As an important equipment of bell annealing furnace, the stable operation of the furnace bearing plate has a significant impact on production safety. Starting from the working environment, stress, installation accuracy, location, material, etc., the paper uses the finite element method to analyze the causes of cracks in the load-bearing plate, and the equipment repair methods and precautions are analyzed, so as to improve the service life of the equipment and reduce the maintenance cost of the equipment. After the crack repair scheme of the load-bearing plate was determined, a total of 8 load-bearing plates were repaired. After repair, the load-bearing plate with the longest use time has been used for more than one year, and the use effect is good, the load-bearing plate surface is smooth and without deformation, and there are no cracks before continuing to occur. The load-bearing plate crack repair scheme has a reference effect on the maintenance methods and inspection methods of similar working conditions and structural workpieces, and provides a reference for maintenance personnel to find the cause of the failure more quickly and efficiently.
Bell annealing furnace generally adopts furnace group production, and the furnace bearing plate is the key equipment of bell annealing furnace. Due to the annealing process, the furnace load-bearing plate will be subjected to repeated alternating thermal stresses during the production cycle [1], which will have a significant impact on the production stability and the safety of the unit operation if it occurs. In the cold rolling mill of a steel company, after 1 or 2 years of commissioning of the load-bearing plates of the bell annealing furnace group, cracks began to appear in some of the load-bearing plates, and the crack length continued to increase with the increase of production times. When the crack length of the load-bearing plate grows to a certain extent, the load-bearing plate will have local serious deformation, collapse and other faults, which is a serious hidden danger of safety production, so it should be forced to be scrapped. However, due to the high cost and large number of spare parts for the furnace load-bearing plate, if all of them are replaced, the annealing furnace mandrel will be scrapped, resulting in a large waste. Therefore, it is necessary to analyze the cause of the fracture of the bearing plate of the furnace, formulate a repair plan, and propose a transformation plan for the structure and material of the new spare parts of the subsequent furnace bearing plate, so as to solve the problem more effectively and at low cost.
Analysis of the cause of cracks
In the whole annealing production cycle, the load-bearing plate mainly bears the pressure of the steel coil in the working environment of high and low temperature alternating, and the reasons for the cracks are judged to be the defects of the structural design of the load-bearing plate and the low heat resistance of the material selected for the load-bearing plate.
Bearing plate structure shape influence
The cracks of the load-bearing plate of the furnace are all in the same position and evenly distributed, and the cracks are installed through the positioning hole position on the load-bearing plate in the process of expanding to the middle after the cracks appear. The installation positioning hole is a counterbore structure, and is in a suspended state, and the bottom plate does not support this part, and under the pressure of 90 100 t steel coil, it is easy to produce stress concentration and form a weak point. From the initial position of the occurrence, the cracks all appear from the edge of the bearing plate overhang, see Figure 1.
In addition, the load-bearing plate bears the temperature change of 30 700 repeated alternation at the same time in the working process, especially in the heating stage, there is a temperature difference between the edge of the furnace floor and the first part, so the thermal expansion rate of the edge and the first part is not the same after heating, the edge expands quickly, the first expansion is slow, the load-bearing plate constantly suffers from the periodic alternating thermal stress, and the weak point of the load-bearing plate suspension - the position of the installation positioning hole, is subjected to the action of repeated change stress, and is easy to produce stress concentration, As a result, cracks first appear on the edges and gradually expand along the location of the locating holes, and the cracks continue to expand until the load-bearing plate is scrapped [2].
Load-bearing plate material influence
In order to analyze the effect of the load-bearing plate manufacturing material on the cracks, a metallographic analysis was carried out after sampling the load-bearing plates. The results show that the Ni content in the material is very small, less than 05%, resulting in a significant reduction in the heat resistance, plasticity and toughness of the load-bearing plate at high temperatures compared with stainless steel, and at a high temperature of 700, the weak areas of the load-bearing plate structure are prone to cracking.
From the analysis of the causes of cracks in the load-bearing plate, it can be seen that the defects in the structural design of the load-bearing plate and the low high temperature heat resistance of the selected material components are the main reasons for the cracks in the load-bearing plate.
Remediation plans and improvement measures.
In order to avoid the scrapping of the cracked load-bearing plate, try to repair the load-bearing plate, and at the same time improve the original structural defects of the load-bearing plate to meet the requirements of the production process. The improvement of the load-bearing plate is mainly to effectively use the original old parts, and the influence of the material cannot be changed, so the main direction is to improve the load-bearing plate structure and reduce thermal deformation and thermal stress.
Determination and implementation of remediation plans.
It can be seen from the crack set ** now the load-bearing plate positioning hole, the cross-sectional area of the positioning hole changes greatly, belongs to the stress concentration part, the thermal stress generated by the repeated heating process makes the thick wall that cools slowly is stretched, and the thin wall that cools faster is compressed, resulting in cracks after heating stress. By changing the structure of the load-bearing plate through processing, removing cracks, and reducing the thermal stress and deformation of the load-bearing plate, so as to eliminate the impact of cracks and improve the life of the load-bearing plate, the repair scheme is carried out according to the following aspects:
1) The intermediate plate of the load-bearing plate is preliminarily leveled after heating, and the height difference between the highest point and the lowest point of the load-bearing plate plane is required to be <5 mm after the leveling is completed.
