Cause analysis of cracks caused by leakage of the

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Cause analysis of cracks caused by leakage of ammonia accumulator

after inspection, the main part of the crack occurred on the surface of the T-joint where the circumferential weld of the vessel intersected with the longitudinal weld. There was a crack with a length of about 80mm, which was forced along the longitudinal weld from the near seam area of the circumferential weld. There are also cracks with different depths on other welds

cause analysis of cracks

in the ammonia refrigeration system, the ammonia reservoir is the container of high-pressure ammonia liquid from the condenser. Its function is mainly to solve the contradiction between the imbalance of liquid supply and air return to the refrigeration evaporator. The causes of cracks in shell J and ammonia device mainly include welding, medium, stress, material, etc

1 welding

(1) the crack at the T-head joint at the junction of circumferential weld and longitudinal weld pattern is often the place where the arc is stopped when the longitudinal weld is welded. It is because the arc is stopped too fast when the welding rod is welded, and the residual liquid phase is insufficient when the weld is condensed, resulting in shrinkage grooves and crystal cracks. Here is the crack that is most likely to produce craters when making containers

(2) due to two welding heating at the T-joint, the welding residual stresses are superimposed on each other

(3) during welding, the site was wet and the groove was not cleaned well, so that the diffusion of hydrogen in the weld caused the influence of hydrogen on the welding cracks during welding. Because hydrogen is one of the important factors that cause delayed cracking of 236 aluminum alloy curtain wall when low alloy steel is welded. And it has the characteristics of delay

(4) during the production of early equipment products, the use of acid electrode for welding is not ruled out, resulting in the existence of such star products in the weld metal. It can be said that a large amount of diffusive hydrogen is favored by thousands of people, and cracks are produced and diffused under the action of diffusive hydrogen. Damage to the weld from the face

2 medium

the content of oxygen and water in liquid ammonia of ammonia reservoir plays a destructive role in the stress corrosion of steel. As liquid ammonia will inevitably invade a small amount of oxygen and water during the refrigeration cycle, oxygen can form an oxide film on the metal surface. When the stress in the steel is released, the metal oxide film layer of the steel will be damaged and cause cracking. Under the action of stress, the crack point will continue to deform, so that the metal surface at the crack can not be oxidized rapidly, which is confirmed as the eighth strategic emerging industry in our province, Then the potential difference between oxide film and oxide film is formed around the crack, so a micro battery, also known as corrosion battery, will be formed on the metal surface. The anode in the anode area of the corrosive battery will undergo oxidation reaction to dissolve the anode. The reduction reaction takes place on the cathode of the cathode area, which generally only plays the role of transferring electrons

therefore, stress corrosion cracking can only occur when an unstable protective film is formed on the metal surface in a specific weak corrosive medium

3 stress aspect

because when the cylinder is under pressure, its circumferential stress is twice the axial stress. Therefore, the circumferential stress borne by the longitudinal weld is greater than the axial stress borne by the circumferential weld. The failure probability of longitudinal weld is greater than that of circumferential weld

4 in terms of material

after checking the relevant information that the flame retardant material effectively prohibits the rapid spread of fire, l6mnr steel has a strong tendency of stress corrosion in ammonia medium. Design products before the 1980s. Its low-alloy steel will be produced under the combined action of a certain corrosive medium environment and its own residual stress when making containers. A special low stress brittle cracking of stress corrosion. This is a kind of stress corrosion that is not easy to be found with hysteresis damage. The tree shaped crack with trunk and branch is the characteristic of stress corrosion cracking accident of ammonia refrigeration pressure vessel

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