In the welding seam or the near seam area, due to the influence of welding, the atomic combination of the material is destroyed, and the seam formed by the formation of a new interface is called a welding crack, which is characterized by a sharp gap and a large aspect ratio.
Cracks can be divided into hot cracks, cold cracks, stress corrosion cracks and lamellar tearing according to the temperature and time of occurrence. In welding production, there are many places where cracks occur. Some cracks appear on the surface of the weld and can be observed with the naked eye; some are hidden inside the weld and can only be found through flaw detection; some occur on the weld; and some occur in the heat-affected zone. It is worth noting that cracks sometimes occur during the welding process, and sometimes appear after the weldment is placed or operated for a period of time after welding. The latter is called delayed cracks, which are more harmful. The locations and types of common cracks are shown in the figure below.
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The location and type of common cracks
2. The hazards of welding cracks
Welding crack is the most harmful defect. In addition to reducing the bearing capacity of the welded joint, the sharp gap at the end of the crack will cause serious stress concentration, promote the expansion of the crack, and eventually lead to the destruction of the welded structure, and the product will be scrapped. cause serious accidents. In general, cracks are an impermissible defect in welded joints. Once found, it should be completely removed and repaired and welded.
3. Causes and prevention measures of welding cracks
Due to the different causes and formation mechanisms of different cracks, the three types of hot cracks, cold cracks and reheated cracks will be discussed separately below.
3.1, hot crack
Thermal cracks generally refer to cracks generated at high temperatures (from near the solidification temperature range to above the A3 line on the iron-carbon balance diagram) as shown in the figure below, also known as high-temperature cracks or crystallization cracks.
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Hot cracks usually occur within the weld, and sometimes can also appear in the heat-affected zone, as shown in the figure.
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reason:
Due to the segregation phenomenon in the welding molten pool during the crystallization process, the low melting point eutectic and impurities form segregation in the liquid interlayer during the crystallization process, and the strength after solidification is also low. When the welding stress is large enough, the liquid interlayer will be released. Layers or newly solidified solid metal pull apart to form cracks.
In addition, if there are low-melting eutectics and impurities on the grain boundaries of the base metal, these low-melting compounds will melt to form a liquid interlayer in the heat-affected zone where the heating temperature exceeds its melting point. When the welding tensile stress is large enough , will also be pulled apart to form liquefaction cracks in the heat-affected zone.
In short, the occurrence of thermal cracks is the result of the combined effects of metallurgical and mechanical factors.
Prevention:
Measures to prevent thermal cracks can start from two aspects of metallurgical factors and mechanical factors.
Control the content of harmful elements and impurities in the base metal and welding consumables
Limit the content of easily segregated elements and harmful impurities in base metal and welding materials (including welding rod, welding wire, flux and shielding gas). In particular, the content of impurity elements such as sulfur and phosphorus should be controlled and the carbon content should be reduced.
Sulfur is practically insoluble in steel, and it forms iron sulfide (FeS), which has a low melting point, with iron. During welding, the presence of iron sulfide will lead to hot cracking of the weld and liquefaction cracks in the heat-affected zone, which will deteriorate the welding performance; the same sulfur exists in the grain boundary in the form of a film, which will reduce the plasticity and toughness of the steel. Generally, the sulfur content in the steel used for welding should not exceed 0.045%. Sometimes tighter controls are required.
Phosphorus will reduce the plasticity and toughness of steel, increase the brittle transition temperature of steel, and cause cracks in welds and heat-affected zones. Phosphorus content should not exceed 0.055%. Sometimes tighter controls are required.
The welding performance of materials is closely related to the carbon content. The higher the carbon content of the steel, the poorer the weldability. It is generally believed that the carbon content in the weld is controlled below 0.10%, and the thermal crack sensitivity can be greatly reduced.
Adjust the chemical composition of the weld metal, improve the structure of the weld, refine the grain of the weld to improve its plasticity, reduce or disperse the degree of segregation, and control the harmful effects of low melting point eutectic.
For example, when welding austenitic stainless steel, the use of austenite plus ferrite dual-phase structure weld can improve its thermal cracking resistance. The single-phase austenitic weld seam is prone to hot cracks.
Use basic welding rod or flux to reduce the impurity content in the weld and improve the degree of segregation during crystallization.
