The quenching process is a heat treatment process in which steel is heated to a temperature above AC3 or AC1 for a certain period of time and then cooled at an appropriate speed to obtain martensite and/or bainite structure.

The purpose of quenching is to improve hardness, strength and wear resistance to meet the performance of the parts. Quenching process is the most widely used, such as tools, measuring tools, molds, bearings, springs and automobiles, tractors, diesel engines, cutting machine tools, pneumatic tools, drilling machinery, agricultural machinery, petroleum machinery, chemical machinery, textile machinery, aircraft and other parts are Use a quenching process.

(1) Quenching heating temperature
The quenching heating temperature is determined according to the composition, structure and different performance requirements of the steel. The hypoeutectoid steel is AC3+ (30-50 °C); the eutectoid steel and the hypereutectoid steel are AC1+ (30-50 °C).
If the quenching heating temperature of the hypoeutectoid steel is lower than the temperature of AC3, then the steel is not completely austenitized, and some unconverted ferrite exists. After quenching, the ferrite remains in the quenched structure. The hardness of the ferrite is low, so that the hardness after quenching is not required, and other mechanical properties are also affected. If the hypoeutectic steel is heated to a temperature much higher than the AC3 temperature, the austenite grains are significantly coarser and the properties after quenching are destroyed. Therefore, the quenching heating temperature of the sub-aluminum steel is AC3+ (30-50 °C), which ensures sufficient austenitization and keeps the austenite grains fine.

The quenching heating temperature of hypereutectoid steel is generally recommended as AC1+ (30 to 50 °C). In actual production, it is also appropriately increased by about 20 °C depending on the situation. It is heated in this temperature range, and its structure is a fine grain of austenite and a part of finely uniformly distributed undissolved carbide. After quenching, except for a small number of retained austenite, the microstructure is a fine carbonized material point uniformly distributed on the sheet martensite matrix. Such a tissue has a high hardness, a wear resistance number, and relatively little brittleness.

The quenching heating temperature of the hypereutectoid steel cannot be lower than AC1 because the steel is not yet austenitized. If heated to a temperature slightly above AC1, the pearlite completely transforms the austenite and a small amount of cementite dissolves into the austenite. At this time, the austenite grains are fine, and the mass fraction of carbon is slightly higher than that of the eutectoid component. If the temperature is continuously increased, the secondary cementite will continue to dissolve into the austenite, causing the austenite grains to grow continuously, and the carbon concentration will increase continuously, which will lead to an increase in the tendency of quenching deformation and an increase in microcracks in the quenched structure. And the brittleness is increased. At the same time, due to the high carbon content of austenite, the amount of retained austenite after quenching increases, which reduces the hardness and wear resistance of the workpiece. Therefore, it is not suitable to have a quenching heating temperature of the hypereutectoid steel higher than AC1, and heating to a completely austenitized ACm or above is more inappropriate.

When selecting the quenching heating temperature of the workpiece in production practice, in addition to complying with the above general principles, the chemical composition, technical requirements, size and shape, original structure, heating equipment, cooling medium and other factors of the workpiece should be considered. Make appropriate adjustments. For example, alloy steel parts usually take the upper limit and take the lower limit for complex shapes.

The quenching heating temperature selected for the new toughening process is different from the usual quenching temperature. For example, sub-temperature quenching is quenching after the austenitic steel is austenitized at a temperature slightly lower than AC3, which can improve the toughness, reduce the brittle transition temperature, and eliminate the temper brittleness. The sub-temperature quenching heating temperature of the workpiece made of materials such as 45, 40Cr, 60Si2 is AC3-(5~10 °C).

High-temperature quenching can obtain more lath martensite or increase the strength and toughness of all lath martensite. For example, 16Mn steel is quenched at 940 °C, 5CrMnMo steel is quenched at 890 °C, and 20CrMnMo steel is quenched at 920 °C, and the effect is better.

High-carbon steel is low-temperature, fast, short-time heating and quenching, appropriately reducing the quenching heating temperature of high-carbon steel, or adopting rapid heating and shortening the holding time, which can reduce the carbon content of austenite and improve the toughness of steel.

(2) Holding time
In order to complete the structural transformation, carbide dissolution and austenite composition homogenization in the inner and outer parts of the workpiece, it must be kept at the quenching heating temperature for a certain period of time, and the holding time is maintained.

(3) Quenching medium
The medium used for quenching 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 above the "nose" of the C curve to reduce the thermal stress generated by quenching; the cooling rate at the "nose" is greater than the critical cooling rate to ensure that non-Martens does not occur in the supercooled austenite. Body transformation; under the "nose", especially when the temperature of Ms is clicked, the cooling rate should be as small as possible to reduce the stress of tissue transformation.

Common quenching media are water, aqueous solution, mineral oil, molten salt, molten alkali and the like.
● Water Water is a quenching medium with strong cooling capacity. Wide source, low price, stable composition and not easy to deteriorate. The disadvantage is that in the "nose" area of ​​the C curve (about 500 ~ 600 ° C), the water is in the vapor film stage, the cooling is not fast enough, it will form a "soft point"; and in the martensite transformation temperature zone (300 ~ 100 ° C), When the water is in the boiling stage, the cooling is too fast, and the martensite transformation speed is too fast to generate a large internal stress, which causes the workpiece to be deformed or even cracked. When the water temperature rises, the water contains more gas or water 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 having a small cross-sectional size and a simple shape.

