Heat Resistant Steel
Heat resistant steel
Heat resistant steel is a special steel with high strength and good chemical stability at high temperature. It includes two kinds of anti-oxidation steel (high temperature steel without skin) and hot strength steel. Oxidation resistant steel is usually used in the temperature range of 550-1250 ℃, which requires better chemical stability (such as oxidation resistance and high temperature corrosion resistance, etc.), but it bears lower load and requires less resistance to creep and creep fracture. The hot strength steel is usually used in the temperature range of 450-900 ℃, which bears a large stress and requires the material to have good creep resistance, breaking resistance and oxidation resistance.
Heat resistant steel is widely used. The main components of boilers, steam turbines and gas turbines in thermal power plants are made of heat-resistant steel. Heat resistant steel is indispensable for the key equipment of oil refining, ammonia synthesis and ethylene plant in petrochemical industry. In addition, it is also widely used as heat-resistant structural materials in various industrial furnaces such as metallurgy, machinery, building materials, light industry, etc. In coal liquefaction, gasification and nuclear energy (such as high temperature gas cooled reactor, fast reactor), heat-resistant steel is also widely used in some core structural materials.
Due to the development of power station and petrochemical industry, molybdenum was found to be an effective element to improve the thermal strength of heat-resistant steel in 1930s. Adding 0.5% molybdenum to low carbon steel can raise the working temperature from 450 ℃ to 500 ℃. In order to solve the problem of graphitization of molybdenum containing steel, a low alloy chromium molybdenum steel was developed. In 1938, the steel containing 0.8% chromium and 0.5% molybdenum (equivalent to 15CrMo) was used in the boiler superheater in Britain, and then 10.25 chromium-0.5 molybdenum and 2.25 chromium-1 molybdenum with higher performance were developed. In order to further improve the thermal strength of steel and save molybdenum resources, a series of chrome molybdenum vanadium steel has been developed. The former Soviet Union's 12CrlMoV is a typical representative, and it is still the main material for the manufacture of power station boilers.
In the 1950s, China began to introduce and produce low alloy chromium molybdenum steel and chromium molybdenum vanadium steel, which were used in power station boilers and petrochemical industry. Low alloy heat-resistant steel has been paid attention to for its low price and good process performance. In order to further improve the performance and working temperature of the steel, since the 1960s, through a large number of research work at home and abroad, multi-element low alloy heat-resistant steel has been developed, with the working temperature of 600-620 ℃ and partially replacing nickel chromium austenitic steel in use. For example, 12Cr2MoWVTiB (102) steel developed in China, whose performance exceeds that of similar steel in other countries, has been widely used as superheater tubes in 200, 300 and 600MW units in China.
The medium and high alloy heat-resistant steel was developed at the same time as stainless steel. In 1914, Strauss et al. Pointed out that the steel containing enough nickel and chromium had the function of oxidation resistance and acid resistance. After 1920, the chromium and chromium nickel stainless heat-resistant steel was developed gradually and formed many branches. For example, the blade steel series with 12 chromium type medium alloy heat-resistant steel as the main part is mainly used for the blades of steam turbines and gas turbines; the valve steel for the inlet and outlet valves of internal combustion engines in automobiles and tractors, including chromium silicon, chromium nickel and chromium manganese nickel nitrogen type steel; the heat-resistant steel and heat-resistant cast steel for the furnaces of metallurgical machinery, petrochemical industry, light industry and building materials industry, including high chromium ferrite steel and high chromium nickel austenitic steel. Due to the higher creep strength of heat-resistant cast steel than deformed steel, it has been widely used in petrochemical and furnace materials in recent 20 years. A series of high chromium nickel austenitic heat-resistant cast steel strengthened by tungsten, molybdenum, niobium, titanium, etc. have been developed. Widely used in ammonia and ethylene cracking plant. In the field of heat-resistant steel, the developed countries in the world are relatively mature and have formed their own special standards. Since the 1950s, China has formed a relatively complete series and standard brand of heat-resistant steel in production practice. For example, in addition to gb1221-90 - hot steel bar and gb413890 basic standard of heat-resistant steel plate, there are also steel tubes and billets for high-pressure boiler (gb5310-85, gb5311-89), turbine blade steel (gb8732-88), gas valve steel for internal combustion engine (GB / t1277391), heat-resistant steel castings (gb8492-87) and other product standards, which basically meet the needs of China's national economic development. Some new heat-resistant steel grades, such as 2cr20mn9ni2si2n, 3Cr24Ni7SiN (RE), 3cr18mn12si2n and so on, have been developed in combination with the characteristics of domestic resources, which are listed in the relevant standards respectively, so that the series of heat-resistant steel standards in China has its own characteristics.
