Materials Used for Valves and Their Heat Treatments
The commonly used materials for valve bodies are as follows:
1. Gray cast iron: it is suitable for low-pressure valves with working temperatures between -15℃ and +200℃ and nominal pressure less than and equal to 1.6MPa.
2. Black heart malleable cast iron: it is applicable for medium and low pressure valves with working temperatures between -15℃ and +250℃ and nominal pressure less than and equal to 2.5MPa.
3. Ductile iron: it is suitable for medium and low pressure valves with working temperatures between -30℃ and +350℃ and nominal pressure less than and equal to 4.0MPa.
4. Carbon steel (WCA, WCB and WCC): it is suitable for medium and high pressure valves with working temperatures between -29℃ and +425℃. Among them, working temperatures of valve made of 16Mn and 30Mn are between -40℃ and +450℃, which are often used to replace ASTM A105.
5. Low-temperature carbon steel (LCB): it is suitable for low-temperature valves with working temperatures between -46℃ and +345℃.
6. Alloy steel (WC6 and WC9): it is suitable for high-temperature and high-pressure valves with non-corrosive media whose working temperatures are between -29 and +595℃; WC5 and WC12 are suitable for high-temperature and high-pressure valves with corrosive media and working temperatures between -29℃ and+650℃.
7. Austenitic stainless steel: it is suitable for valves with corrosive media and operating temperatures between -196℃ and +600℃.
8. Monel alloy: it is mainly suitable for media containing hydrogen and fluorine.
9. Cast copper alloy: it is mainly suitable for valve for oxygen pipelines with working temperatures between -273℃ and +200℃.
The materials listed above are the main categories of commonly used materials for valve bodies. There are many different grades for each type of material, and different grades are suitable for different pressure. Therefore, when selecting the valve body material for the valve, determine the material suitable for the working conditions according to different uses and different pressure. In addition, titanium alloys, aluminum alloys and plastic are also used for manufacturing valves.
The heat treatment processes of the valve body are as follows according to different materials:
1. Heat treatment of grey cast iron
In order to achieve different purposes, gray cast iron can be subjected to different heat treatments after casting. In valve production, the heat treatment processes often used for parts such as gray cast iron valve bodies after casting are thermal aging to eliminate casting stress and high-temperature annealing to eliminate free cementite. Thermal aging is a necessary process. High-temperature annealing is only used to replace thermal aging when there is primary cementite in the structure after casting due to improper control of chemical composition and casting cooling rate in the casting process.
2. Heat treatment of carbon cast steel
Steel castings have great casting residual stress after casting. Sometimes, the structure of steel castings is coarse and even overheated. These all affect the dimensional stability of steel castings, reduce the mechanical properties of steel and are not conducive to cutting. In order to eliminate the casting stress, refine the structure, improve the mechanical properties and machinability, when the valve is manufactured, the carbon steel valve body and other parts are often annealed or normalized plus tempered after casting.
3. Heat treatment of austenitic stainless and acid-resistant steel
The main defect of austenitic stainless and acid-resistant steel is that it is prone to intergranular corrosion. Generally, preventive measures such as heat treatment can be taken for the steel. In valve production, the commonly used heat treatment processes for austenitic stainless and acid-resistant steel valve bodies and other parts are solution treatment (quenching), stabilization treatment and cryogenic treatment.
4. Heat treatment of martensitic heat-resistant steel
The martensitic heat-resistant steel should be annealed in time after casting to prevent cracks, and the annealing and holding time should be sufficient (usually 4 to 8 hours). The purpose of annealing martensitic heat-resistant steel is to relieve stress, recrystallize, refine grains, reduce hardness, improve machinability, and prepare for the final heat treatment. The final heat treatment of martensitic heat-resistant steel is normalizing plus tempering.
5. Heat treatment of high-quality carbon steel
For the heat treatment of high-quality carbon steel, take the No.35 forged steel valve body as an example. After forging, the No.35 steel valve body should be normalized, and its final heat treatment should be carried out according to the requirements of the valve's manufacturing technical documents. Generally, quenching and tempering should be performed.
