Quenching medium for heat treatment
Quenching medium for heat treatment
Knowledge summary of heat treatment quenching medium
The medium used for quenching and cooling the workpiece is called quenching cooling medium (or quenching medium). The ideal quenching medium should have the condition that the workpiece can be quenched into martensite without causing too much quenching stress. This requires slow cooling at the temperature above the "nose" of the C curve to reduce the thermal stress generated by the rapid cooling; the cooling rate at the "nose" is greater than the critical cooling rate to ensure that non-martensitic austenite does not occur in supercooled austenite Body transformation; below the "nose", especially when the temperature of Ms is lowered, the cooling rate should be as small as possible to reduce the stress of tissue transformation.
Commonly used quenching media are water, aqueous solutions, mineral oil, molten salt, molten alkali, etc.
● Water
Water is a quenching medium with strong cooling capacity. Wide source, low price, stable ingredients and not easy to deteriorate. The disadvantage is that in the "nose" area of the C curve (about 500 ~ 600 ℃), the water is in the steam film stage, the cooling is not fast enough, it will form a "soft spot"; and in the martensite transition temperature area (300 ~ 100 ℃), The water is in the boiling stage, and the cooling is too fast. It is easy to make the martensite transformation rate too fast and generate a large internal stress, which causes the workpiece to deform or even crack. When the water temperature rises, the water contains more gas or the water is mixed with insoluble impurities (such as oil, soap, mud, etc.), which will significantly reduce its cooling capacity. Therefore, water is suitable for quenching and cooling of carbon steel workpieces with small cross-sectional dimensions and simple shapes.
● Salt water and alkaline water
Add appropriate amount of salt and alkali to the water to immerse the high-temperature workpiece in the cooling medium, then the crystals of salt and alkali are precipitated at the steam film stage and immediately burst, destroying the steam film, and the oxide scale on the surface of the workpiece is also broken, which can improve The cooling capacity of the medium in the high temperature area. The disadvantage is that the medium is very corrosive.
In general, the concentration of brine is 10%, and the concentration of aqueous caustic soda is 10% to 15%. It can be used as the quenching medium for carbon steel and low-alloy structural steel workpieces. The use temperature should not exceed 60°C. After quenching, it should be cleaned and rust-proofed in time.
● Oil
The cooling medium generally uses mineral oil (mineral oil). Such as motor oil, transformer oil and diesel. Motor oil is generally No. 10, No. 20 and No. 30. The larger the oil, the greater the viscosity, the higher the flash point, the lower the cooling capacity, and the higher the temperature.
The new quenching oils currently used mainly include high-speed quenching oil, bright quenching oil and vacuum quenching oil.
High-speed quenching oil is quenching oil with improved cooling rate in the high temperature area. There are two basic ways to obtain high-speed quenching oil. One is to select mineral oils of different types and different viscosities, mix with each other at an appropriate ratio, and improve the cooling capacity of the high-temperature region by increasing the characteristic temperature; the other is to use ordinary quenching. Add additives to the oil to form fly ash floats in the oil. Additive barium, sodium, calcium, phosphate, stearate, etc. Production practice shows that the cooling rate of high-speed quenching oil in the supercooled austenite unstable zone is significantly higher than that of ordinary quenching oil, while the cooling rate in the low-temperature martensite transformation zone is close to that of ordinary quenching oil. In this way, high hardenability and hardenability can be obtained, and the deformation is greatly reduced, which is suitable for the quenching of alloy steel workpieces with complex shapes.
Bright quenching oil can keep the workpiece bright surface after quenching. By adding polymer additives with different properties to mineral oil, bright quenching oils with different cooling rates can be obtained. The main component of these additives is a brightener. Its function is to suspend aging products that are insoluble in oil to prevent accumulation and precipitation on the workpiece. In addition, bright quenching oil additives also contain antioxidants, surfactants and coolants.
