Results of Stainless Steel 304 Flanges in Propylene

3. Results
Stainless steel has good corrosion resistance, especially intergranular corrosion resistance, so it is an ideal material for chemical equipment such as acid-resistant containers, pipes and heat exchangers. The raw material of the failed flange is severely sensitized, and carbides are precipitated on the grain boundary. Cracks propagate along the grain from the outside to the inside, and there is Cl on the fracture, stress corrosion cracking of Cl- exists.
 
From the analysis of the chemical composition of the flange, it can be seen that the C content of the flange material is close to the maximum value required by the standard, while the content of Cr is lower than the standard requirement. The intergranular corrosion of austenitic stainless steel is mainly caused by C in the steel. When the C content in the steel exceeds 0.03%, the higher the content of C is, the intergranular corrosion will easily occur after welding or heating at 550 to 800°C. The tendency to corrosion is more serious. The content of C in the failed flange is 0.075% and it has been welded, so intergranular corrosion is more likely to occur. When the flange material stays at 450 to 850°C, the material will be sensitized and carbide will be formed on the grain boundary, which will reduce 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 is just in the sensitization temperature range (at a temperature of 450 to 850°C) for a long time, which makes the sensitization more serious, and the precipitation of carbides on the grain boundary makes the chromium-poor area appear in the nearby area in the grain boundary. When the area is in contact with corrosive media such as Cl and S, intergranular corrosion will soon occur.
 
The outer wall is corroded by Cl - due to the residual stress of the flange after processing and welding, resulting in the formation of multi-point crack sources on the outer wall of the flange. At the same time, the constantly changing pressure and the tensile stress generated by the outer wall make 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 after being in contact with corrosive media such as chlorine and sulfuric acid. Under the interaction of intergranular corrosion and stress corrosion, the formed cracks rapidly expand inward along the grain boundaries. Under the same environment, the side of the heat exchange tube in the heat-affected zone of the welding seam was intact, indicating that the main cause of corrosion cracking in the heat-affected zone of the flange welding seam was the problem of the raw material of the flange.
 
4. Conclusion
According to the analysis, the reason for flange cracking is the overall sensitization of the raw material, which precipitates a large number of dispersed carbon-chromium compounds, resulting in the lack of chromium in the grain boundary, which reduces the grain boundary strength and corrosion resistance of the material. Under the action of Cl-, stress corrosion cracking appears for the flange. It is recommended to perform solution treatment on the flange after forging, heat the forging to a certain temperature, and cool it rapidly to obtain a single-phase austenite structure. When purchasing, the quality of the original parts of the pipes should be controlled, and the quality of the materials should be controlled from the links of macro inspection, chemical analysis and metallographic inspection. Ultra-low carbon electrodes should be used for welding to reduce the sensitization degree of subsequent material welding.