What are the cracks that occur in steel pipe welding?
Posted by zora li on November 21st, 2023
What are the cracks that occur in steel pipe welding?
Welding cracks are the most common serious defects in steel pipe welding (such as seamless pipes and welded pipes). Under the combined action of welding stress and other brittle factors, the binding force of metal atoms in the local area of the welded joint is destroyed, forming a gap formed by a new interface. It features sharp notches and a large aspect ratio. Cracks affect the safe use of welded steel pipes and are very dangerous process defects.
Welding cracks not only occur during the welding process, some have a certain incubation period, and some occur during the reheating process after welding. Welding cracks can be classified differently based on their location, size, cause and mechanism of formation. According to the crack formation conditions, it can be divided into four categories: hot cracks, cold cracks, reheat cracks and lamellar tears.
It is mostly produced at high temperatures close to the solidus line and has the characteristics of being distributed along grain boundaries; but sometimes it can also be formed along "polygonal boundaries" at temperatures lower than the solidus line. Hot cracks usually occur in the weld metal, but can also form in the weld metal (base metal) near the weld fusion line. According to the characteristics of its formation process, it can be divided into the following three situations.
(1) Crystal cracks
It occurs in the "brittle temperature" interval at the end of the weld metal crystallization process, when a thin layer of liquid exists between the grains. Therefore, the plasticity of the metal is extremely low, and when the tensile deformation caused by uneven cooling shrinkage exceeds the allowable value, the liquid layer breaks along the grain boundaries. The main metallurgical measures to eliminate crystallization cracks are to adjust the composition, refine the grains, and strictly control the impurity elements that form low-melting point eutectics, etc., to improve the plasticity of the material in the brittle temperature range. In addition, the internal tensile deformation in this temperature range should be minimized from the design and process.
It mainly occurs in the base metal near the fusion line of the weld, and sometimes also occurs in the first pass of multi-layer welding. The reason for the formation is that the metal outside the weld fusion line is partially melted along the grain boundary under the action of welding heat, and the subsequent cooling shrinkage causes the cracking of the liquefied layer along the grain boundary. The principle to prevent such cracks is to strictly control the impurity content, rationally select welding materials, and minimize the impact of welding heat.
(3) Multilateral cracks
Formed at temperatures below the solidus. Its characteristic is that it is distributed along the "polygonal grain boundary" and has no obvious relationship with the primary grain boundary. The way to eliminate this defect is to add alloy elements that can increase the activation energy of polygonalization, such as adding W, Mo, Ta, etc. to Ni-Cr alloy; on the other hand, it is to reduce overheating and welding stress during welding.
According to the main causes, it can be divided into quenching cracks, hydrogen-induced delayed cracks and deformation cracks.
Cracks occur near the martensitic transformation point (Ms) of the steel pipe or below 200°C. It mainly occurs in medium and high carbon steel pipes, low alloy high strength steel pipes, titanium alloy steel pipes, etc. The main parts are in the heat affected zone and weld metal. The crack direction is intergranular or transgranular.
The main factors causing cold cracks are:
①Steel pipes have high hydrogen content;
② Brittle tissue or tissue sensitive to hydrogen embrittlement;
③Welding restraint stress (or strain).
(2) Hydrogen-induced delayed cracking
During the welding process, the dissolved hydrogen in the weld metal diffuses and segregates to the heat-affected zone, and is especially enriched in the triaxial tensile stress concentration zone that is prone to cracking, causing hydrogen embrittlement. That is, the critical stress of the metal at the opening position (or crack front) is reduced. When the local stress here exceeds the critical stress, cracking will occur.
Preventive measures include:
① Reduce the hydrogen content in the weld, such as using low-hydrogen welding rods, strictly drying welding materials, etc.;
② Reasonable preheating and postheating;
③ Choose raw materials with lower carbon equivalent;
④ Reduce restraint stress and avoid stress concentration.
(3) Deformation cracks
The formation of such cracks is not necessarily due to the high hydrogen content. In the case of strain concentration in multi-layer welds or fillet welds, the tensile strain exceeds the plastic deformation capacity of the metal.
Produced in the reheating process of some low-alloy high-strength steel pipes, pearlitic heat-resistant steel pipes, austenitic stainless steel pipes, and nickel-based alloy steel pipes after welding. Such cracks have the characteristics of intergranular cracking, and all occur in the coarse-grained zone of the heat-affected zone with severe stress concentration. In order to prevent the occurrence of such cracks, firstly, materials with low reheating crack sensitivity should be selected during design, and secondly, internal stress and stress concentration problems near the fracture area should be minimized in terms of technology.
Mainly occurs in thick plate fillet welding. It is characterized by being parallel to the surface of the steel plate and developing in a stepped shape along the rolling direction. Such cracks are often not limited to the heat-affected zone, but can also appear in the base metal away from the surface. The main reason is that due to the layered distribution of non-metallic inclusions in the metal, the plasticity of the steel plate along the thickness direction is lower than that along the rolling direction. In addition, due to the large welding stress in the direction of plate thickness during fillet welding of thick plates, lamellar tearing is caused. To prevent such defects, the quantity and distribution of inclusions should be strictly controlled during the metallurgical process. In addition, improving the joint design and welding process also has a certain effect.