the increase of carbon, sulfur, phosphorus and other impurity elements will expand the sensitive temperature zone and increase the chance of crystal cracks; under the influence of cooling rate, the increase of cooling rate will aggravate the crystallization segregation and increase the crystallization temperature range, both of which will increase the probability of crystal cracks; the influence of crystallization stress and restraint stress is at the brittle temperature When the tensile stress reaches a certain degree, the crystal cracks will appear.

Preventive measures:

Reduce the content of sulfur, phosphorus and other harmful elements, and weld with materials with low carbon content; add certain alloy elements to reduce columnar crystal and segregation, such as molybdenum, vanadium, titanium, niobium and so on, which can refine the grain, adopt the weld with shallow penetration, improve the heat dissipation conditions, and make the low melting point materials float on the weld surface but not exist in the weld; reasonably select the welding specification, and adopt preheating and post heating The reasonable assembly sequence is adopted to reduce the welding stress.

Mechanism of reheat crack formation and preventive measures

The characteristics of reheat cracks are as follows

The results show that the heat affected zone of welding produces the superheated coarse crystal region during the reheating process, and the temperature is 550-650 ℃, and the intergranular cracking is easy to occur, and the precipitation strengthened steel is easy to produce, which is related to the welding residual stress.

Preventive measures:

Reasonable preheating or afterheating, control cooling rate, reduce residual stress, avoid stress concentration, avoid or shorten sensitive temperature zone.