[[TitleIndustry]]

Applications of Duplex Stainless Steel

Date:Jun 14, 2019

3RE60 (00Cr18Ni8Mo3Si2) is austenitic-ferritic duplex stainless steel. Some foreign factories use this stainless steel for the internals of towers, such as grilles, trays, etc. The distillation section of the mother liquor distillation column, but after the operation, stress corrosion cracking occurs. After inspection and analysis, the stress corrosion damage site is at the highest temperature. Although duplex stainless steel has better resistance to intergranular corrosion than austenitic stainless steel with comparable carbon content, the resistance to chloride stress corrosion of duplex stainless steel is lower than that of ordinary 18-8 austenitic stainless steel only under low stress. It shows a certain superiority, and the difference is not big or basically the same under the action of high stress. Therefore, the mother liquid distillation column made of this material has cracks on the wall of the tower after 2 years of use, and leakage occurs. 

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After 5 years of use, there are more than 30 leak points in each tower. Cracks and cracks occur in the weld and heat-affected zone of the tower body. Among them, the cracks in the heat-affected zone of the ring weld are the most, and the cracks in the serious area are outward radial, which leads to the inner and outer perforations. The cracks in the heat affected zone of the weld are concentrated. Parallel to the weld and the length corresponding to it, the local cylinder base material is pitting corrosion in many places. The reason for the analysis is that the stress is mainly caused by the stress corrosion damage of the chloride. The cause of the stress corrosion cracking is mainly the welding stress, especially the stress source caused by the residual stress caused by the repair welding and the structural stress generated during the manufacture of the equipment. Scaling and dry-wetting on the wall of the tower alternately concentrates the chloride ion medium to cause stress corrosion damage. Finally, it is concluded that the duplex stainless steel can not withstand the conditions of the mother liquor distillation in the alkali plant. For the application of the material in soda ash, the recommended temperature should not exceed 80 °C, and attention should be paid to avoid stress concentration during design and manufacture. This also shows that in industrial production, the selection of suitable materials is extremely important for the safety, stability and cost reduction of production.

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The corrosion resistance of stainless steel is mainly achieved by the passivation of Fe-Cr alloy. The passivation film of stainless steel is extremely thin, about 1-10 nm, the film is not very uniform, and the local is always defective. Therefore, the passivation film is easy. Suffering from damage, such as chemical damage and mechanical damage, the chemical destruction of the passivation film is mainly due to the interaction of aggressive anions such as chloride and the passivation film, resulting in local corrosion damage such as pitting, crevice corrosion and intergranular corrosion. 

As for the mechanism of damage to the passivation film such as chloride ions, it is also a problem to be explored. The mechanical destruction of the passivation film is the result of the interaction of the corrosive environment and the mechanical force. In this form of failure, the halide ion plays an important role, so it is extremely important to select a suitable material for different working conditions.


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