The role of alloy elements (1) chromium, aluminum, silicon. These elements form and stabilize ferrite, which can promote the formation of dense oxide film on the surface of steel at high temperature and prevent continuous oxidation. They are the main elements to improve the oxidation resistance and high temperature gas corrosion resistance of steel. However, the high content of aluminum and silicon in steel will lead to the serious deterioration of room temperature plasticity and thermoplasticity. Chromium can significantly increase the recrystallization temperature of low alloy steel. When the content is 2%, the strengthening effect is the best. (2) nickel and manganese. Austenite can be formed and stabilized. The high temperature strength and carburizing resistance of austenitic steel are improved. Although manganese can replace nickel to form austenite, it is harmful to the oxidation resistance of heat-resistant steel. (3) molybdenum and tungsten. It's a ferrite forming element. It can increase the recrystallization temperature of the steel and the creep strength of the heat-resistant steel. In low alloy steel, molybdenum is the key element to improve the thermal strength, and the effect of tungsten molybdenum composite is more obvious than that of adding alone. Because molybdenum and tungsten are easy to volatilize and oxidize at high temperature, adding too much molybdenum and tungsten to steel will bring adverse effect on high temperature oxidation resistance. (4) vanadium, niobium and titanium. It is a ferrite forming element and a strong carbide forming element, which can form fine dispersed carbide and improve the high temperature strength of steel. The combination of titanium, niobium and carbon can also prevent intergranular corrosion of austenitic steel at high temperature or after welding. (5) carbon and nitrogen. It can enlarge and stabilize austenite, so as to improve the high temperature strength of heat-resistant steel. When there are more chromium and manganese in steel, it can significantly improve the melting degree of nitrogen, add nitrogen or increase the carbon content, which can replace the precious metal nickel. The formation of carbides or carbonitrides, the control of their morphology and quantity can further strengthen the effect. (6) boron and rare earth. All of them are trace elements in heat-resistant steel. When boron dissolves into the solid solution, the crystal lattice will be distorted, and the boron on the grain boundary can prevent element diffusion and grain boundary migration, so as to improve the high temperature strength of steel. The addition of Trace Rare Earth in steel can improve the oxidation resistance and thermoplasticity of steel; the enrichment of rare earth in grain boundary can purify and strengthen grain boundary, which is beneficial to improve the thermal strength of steel. However, the type, quantity and% 26lsquo; addition method of rare earth are very important. The existing state of rare earth in steel should be controlled properly, otherwise it is difficult to achieve the expected effect.
According to the structure, the classified heat-resistant steel can be divided into four categories: pearlitic heat-resistant steel, martensitic heat-resistant steel, ferritic heat-resistant steel and austenitic heat-resistant steel.
Precipitation hardening stainless steel is usually included in the heat-resistant steel, because this kind of steel has high thermal strength and enough oxidation resistance in the range of 540-650 ℃.
In addition, heat-resistant steel is often divided into: low alloy steel for utility boiler; blade steel for steam turbine and gas turbine; valve steel for automobile and ship; heat-resistant steel and heat-resistant cast steel for petrochemical industry and furnace.
The production process of heat-resistant steel mainly includes smelting, rolling and heat treatment.
