Causes and prevention of manufacturing deformation

2022-08-14
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Causes and prevention of pressure vessel manufacturing deformation

pressure vessel manufacturing deformation refers to that the geometric dimension of pressure vessel or one of its components is inconsistent with the drawing requirements and standard specifications, and the error exceeds the provisions of the drawing and standard specifications. There are two reasons for the deformation of pressure vessels: 1. The deformation caused by stress, including flame cutting deformation, processing instability deformation, welding deformation and heat treatment deformation; 2. The deformation caused by machining error includes blanking error deformation, forming error deformation and assembly error deformation

some pressure vessel manufacturing deformation can be corrected by difficult orthopedics, while others cannot be changed and can only become unqualified products and cause waste. Therefore, great attention should be paid to the manufacturing deformation of pressure vessels, and the manufacturing process must be carefully formulated and followed, so as to avoid deformation and ensure that the manufacturing quality of pressure vessels meets the requirements of drawings, standards and specifications

I. stress deformation and prevention

1. Flame cutting deformation

(1) barrel section: when the short barrel section of the large-diameter shell is unloaded (the material is long and therefore narrow), the flame cutting of its port and if the comparison voltage of the two operational amplifiers is abnormal, the work edge is prone to deformation. After cutting and cooling at high temperature, the processing edge shrinks, and the straight edge becomes an "arc" edge. After the barrel roll is round, its port is not on a horizontal plane. When the error is large, it cannot meet the requirements of assembly and welding. Symmetrical cutting or machining shall be adopted to avoid deformation

(2) head: after the flame net material of the formed head is cut, the periphery of its port will shrink, making the diameter of the head smaller. In serious cases, the diameter of the contracted head cannot meet the dimensional requirements. For the processing of integrally formed head end, if flame cutting is adopted, the shrinkage after cutting should be considered in the design of its forming die; If flame cutting is adopted for the processing of the end of the combined flap head, the diameter of the head shall be appropriately enlarged during assembly to make up for the shrinkage after cutting. Mechanical processing can also be adopted to avoid deformation

(3) blank of machined parts (mainly steel plate blank): this blank is mostly used for large flanges or sealing rings on pressure vessels. Due to the mixed composite materials containing half of carbon fiber of all carbon materials, etc. After flame cutting, due to uneven expansion and contraction of the steel plate, the blank surface is uneven, and in serious cases, the processing volume of the blank surface is insufficient. The blank plate should be leveled and straightened after cutting. For the blank plate that is difficult to straighten, the machining allowance can be appropriately increased

2. Processing instability deformation

processing instability deformation is often the opening of large holes (such as the loading and unloading holes of containers) in the formed head or cylinder section. Due to the weakening of the stability in the opening area and its vicinity, the deformation of local parts or components of the shell is caused. Try to avoid opening large holes directly on a single cylinder section or a single head. Assemble the shell into a large section according to the situation, or the electronic tensile testing machine produced by Jinan Shijin can meet the requirements, and then open large holes as a whole; Before opening a large hole, the opening area shall be strengthened with a rib plate close to the shell. When the shell is in an overall stable state after assembling and welding pipes, the reinforcing plate shall be removed

3. Welding deformation

welding process is the technical requirements and operating regulations of vessel welding, including: the welding method, welding groove, type and diameter of welding rod, welding process parameters, welding sequence, number of weld bead layers, pre welding and post welding treatment, welding environment requirements, anti deformation and anti deformation measures, etc. The welding process must be qualified through process evaluation, and the process requirements must be strictly implemented during the welding operation

according to the welding conditions and welding volume of pressure vessels and large components, analyze the deformation size and shape that will be produced by welding in advance, and formulate targeted control measures:

(1) for large pressure vessels with multiple weld passes, such as spherical vessels, they should be assembled and connected into a whole before welding. The welding should be carried out symmetrically, and the specified welding sequence should be observed

(2) for large parts with multiple weld passes, such as melon flap combined heads and shell transition sections combined by flaps, in addition to the above requirements, mouth shaped fixed fixtures should also be set at the welding site

(3) for pressure vessels that are long and welded in multiple sections, the welding shrinkage shall be properly released during the blanking of the barrel section to avoid the shortening of the shell after welding

(4) for pressure vessels, especially for the assembly and welding of pressure vessels with complex structures, reasonable assembly sequence and welding deformation prevention measures should be taken to ensure that they are not deformed during manufacturing

(5) anti deformation measures: according to practical experience or calculation, deformation is given to the welding parts in the opposite direction of welding deformation in advance, and this pre deformation amount is just offset after welding. The specific method is: when pressing the arc at both ends of the longitudinal joint of the pressure vessel section, reserve the anti deformation amount in the opposite direction of the welding deformation direction; The reverse deformation to offset the welding deformation shall be considered for the die size of the combined flap head and transition section

4. Preventive measures for heat treatment deformation

(1) the heat treatment furnace must meet the specification requirements, and the temperature in the furnace must be uniform and accurate. A fire retaining wall should be set at the flame nozzle of the furnace wall, and the flame is strictly prohibited from directly contacting or approaching the heat treatment parts

(2) after the pressure vessel with large length is put into the furnace, temporary support pads shall be added, and the quantity used depends on the specific size of the vessel

(3) shells with larger diameter and thinner thickness should generally be strengthened internally

(4) for segmental prefabricated pressure vessels, reinforced supports should be set at the segmental ports

(5) pressure vessel components that are prone to lose stability under high temperature should also be reinforced according to specific conditions

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