What is duplex stainless steel
Duplex stainless steel is a kind of stainless steel composed of any two phases of austenite, ferrite, and martensite in a ratio of about half. But it usually refers to austenitic-ferritic duplex stainless steel.

In the case of low C content, the Cr content is 18%~28% and the Ni content is 3%~10%. Some steels also contain alloying elements such as Mo, Cu, Nb, Ti, and N.

This type of steel has the characteristics of both austenitic and ferritic stainless steel. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance, while maintaining iron content. The brittleness of solid stainless steel at 475°C and the high thermal conductivity make it superplastic. Compared with austenitic stainless steel, it has high strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-saving stainless steel.

Duplex stainless steel seamless pipe characteristics
1.Good welding performance
Compared with ferritic stainless steel and austenitic stainless steel seamless pipes, it does not have the welding heat affected zone of ferritic stainless steel, and the plastic toughness is greatly reduced due to severe grain coarsening. It is also not like austenite. Stainless steel seamless pipes are more sensitive to welding heat cracks. It has good weldability and low tendency of hot cracking. Generally, no preheating is required before welding, and no heat treatment is required after welding. It can be welded with dissimilar types such as 18-8 austenitic stainless steel or carbon steel.

2. Good corrosion fatigue and wear corrosion resistance.
Under the conditions of certain corrosive media, it is suitable for making power equipment such as pumps and valves.

3. Good comprehensive mechanical properties.
It has high strength and fatigue strength, and its yield strength is twice that of 18-8 austenitic stainless steel. The elongation in the solid solution state reaches 25%, and the toughness value AK (V-shaped notch) is above 100J.

4. Difficulty in deformation
The work hardening effect is greater than that of 18-8 type austenitic stainless steel during cold working. In the early stages of deformation of tubes and plates, greater stress must be applied to deform.

Safety requirements for duplex stainless steel seamless pipes
1. The phase ratio needs to be controlled. The most appropriate ratio is about half of the ferrite phase and half of the austenite phase. The amount of a certain phase cannot exceed 65% at most, so as to ensure the best overall performance. If the proportion of the two phases is out of balance, for example, the number of ferrite phases is too much, it is easy to form single-phase ferrite in the welding HAZ, which is sensitive to stress corrosion cracking in some media.

2. It is necessary to master the structural transformation rules of duplex stainless steel seamless pipes and be familiar with the TTT and CCT transformation curves of each steel type. This is the key to correctly guide the formulation of duplex stainless steel heat treatment, hot forming and other processes. The brittle phase of duplex stainless steel It is much more sensitive to precipitation than austenitic stainless steel.

3. The continuous use temperature range of duplex stainless steel seamless pipes is -50~250℃. The lower limit depends on the brittle transition temperature of the steel. The upper limit is limited by the brittleness of 475℃. The upper limit temperature cannot exceed 300℃.

4. Duplex stainless steel seamless pipes need to be cooled quickly after solution treatment. Slow cooling will cause the precipitation of brittle phases, which will lead to a decrease in the toughness of the steel, especially the resistance to local corrosion.

5. The lower limit temperature of hot processing and hot forming of high chromium molybdenum duplex stainless steel seamless pipes cannot be lower than 950°C, super duplex stainless steel cannot be lower than 980°C, and low chromium molybdenum duplex stainless steel cannot be lower than 900°C to avoid The precipitation of brittle phases causes surface cracks during processing.

6. The commonly used 650-800°C stress relief treatment for austenitic stainless steel cannot be used. Solution annealing treatment is generally used. When the overall stress relief treatment at 600-650℃ is required after overlaying duplex stainless steel on the surface of low alloy steel, the toughness and corrosion resistance caused by the precipitation of the brittle phase must be taken into consideration, especially the local corrosion resistance. To reduce the drop problem, shorten the heating time in this temperature range as much as possible. The heat treatment issues of low alloy steel and duplex stainless steel composite plates should also be considered.

7. You need to be familiar with the welding rules of duplex stainless steel seamless pipes. You cannot apply all the welding of austenitic stainless steel. The safe use of duplex stainless steel equipment has a lot to do with the correct mastery of the steel welding process. The failure of some equipment Often related to welding. The key lies in the control of line energy and interlayer temperature, and the correct selection of welding materials is also important. The proportion of the two phases in the welded joint (weld metal and welding HAZ), especially the welding HAZ, maintains the necessary austenite quantity, which is important to ensure that the welded joint has the same properties as the base metal.

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