(1) smelting. Generally, the heat-resistant steel adopts the process of electric arc furnace and external refining, while the blade steel adopts the process of electric arc furnace plus electroslag. The high alloy heat-resistant steel with high requirements can be smelted in vacuum induction furnace and remelted by electroslag. Continuous casting process can be used for low alloy heat-resistant steel or austenitic heat-resistant steel (Cr18Ni9 type).
(2) rolling. Low alloy pearlitic steel and austenitic steel such as Cr18Ni9 and cr25ni20 can be bloomed by primary rolling, while martensitic or austenitic steel with more alloying elements can be bloomed by forging. Due to the poor thermal conductivity of austenitic steel, it is necessary to heat and burn through during heating. For martensitic steel and pearlitic steel with high alloying, it is necessary to pay attention to the annealing and slow cooling of ingots and billets to prevent cracks.
(3) heat treatment. Pearlitic steel is generally used after normalizing, tempering or tempering, while martensitic heat-resistant steel is treated by tempering to stabilize the structure and obtain good comprehensive mechanical properties.
Ferritic steel can not be strengthened by heat treatment. In order to eliminate the internal stress caused by cold deformation or welding, it can be annealed at 650-830 ℃ and cooled rapidly after annealing to quickly pass the 475 ℃ brittleness temperature range.
Austenitic oxidation resistant steel is treated with high temperature solution to obtain good cold deformation. The austenitic hot strength steel is first treated by high temperature solution treatment, and then aged at 60-100 ℃ higher than the service temperature, so as to stabilize the structure and precipitate the second item, so as to achieve the purpose of dispersion strengthening. Austenitic heat-resistant cast steel is mostly used in as cast condition.
In heat-resistant steel, heat-resistant cast steel accounts for a large proportion, especially austenitic heat-resistant cast steel. In addition to sand casting, the common furnace heat-resistant components adopt the precision casting technology such as wax loss or resin sand for the fine parts with high surface requirements to obtain the products with smooth surface and accurate size. For ammonia and ethylene cracking furnace, high temperature furnace tube, furnace bottom roller and radiation tube are produced by centrifugal casting.
Heat resistant steel is a special steel with high strength and good chemical stability at high temperature. It includes two kinds of anti-oxidation steel (high temperature steel without skin) and hot strength steel. Oxidation resistant steel is usually used in the temperature range of 550-1250 ℃, which requires better chemical stability (such as oxidation resistance and high temperature corrosion resistance, etc.), but it bears lower load and requires less resistance to creep and creep fracture. The hot strength steel is usually used in the temperature range of 450-900 ℃, which bears a large stress and requires the material to have good creep resistance, breaking resistance and oxidation resistance.
Heat resistant steel is widely used. The main components of boilers, steam turbines and gas turbines in thermal power plants are made of heat-resistant steel. Heat resistant steel is indispensable for the key equipment of oil refining, ammonia synthesis and ethylene plant in petrochemical industry. In addition, it is also widely used as heat-resistant structural materials in various industrial furnaces such as metallurgy, machinery, building materials, light industry, etc. In coal liquefaction, gasification and nuclear energy (such as high temperature gas cooled reactor, fast reactor), heat-resistant steel is also widely used in some core structural materials.
Due to the development of power station and petrochemical industry, molybdenum was found to be an effective element to improve the thermal strength of heat-resistant steel in 1930s. Adding 0.5% molybdenum to low carbon steel can raise the working temperature from 450 ℃ to 500 ℃. In order to solve the problem of graphitization of molybdenum containing steel, a low alloy chromium molybdenum steel was developed. In 1938, the steel containing 0.8% chromium and 0.5% molybdenum (equivalent to 15CrMo) was used in the boiler superheater in Britain, and then 10.25 chromium-0.5 molybdenum and 2.25 chromium-1 molybdenum with higher performance were developed. In order to further improve the thermal strength of steel and save molybdenum resources, a series of chrome molybdenum vanadium steel has been developed. The former Soviet Union's 12CrlMoV is a typical representative, and it is still the main material for the manufacture of power station boilers.
