Introduction of P91 steel and steel pipe

By Published On: May 12th, 2023Categories: Pipe Knowledge0 Comments
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P91 steel

At present, P91 steel has been used to replace P22 steel in the hot section of main steam pipeline and reheat steam pipeline in the newly built sub-critical unit. Therefore, the comparison of P91 steel and P22 steel from the aspects of chemical elements, mechanical properties at room temperature, allowable stress and creep strength, coefficient of linear expansion, thermal conductivity and welding performance shows that P91 steel is more superior. Through the practical application in the project, it is further explained that P91 steel can save pipe, convenient design, installation, economic benefits are very significant.

High temperature and high pressure pipeline material is one of the most important materials used in thermal power plants. It not only plays an important role in the safe operation of power plants, but also affects the construction investment of power plants. In the past 600 MW unit project in China, A335P22 pipes were used for the main steam pipeline and the hot section of the reheated steam pipeline. A335P22 steel is characterized by good process performance, not too sensitive to heat treatment heating temperature, welding performance is good, with good plasticity, the disadvantage of *** is low strength, especially the high temperature lasting strength is lower than 12Cr1MoV steel, under the same temperature, stress, The tube wall thickness is about 30% thicker than 12Cr1MoV steel. The wall thickness of soda pipe is too large, which not only increases the difficulty of manufacture, but also brings inconvenience to design, installation and operation. In the 1980s, the United States launched the A335P91 pipe, because the pipe has better high-temperature performance than P22 steel, its launch immediately received the attention of the world. When the pressure, temperature and inner diameter are the same, P91 steel is used instead of P22 steel in the main steam pipeline and reheating hot section pipeline, the wall thickness can be reduced by about half, and the dosage of pipe fittings such as right-angle tee can be reduced by about 65%.

Characteristics of P91 steel

In previous projects, the main steam pipe of the ultra high pressure 125 MW and 200 MW units uses 10 CrMo910 steel; In the late 1970s to the 1980s, the introduction of subcritical 300 MW and 600 MW units, the main steam pipe selection of A335P22 steel, the two kinds of steel performance is comparable, they belong to pearlite heat-resistant steel, working temperature of 580 ~ 590 ℃, then high temperature can only choose austenitic heat-resistant steel, Austenitic steel working temperature up to 700 ℃, but this kind of steel thermal expansion is high, sensitive to stress corrosion, dissimilar steel joint life is short, not suitable for high temperature, high pressure soda pipe.

P91 steel is a new steel used for steam water pipeline to fill in the 600 ~ 650 ℃ temperature zone between pearlite heat-resistant steel and austenitic heat-resistant steel. It belongs to martensitic heat-resistant steel, and its first service temperature is 650 ℃. In fact, V, Nb, N and other strengthening elements are added to the original 9Cr-1Mo steel to form a metamorphic new steel.

Mass fraction of P91 and P22 steel elements

The mass fractions of main elements in P91 and P22 steel are shown in Table 1. Element S is the main source of sulfide inclusion in steel, which is easy to produce red heat brittleness. Element P mainly affects tempering brittleness and thermal brittleness, so the mass fraction of S and P in steel is lower. V, Nb and N are strengthening elements, which can improve the strength of steel. Cr, Si and other elements can make the oxide film produced by metal in high temperature operation dense and firm, improve the oxidation resistance of steel. As can be seen from Table 1, P91 steel has lower mass fraction of harmful elements, higher oxidation resistance, and the addition of strengthening elements. It is a high temperature and high pressure pipe better than P22 steel.

Room temperature mechanical properties

The room temperature mechanical properties of P91 and P92 steels are shown in Table .

As can be seen from Table 2, the yield limit at room temperature of P91 steel is twice that of P22 steel, and the tensile strength is 41% higher than that of P22 steel.

Allowable stress and creep strength

The allowable stress and creep strength of P91 and P22 steel are shown in Table.

As can be seen from Table 3, the allowable stress of P91 steel is higher than that of P22 steel at all temperatures below 650 ℃. In pipe design, the allowable stress directly affects the selection of pipe wall thickness. It is precisely because P91 steel is used instead of P22 steel as the main steam pipe, its wall thickness can be almost reduced by half, so that the main steam pipe system using P91 steel has the following advantages: the pipe system flexibility increases, reducing the expansion force; The load of support and hanger is reduced; End thrust and torque decrease; Allow the unit load to change faster, shorten the starting time; Lower investment costs.

In thermal power plants, in order to ensure the safe operation of the main steam pipeline, creep monitoring should be carried out on every main steam pipeline whose medium temperature is 500 ℃ or above. The main factors affecting creep include temperature, stress and the steel itself. The higher the temperature, the greater the stress and the faster the creep rate. According to the test data of the manufacturer, the creep strength of P91 steel is almost twice that of P22 steel at 105 h and 550 ℃.

Coefficient of linear expansion and thermal conductivity

Table shows the linear expansion coefficient and thermal conductivity of P91 and P22 steel.

As can be seen from Table 4, the linear expansion coefficient and thermal conductivity of P91 steel are close to that of P22 steel, which can avoid the creep fatigue crack in operation when P91 steel and pearlite steel are connected. This crack is the main reason that affects the short life of dissimilar steel joints when austenitic heat-resistant steel and pearlite heat-resistant steel are connected. Because the linear expansion coefficient of P91 steel is slightly lower than that of P22 steel, the end thrust and torque of the pipeline can be reduced.

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