Causes, measures and methods of reheat cracks in welded joints

In recent years, the application of low alloy and high strength materials in special equipment has become more and more common, which is related to the working condition of high temperature and high pressure of boiler and pressure vessel. However, cracks are often found in the welding seam after heat treatment in the manufacturing process of special equipment, especially 2.25Cr-1Mo, 13Monimor and other materials, which has attracted the attention of manufacturers. 01 There are many types of cracks in welded joints Crystallization crack: During the solidification and crystallization of welding pool, the crack formed by the weld metal along the grain boundary of primary crystallization due to the action of crystallization segregation and shrinkage stress and strain in the temperature range where the liquid phase and solid phase coexist. Such cracks occur only in welds (including arc craters). Liquifaction crack: in the welding process, under the action of peak temperature of welding thermal cycle, in the interlaminar metal of multi-layer weld and the metal near the seam area of the base metal, due to the re-melting of intergranular metal/heated, under the action of certain shrinkage stress, the phenomenon of cracking along the austenite grain boundary is referred to as “hot tear” in some literature. High temperature and low plastic crack, in after the completion of the liquid crystal, weld metal from the material of plastic recovery temperature started to cool, for some special materials, when cooled to a certain temperature range, due to the strain rate and the interaction of some metallurgical factors, causes plastic drop, result in the crack of the weld metal along the grain boundary. Generally occurs in the heat affected zone farther from the fusion line than the liquefaction crack. Reheating crack: crack developed along the austenitic grain boundary under certain conditions after welding, during residual stress relief heat treatment or during service at certain temperatures without any heat treatment. In fact high strength steel welding hot crack is low alloy sex again one of the major problems to solve, especially some contained more carbide forming elements such as Cr, Mo, V, and can produce precipitated carbide heat intensity steel of high strength steel and low alloy thick plate welding, tends to be produced in the process of eliminating stress after welding heat treatment and hot crack, deal with these defects both work and time consuming, big influence on the production. The following is a brief analysis of the formation mechanism of reheat crack and the preventive measures and inspection methods in the manufacturing process. 02 Mechanism of reheat crack To put it simply, the formation of reheat cracks means that due to the high strengthening strength in the grain, the grain boundary strength is weak. During the post-welding heat treatment, the deformation during stress relaxation is concentrated on the grain boundary. Once the grain boundary strain exceeds the strength limit of the grain boundary, it will lead to crack and crack along the grain boundary. (1) the reheat internal cause of crack formation, when welding heat affected zone near the fusion line is heated to a temperature of about 1200 ℃, especially of thick plate have been heated, coarse grains, and strong carbide precipitation during cooling slowly, also in submerged arc welding, because of its high linear energy, weld in the middle of the grain is bulky, in subsequent SR treatment (480 ~ 680 ℃), in the process of carbide (V4C3, NbC, MoC, etc.) in the intracrystalline diffusion deposition, thus strengthening the intracrystalline (intracrystalline thermal strong sex), make the heat treatment, the stress relaxation of strain when concentrated load on the grain boundary; The coarse grain makes the number of grain boundary of bearing strain decrease sharply, and the strain per grain boundary increases greatly. In addition, during post-weld SR treatment, low melting point impurity and trace elements such As B, Sb, Sn and As segregate at grain boundaries, weakening the plasticity of grain boundaries, and forming cracking when strain exceeds the plastic limit of grain boundaries. (2) External cause of reheat crack formation The internal cause of reheat crack is described above, but the existence of external cause is needed to generate reheat crack, the generation of external cause should be considered from welding residual stress and expansion stress. During the post-welding stress reduction heat treatment, the welding residual stress can be reduced by relaxing the creep deformation. When the deformation of the material cannot meet the requirements of such deformation, cracks will occur. In the welding zone, the existence of low melting point compound, segregation and coarse grain embrittlement zone, because of the lack of grain boundary strength and toughness, can not resist the creep expansion deformation and crack failure. Creep deformation, in fact, is a process of thermal expansion, in which expansion tensile stress is generated to offset part of the compressive stress generated in the welding process, and contraction force is generated when cooling contraction to offset part of the tensile stress generated in the welding process, so as to reduce the peak stress. Therefore, in the stress concentration area such as micro-defect pores and slag in the welding area, when the expansion force and the stress in the area are superimposed, the peak tensile stress is generated, and the peak value is greater than the strength value of the material, the original balance that maintains no failure will be broken and crack will occur. The state of stress distribution in these stress concentrated areas is very complex, which varies with the position of thickness and is constrained by whether there are nozzles around. For example, the defects in the lower part of the V-groove welding are subjected to tensile stress, and the defects in the upper part are subjected to compressive stress. This is also the reason why many reheat cracks exist at the root of the welding zone. Composite surfacing transition layer is a dissimilar steel welding, the structure is very complex, and in the region of tensile stress, so the tendency of reheat crack is very large. Preventive measures: