Safety requirements for duplex stainless steel pipes

1. The phase ratio needs to be controlled. The most suitable ratio is that the ferrite phase and the austenite phase account for about half each, and the number of one phase cannot exceed 65% at most, so as to ensure the best comprehensive performance. If the two phases are out of proportion, for example, the number of ferrite phases is too large, it is easy to form single-phase ferrite in the welded HAZ, which is sensitive to stress corrosion cracking in some media.

2. It is necessary to master the structure transformation law of duplex stainless steel pipes, and be familiar with the TTT and CCT transformation curves of each steel grade. This is the key to correctly guiding the formulation of duplex stainless steel heat treatment, hot forming and other processes. The precipitation of duplex stainless steel brittle phase should be Much more sensitive than austenitic stainless steel.

3. The continuous service temperature range of the duplex stainless steel pipe is -50 to 250 °C, the lower limit depends on the brittle 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. The duplex stainless steel pipe needs to be cooled quickly after solution treatment. Slow cooling will cause the precipitation of the brittle phase, which will lead to the decline of the toughness of the steel, especially the local corrosion resistance.

5. The lower limit temperature of hot working and hot forming of high chromium molybdenum duplex stainless steel pipe should not be lower than 950℃, super duplex stainless steel should not be lower than 980℃, low chromium molybdenum duplex stainless steel should not be lower than 900℃, to avoid the brittle phase Precipitation causes surface cracks during processing

6. Stress relief treatment at 650-800°C commonly used in austenitic stainless steel cannot be used, and solution annealing treatment is generally used. For the surfacing of duplex stainless steel on the surface of low-alloy steel, when the overall stress reduction treatment at 600-650°C is required, the toughness and corrosion resistance caused by the precipitation of brittle phase must be considered, especially the local corrosion resistance. To reduce the problem of falling, the heating time in this temperature range should be shortened as much as possible. The same is true for the heat treatment of low-alloy steel and duplex stainless steel clad plates.

7. It is necessary to be familiar with the welding rules of duplex stainless steel pipes. It is not possible to apply the welding of austenitic stainless steel. The safe use of duplex stainless steel equipment has a lot to do with the correct grasp of the welding process of steel. The failure of some equipment is often caused by related to welding. The key lies in the control of line energy and interlayer temperature, and the correct selection of welding materials is also very important. The two-phase ratio of the welded joint (weld metal and welded HAZ), especially the welded HAZ, maintains the necessary amount of austenite, which is important to ensure that the welded joint has the same properties as the base metal.

8. When choosing duplex stainless steel in different corrosive environments, it should be noted that the corrosion resistance of steel is always relative. Although duplex stainless steel has good local corrosion resistance, as for a certain duplex stainless steel, it is also There is an applicable range of medium conditions, including temperature, pressure, medium concentration, pH value, etc., which need to be carefully selected. Many of the data obtained from literature and manuals are the results of laboratory corrosion tests, which are often different from the actual conditions of the project. Therefore, attention should be paid when selecting materials. If necessary, corrosion tests in actual media or field conditions should be carried out. Coupon tests, and even tests of simulated devices.