What are the damage forms of pressure vessels?
Time:2022-09-15
Views:75
What are the damage forms of pressure vessels?
A pressure vessel is a closed equipment that contains gas or liquid and bears a certain pressure. According to the methods proposed by the International Pressure Vessel, Boiler and Pipeline Commission, and according to the damage forms and causes, the damage forms of pressure vessels can be divided into the following types:
1. Excessive elastic deformation: elastic deformation is a behavior of a solid under the action of an external force. When the external force is withdrawn, the ability of the object to restore its original shape is called elastic property, while the deformation with this reversibility is called elastic deformation. Excessive elastic deformation can make the container appear unstable, even to the extent of instability.
2. Excessive plastic deformation: when the pressure load greatly exceeds the design value, the vessel wall becomes thinner and finally reaches the unstable point, that is, when the pressure increases slightly, the vessel will break due to excessive plastic deformation. When the container is cracked due to excessive plastic deformation, the fracture is in a tear off state. When the container is damaged, no fragments or only a small amount of fragments are produced. The size of the blasting opening depends on the expansion energy of the container blasting. In addition to the influence of pressure, the creep of metal materials at high temperatures is also an important reason for plastic deformation. During the creep process, materials undergo continuous plastic deformation, which will end in fracture after plastic deformation accumulates for a long time.
3. Large strain fatigue: under the action of alternating stress, the metal grains with the maximum stress in some local areas of the pressure vessel (such as around the opening nozzle, local structural discontinuities, etc.) will slip and gradually develop into tiny cracks, and both ends of the cracks will continue to expand, eventually leading to the fatigue damage of the vessel. Fatigue first occurs in the above local areas of high stress, that is, in the places of large strain caused by these high stresses. This kind of failure is called large strain fatigue. The fatigue failure of pressure vessels generally has the following characteristics: (1) The vessels have no obvious deformation. (2) There are two areas in the fracture surface: fatigue crack initiation to propagation area and final fracture area. (3) Containers often fail due to cracking and leakage. (4) Fatigue failure always occurs after repeated loading and unloading of the container.
4. Brittle fracture: In engineering, the fracture without obvious plastic deformation is collectively referred to as brittle fracture or fracture, while the brittle fracture of pressure vessel refers to the pressure vessel made of plastic materials, which is characterized by brittle fracture. The working stress of the cracked container is far lower than the strength limit of the material, even lower than the yield limit of the material. The characteristics of brittle fracture of pressure vessels are: (1) The vessel wall has no obvious elongation deformation, and the thickness of the vessel has not changed generally. (2) The fracture surface is metallic crystalline, and the fracture surface is flush and perpendicular to the main stress direction. (3) Brittle cracked containers are often in the form of fragments, and fragments often fly out. (4) Fracture accidents mostly occur at low temperature. (5) Brittle fracture is more likely to occur on pressure vessels made of high-strength steel and thick walled vessels made of medium and low strength.
5. Hydrogen corrosion damage: under high temperature and high pressure, the part of hydrogen molecule adsorbed on the steel surface decomposes into hydrogen atoms or ions, which are dissolved in the steel surface layer and diffused into the steel. It affects the performance of steel in two ways: hydrogen embrittlement and hydrogen corrosion. Hydrogen embrittlement is due to the diffusion and dissolution of hydrogen in the metal lattice, which makes the steel brittle during slow deformation. At this time, the plasticity of the steel is significantly reduced. Hydrogen corrosion refers to the diffusion of hydrogen atoms or ions into steel, which will combine into hydrogen molecules, and partially react with carbon or carbide and non-metallic inclusions on the microporous wall. These insoluble gas products accumulate in the original micro gap of the grain boundary, forming local high pressure, causing stress concentration, widening the grain boundary, developing into microcracks, and reducing the mechanical properties of the steel.
6. Corrosion fatigue: Corrosion fatigue is a form of damage caused by the joint action of corrosion and stress of metal materials. In the corrosion fatigue of materials, on the one hand, corrosion causes local damage to the metal surface and promotes the generation and development of fatigue cracks; On the other hand, the alternating tensile stress destroys the protective film on the metal surface and promotes surface corrosion. Under the action of alternating stress, the damaged protective film cannot be formed again, and the corrosion products deposited in the corrosion pit prevent the diffusion of oxygen, making it difficult to restore the protective film. Therefore, the bottom of the corrosion pit is always in the active state and constitutes the anode of the corrosion battery. In this way, under the combined action of corrosion and alternating stress, cracks continue to develop until the metal finally breaks.
7. Stress corrosion: Stress corrosion is a form of damage caused by the joint action of metal corrosion medium and tensile stress. When stress corrosion occurs in metals, corrosion and stress are mutually reinforcing. On the one hand, corrosion reduces the effective sectional area of the metal and forms a notch on the surface, resulting in stress concentration; On the other hand, the existence of stress accelerates the progress of corrosion, making the corrosion notch on the surface expand to the depth, and finally leading to fracture.
.png)
.png)
Mailme
18672222184