Technological Countermeasures for Reducing Welding Deformation of Large Structural Parts
Large structure is the skeleton of construction machinery products, and also reflects the design level and manufacturing level of manufacturers. Due to the important role of structural members in bearing capacity, appearance modeling and product function realization, special attention should be paid to the design and manufacturing of construction machinery products.
Large structural parts are difficult to support because of their large size, many welding parts, high requirements for part size accuracy and location accuracy, and especially the overall deformation, which has been a major problem for technologists. In this paper, based on the process practice of typical structural parts of graders and loaders produced by Sinoforeign Construction Development Co., Ltd., the welding deformation of large structural parts and its causes are analyzed, and some process countermeasures to control the deformation are proposed.
1 Typical examples and hazards of deformation of large structural members
1.1 Deformation of rear frame of PY160C grader
The rear frame is the largest structural part of PY160C grader (see Figure 1), and its overall dimension is 3300 mm × 1 100 mm × 800 mm, consisting of 72 parts, is a large structural member with frame features, which is manufactured by assembling, splicing and welding multiple parts. This part shall be connected with engine, transmission, cab, drive axle and other components during assembly of the whole machine. The process requirements are: flatness of foundation surface ≤ 3 mm, twist ≤ 5 mm, and verticality ≤ 2.5 mm. After welding in the traditional way, the deformation is generally 10~30 mm in distortion, 5~8 mm in flatness, 5~10 mm in perpendicularity, and the center distance of the transverse hole group is out of tolerance. For example, the installation distance of the rear axle is 720 mm ± 0.5 mm, and it becomes 720 mm+2 mm after welding. Although the reshaping process was carried out after welding, the deformation was still relatively large compared with the assembly requirements, resulting in a large repair rate of the piece, so that the backward processes such as “matching welding” and “base plate adjustment” were later used for adjustment.
1.2Deformation of ZL50C loader arm
This is a large H-shaped structural member (see Figure 2) with an overall dimension of 2800 mm × 1 200 mm × 800 mm. The manufacturing method is to weld the large section corner seam after the support assembly is closed with the two boom plates. The welding method is continuous welding of 3 layers and 6 welds. This piece is the key support for bucket movement during loader operation, and its process specification requires that the symmetry of dimensions a and b relative to the centerline is 1.5 mm; A. The parallelism of center lines of holes in groups B, C and D is Φ 1.0 mm； The coaxiality of each group of holes is Φ 0.5 mm。 The main deformation after welding in the above way is the side bending of the support, which usually causes the size b to be 8~15 mm smaller, the symmetry of size a and b to exceed the tolerance of 1.5~2 mm, the respective center lines of size a and b are not parallel, and the two center lines have an included angle of 2 °~3 °, causing boom deformation, and the whole machine will lose stability when operating for a long time.
2 Theoretical analysis of structural member deformation
The structural members of construction machinery are mainly assembled and welded from cold and hot rolled steel plates, sections and their formed parts, which are mainly made of low alloy structural steel. From the perspective of the manufacturing process of structural parts, the causes of deformation of large structural parts mainly come from three aspects: welding thermal stress, residual stress and external force.
2.1 Welding thermal stress and deformation
In the process of welding, the workpiece heats and cools the metal materials unevenly. During welding, the heat source of heating is the moving high-temperature arc. The metal temperature in the weld and heat affected zone is very high, and the metal expands when heated, but it is hindered and restrained by the metal at normal temperature, which results in compression plastic deformation. The welding deformation degree of structural members is proportional to the input energy of heat source during welding.
2.2 Residual stress and deformation
The residual stress is mainly welding residual stress and forming residual stress. When a part of the workpiece is welded, the weld metal changes from expansion to contraction, but it is limited by the metal at room temperature. At this time, welding residual stress is generated. The residual stress in forming process is mainly caused by the external force of processing, for example, the free bending forming of the workpiece is not feasible; The number of steel plate leveling and rolling is less; Excessive machining cut can cause residual stress in forming process.
2.3 Deformation caused by external force
It mainly refers to abnormal deformation caused by knock, bump, fall, bump or overload during assembly and welding.
Combined with the theoretical analysis of the deformation stress, it can be seen that the deformation of the rear frame of the PY160C grader mentioned above is a comprehensive welding deformation of hundreds of welds. The deformation of ZL50C loader arm is the welding thermal stress deformation caused by the typical large thermal field weld. The thickness of the boom plate of this piece is 50 mm. Since the temperature of the welding heat affected zone can reach 850 ℃ during welding, with the phase change of the metal at the heating part, the residual stress is generated. It remains in the base metal, which not only causes the deformation of the component, but also affects the service performance and quality of the component.
