E-book material ~ Lecture on Basic Knowledge of Pressure Vessels (Chapter 7)
Thread Content
Chapter 7 Basic Knowledge of Ultra-High Pressure Vessels 1. Main characteristics of ultra-high pressure vessels: A. High pressure (100 MPa–1000 MPa), with relatively small sizes. B. belongs to thick-walled vessels (D outside/D inside > 1.5), with large differences in stress levels between the inner and outer walls, making simplification impossible. C. It is manufactured using forging methods; the material (forging) is required to have high strength, excellent plasticity and toughness, with no requirement for weldability. D. The inner and outer walls require precision machining; flanges and threads are commonly used for connections between components, resulting in high machining requirements and complex processes. E. There is no unified standard yet, and many issues remain to be discussed. 2. Design requirements: Stress analysis and calculations shall be carried out to verify stress concentration areas such as openings and shape transition zones. 3. Manufacturing requirements: The quality of the raw materials (forgings) is crucial. Advanced metallurgical techniques such as vacuum degassing and powder spraying, secondary refining, and electroslag remelting are required to ensure the purity of the steel and to attain excellent mechanical properties (strength, plasticity, toughness, fracture toughness, etc.). The forging ratio should generally be greater than 3; during manufacturing, 100% ultrasonic testing (on the base material) must be carried out at least twice (once before and once after heat treatment). The surface of the cylinder should undergo 100% magnetic particle or penetrant testing. Both the inner and outer surfaces need to be finely processed, with strict requirements regarding surface roughness in order to prevent stress concentration. 4. Ways to improve pressure resistance strength (load-bearing capacity): A. Use a multi-layer heat-shrinkable structure: By utilizing the interference between layers, the outer tube exerts a pre-compressive stress on the material of the inner tube, which helps to equalize the stress levels across various layers when internal pressure is applied, thereby improving the utilization efficiency of the outer layer material. Three key differences between ultra-high pressure heat sleeves and high pressure heat sleeve containers: selection of inter-layer interference fit; the former is based on precise mechanical calculations ； The latter is selected according to the fitting process. Fitting surface: The former requires precision machining (to ensure an accurate interference fit) ； The latter requires no processing or only rough processing. The latter requires heat treatment to eliminate fitting stress ； The former is not allowed. B. Self-reinforcing treatment: Pressure is applied to cause the inner wall material to yield, while the outer wall remains elastic; this results in the inner wall material being subjected to pre-compressive stress, thereby increasing its initial yield pressure. Self-reinforcing stress should be carefully calculated and controlled, with attention paid to the material’s yield ratio. C. Adoption of a wire-winding structure: High-strength stainless steel wires are wound around the inner cylinder; during winding, the winding prestress can be precisely controlled using methods such as heating, thereby putting the material of the inner cylinder in a pre-compressed state.
I wonder what the purpose is of requiring a forging ratio greater than 3 in the manufacturing specifications? ?
As the forging ratio increases during the forging process, the internal pores are compressed and the as-cast dendrites are broken, resulting in a significant improvement in both the longitudinal and transverse mechanical properties of the forged part. However, when the elongation-to-forging cross-sectional ratio exceeds 3-4, as this ratio increases, a distinct fibrous structure forms, causing the plasticity indices of the transverse mechanical properties to drop sharply, resulting in anisotropy in the forged part. If the forging cross-sectional ratio is chosen too small, the forgings will not meet the required performance standards; if it is too large, it increases the forging workload and also leads to anisotropy. Therefore, choosing an appropriate forging ratio is an important issue, and the problem of uneven deformation during forging should also be taken into consideration here. The forging ratio is usually measured by the degree of deformation during drawing. It refers to the ratio of the length to the diameter of the material that is to be shaped, or the ratio of the cross-sectional area of the raw material (or pre-formed piece) before forging to the cross-sectional area of the finished product after forging. The size of the forging ratio affects the mechanical properties of the metal and the quality of the forged part. Increasing the forging ratio helps to improve the microstructure and properties of the metal, but an excessively large forging ratio is also not beneficial. The principle for selecting the forging ratio is to choose a value as small as possible, while ensuring that all requirements of the forged part are met. The forging ratio is generally determined as follows: 1. For high-quality carbon structural steel and alloy structural steel when forged freely on a hammer: for shaft-type forgings, when forged directly from the steel ingot, the forging ratio calculated based on the main cross-section should be ≥3 ； The forging ratio calculated based on the flange or other protruding parts shall be ≥ 1.75 ； When using billets or rolled products, the forging ratio calculated based on the major cross-section is ≥1.5 ； The forging ratio calculated based on the flange or other protruding parts shall be ≥ 1.3. For ring-shaped forgings, the forging ratio should generally be ≥3. For disc-shaped forgings, which are forged directly from steel ingots, their upsetting forging ratio is ≥3 ； In other cases, the swaging forging ratio should generally be >3, but the final stage should be >2. 2. For the feeding of high-alloy steel billets, it is necessary not only to eliminate its structural defects but also to ensure a more uniform distribution of the carbides within it; therefore, a larger forging ratio must be employed. The forging ratio for stainless steel can be set between 4 and 6, while that for high-speed steel requires a range of 5 to 12.
The forging ratio refers to a processing method in which forging machinery is used to apply pressure to a metal billet, causing it to undergo plastic deformation in order to produce forgings with specific mechanical properties as well as certain shapes and dimensions. The size of the forging ratio affects the mechanical properties of the metal and the quality of the forged part. Increasing the forging ratio helps to improve the microstructure and properties of the metal, but an excessively large forging ratio is also not beneficial. Forging at different deformation temperatures can be further divided into hot forging (where the processing temperature is above the recrystallization temperature of the metal stock), warm forging (below the recrystallization temperature), and cold forging (at room temperature). F1, H1—the cross-sectional area and height of the ingot or billet after upsetting. The forging ratio is usually measured by the degree of deformation during drawing. The principle for selecting the forging ratio is to choose a value as small as possible, while ensuring that all requirements of the forged part are met.