1. It is necessary to control the phase ratio, the most suitable ratio is about half of the ferrite phase and austenite phase, and the number of a certain phase cannot exceed 65% at most, so as to ensure the best comprehensive performance. If the ratio of the two phases is out of balance, for example if the number of ferrite phases is excessive, it is easy to form single-phase ferrite in the welding AZ, which is sensitive to stress corrosion cracking in some media.
2. Need to master the microstructure transformation law of duplex stainless steel pipe, familiar with the TTT and CCT transition curve of each steel grade, which is the key to correctly guiding the development of duplex stainless steel heat treatment, thermoforming and other processes, duplex stainless steel brittle phase precipitation is much more sensitive than austenitic stainless steel.
3. The continuous use temperature range of duplex stainless steel pipe is -50~250 °C, the lower limit depends on the brittleness transition temperature of the steel, the upper limit is limited by the brittleness of 475 °C, and the upper limit temperature cannot exceed 300 °C.
4. Duplex stainless steel pipe needs to be quickly cooled after solution treatment, and slow cooling will cause the precipitation of brittle phase, resulting in the toughness of steel, especially the decline of local corrosion resistance.
5. The lower limit temperature of hot working and thermoforming of high chromium molybdenum duplex stainless steel pipe cannot be lower than 950 °C, super duplex stainless steel cannot be lower than 980 °C, low chromium molybdenum duplex stainless steel cannot be lower than 900 °C, to avoid surface cracks caused by the precipitation of brittle phase in the processing process.
6. The stress relief treatment of 650-800 °C commonly used in austenitic stainless steel cannot be used, and the solid solution annealing treatment is generally used. For the surface of low-alloy steel after overlay welding duplex stainless steel, when 600-650 °C overall stress treatment is required, it is necessary to consider the toughness and corrosion resistance caused by the precipitation of the brittle phase, especially the decline of local corrosion resistance, and shorten the heating time in this temperature range as much as possible. The heat treatment of low-alloy steel and duplex stainless steel composite panels should also be considered in the same way.