● The salt water and the alkaline water are added with appropriate salt and alkali in the water to make the high temperature workpiece immersed in the cooling medium. The salt and alkali crystals are precipitated in the vapor film stage and immediately burst, the vapor film is destroyed, and the scale on the surface of the workpiece is also fried. Broken, this can improve the cooling capacity of the medium in the high temperature zone. The disadvantage is that the medium is highly corrosive. In general, the concentration of the brine is 10%, and the concentration of the aqueous caustic soda is 10% to 15%. It can be used as quenching medium for carbon steel and low-alloy structural steel workpieces. The temperature should not exceed 60 °C. After quenching, it should be cleaned and rust-proofed.

● Oil The cooling medium is generally made of mineral oil (mineral oil). Such as oil, transformer oil and diesel. The engine oil is generally made of No. 10, No. 20 and No. 30 engine oil. The larger the oil number, the greater the viscosity, the higher the flash point, the lower the cooling capacity, and the higher the use temperature.

The new quenching oils currently used are mainly high-speed quenching oil, bright quenching oil and vacuum quenching oil.
The high-speed quenching oil is a quenching oil in which the cooling rate is increased in a high temperature region. There are two basic ways to obtain high-speed quenching oil. One is to select mineral oils of different types and different viscosities, and mix them with appropriate ratios to improve the cooling capacity of the high temperature zone by increasing the characteristic temperature. The other is to quench the ordinary heat. An additive is added to the oil to form a ash-like float in the oil. The additive is a sulfonium salt, a sodium salt, a calcium salt, and a phosphate, a stearate, and the like. Production practice shows that the cooling rate of high-speed quenching oil in the unstable region of supercooled austenite 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 deformation is greatly reduced, which is suitable for quenching of alloy steel workpieces with complicated shapes.
Bright quenching oil keeps the workpiece on a shiny surface after quenching. Brightening quenching oils with different cooling rates can be obtained by adding polymer additives of different properties to the mineral oil. The main component of these additives is a brightener, which functions to suspend the aged product that is insoluble in the oil to prevent accumulation and precipitation on the workpiece. In addition, the bright quenching oil additive also contains an antioxidant, a surfactant, a refrigerant, and the like.
Vacuum quenching oil is a cooling medium used for vacuum heat treatment quenching. Vacuum quenching oil must have a low saturated vapor pressure, high and stable cooling capacity, and good gloss and thermal stability, otherwise it will affect the effect of vacuum heat treatment.
Salt bath and alkali bath quenching media are generally used in fractional quenching and isothermal quenching.

● New quenching agent There are polyvinyl alcohol aqueous solution and trinitrogen aqueous solution.
Polyvinyl alcohol is usually used in an aqueous solution having a mass fraction of 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 cools faster. When the low temperature is reached, the polyvinyl alcohol gel film is formed again, and the cooling rate of the workpiece decreases. Therefore, the solution has low cooling capacity in the high and low temperature regions and is cooled in the middle temperature region. High capacity and good cooling characteristics.
The aqueous trinitrogen solution consists of 25% sodium nitrate + 20% sodium nitrite + 20% potassium nitrate + 35% water. At high temperatures (650 to 500 ° C), due to the precipitation of salt crystals, the formation of a vapor film is broken, and the cooling ability is close to that of water. At low temperatures (300 to 200 ° C), the concentration is extremely high, the fluidity is poor, and the cooling capacity is close to that of oil, so it can replace the water-oil double medium quenching.

(4) Cooling method
The most widely used quenching classification in production practice is divided by the cooling method. There are mainly single-liquid quenching, two-liquid quenching, fractional quenching and isothermal quenching.

● Single-liquid quenching is a quenching operation method in which an austenitizing workpiece is immersed in a quenching medium and cooled to room temperature. Single-liquid quenching medium is water, brine, alkaline water, oil and specially prepared quenching agent. In general, carbon steel is quenched and alloy steel is quenched.
Single-liquid quenching is simple and easy to mechanize and automate. 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 carbon steels for workpieces with a simple shape.

● Two-liquid quenching is to first immerse the austenitic workpiece in a medium with strong cooling capacity. When the steel piece has not reached the temperature of the quenching medium, it is taken out and immediately immersed in another medium with weak cooling capacity to cool. After the water, the oil, the first water and the air. Two-liquid quenching reduces the tendency of deformation and cracking, and the operation is not well mastered, and has certain limitations in application.

● Martensite grade quenching is to immerse the austenitized workpiece in a liquid medium (salt bath or alkali bath) with a slightly higher temperature or slightly lower than the martensite point of the steel, and keep it for a suitable time. After the outer layer reaches the medium temperature, the air cooling is taken out to obtain a quenching process of the martensite structure, which is also called step quenching.
The staged quenching can effectively reduce the phase transformation stress and thermal stress and reduce the tendency of quenching deformation and cracking because it is air-cooled after the temperature at the classification temperature stays inside and outside the workpiece. Graded quenching is suitable for alloy steel and high alloy steel workpieces with high deformation requirements, and for carbon steel workpieces with small cross-section and complex shapes.

● Bainite austempering is a quenching process in which austenite is formed into austenite, which is cooled to the bainite transformation temperature range (260-400 °C), and the austenite is transformed into bainite. Called austempering. The general holding time is 30 to 60 minutes.

● Composite quenching The workpiece is quenched to below 10 m to obtain 10% to 20% martensite, and then isothermal in the lower bainite temperature zone. This cooling method allows the workpiece to obtain tissue M+B tissue in a larger cross section. Martensite formed during pre-quenching promotes bainite transformation and temper martensite when isothermal. Composite quenching is used for alloy tool steel workpieces to avoid the first type of temper brittleness and reduce the amount of retained austenite, ie the tendency of deformation and cracking.
Special workpieces are also quenched by compressed air, spray quenched, and jet quenched.

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