In the 1950s, China began to introduce and produce low alloy chromium molybdenum steel and chromium molybdenum vanadium steel, which were used in power station boilers and petrochemical industry. Low alloy heat-resistant steel has been paid attention to for its low price and good process performance. In order to further improve the performance and working temperature of the steel, since the 1960s, through a large number of research work at home and abroad, multi-element low alloy heat-resistant steel has been developed, with the working temperature of 600-620 ℃ and partially replacing nickel chromium austenitic steel in use. For example, 12Cr2MoWVTiB (102) steel developed in China, whose performance exceeds that of similar steel in other countries, has been widely used as superheater tubes in 200, 300 and 600MW units in China.
The medium and high alloy heat-resistant steel was developed at the same time as stainless steel. In 1914, Strauss et al. Pointed out that the steel containing enough nickel and chromium had the function of oxidation resistance and acid resistance. After 1920, the chromium and chromium nickel stainless heat-resistant steel was developed gradually and formed many branches. For example, the blade steel series with 12 chromium type medium alloy heat-resistant steel as the main part is mainly used for the blades of steam turbines and gas turbines; the valve steel for the inlet and outlet valves of internal combustion engines in automobiles and tractors, including chromium silicon, chromium nickel and chromium manganese nickel nitrogen type steel; the heat-resistant steel and heat-resistant cast steel for the furnaces of metallurgical machinery, petrochemical industry, light industry and building materials industry, including high chromium ferrite steel and high chromium nickel austenitic steel. Due to the higher creep strength of heat-resistant cast steel than deformed steel, it has been widely used in petrochemical and furnace materials in recent 20 years. A series of high chromium nickel austenitic heat-resistant cast steel strengthened by tungsten, molybdenum, niobium, titanium, etc. have been developed. Widely used in ammonia and ethylene cracking plant. In the field of heat-resistant steel, the developed countries in the world are relatively mature and have formed their own special standards. Since the 1950s, China has formed a relatively complete series and standard brand of heat-resistant steel in production practice. For example, in addition to gb1221-90 - hot steel bar and gb413890 basic standard of heat-resistant steel plate, there are also steel tubes and billets for high-pressure boiler (gb5310-85, gb5311-89), turbine blade steel (gb8732-88), gas valve steel for internal combustion engine (GB / t1277391), heat-resistant steel castings (gb8492-87) and other product standards, which basically meet the needs of China's national economic development. Some new heat-resistant steel grades, such as 2cr20mn9ni2si2n, 3Cr24Ni7SiN (RE), 3cr18mn12si2n and so on, have been developed in combination with the characteristics of domestic resources, which are listed in the relevant standards respectively, so that the series of heat-resistant steel standards in China has its own characteristics.
The role of alloy elements (1) chromium, aluminum, silicon. These elements form and stabilize ferrite, which can promote the formation of dense oxide film on the surface of steel at high temperature and prevent continuous oxidation. They are the main elements to improve the oxidation resistance and high temperature gas corrosion resistance of steel. However, the high content of aluminum and silicon in steel will lead to the serious deterioration of room temperature plasticity and thermoplasticity. Chromium can significantly increase the recrystallization temperature of low alloy steel. When the content is 2%, the strengthening effect is the best. (2) nickel and manganese. Austenite can be formed and stabilized. The high temperature strength and carburizing resistance of austenitic steel are improved. Although manganese can replace nickel to form austenite, it is harmful to the oxidation resistance of heat-resistant steel. (3) molybdenum and tungsten. It's a ferrite forming element. It can increase the recrystallization temperature of the steel and the creep strength of the heat-resistant steel. In low alloy steel, molybdenum is the key element to improve the thermal strength, and the effect of tungsten molybdenum composite is more obvious than that of adding alone. Because molybdenum and tungsten are easy to volatilize and oxidize at high temperature, adding too much molybdenum and tungsten to steel will bring adverse effect on high temperature oxidation resistance. (4) vanadium, niobium and titanium. It is a ferrite forming element and a strong carbide forming element, which can form fine dispersed carbide and improve the high temperature strength of steel. The combination of titanium, niobium and carbon can also prevent intergranular corrosion of austenitic steel at high temperature or after welding. (5) carbon and nitrogen. It can enlarge and stabilize austenite, so as to improve the high temperature strength of heat-resistant steel. When there are more chromium and manganese in steel, it can significantly improve the melting degree of nitrogen, add nitrogen or increase the carbon content, which can replace the precious metal nickel. The formation of carbides or carbonitrides, the control of their morphology and quantity can further strengthen the effect. (6) boron and rare earth. All of them are trace elements in heat-resistant steel. When boron dissolves into the solid solution, the crystal lattice will be distorted, and the boron on the grain boundary can prevent element diffusion and grain boundary migration, so as to improve the high temperature strength of steel. The addition of Trace Rare Earth in steel can improve the oxidation resistance and thermoplasticity of steel; the enrichment of rare earth in grain boundary can purify and strengthen grain boundary, which is beneficial to improve the thermal strength of steel. However, the type, quantity and% 26lsquo; addition method of rare earth are very important. The existing state of rare earth in steel should be controlled properly, otherwise it is difficult to achieve the expected effect.
