If the stainless steel storage tank leaks, we can analyze it from the following points:
1. Visual inspection. After a leak test with water, it was found that there was a leak point at the connection between the nozzle and the shell. After careful observation, there were tiny cracks in the weld.
2. Macroscopic inspection of fracture. Observing the cracked parts of the shell, it was found that the cracks in the shell were located in the welding area near the welding seam with the nozzle, showing a zigzag shape.
3. Chemical composition inspection. The chemical composition of the shell and the nozzle is analyzed, and the chemical composition of the shell meets the corresponding standards, the chemical composition of the nozzle meets the corresponding standards, and the data meets the standard requirements.
4. Metallographic inspection. The metallographic sample is prepared by cutting the test block at the cracked part of the shell. The sample is first observed under the microscope without corrosion. The level of non-metallic inclusions in the shell test block is relatively low. Further observation shows that the cracks spread inward from the surface of the shell in an intergranular manner, the cracks appear as a network as a whole, and the combined cracks appear near the weld area, which should belong to the welding heat-affected zone, so the cracks are in the welding heat cracks. crystal cracks.
After further erosion, the matrix structure of the shell is found to be austenite + a small amount of carbides. Continuing to observe, it can be found that the crack starts in the heat-affected zone of the weld, and the black part is the place where the crystal falls off; and black carbides can be seen near the fall off of the grain and at the grain boundary.
5. Hardness testing. The hardness test block was intercepted at the cracked part of the shell, and 4 points were detected, and 3 points were greater than the standard value of 210HV.
Based on the above analysis, during the welding process of the shell and the nozzle, the temperature of the heat-affected zone close to the shell is 500~850℃, forming a welding sensitization zone. At the same time, the hardness value of the shell steel plate exceeds the standard, and the solution treatment effect is not good. Carbon in solid solution and austenite also increases the tendency to form welding sensitization. The combined action of the two causes intergranular corrosion and cracking of the shell, thus causing leakage of stainless steel storage tanks.