3. Process measures to overcome structural deformation
3.1 Deformation correction of rear frame of grader
The traditional method uses flame plus external force to solve the distortion of the rear frame of PY160C grader. Place the piece flat on the working platform, pad its two or three corners, and fasten it to the working platform. Heat the stress concentration area with flame, and then mechanically pull the suspended corner to correct the distortion. However, when we are correcting other secondary deformations, the distorted deformation reappears. This method of repeated correction not only consumes a lot of manpower and material resources, but also causes new residual stress in the workpiece, leaving a potential danger of late deformation of the product.
Through continuous practice and exploration, we re examined and approved the process of the rear frame, and implemented a set of “components first, then assembly”, that is, the whole rear frame is divided into three parts: bearing pedestal, left beam, and right beam. Each part is divided into several parts, and the next layer is parts. The order of assembly, welding, and reshaping is: parts → sub parts → parts → rear frame assembly. In fact, this method is to disperse all the heat and deformation that the rear frame is subjected to at a time of welding to the front sequence to weaken step by step, and rectify the deformation by component to reduce the deformation after final assembly. The deformation of the rear frame made by this process is very small. Because we have eliminated the most serious distortion, the part can easily meet the requirements of the drawing when shaping.
3.2 Overcoming the deformation of loader arm assembly
We take measures to overcome the welding deformation of the boom from three aspects: first, we take the method of increasing constraints to limit the deformation of the boom, that is, two adjustable supporting rods are set 200 mm away from the support, and two boom plates are supported from both sides to limit the inward deformation of the boom plate during welding; The second is to reduce the welding line energy. For the weld with a sectional area of 200 mm2, the process of four layers and 12 times of alternating welding is adopted, as shown in Figure 3. The number in the figure is the welding sequence. Manual CO2 gas shielded arc welding is adopted, which has a good effect on reducing the deformation of the boom; Third, adopt an economical and simple flame correction method.
The operation method is as follows: heat with a gas welding gun, use a carbonization flame, and gradually heat the outside of the boom plate corresponding to the weld of the support and the boom plate in a linear form from top to bottom. The heating speed is 3~5 mm/min, the heating temperature is 750~800 ℃, and use clean water to cool it from bottom to top. When the deformation of size b is greater than 10 mm, the heating width is 10~20 mm, and the heating depth is 15~20 mm; When the deformation is less than 10 mm, the heating width is 10-15 mm, and the heating depth is 10-15 mm. With the above methods, the shape and position tolerance and dimensions of the boom can meet the standard requirements.
4. Technological measures to reduce deformation of large structural parts
Large structural parts of construction machinery are much more complex than those described in general textbooks or technical materials, and the performance requirements for different structural parts are also different. According to practical experience, we summarized several technological countermeasures to overcome the deformation of large structural parts of construction machinery.
(1) The structural design of the structure itself should be reasonable, that is, the three principles of weld design should be followed as far as possible: the number of welds should be as small as possible, the weld section should be as small as possible, and the weld position should be symmetrical.
(2) Welding methods with low line energy shall be selected, including multi-layer welding and CO2 automatic welding instead of gas welding or manual arc welding.
(3) Select a reasonable welding sequence to make the workpiece heated evenly.
(4) Multi layer assembly and welding method is adopted for structural members with complex shape and many components, multi-step assembly, multi-step welding and multiple sorting.
(5) Eliminate residual stress and ensure long-term stability of structural members:
a. For parts with pressing and leveling processes, measures shall be taken to make the metal structure at the force application place uniform.
b. Heating above 800 ℃ should be careful to avoid causing phase transition of materials.
c. It is forbidden to use flame forming method to process parts or a part after assembly and welding.
d. Structural members shall be tempered or naturally aged before processing.
(6) Mechanical correction shall be adopted for simple parts as far as possible.
(7) For the deformation between parts with constraints at both ends, mechanical means shall be used to adjust and maintain a certain force application time.
In a word, overcoming the deformation of large structural parts of construction machinery is a subject with strong theory and practice. The measure to overcome the deformation is to ensure the thermal field of structural parts is balanced and avoid the generation of stress as much as possible. With this guiding ideology, we can constantly formulate more perfect process countermeasures in production practice.