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1. Gray cast iron: it is suitable for low-pressure valves with working temperatures between -15℃ and +200℃ and nominal pressure less than and equal to 1.6MPa.
2. Black heart malleable cast iron: it is applicable for medium and low pressure valves with working temperatures between -15℃ and +250℃ and nominal pressure less than and equal to 2.5MPa.
3. Ductile iron: it is suitable for medium and low pressure valves with working temperatures between -30℃ and +350℃ and nominal pressure less than and equal to 4.0MPa.
4. Carbon steel (WCA, WCB and WCC): it is suitable for medium and high pressure valves with working temperatures between -29℃ and +425℃. Among them, working temperatures of valve made of 16Mn and 30Mn are between -40℃ and +450℃, which are often used to replace ASTM A105.
5. Low-temperature carbon steel (LCB): it is suitable for low-temperature valves with working temperatures between -46℃ and +345℃.
6. Alloy steel (WC6 and WC9): it is suitable for high-temperature and high-pressure valves with non-corrosive media whose working temperatures are between -29 and +595℃; WC5 and WC12 are suitable for high-temperature and high-pressure valves with corrosive media and working temperatures between -29℃ and+650℃.
7. Austenitic stainless steel: it is suitable for valves with corrosive media and operating temperatures between -196℃ and +600℃.
8. Monel alloy: it is mainly suitable for media containing hydrogen and fluorine.
9. Cast copper alloy: it is mainly suitable for valve for oxygen pipelines with working temperatures between -273℃ and +200℃.
The materials listed above are the main categories of commonly used materials for valve bodies. There are many different grades for each type of material, and different grades are suitable for different pressure. Therefore, when selecting the valve body material for the valve, determine the material suitable for the working conditions according to different uses and different pressure. In addition, titanium alloys, aluminum alloys and plastic are also used for manufacturing valves.
The heat treatment processes of the valve body are as follows according to different materials:
1. Heat treatment of grey cast iron
In order to achieve different purposes, gray cast iron can be subjected to different heat treatments after casting. In valve production, the heat treatment processes often used for parts such as gray cast iron valve bodies after casting are thermal aging to eliminate casting stress and high-temperature annealing to eliminate free cementite. Thermal aging is a necessary process. High-temperature annealing is only used to replace thermal aging when there is primary cementite in the structure after casting due to improper control of chemical composition and casting cooling rate in the casting process.
2. Heat treatment of carbon cast steel
Steel castings have great casting residual stress after casting. Sometimes, the structure of steel castings is coarse and even overheated. These all affect the dimensional stability of steel castings, reduce the mechanical properties of steel and are not conducive to cutting. In order to eliminate the casting stress, refine the structure, improve the mechanical properties and machinability, when the valve is manufactured, the carbon steel valve body and other parts are often annealed or normalized plus tempered after casting.
3. Heat treatment of austenitic stainless and acid-resistant steel
The main defect of austenitic stainless and acid-resistant steel is that it is prone to intergranular corrosion. Generally, preventive measures such as heat treatment can be taken for the steel. In valve production, the commonly used heat treatment processes for austenitic stainless and acid-resistant steel valve bodies and other parts are solution treatment (quenching), stabilization treatment and cryogenic treatment.
4. Heat treatment of martensitic heat-resistant steel
The martensitic heat-resistant steel should be annealed in time after casting to prevent cracks, and the annealing and holding time should be sufficient (usually 4 to 8 hours). The purpose of annealing martensitic heat-resistant steel is to relieve stress, recrystallize, refine grains, reduce hardness, improve machinability, and prepare for the final heat treatment. The final heat treatment of martensitic heat-resistant steel is normalizing plus tempering.
5. Heat treatment of high-quality carbon steel
For the heat treatment of high-quality carbon steel, take the No.35 forged steel valve body as an example. After forging, the No.35 steel valve body should be normalized, and its final heat treatment should be carried out according to the requirements of the valve's manufacturing technical documents. Generally, quenching and tempering should be performed.