Vacuum quenching oil is a cooling medium for vacuum heat treatment quenching. Vacuum quenching oil must have low saturated vapor pressure, high and stable cooling capacity, and good brightness and thermal stability, otherwise it will affect the effect of vacuum heat treatment.
Salt bath and alkali bath quenching media are generally used in graded quenching and isothermal quenching.
● New quenching agent
There are polyvinyl alcohol aqueous solution and trinitrate aqueous solution.
Polyvinyl alcohol is usually used in an aqueous solution with a mass fraction between 0.1% and 0.3%, and the co-cooling capacity is between water and oil. When the workpiece is quenched into the solution, a vapor film and a gel film are formed on the surface of the workpiece, and the two films cool the heated workpiece. After entering the boiling stage, the film ruptures and the workpiece cooling speeds up. When it reaches a low temperature, the polyvinyl alcohol gel film is formed again, and the workpiece cooling rate decreases again. Therefore, this solution has a low cooling capacity in the high and low temperature regions, and cools in the middle temperature region. High capacity and good cooling characteristics.
The trinitrate aqueous solution is composed of 25% sodium nitrate + 20% sodium nitrite + 20% potassium nitrate + 35% water. At high temperature (650 ~ 500 ℃), due to the precipitation of salt crystals, the steam film is broken, and the cooling capacity is close to water. At low temperature (300～200℃), the concentration is extremely high, the fluidity is poor, and the cooling capacity is close to oil, so it can replace the water-oil dual medium quenching.
Cooling method
The most widely used quenching classification in production practice is divided into different cooling methods. There are mainly single liquid quenching, double liquid quenching, graded quenching and isothermal quenching.
● Single liquid quenching
It is a quenching operation method of immersing the austenitized workpiece into a certain quenching medium and cooling it to room temperature. The single-liquid quenching medium includes water, brine, alkaline water, oil and specially formulated quenching agent. Generally, carbon steel is quenched and alloy steel is quenched with oil.
The single-liquid quenching operation is simple, which is conducive to mechanization and automation. The disadvantage is that the cooling rate is limited by the cooling characteristics of the medium and affects the quenching quality. Single liquid quenching is only suitable for workpieces with simpler shapes for carbon steel.
● Double liquid quenching
It is to immerse the austenitized workpiece into a medium with strong cooling ability, take it out before the steel parts reach the temperature of the quenching medium, and immediately immerse it in another medium with weak cooling ability, such as water first and then oil , Water first, then air, etc. Double-liquid quenching reduces the tendency of deformation and cracking, the operation is not easy to master, and has certain limitations in application.
● Martensite grade quenching
It is to immerse the austenitized workpiece first in the liquid medium (salt bath or alkali bath) whose temperature is slightly higher or slightly lower than the martensite point of the steel, and maintain it for an appropriate time, until the inner and outer layers of the steel parts reach the medium After the temperature is taken out, air-cooling is adopted to obtain a quenching process of martensite structure, also called graded quenching.
Classification quenching can stay at the classification temperature until the temperature inside and outside the workpiece is the same and air-cooled, so it can effectively reduce the phase transformation stress and thermal stress, and reduce the quenching deformation and cracking tendency. Graded quenching is suitable for alloy steel and high alloy steel workpieces with high deformation requirements, and can also be used for carbon steel workpieces with small cross-sectional dimensions and complex shapes.
● Bainite isothermal quenching
It is the quenching process of austenitizing steel parts, making them cool to the temperature range of bainite transformation temperature (260～400℃), and transforming austenite to bainite, sometimes called isothermal quenching. The general holding time is 30～60min.
● Compound quenching
The workpiece is quenched below Ms to obtain 10%-20% martensite, and then isothermal in the lower bainite temperature zone. This cooling method can obtain the M+B structure of the workpiece with larger cross section. The martensite formed during pre-quenching can promote the transformation of bainite and temper the martensite at isothermal. Compound quenching is used for alloy tool steel workpieces, which can avoid the first type of temper brittleness and reduce the amount of residual austenite, that is, the tendency of deformation and cracking.
Special workpieces also use compressed air quenching, spray quenching, and jet quenching.