Control the welding specification, appropriately increase the shape factor of the weld, adopt multi-layer multi-pass welding method, avoid centerline segregation, and prevent centerline cracks. When welding, the ratio of the weld width to the weld thickness on the single-pass weld section is called The shape factor, or weld form factor, of the weld. When the shape factor of the weld seam is too small, the weld seam is narrow and deep, and impurities with low melting point will gather in the center of the weld seam, which greatly increases the possibility of thermal cracks. When the shape factor of the weld seam is large, the weld seam is wide and shallow, Low-melting eutectics and impurities collect in the near-surface region of the weld, greatly reducing the propensity for centerline cracking.
Take measures to reduce welding stress
Take various technological measures to reduce welding stress, such as adopting reasonable welding sequence and method, using smaller welding input energy, overall preheating and hammering method, etc.
Filling the arc crater during arc closing can avoid arc crater cracks.
3.2, cold crack
Cold cracks generally refer to the cracks generated by the weld below the A3 temperature during the cooling process. The temperature at which cracks are formed is usually below 300~200°C, which is within the range of martensitic transformation temperature, so it is called cold crack.
Cold cracks can appear immediately after welding, or after a long time after welding, so they are also called delayed cracks. Since the generation of cold cracks is related to hydrogen, it is also called hydrogen-induced cracks. The generation of cold cracks has a delayed nature, which may cause unexpected serious accidents. Therefore, it is more dangerous and must be given full attention.
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Causes of cold cracks
The basic conditions for the formation of cold cracks are: the formation of hardened structure in welded joints; the existence and concentration of diffusible hydrogen; and the existence of large welding tensile stress. These three conditions influence each other and promote each other. Under different circumstances, any of the three factors may lead to the generation of cold cracks, among which diffusible hydrogen is the most active factor inducing cold cracks.
Cold crack prevention measures
1) Use basic electrodes or fluxes to reduce the diffusible hydrogen content in the weld metal. Alkaline electrodes are also called low-hydrogen electrodes, which can reduce the hydrogen content in the weld metal.
2) Electrodes and flux should be dried in strict accordance with the specified requirements before use. In addition, the groove and welding wire should be carefully cleaned to remove oil, water and rust spots to reduce the source of hydrogen.
3) Choose reasonable welding specifications and heat input, such as preheating before welding, controlling interlayer temperature, slow cooling after welding, etc., to improve the organizational state of the weld and heat-affected zone.
4) Carry out heat treatment in time after welding. One is to carry out annealing treatment to eliminate internal stress, temper the quenched structure and improve its toughness; the other is to carry out hydrogen elimination treatment to fully escape hydrogen from the welded joint.
5) Improve the quality of the steel, reduce the layered inclusions in the steel, and take measures from the structural design and welding process to reduce the welding tensile stress in the direction of the thickness of the plate, which can prevent layered tearing.
6) Take various technological measures to reduce welding stress (see thermal cracks, preventive measures for details)
3.3, reheat crack
Reheat cracks originate from the coarse-grained zone in the heat-affected zone of welding, which is characterized by grain boundary fracture. Most of the cracks occur in the stress concentration parts. Generally, it is formed when the weld area is heated again, so it is called reheat crack.
Causes of reheat cracks
The reason for reheating cracks is generally believed to be that during reheating, supersaturated solid-solution carbides (mainly carbides of vanadium and molybdenum) precipitate again during the first heat process, resulting in intragranular strengthening and slippage. The strain concentrates on the former austenite grain boundaries. Reheat cracks form when the plastic strain capacity of grain boundaries is insufficient to withstand the strains induced during stress relaxation.
Reheat crack prevention measures
1) Reduce residual stress and stress concentration, such as increasing the preheating temperature, slow cooling after welding, and smooth transition between the weld and the base metal.
2) Under the premise of meeting the design requirements, select the appropriate welding material so that the high-temperature strength of the weld metal is slightly lower than that of the base metal, allowing the stress to relax in the weld and avoiding cracks in the heat-affected zone.
3) In the case of ensuring the joint strength at room temperature, increase the annealing temperature for stress relief, resulting in the precipitation of relatively coarse carbide particles to improve high temperature ductility.
