Alloy steel accessories are the cornerstone of industry

1. Unique composition of heat-resistant alloy steel

The key to the excellent performance of heat-resistant alloy steel in high temperature environment lies in its unique composition. The key elements such as chromium, molybdenum, nickel, vanadium, tungsten, carbides and solid solutions work together to give alloy steel excellent performance.

Chromium is the main alloying element in stainless steel. It has excellent corrosion resistance at high temperatures. The oxide layer formed on the surface of the steel can effectively prevent further oxidation and scaling. For example, when the chromium content in steel reaches 12%, the steel's resistance to high-temperature oxidation is significantly improved. When the content reaches 22%, the high-temperature oxidation resistance at 1000℃ is extremely good.

Molybdenum can improve the strength of steel at high temperatures, improve the ability to resist pitting and crevice corrosion, and help improve creep resistance. Molybdenum is a refractory metal with a high melting point (2625℃). Solid solution in the base metal can increase the recrystallization temperature of the solid solution. In low-alloy heat-resistant steel, the role of molybdenum is to strengthen the solid solution and form fine carbide phases, such as the common chromium-molybdenum low-alloy steel (9/4Cr1Mo steel).

Nickel can improve the toughness and strength of steel at high temperatures, especially it can reduce the embrittlement that may be caused by the chromium content. Nickel is an element that expands the γ phase region and forms an infinite solid solution. In heat-resistant steel, when the nickel content is not less than 25%, the steel can obtain pure austenite structure at room temperature.

Vanadium can improve the strength of steel, especially under high temperature service, and can also improve hardenability and wear resistance. Adding 0.5% vanadium to steel can refine the grain size, improve strength and toughness, and the carbide formed by vanadium and carbon can improve the resistance to hydrogen corrosion under high temperature and high pressure.

Tungsten can improve high temperature strength and creep resistance. It is a refractory metal with a high melting point (3380℃). Solid solution into the base metal can increase the recrystallization temperature of the solid solution. In pearlite heat-resistant steel, adding tungsten can improve the thermal strength of the solid solution; in ferrite heat-resistant steel, adding tungsten can strengthen the α phase solid solution and produce dispersion strengthening.

Carbides formed by elements such as tungsten, molybdenum and chromium are stable at high temperatures, providing strength and wear resistance. Solid solutions formed by elements such as nickel and manganese in the solid solution enhance the mechanical properties of the alloy, improve its creep resistance and maintain strength at high temperatures. The synergistic effect of these components makes heat-resistant alloy steel an ideal choice for high-temperature environments.

2. Common grades and characteristics

Different grades of heat-resistant alloy steel

P11 and P22: P11 high-pressure alloy pipe is made of P11 alloy steel, which has excellent high-temperature resistance and corrosion resistance. It contains a higher proportion of chromium and molybdenum, which can still maintain high strength and oxidation resistance at high temperatures. P11 and P22 are chromium-molybdenum alloy steels, which are widely used in boilers, pressure vessels and pipelines in the power generation industry. For example, P22 steel is a 2.25Cr-1Mo low-alloy steel with high thermal strength, good oxidation resistance, hydrogen corrosion resistance and welding performance, and is widely used in high-temperature equipment such as power station boilers.

F91: F91 is a high-grade alloy steel containing chromium, molybdenum and vanadium. It is used in steam piping and other high-temperature applications and has excellent strength and durability at high temperatures. F91 not only has high resistance to oxidation and high-temperature steam corrosion, but also has good impact toughness and high and stable long-lasting plasticity and hot strength. Its service stress is higher than that of austenitic stainless steel when the service temperature is below 620°C.

310 and 321 stainless steel: 310 stainless steel has strong resistance to oxidation and scaling at temperatures up to 1100°C and can be used in furnaces and reactors. 321 stainless steel has titanium added and can be used in applications that require resistance to intergranular corrosion at high temperatures. For example, 321 stainless steel is 18/8 austenitic stainless steel (304) stabilized with titanium, and will not undergo intergranular corrosion after heating in the carbide precipitation temperature range of 425-900°C.

Characteristics of heat-resistant alloy steel

Heat-resistant alloy steel has high-temperature strength and can maintain high strength in high-temperature environments. This is one of its important characteristics that distinguishes it from ordinary steel. For example, F91 has excellent material properties at high temperatures, and its tensile strength should reach more than 585 MPa and its yield strength should reach more than 415 MPa.

Chemical stability is also an important characteristic of heat-resistant alloy steel. Elements such as chromium, aluminum, and silicon can promote the formation of dense oxide films on the metal surface at high temperatures to prevent further oxidation and improve the steel's oxidation resistance and resistance to high-temperature gas corrosion. For example, chromium forms a protective oxide layer on the steel surface to prevent further oxidation and scaling at high temperatures.