Knowledge summary of heat treatment quenching medium
The medium used for quenching and cooling the workpiece is called quenching cooling medium (or quenching medium). The ideal quenching medium should have the condition that the workpiece can be quenched into martensite without causing too much quenching stress. This requires slow cooling at the temperature above the "nose" of the C curve to reduce the thermal stress generated by the rapid cooling; the cooling rate at the "nose" is greater than the critical cooling rate to ensure that non-martensitic austenite does not occur in supercooled austenite Body transformation; below the "nose", especially when the temperature of Ms is lowered, the cooling rate should be as small as possible to reduce the stress of tissue transformation.
Commonly used quenching media are water, aqueous solutions, mineral oil, molten salt, molten alkali, etc.
● Water
Water is a quenching medium with strong cooling capacity. Wide source, low price, stable ingredients and not easy to deteriorate. The disadvantage is that in the "nose" area of the C curve (about 500 ~ 600 ℃), the water is in the steam film stage, the cooling is not fast enough, it will form a "soft spot"; and in the martensite transition temperature area (300 ~ 100 ℃), The water is in the boiling stage, and the cooling is too fast. It is easy to make the martensite transformation rate too fast and generate a large internal stress, which causes the workpiece to deform or even crack. When the water temperature rises, the water contains more gas or the water is mixed with insoluble impurities (such as oil, soap, mud, etc.), which will significantly reduce its cooling capacity. Therefore, water is suitable for quenching and cooling of carbon steel workpieces with small cross-sectional dimensions and simple shapes.
● Salt water and alkaline water
Add appropriate amount of salt and alkali to the water to immerse the high-temperature workpiece in the cooling medium, then the crystals of salt and alkali are precipitated at the steam film stage and immediately burst, destroying the steam film, and the oxide scale on the surface of the workpiece is also broken, which can improve The cooling capacity of the medium in the high temperature area. The disadvantage is that the medium is very corrosive.
In general, the concentration of brine is 10%, and the concentration of aqueous caustic soda is 10% to 15%. It can be used as the quenching medium for carbon steel and low-alloy structural steel workpieces. The use temperature should not exceed 60°C. After quenching, it should be cleaned and rust-proofed in time.
● Oil
The cooling medium generally uses mineral oil (mineral oil). Such as motor oil, transformer oil and diesel. Motor oil is generally No. 10, No. 20 and No. 30. The larger the oil, the greater the viscosity, the higher the flash point, the lower the cooling capacity, and the higher the temperature.
The new quenching oils currently used mainly include high-speed quenching oil, bright quenching oil and vacuum quenching oil.
High-speed quenching oil is quenching oil with improved cooling rate in the high temperature area. There are two basic ways to obtain high-speed quenching oil. One is to select mineral oils of different types and different viscosities, mix with each other at an appropriate ratio, and improve the cooling capacity of the high-temperature region by increasing the characteristic temperature; the other is to use ordinary quenching. Add additives to the oil to form fly ash floats in the oil. Additive barium, sodium, calcium, phosphate, stearate, etc. Production practice shows that the cooling rate of high-speed quenching oil in the supercooled austenite unstable zone is significantly higher than that of ordinary quenching oil, while the cooling rate in the low-temperature martensite transformation zone is close to that of ordinary quenching oil. In this way, high hardenability and hardenability can be obtained, and the deformation is greatly reduced, which is suitable for the quenching of alloy steel workpieces with complex shapes.
Bright quenching oil can keep the workpiece bright surface after quenching. By adding polymer additives with different properties to mineral oil, bright quenching oils with different cooling rates can be obtained. The main component of these additives is a brightener. Its function is to suspend aging products that are insoluble in oil to prevent accumulation and precipitation on the workpiece. In addition, bright quenching oil additives also contain antioxidants, surfactants and coolants.
Vacuum quenching oil is a cooling medium for vacuum heat treatment quenching. Vacuum quenching oil must have low saturated vapor pressure, high and stable cooling capacity, and good brightness and thermal stability, otherwise it will affect the effect of vacuum heat treatment.