from the formation mechanism of reheat crack analysis, preventive measures have the following aspects: Strict control of raw materials: in the procurement of raw materials, steel in Cr, Mo, V, Nb, Ti and B strong carbide forming elements, has a great influence on the reheat crack formation, must strictly control, also can form sulfur phosphorus eutectic content of S and P content, purchasing welding material also should have the same requirements, such measures is to address the internal cause of the reheat cracking formation is one of effective measures. Select the welding materials with low thermal crack sensitivity (strictly control the content of S, P, V, Nb and other elements), and take the lower limit of weld metal strength. Develop reasonable welding specifications: (1) As far as possible to reduce the energy of the welding line, control the preheating temperature between layers. These two factors determine the cooling conditions of the weld metal and have a great influence on the microstructure of the weld zone. Generally speaking, it is beneficial to improve the microstructure, increase the impact toughness and prevent the thermal crack by using small line energy multi-channel multi-layer and appropriately increasing the cooling rate in the weld zone. However, if the interlayer temperature is too low, it is not conducive to the escape of hydrogen and may lead to the risk of cold cracks. Therefore, to control the cooling rate and obtain the refined grains, we should focus on the energy of the control line. (2) take appropriate preheating measures. Appropriate preheating measures can soften the hardness, improve the toughness and improve the crack resistance of the hardened layer. Control the welding process, reduce the amount of small defects: carefully implement welding specifications, reduce small defects, reduce the amount of cladding metal, narrow gap welding is also an effective measure to control the reheat crack. Through the above discussion, these tiny defects, not exceeding the standard of the defects, because is the stress concentration point, therefore, in the process of heat treatment to release stress, there is the cause of stress overlap, resulting in reheat cracks. Therefore, it is necessary to control these defects. Control welding residual stress: under the action of heat treatment creep expansion force, especially in the case of stress superposition as tensile stress, the stress concentration point in the weld, the weak link behind the grain boundary of the precipitation hardening zone produced by carbide, can not resist the strain and cause cracking. Therefore, the method of reducing residual stress before heat treatment can also reduce the generation of reheat cracks. (1) the use of half intermediate heat treatment. ② Using high frequency ultrasonic shock method. Both of these methods can effectively reduce the welding residual stress. Post-welding heat treatment: in the process of post-welding heat treatment, the speed of heating and cooling is controlled to expand and contract slowly and evenly to reduce the generation of reheat cracks. 03 Inspection, inspection and identification of defects Generally used surface inspection can only specify the presence of defects, in order to determine the true cause of defects also need to use the following methods of inspection: Complex metallography: The complex metallography is often used in the field for non-destructive inspection. When the workpiece is in vibration or small parts, the complex metallographic method can be used. The obtained replicates are easy to be preserved for a long time, and can be observed, analyzed and photographed in the laboratory with microscope. It is better to use the large workpiece metallographic inspection instrument and the duplicated metallographic method. 04 Preparation of samples on the surface of the inspected site Replicate material can be 1 ~ 2mm thick plexiglass sheet, can also be acetic acid fiber or nitric acid fiber film (AC paper). Organic solvent can use chloroform, acetone, acetic acid ethyl fat. First cut the film into small pieces of the desired size. During the operation, the sample surface has been prepared with an appropriate amount of organic solvent dripping, and quickly covered with plexiglass or film, with fingers or rubber gently pressed, so that the bubble during the escape. When it is fully dry, it is removed for observation and photography. In order to increase the contrast of the tissue, the etched surface can be slightly deeper, or an appropriate amount of colorant can be added in an organic solvent. 05 Microscopic inspection with large work – piece metallographic tester The microscopic inspection includes the inspection before and after the etching: before the etching mainly check whether the sample has cracks, non-metallic inclusions and defects caused by the sample preparation process; After etching, the microstructure of the sample is mainly examined. When observing, the general first with the microscope 75 ~ 100 times to observe the whole low-power tissue. When it is necessary to observe fine tissues, the appropriate high magnification rate is selected. 06 Microscopic inspection of pipes and components A. Identify the types of non-metallic inclusions and microcracks in materials, observe their morphology and distribution, and measure their quantity and size. B. Identify the composition of the microstructure, morphology, distribution and quantity of various tissues. The grain size, banded structure, nonmetallic inclusions, Widmannian structure, spheroidized structure and decarburized layer were evaluated. C. Identify the structure characteristics, determine the state of the heat treatment process, and provide the basis for reformulating the heat treatment process if necessary. D. Identify whether there is any correlation between the above defects and the detected cracks. Because the reheat crack is not produced in the welding process, but during the heat treatment or operation, so the reheat crack has a certain concealment, and then the accident is unpredictable, which will cause greater losses. Therefore, the appearance of reheat cracks must be considered in advance in the early design, manufacturing, inspection and other links of special equipment.铜焊丝,Copper Welding Rods,铝焊丝,aluminium welding wire,镍焊条,Nickel electrode,药皮焊条,Flux Coated Brazing Welding Rods