Heat-resistant alloy steel is divided into two categories: oxidation-resistant steel and heat-resistant steel. The diversified classification meets the needs of different fields. Oxidation-resistant steel has good chemical stability, but the load it bears is relatively low; heat-resistant steel requires higher high-temperature strength and oxidation resistance. Heat-resistant steel is often used in the manufacture of industrial equipment such as boilers, steam turbines, and power machinery, as well as high-temperature working parts in industrial sectors such as aviation and petrochemicals, and plays an important role in these fields.

3. Wide application in high temperature industry

Power generation field

In thermal power plants, heat-resistant alloy steel accessories play a vital role. As one of the core equipment, the boiler needs to withstand a high temperature and high pressure steam environment. Heat-resistant alloy steel can maintain stable performance under such harsh conditions to ensure the safe operation of the boiler. For example, in 700℃ ultra-supercritical thermal power plants, ultra-supercritical heat-resistant alloys are used to manufacture turbine rotors, boilers and other equipment. In 2021, my country's thermal power generation accounted for 71.13% of the total power generation of the whole society. In order to achieve "carbon neutrality", it is very important to improve the thermal efficiency of thermal power units and reduce energy consumption and carbon emissions, and heat-resistant alloy steel just meets this demand.

The superheater is responsible for heating saturated steam to superheated steam, and its operating temperature is extremely high. Heat-resistant alloy steel can withstand the scouring of high-temperature steam here, maintain structural integrity, and improve heat exchange efficiency. The heat exchanger requires efficient heat transfer between fluids at different temperatures. The good thermal conductivity and high temperature resistance of heat-resistant alloy steel make it an ideal material choice. During the operation of the steam turbine, the blades and other components rotate at high speed and are subjected to the action of high-temperature steam, which places extremely high demands on the strength and heat resistance of the material. Heat-resistant alloy steel can ensure the stable operation of the steam turbine under high temperature and high pressure, reduce the frequency of shutdown maintenance, and improve power generation efficiency.

Petrochemical Industry

Equipment such as reactors, furnaces, and distillation units in refineries and chemical processing facilities need to work in an environment of high temperature and corrosive chemicals. Heat-resistant alloy steel accessories can meet these demanding requirements. Various chemical reactions are taking place in the reactor, and the temperature and pressure change frequently. The creep resistance, oxidation resistance, and scaling resistance of heat-resistant alloy steel enable it to withstand such complex working conditions and ensure the smooth progress of the reaction.

The furnace is a key equipment for high-temperature processing in the petrochemical industry, and the temperature is usually above thousands of degrees. Heat-resistant alloy steel can maintain strength and stability at such high temperatures to prevent deformation and damage to the equipment. The distillation unit needs to separate substances with different boiling points at high temperatures, which requires high heat resistance and chemical stability of the material. The unique composition of heat-resistant alloy steel enables it to resist the erosion of corrosive chemicals, ensuring the efficiency and safety of the distillation process.

For example, 2535Nb alloy, as an iron-nickel-based heat-resistant alloy, is mainly used for welding of centrifugal casting reforming furnaces in the petrochemical industry. It can effectively resist high-temperature oxidation and corrosion, ensuring welding quality and safety. In addition, the supply of heat-resistant alloy steel castings for petrochemical heating furnace tube hangers, such as intermediate tube plates, hooks, and draw hooks, can provide stable support for heating furnaces under high temperature environments.

Aerospace

In the field of aerospace, jet engines and turbine components are highly dependent on heat-resistant alloy steels. When a jet engine is working, the combustion chamber temperature can reach thousands of degrees, and components such as turbine blades and guide vanes need to withstand huge thermal and mechanical stresses. The high-temperature strength, oxidation resistance and creep resistance of heat-resistant alloy steel enable it to maintain structural integrity and performance in such extreme environments.

For example, the application of high-temperature alloys in aircraft engine turbines is very critical. Aircraft engine turbine blades are one of the components that bear the most severe temperature loads and the worst working environment, and the requirements for materials are very strict. The high-temperature alloy for the guide needs to have sufficient high-temperature strength, long-lasting creep performance and good thermal fatigue performance, high oxidation resistance and thermal corrosion performance, etc. The high-temperature alloy used for turbine blades should have high creep resistance and long-lasting fracture strength at the use temperature, sufficient plasticity and impact toughness, etc. As the rotating load-bearing part with the greatest force, the turbine disk requires the alloy to have the highest yield strength, impact toughness and other properties at the use temperature. The excellent performance of heat-resistant alloy steel provides a reliable guarantee for the development of the aerospace industry.