Analysis and application of high pressure bolts

The crack treatment on the threaded hole is extraordinarily cautious. Under the premise that the welding quality, performance and construction period must be ensured, the thread profile should be correct and the contact area should be greater than 90%. It is necessary to ensure that the bolt can enter and exit freely to prevent jamming. At the same time, ensure that the thread has sufficient strength. The welding method is selected because ZG20CMrOV has poor weldability and is highly sensitive to the cooling rate. In a certain cooling range, the transformation basically occurs in the middle temperature region, and the bainite structure is formed, and the cold brittle transition temperature is above room temperature. Due to the quenching sensitivity and embrittlement tendency of the material itself, under the action of stress, the base metal adjacent to the weld is prone to cold cracks. When welding with Cr-M.-V pearlitic heat-resistant steel welding rod, the weld metal is prone to re-quenching and stress when it is cooled, and becomes sensitive to cracks. In order to eliminate the risk of cracking in the weld zone metal, the workpiece must be preheated to above 270-320 °C before welding, and tempered at 690-710 °C after welding. However, when the power plant is repairing the cylinder, the pre-weld preheating to avoid welding cracks, and the weld metal have the performance requirements and lower stress levels to meet the operational requirements, and the post-weld high-temperature tempering treatment is required, which is related to the cylinder. Requires small deformation contradictions.

The method of using Cr-M.-V pearlite electrode is characterized by power frequency induction whole-stage preheating, layered hammering and tracking tempering (insulation during welding) to avoid crack reduction stress and Deformation. The application of this method has certain limitations. This method is difficult to apply when the location of the defect of the weld repair is difficult to achieve stratified hammering and tracking tempering. When using austenitic steel welding rod to repair the cylinder, the heating method of preheating surfacing welding layer is not limited, allowing local heating by gas flame, the cylinder has small cumulative deformation, simple construction, easy to grasp, short cycle, low cost and low labor intensity. Etc. Feasibility analysis of austenitic steel welding rod ZG20CrMoV cylinder, the corrosion resistance of the weld metal of the austenitic steel has good cold crack resistance. From the mechanism of cold crack formation, avoiding the generation of cold crack, the choice of austenitic welding rod is better than Cr-MO A V pearlite electrode is more advantageous. In the case of cylinder welding, when using austenitic welding rods (referring to the deposited welding layer), the preheating temperature can be appropriately reduced by 100% and 0 °C without cracking. The smaller welding stress and deformation welding are carried out in a discontinuous state. After the previous welding wave is cooled to not hot, the next welding wave is not welded. No tissue stress is generated during the whole welding process, which is caused by each welding wave. The cumulative residual stress is also small.

The possibility of hot cracking of austenitic weld metal is only due to the use of austenitic electrodes of different compositions or grades, which have good resistance to thermal cracking, and the tendency to form hot cracks is small. of. Diluting should use the electrode with higher nickel equivalent to obtain the mixed weld metal with high Cr-Ni equivalent, so as to reliably prevent the formation of martensite. In addition, the degree of dilution of the weld metal is related to the welding process, for example, reducing the diameter of the electrode and using a small current. High voltage and fast soldering are effective in reducing dilution. Thermal stresses For pearlitic steels and austenitic steel joints, micro-cracks may occur if they are periodically heated and cooled, ie, under thermal fatigue conditions, causing premature fracture of the weld. However, the units above SOMW in China's thermal power plants are under basic load, and there is no periodic operation starting and stopping every day, so as to avoid the risk of cracks due to thermal fatigue. The effect of carbon diffusion on carbon diffusion pearlitic steel and austenitic steel dissimilar metal joints, there is no difference in performance between normal temperature and high temperature strength in the short-term tensile test compared with similar pearlitic steel joints, within a certain range (500 Below °C) and the similar pearlite steel joints have similar strength levels, and the fracture usually occurs on the base metal away from the fusion zone, accompanied by high plasticity.

Welding scheme and implementation In summary, for the welding of more serious cylinder cracks, the austenitic welding rod with higher nickel equivalent is used to meet the performance requirements. Studies have shown that the thickness of the nickel-based electrode is more than 6mm, the weld is prone to hot cracks, the thickness of the weld exceeds IOmm, and the hot crack is very obvious. Under normal circumstances, for deeper and larger defect welding, a nickel-based electrode should be used to weld the weld layer, and a high-nickel austenitic electrode should be used to weld the filling layer. According to the existing research results, the welding layer electrode selects ENICrFe-3, and the high nickel content ensures good crack resistance and high temperature performance, and the martensite structure is not formed in the fusion zone. Austenite with higher nickel equivalent, such as E16-25MoN-15 welding rod, guarantees strength and good crack resistance. In order to reduce the hardness of the fusion zone, the local preheating of the weld layer is 100 °C, and the weld layer is slowly cooled after welding, which can reduce the hardening degree of the fusion zone and improve the performance of the weld zone. The welding of the filling layer should control the temperature between the layers to 60 ° C to avoid large welding stress caused by excessive heat input. Each layer of welds should be carefully inspected and stained as necessary to determine no defects. For cracks in which the bolt holes are cracked, the threaded side is forcibly formed by a copper rod or other method, and a margin should be left for modification. Use special tools to grind the threads to ensure free entry and exit of the bolts. In short, through the welding and repair of the bolt hole penetrating cracks in power plants such as Dengfeng and Rongyang in Henan, the results are satisfactory. After a large repair period, no defects have been found in the welding area. Technical requirements of the unit.

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