According to the structure, the classified heat-resistant steel can be divided into four categories: pearlitic heat-resistant steel, martensitic heat-resistant steel, ferritic heat-resistant steel and austenitic heat-resistant steel.
Precipitation hardening stainless steel is usually included in the heat-resistant steel, because this kind of steel has high thermal strength and enough oxidation resistance in the range of 540-650 ℃.
In addition, heat-resistant steel is often divided into: low alloy steel for utility boiler; blade steel for steam turbine and gas turbine; valve steel for automobile and ship; heat-resistant steel and heat-resistant cast steel for petrochemical industry and furnace.
The production process of heat-resistant steel mainly includes smelting, rolling and heat treatment.
(1) smelting. Generally, the heat-resistant steel adopts the process of electric arc furnace and external refining, while the blade steel adopts the process of electric arc furnace plus electroslag. The high alloy heat-resistant steel with high requirements can be smelted in vacuum induction furnace and remelted by electroslag. Continuous casting process can be used for low alloy heat-resistant steel or austenitic heat-resistant steel (Cr18Ni9 type).
(2) rolling. Low alloy pearlitic steel and austenitic steel such as Cr18Ni9 and cr25ni20 can be bloomed by primary rolling, while martensitic or austenitic steel with more alloying elements can be bloomed by forging. Due to the poor thermal conductivity of austenitic steel, it is necessary to heat and burn through during heating. For martensitic steel and pearlitic steel with high alloying, it is necessary to pay attention to the annealing and slow cooling of ingots and billets to prevent cracks.
(3) heat treatment. Pearlitic steel is generally used after normalizing, tempering or tempering, while martensitic heat-resistant steel is treated by tempering to stabilize the structure and obtain good comprehensive mechanical properties.
Ferritic steel can not be strengthened by heat treatment. In order to eliminate the internal stress caused by cold deformation or welding, it can be annealed at 650-830 ℃ and cooled rapidly after annealing to quickly pass the 475 ℃ brittleness temperature range.
Austenitic oxidation resistant steel is treated with high temperature solution to obtain good cold deformation. The austenitic hot strength steel is first treated by high temperature solution treatment, and then aged at 60-100 ℃ higher than the service temperature, so as to stabilize the structure and precipitate the second item, so as to achieve the purpose of dispersion strengthening. Austenitic heat-resistant cast steel is mostly used in as cast condition.
In heat-resistant steel, heat-resistant cast steel accounts for a large proportion, especially austenitic heat-resistant cast steel. In addition to sand casting, the common furnace heat-resistant components adopt the precision casting technology such as wax loss or resin sand for the fine parts with high surface requirements to obtain the products with smooth surface and accurate size. For ammonia and ethylene cracking furnace, high temperature furnace tube, furnace bottom roller and radiation tube are produced by centrifugal casting.
