Restriction requirements for duplex stainless steel

1. It is necessary to control the phase ratio, and the most suitable ratio is about half of the ferrite phase and half of the austenite phase. The maximum number of any one phase should not exceed 65%, in order to ensure the best comprehensive performance. If the proportion of the two phases is imbalanced, for example, if there are too many ferrite phases, it is easy to form single-phase ferrite in the welding HAZ, which is sensitive to stress corrosion cracking in certain media.
2. It is necessary to master the microstructure transformation law of duplex stainless steel and be familiar with the TTT and CCT transformation curves of each steel grade. This is the key to correctly guiding the development of duplex stainless steel heat treatment, hot forming and other processes. The precipitation of brittle phases in duplex stainless steel is much more sensitive than that of austenitic stainless steel.
The continuous use temperature range of duplex stainless steel is -50~250 ℃, with the lower limit depending on the brittle transition temperature of the steel, the upper limit limited by the brittleness of 475 ℃, and the upper limit temperature cannot exceed 300 ℃.
4. After solid solution treatment of duplex stainless steel, fast cooling is required. Slow cooling can cause the precipitation of brittle phases, leading to a decrease in the toughness of the steel, especially its resistance to local corrosion.
5. The lower limit temperature for hot working and hot forming of high chromium molybdenum duplex stainless steel should not be lower than 950 ℃, and super duplex stainless steel should not be lower than 980 ℃. Low chromium molybdenum duplex stainless steel should not be lower than 900 ℃ to avoid surface cracks caused by the precipitation of brittle phases during the processing