Salt bath and alkali bath quenching media are generally used in graded quenching and isothermal quenching.
● New quenching agent
There are polyvinyl alcohol aqueous solution and trinitrate aqueous solution.
Polyvinyl alcohol is usually used in an aqueous solution with a mass fraction between 0.1% and 0.3%, and the co-cooling capacity is between water and oil. When the workpiece is quenched into the solution, a vapor film and a gel film are formed on the surface of the workpiece, and the two films cool the heated workpiece. After entering the boiling stage, the film ruptures and the workpiece cooling speeds up. When it reaches a low temperature, the polyvinyl alcohol gel film is formed again, and the workpiece cooling rate decreases again. Therefore, this solution has a low cooling capacity in the high and low temperature regions, and cools in the middle temperature region. High capacity and good cooling characteristics.
The trinitrate aqueous solution is composed of 25% sodium nitrate + 20% sodium nitrite + 20% potassium nitrate + 35% water. At high temperature (650 ~ 500 ℃), due to the precipitation of salt crystals, the steam film is broken, and the cooling capacity is close to water. At low temperature (300～200℃), the concentration is extremely high, the fluidity is poor, and the cooling capacity is close to oil, so it can replace the water-oil dual medium quenching.
Cooling method
The most widely used quenching classification in production practice is divided into different cooling methods. There are mainly single liquid quenching, double liquid quenching, graded quenching and isothermal quenching.
● Single liquid quenching
It is a quenching operation method of immersing the austenitized workpiece into a certain quenching medium and cooling it to room temperature. The single-liquid quenching medium includes water, brine, alkaline water, oil and specially formulated quenching agent. Generally, carbon steel is quenched and alloy steel is quenched with oil.
The single-liquid quenching operation is simple, which is conducive to mechanization and automation. The disadvantage is that the cooling rate is limited by the cooling characteristics of the medium and affects the quenching quality. Single liquid quenching is only suitable for workpieces with simpler shapes for carbon steel.
● Double liquid quenching
It is to immerse the austenitized workpiece into a medium with strong cooling ability, take it out before the steel parts reach the temperature of the quenching medium, and immediately immerse it in another medium with weak cooling ability, such as water first and then oil , Water first, then air, etc. Double-liquid quenching reduces the tendency of deformation and cracking, the operation is not easy to master, and has certain limitations in application.
● Martensite grade quenching
It is to immerse the austenitized workpiece first in the liquid medium (salt bath or alkali bath) whose temperature is slightly higher or slightly lower than the martensite point of the steel, and maintain it for an appropriate time, until the inner and outer layers of the steel parts reach the medium After the temperature is taken out, air-cooling is adopted to obtain a quenching process of martensite structure, also called graded quenching.
Classification quenching can stay at the classification temperature until the temperature inside and outside the workpiece is the same and air-cooled, so it can effectively reduce the phase transformation stress and thermal stress, and reduce the quenching deformation and cracking tendency. Graded quenching is suitable for alloy steel and high alloy steel workpieces with high deformation requirements, and can also be used for carbon steel workpieces with small cross-sectional dimensions and complex shapes.
● Bainite isothermal quenching
It is the quenching process of austenitizing steel parts, making them cool to the temperature range of bainite transformation temperature (260～400℃), and transforming austenite to bainite, sometimes called isothermal quenching. The general holding time is 30～60min.
● Compound quenching
The workpiece is quenched below Ms to obtain 10%-20% martensite, and then isothermal in the lower bainite temperature zone. This cooling method can obtain the M+B structure of the workpiece with larger cross section. The martensite formed during pre-quenching can promote the transformation of bainite and temper the martensite at isothermal. Compound quenching is used for alloy tool steel workpieces, which can avoid the first type of temper brittleness and reduce the amount of residual austenite, that is, the tendency of deformation and cracking.
Special workpieces also use compressed air quenching, spray quenching, and jet quenching.