2) Along the direction of the crack in the middle plate of the load-bearing plate, the anti-crack groove that completely penetrates the load-bearing plate is processed, and after all the cracked areas are removed, the stress hole is machined at the end of the anti-crack groove, so that the workpiece can form an expansion joint and release the stress in the heated state after processing the crack relief groove, and effectively prevent the extension of the fracture line [3].
3) Reposition the load-bearing plate mounting position. After the completion of the crack relief processing, the edge of the relief groove is still in the suspended state after the load-bearing plate is assembled, and the stress is poor, in order to improve the stress condition of the load-bearing plate, the load-bearing plate is installed with the original design installation position as the benchmark, and rotates clockwise about 8 ° 9 ° for installation (see Fig. 2). The relief groove and the positioning hole are within the support range of the bottom plate, so that in the production process, the area around the relief groove can be effectively supported, and at the same time, the thermal expansion direction of the bearing plate is limited after heating, so that it can shrink along the radial direction of the bearing plate, improve the stress condition of the weak area of the bearing plate, and effectively eliminate the stress concentration.
Finite element analysis of a load-bearing plate repair scheme
According to the analysis of the causes of the cracks of the load-bearing plate and the corresponding repair scheme, the finite element analysis software was used to analyze the force and deformation of the load-bearing plate before repair, and the force and deformation before and after the adjustment of the installation position after repair, so as to verify whether the cause of the crack and the adjustment of the installation position of the load-bearing plate were reasonable.
1) Preparatory model work for load-bearing plates.
According to the analysis of the alloy composition of the load-bearing plate material, the material is close to 40cro steel, and the mechanical properties of the material are as follows: b=757 MPa, s=540 MPa, e=206 GPA.
It is known that the maximum steel coil weight borne by the load-bearing plate is 1 mn, the force is concentrated at the backing plate on the load-bearing plate, and is supported by the bottom plate, according to the actual working situation, the forced position constraint option is selected, the load-bearing plate is prohibited to move in the normal direction, and the correction is carried out according to the area of the structural constraint of the load-bearing plate supporting the bottom plate at the same time, and the safety in the practical application is considered simultaneously, and the maximum value of the load is taken in the actual work, which is used to judge the situation in the most dangerous time.
2) Analysis of the stress situation of the load-bearing plate before repair.
After meshing the load-bearing plate model before repair, ANSYS finite element force analysis software was used to analyze and verify that the weak part of the force (see Figure 4) was consistent with the actual crack part, and the deformation direction was consistent with the actual crack extension direction. Among them, the deformation at the positioning pin hole is the largest, and the reason for the crack is that the pin hole is under prestress. In the actual working conditions, the load-bearing plate is subjected to both gravity and thermal expansion forces, and the stress deformation is aggravated due to superposition, which verifies the previous analysis of the causes of cracks [4].
3) Analysis of the stress of the installation position adjustment after the load-bearing plate is repaired.
The finite element calculation of the repaired load-bearing plate is mainly aimed at the analysis of the deformation and stress of the load-bearing plate. The state of the load-bearing plate before and after the adjustment of the working position was analyzed, and after the meshing of the repaired load-bearing plate model, the stress and deformation of the load-bearing plate before and after the adjustment were calculated by ANSYS finite element force analysis software, and the results are shown in Figure 5.
From the finite element method calculation results, it can be seen that the stress of the load-bearing plate is mainly concentrated at the edge of the load-bearing plate, and the place where the maximum stress is at the edge of the support contact at the bottom of the load-bearing plate. The deformation is mainly the contact position with the steel coil edge, and the maximum deformation is on the outermost side of the load-bearing plate.
According to the calculation results, after adjusting the position of the load-bearing plate, the maximum deformation of the load-bearing plate is reduced by 142%, the maximum stress value is reduced by 2134%, indicating that after the repair of the load-bearing plate is completed, adjusting the original installation position can improve the strength of the load-bearing plate after increasing the stress relief groove, reduce the deformation and stress, and achieve the purpose of improving the stress condition of the load-bearing plate and improving the service life of the load-bearing plate [5].
Wrapping up
1) After the load-bearing plate crack repair scheme is determined, a total of 8 load-bearing plates are repaired, and the repaired load-bearing plates have been used for more than one year at most, and the current use effect is good, and the load-bearing plate surface is smooth and without deformation, and there are no cracks before continuing to occur. Therefore, continue to process and repair the remaining deformed load-bearing plate according to this repair scheme.
2) The repair cost of the load-bearing plate is only 10% of that of the new parts, which not only ensures the continuous and stable production, but also greatly reduces the equipment maintenance cost.
3) The solution of the crack problem of the load-bearing plate makes the maintenance methods and inspection methods of similar working conditions and structural workpieces in the future have a reference, and provides a reference for the maintenance personnel to find the cause of the failure more quickly and efficiently.
References. 1] Zhang Jingjin. Cold rolling production of strip and plate. Beijing: Metallurgical Industry Press, 2006
2] Song Tingxin, Luo Yuanwen, Zhu Ruoyan. Research on improving the thermal fatigue life of disc construction for annealing furnaces. Journal of Beijing Institute of Technology,2003(8):14
3] Cheng Daxian. Mechanical Design Handbook, Beijing: Chemical Industry Press, 2009
4] Cao Huajun, Shu Linsen, Xu Lei, et al. Hexahedral finite element mesh generation method for complex mechanical parts. Journal of Mechanical Engineering, 2014(15): 114
5] Fang Qinghua. Ansys-based study of the spindle structure of an uncoiler. Science and Technology Innovation and Productivity, 2013(9): 107
Article** — Metal World.