Stress corrosion cracking caused by an unreasonable equipment design
Thread Content
The R-3101A reactor used for wastewater treatment at a steel plant was manufactured from Hastelloy B-2, and leaks frequently occurred after it went into operation. It had to be shut down for welding repairs 10 times, with the shortest maintenance period lasting over a month. The reactor is a cylindrical tank with an elliptical head. To avoid welding of dissimilar materials on the body, a transition ring is added at the weld between the support and the lower head. The body and transition ring are made of Hastelloy B-2, while the support is made of SS4. The media are steam and 1% fluorine-containing mud water, which are highly corrosive. Cracks mainly occur in the lower annular crack and the middle annular crack, especially the lower annular crack. This reactor is subjected to frequent alternating stresses, especially at the lower ring seam, where it has to endure not only service stresses and thermal stresses but also the centrifugal force generated by mixing the slurry, as well as the alternating stresses resulting from frequent start-up and shutdown operations. But the main cause of corrosion is the unreasonable design of the equipment structure. The straight edges of the head are too narrow (5–10 mm), which does not meet the design specifications. Therefore, the weld connecting the transition ring to the head is too close to the circumferential weld of the lower head, at only 45 mm. This adds additional welding residual stress to the lower ring seam area, which already has a high stress level. Therefore, the stress at the lower ring seam is the highest. Corrosion fatigue cracks occur under the combined action of alternating stress and a corrosive medium. During patch welding, the working conditions are poor, making it difficult to ensure quality; the material in the lower weld area deteriorates increasingly, cracks continue to develop, leading to frequent leakage and catastrophic failures.
Due to an unreasonable design of the equipment, the R-3101A reactors manufactured from Hastelloy B-2 experienced frequent leaks after being put into operation. The main problems lie in the excessively narrow straight edges of the head and the inappropriate location of the welds; these design flaws lead to the formation of high-stress areas, especially in the lower ring weld area. This area is subjected not only to operational stress and thermal stress, but also to centrifugal force caused by the stirred slurry and alternating stress resulting from frequent start-up and shutdown operations; these factors combined lead to the formation of corrosion fatigue cracks. During the patch welding process, poor working conditions made it difficult to ensure welding quality, which further deteriorated the material in the lower ring weld area, allowing cracks to continue to develop and ultimately leading to frequent leakage incidents. To address this issue, it is recommended to redesign the structure of the reactor, particularly by improving the design of the head and the welding methods, to ensure that the weld positions are appropriate and stress concentration is reduced. At the same time, consider using materials more suitable for corrosive environments or improving the corrosion resistance of existing materials, as well as optimizing the mixing and operation processes to reduce unnecessary stress. .