Brief Introduction of Duplex Stainless Steel

Duplex stainless steel pipe has good weldability. Compared with ferritic stainless steel and austenitic stainless steel, it is neither like the weld heat affected zone of ferritic stainless steel, nor is it sensitive to weld hot crack as austenitic stainless steel.

Duplex stainless steel is widely used in petrochemical equipment, seawater and wastewater treatment equipment, oil pipeline, paper machinery and other industrial fields because of its special advantages. In recent years, duplex stainless steel has also been studied and applied in the field of bridge load-bearing structure, which has good prospects for development.

Duplex stainless steel consists of ferrite phase and austenite phase in its quenched structure, and the content of the minimum phase may reach 30%.

Safety Limitations and Requirements for Duplex Stainless Steel Pipes

1.It is necessary to control the ratio of ferrite phase to austenite phase. The most suitable ratio is about half of ferrite phase and half of austenite phase. The number of one phase can not exceed 65%, so as to ensure the best comprehensive performance. If the ratio of the two phases is out of balance, such as too many ferrite phases, it is easy to form single phase ferrite in welded HAZ, which is sensitive to stress corrosion cracking in some media.

2.It is necessary to master the rules of structure transformation of duplex stainless steel pipes and to be familiar with the TTT and CT transformation curves of each kind of steel. This is the key to correctly guide the formulation of heat treatment and hot forming process of duplex stainless steel. The precipitation of brittle phase of duplex stainless steel is more sensitive than that of austenitic stainless steel.

3.The continuous service temperature of duplex stainless steel pipes ranges from - 50 to 250 C. The lower limit depends on the brittle transition temperature of the steel. The upper limit is limited by the brittle transition temperature of 475 C, and the upper limit can not exceed 300 C.

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

5.The lower limit temperature of hot-working and hot-forming of high chromium-molybdenum duplex stainless steel pipes should not be lower than 950 and that of super duplex stainless steel should not be lower than 980 and that of low chromium-molybdenum duplex stainless steel should not be lower than 900 so as to avoid surface crack

6.It is not possible to use the commonly used stress relief treatment of austenitic stainless steel at 650-800 C, but the solution annealing treatment is generally adopted. After surfacing duplex stainless steel on the surface of low alloy steel, the toughness and corrosion resistance caused by the precipitation of brittle phase must be taken into account, especially the decrease of local corrosion resistance, so as to shorten the heating time in this temperature range as far as possible. The heat treatment of low alloy steel and duplex stainless steel clad plate should also be considered in the same way.

7.It is necessary to be familiar with the welding rules of duplex stainless steel pipes, and not all austenitic stainless steel pipes can be welded. Whether the equipment of duplex stainless steel can be used safely or not is closely related to the correct grasp of the welding process of steel. The failure of some equipment is often related to welding. The key lies in the control of line energy and interlayer temperature. It is also important to select welding materials correctly. The comparison of welded joints (welded metal and welded HAZ), especially the maintenance of the necessary austenite quantity in welded HAZ, is very important to ensure that the welded joints have the same performance 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 better local corrosion resistance, for a duplex stainless steel, it also has a suitable range of medium conditions, including temperature, pressure, medium concentration, pH value and so on, which need to be carefully selected. Many of the data obtained from literature and manuals are laboratory corrosion test results, which often lag behind the actual conditions of the project. Therefore, it is necessary to pay attention to the selection of materials, such as corrosion test in real media or in-situ patch test, or even simulation device test.

Characteristics of Duplex Stainless Steel

Due to the characteristics of two-phase structure, DSS has the advantages of both ferritic stainless steel and austenitic stainless steel by correctly controlling chemical composition and heat treatment process.

Advantages of Duplex Stainless Steel over Austenitic Stainless Steel

(1)  The yield strength is more than twice as high as that of common austenitic stainless steel, and it has enough plasticity and toughness for forming. The wall thickness of tanks or pressure vessels made of duplex stainless steel is 30-50% less than that of austenite commonly used, which is conducive to reducing costs.

(2)  Even the duplex stainless steel with the lowest alloy content has higher stress corrosion cracking resistance than the austenitic stainless steel, especially in the environment containing chloride ions. Stress corrosion is an outstanding problem that ordinary austenitic stainless steel is difficult to solve.

(3)  In many media, the corrosion resistance of 2205 duplex stainless steel is better than 316L austenitic stainless steel. Super duplex stainless steel has very high corrosion resistance. In some media, such as acetic acid, formic acid and so on, it can even replace high alloy austenitic stainless steel and even corrosion resistant alloy.

(4)  Compared with austenitic stainless steel with the same alloy content, its wear resistance, corrosion resistance and fatigue corrosion resistance are better than those of austenitic stainless steel.

(5)  The linear expansion coefficient of austenitic stainless steel is lower than that of austenitic stainless steel, which is close to that of carbon steel. It is suitable for joining with carbon steel and has important engineering significance, such as producing composite plates or linings.

(6)  Under both dynamic and static loads, the energy absorption capacity of duplex stainless steel is higher than that of austenitic stainless steel, which has obvious advantages in dealing with unexpected accidents such as collision and explosion, and has practical application value.

The disadvantage of duplex stainless steel compared with austenitic stainless steel

(1)  The universality and versality of application are not as good as austenitic stainless steel. For example, the service temperature of austenitic stainless steel must be controlled below 250 C.

(2)  Its plastic toughness is lower than that of austenitic stainless steel, and its cold working process and forming performance are inferior to those of austenitic stainless steel.

(3)  In order to avoid the appearance of harmful phases and damage the properties, the process of heat treatment and welding should be strictly controlled in the presence of medium temperature brittle zone.

The advantages of duplex stainless steel compared with ferritic stainless steel

(1)  The comprehensive mechanical properties of ferritic stainless steel are better than that of ferritic stainless steel, especially its plasticity and toughness. It is not as sensitive to brittleness as ferritic stainless steel.

(2)  Except the stress corrosion resistance, the local corrosion resistance of other stainless steels is better than that of ferritic stainless steels.

(3)  The cold working process and cold forming properties are much better than those of ferritic stainless steel.

(4)  Welding performance is also much better than that of ferritic stainless steel. Generally, there is no need for preheating before welding and no need for heat treatment after welding.

(5)  The application range is wider than that of ferritic stainless steel.

The disadvantage of duplex stainless steel compared with ferritic stainless steel

Alloy element content is high, the price is relatively high, generally the ferrite does not contain nickel.