Analysis Results of 304 Stainless-steel Flanges in Propylene
Discussion of results
0Cr18Ni9 stainless steel has good corrosion resistance, especially excellent resistance to intergranular corrosion, so it is an ideal material for chemical equipment such as acid-resistant containers, pipes and heat exchangers. The material of the failed flange was severely sensitized, carbides were precipitated on the grain boundaries; the cracks extended along the grain from the outside to the inside; the fracture contained Cl, and there was stress corrosion cracking of Cl-. From the chemical composition analysis of the flange, it can be seen that the C content of the flange material is close to the maximum required by the standard, while the Cr content is already the lower limit of the requirement. The intergranular corrosion of austenitic stainless steel was mainly caused by C in the steel. When the C content in the steel exceeded 0.03%, the higher the C content was, the tendency of intergranular corrosion after welding or heating at 550 to 800℃ became more serious. The C content of the failed flange was 0.075%, and after welding, it was more prone to intergranular corrosion. When the flange material stayed at temperatures between 450 to 850℃, the material was sensitized and carbides were formed on the grain boundary, which reduced the strength and corrosion resistance of the grain boundary. In the welding process of the flange and the heat exchange tube, a certain area of the heat affected zone was in the sensitization temperature range for a long time (450 to 850℃), which made the sensitization more serious, and the precipitation of carbides on the grain boundary made chromium-depleted area appeared at the vicinity of the grain boundary. When the area was in contact with corrosive media such as Cl and S, intergranular corrosion would quickly occur.
The outer wall was corroded by Cl- due to residual stress in the flange after processing and welding, resulting in the formation of multi-point crack sources on the outer wall of the flange. The pipelines connected to the flange were in a suspended state, which would form sufficient tensile stress on the weak area of the flange. At the same time, the constantly changing pressure and the tensile stress generated by the outer wall made the multi-point crack source formed in the heat-affected zone of the flange neck continue to expand inward. Austenitic stainless steel in a sensitized state and under stress may cause stress corrosion when exposed to corrosive media such as chlorine and sulfuric acid. Under the interaction of intergranular corrosion and stress corrosion, the cracks that have formed rapidly extended inward along the grain boundaries. In the same environment, one side of the heat-exchange tube in the heat-affected zone of the welding seam is intact, indicating that the main reason for the corrosion and fracture of the heat-affected zone of the flange welding seam was the problem of the flange material.
Conclusion
After analysis, it can be seen that the reason for cracking of the flange is the overall sensitization of the raw material itself, and the precipitation of a large number of dispersed carbon-chromium compounds resulted in the depletion of chromium at the grain boundary, which reduced the strength and corrosion resistance of the grain boundary of the material. Under the action of Cl-, stress corrosion cracking appeared for the flange. It is recommended to carry out solution treatment after forging the flange. Heat the forging to a certain temperature, and cool it quickly to obtain a single-phase austenite structure. When purchasing, the quality of the original pipe fitting should be controlled, and the quality of materials should be controlled by macro inspection, chemical analysis, and metallographic inspection, and ultra-low carbon electrodes should be used for welding to reduce the sensitization of subsequent welding of the material.
0Cr18Ni9 stainless steel has good corrosion resistance, especially excellent resistance to intergranular corrosion, so it is an ideal material for chemical equipment such as acid-resistant containers, pipes and heat exchangers. The material of the failed flange was severely sensitized, carbides were precipitated on the grain boundaries; the cracks extended along the grain from the outside to the inside; the fracture contained Cl, and there was stress corrosion cracking of Cl-. From the chemical composition analysis of the flange, it can be seen that the C content of the flange material is close to the maximum required by the standard, while the Cr content is already the lower limit of the requirement. The intergranular corrosion of austenitic stainless steel was mainly caused by C in the steel. When the C content in the steel exceeded 0.03%, the higher the C content was, the tendency of intergranular corrosion after welding or heating at 550 to 800℃ became more serious. The C content of the failed flange was 0.075%, and after welding, it was more prone to intergranular corrosion. When the flange material stayed at temperatures between 450 to 850℃, the material was sensitized and carbides were formed on the grain boundary, which reduced the strength and corrosion resistance of the grain boundary. In the welding process of the flange and the heat exchange tube, a certain area of the heat affected zone was in the sensitization temperature range for a long time (450 to 850℃), which made the sensitization more serious, and the precipitation of carbides on the grain boundary made chromium-depleted area appeared at the vicinity of the grain boundary. When the area was in contact with corrosive media such as Cl and S, intergranular corrosion would quickly occur.
The outer wall was corroded by Cl- due to residual stress in the flange after processing and welding, resulting in the formation of multi-point crack sources on the outer wall of the flange. The pipelines connected to the flange were in a suspended state, which would form sufficient tensile stress on the weak area of the flange. At the same time, the constantly changing pressure and the tensile stress generated by the outer wall made the multi-point crack source formed in the heat-affected zone of the flange neck continue to expand inward. Austenitic stainless steel in a sensitized state and under stress may cause stress corrosion when exposed to corrosive media such as chlorine and sulfuric acid. Under the interaction of intergranular corrosion and stress corrosion, the cracks that have formed rapidly extended inward along the grain boundaries. In the same environment, one side of the heat-exchange tube in the heat-affected zone of the welding seam is intact, indicating that the main reason for the corrosion and fracture of the heat-affected zone of the flange welding seam was the problem of the flange material.
Conclusion
After analysis, it can be seen that the reason for cracking of the flange is the overall sensitization of the raw material itself, and the precipitation of a large number of dispersed carbon-chromium compounds resulted in the depletion of chromium at the grain boundary, which reduced the strength and corrosion resistance of the grain boundary of the material. Under the action of Cl-, stress corrosion cracking appeared for the flange. It is recommended to carry out solution treatment after forging the flange. Heat the forging to a certain temperature, and cool it quickly to obtain a single-phase austenite structure. When purchasing, the quality of the original pipe fitting should be controlled, and the quality of materials should be controlled by macro inspection, chemical analysis, and metallographic inspection, and ultra-low carbon electrodes should be used for welding to reduce the sensitization of subsequent welding of the material.
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