Deformation and prevention

Stress deformation and prevention
1．Flame cutting deformation

(1) Cylinder section: When the short cylinder section of the large-diameter shell is blanked (the material is longer and narrower), the flame cutting edge of the port is prone to deformation. Due to the high temperature of cutting and cooling, the processing edge shrinks, and the straight edge becomes an “arc” edge. After the cylindrical section roll is rounded, its port is not on the same horizontal plane. When the error is large, it cannot meet the requirements of assembly and welding. Symmetrical cutting or machining should be adopted to avoid deformation.

(2) Head: After the flame net of the formed head is cut, the periphery of the port will shrink, making the diameter of the head smaller. In severe cases, the caliber of the shrunk head cannot meet the size requirement. For the port processing of the integrally formed head, if flame cutting is adopted, the shrinkage after cutting should be considered in the design of the forming mold; for the port processing of the flap-type combined head, if flame cutting is adopted, the caliber of the head is assembled It should be enlarged appropriately to compensate for the shrinkage after cutting. Machining can also be used to avoid deformation.

(3) Machining blanks (mainly steel blanks): This blank is mostly used for large flanges or sealing rings on pressure vessels. After flame cutting, due to uneven expansion and contraction of the steel plate, the blank surface is uneven, and in severe cases, the processing amount of the blank surface is insufficient. Flattening and correction should be carried out after the blank plate is cut. For blank plates that are difficult to be corrected, the machining allowance can be appropriately increased.
2．Processing instability and deformation

Processing instability and deformation are often caused by opening large holes (such as container loading and unloading holes) on the formed head or tube section. Due to the weakening of the stability of the opening area and its vicinity, the deformation of the shell or parts is caused. Try to avoid directly opening large holes on individual cylinder sections or separate heads, and then open large holes after assembling the shell into a large section or a whole according to the situation; before opening the large holes, use the ribs of the shell close to the opening area For reinforcement, when the shell is in an overall stable state after the assembled welded pipe, the reinforcement plate is removed.
3．Welding deformation

Welding process is the technical requirements and operating regulations for vessel welding, including: welding method, welding groove, electrode type and diameter, welding process parameters, welding sequence, number of weld bead layers, treatment before and after welding, and welding environment Requirements and anti-deformation and anti-deformation measures. The welding process must be qualified through process qualification, 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 parts, analyze the size and shape of the deformation that will be produced by welding in advance, and develop targeted control measures:

(1) For large pressure vessels with multiple welding passes, such as spherical vessels, the welding should be carried out after assembly and connection. The welding should be carried out symmetrically and the prescribed welding sequence should be observed.

(2) For large parts with multiple weld passes, such as the melon-flap combined head and the transition section of the shell combined by the petals, in addition to the implementation of the above requirements, a mouth-shaped fixing fixture should be set at the welding site.

(3) For pressure vessels that are long and welded in multiple sections, the size of the barrel section should be appropriately released when the material is blanked to avoid the shrinkage of the shell after welding.

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

(5) Anti-deformation measures: According to practical experience or calculations, the welded parts are deformed in the opposite direction to the welding deformation in advance, and this pre-deformation amount is just offset after welding. The specific method is: the butt joint of the longitudinal seam of the pressure vessel tube section When the arc is pressed at both ends, the reverse deformation is reserved in the opposite direction of the welding deformation; the combined petal-shaped head and the transition section die size are considered to offset the reverse deformation of the welding deformation.
4．Preventive measures for heat treatment deformation

(1) The heat treatment furnace must meet the requirements of the specification, the temperature in the furnace is uniform and accurate, and the fire wall should be installed at the flame nozzle of the furnace wall. It is strictly forbidden for the flame to directly contact or approach the heat treatment parts.

(2) After the pressure vessel with a larger length enters the furnace, temporary support pads should be added, and the number used depends on the specific dimensions of the vessel.

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

(4) For pressure vessels prefabricated in sections, reinforced supports shall be provided at the ports of the sections.

(5) Pressure vessel parts that are liable to lose stability under high temperature should also be reinforced according to specific conditions.

Two Machining error deformation

1. Cutting error deformation

Due to the inaccurate size of the blank, the shape of the molded part exceeds the standard. The inaccuracy of the blanking size is mainly due to errors in the calculation or enlargement of the sample. In addition to improving the technical level of the blanking personnel, the blanking size calibration system should be implemented, and the blanking size computer software management should be adopted as much as possible.
2．Forming error deformation

During the processing and molding of pressure vessel components, deformation due to improper operation or non-standard molds: The mold release temperature of the thermoforming head has certain requirements. If the temperature is still high, the premature release of the mold will cause the head to shrink. Make its geometric dimensions exceed the standard; mechanically rolled or pressed container parts are deformed due to improper operation; mold design is improperly considered or incorrect, so that the geometric dimensions of the pressure container parts after forming do not meet the requirements. The main preventive measures are:

(1) The forming operation is carried out in strict accordance with the technological requirements.

(2) Strictly control the shape of the processed parts with inspection templates.

(3) The mold design is based on the theoretical size and shape of the processed part, and fully considers the changes that occur during and after the forming of the pressure vessel component. The cold forming mold should consider the springback of the molded part, and the hot forming mold should consider the forming The shrinkage of the part after cooling.
3．Assembly error deformation

When the pressure vessel shell is assembled, the deformation caused by the misalignment or non-straightness error that exceeds the standard is called assembly deformation. Its preventive measures:

(1) The shell assembly should use positioning fixtures, the shell with larger diameter and thinner thickness, and the cylinder section should be supported during assembly to strictly limit the misalignment of the shell butting edge.

(2) The horizontal assembly of the shell shall be carried out on the roller, and the straightness shall be checked with a straight line.

(3) For prefabricated pressure vessels in sections, set the clamps when installing, and check the straightness with theodolite.