Analysis of Cracking and Fracture of Welded Metal Bellows

Writer： admin Time：2021-09-10 09:17 Browse：℃

Analysis of the causes of cracks and fractures of the welded Metal bellows
From the analysis of cracks and fracture morphology, it can be known that the occurrence of bellows cracks is the result of the combined effect of intergranular corrosion and stress corrosion, and intergranular corrosion occurs preferentially; the fracture of the bellows is mainly along the edge. Crystal fracture characteristics, high sulfur content in fracture corrosion products. In the petrochemical industry, polythothionic acid environment is likely to cause stress corrosion cracking in equipment. Polythothionic acid is generally formed by the reaction of sulfur-containing corrosion products remaining in the equipment with water and oxygen during the shutdown of the equipment processing sour crude oil. The stress corrosion cracks caused by polythionic acid have intergranular characteristics, and the working medium of the bellows contains hydrocarbons, water vapor, and a certain amount of sulfur-containing substances. This provides conditions for the formation of polythionic acid, so the corrugation The reason for the failure of the pipe was stress corrosion cracking caused by polythionic acid. In the polythionic acid medium, after chromium is enriched on the grain boundary to form a chromium-depleted area, the microbattery formed between the matrix and the chromium carbide on the grain boundary causes the chromium-depleted area to preferentially dissolve, thereby causing intergranular corrosion. Compared with ordinary pressure vessels, the working condition of the bellows is worse. In addition to withstanding the working temperature, pressure, and medium, it will also produce larger deformations. The corrugated pipe deforms to provide the displacement required for pipeline compensation. The displacement causes the corrugated pipe to produce higher axial stress and bending stress, making the cracks mainly transverse cracks, and there are some oblique cracks at a certain angle to the transverse cracks. In many cases, the bellows will also be subjected to mechanical vibration and the vibration caused by the flow of the medium in the pipeline. At the same time, the self-weight of the pipeline, medium, and insulation material will cause the bellows to produce a certain bending moment, so the bellows is working. The stress state of is very complicated. It can be seen that the microcracks produced by intergranular corrosion are the source of cracks. Under the action of stress, the microcracks propagate and lead to the formation of stress corrosion cracks. In addition, the grains of the bellows structure are relatively coarse, which will have an adverse effect on the properties of the material, and the strength of the grain boundaries will also drop sharply. Therefore, under the combined action of stress and corrosive media, coarse grains will increase the tendency of intergranular corrosion and accelerate the propagation of cracks.