heat treatment commonly used quenching medium water, aqueous solution, mineral oil, molten salt, molten alkali, etc. 

water 

Water is a quenching medium with strong cooling capacity. The source is wide, the price is low, the composition is stable and not easy to deteriorate. The disadvantage is that in the c curve & ldquo; Nose & rdquo; In the area (about 500~600 ℃), water is in the steam film stage, and if cooling is not fast enough, it will form "soft spots"; In the martensite transformation temperature zone (300~100 ℃), water is in the boiling stage, and cooling too fast is easy to cause martensite transformation speed to generate large internal stress, cause the workpiece deformation and even cracking. When the water temperature rises, the water contains more gas or water mixed with insoluble impurities (such as oil, soap, mud, etc.), 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. 

brine and alkaline water 

add appropriate amount of salt and alkali to the water, after the high-temperature workpiece is immersed in the cooling medium, the crystals of salt and alkali are precipitated and burst immediately, and the steam film is destroyed, 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 zone. The disadvantage is that the medium is corrosive. In general, the concentration of brine is 10%, and the concentration of caustic soda aqueous solution is 10% to 15%. Can be used as carbon steel and low alloy structural steel workpiece quenching medium, the use temperature should not exceed 60 ℃, after quenching should be timely cleaning and anti-rust treatment. & nbsp;

oil 

The cooling medium is generally mineral oil (mineral oil). Such as engine oil, transformer oil and diesel oil. Engine oil is generally No. 10, No. 20, and No. 30 engine oil. The larger the number of oil, the greater the viscosity, the higher the flash point, the lower the cooling capacity, and the corresponding increase in operating temperature. & nbsp;

The new quenching oils currently used mainly include high-speed quenching oil, bright quenching oil and vacuum quenching oil. & Nbsp;

High-speed quenching oil is a quenching oil with an increased cooling rate in the high temperature zone. There are two basic ways to obtain high-speed quenching oil, the 1 is to select different types and different viscosity of mineral oil, to mix with each other in the appropriate ratio, by increasing the characteristic temperature to improve the cooling capacity of the high temperature zone; the other 1 is to add additives to the ordinary quenching oil, the formation of ash-like floating matter in the oil. Additives include barium salt, sodium salt, calcium salt, phosphate salt and stearate salt of sulfonic acid. Production practice shows that the cooling rate of high-speed quenching oil in the unstable zone 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 complex shapes. 

Bright quenching oil can keep the workpiece bright after quenching. The bright quenching oil with different cooling rates can be obtained by adding polymer additives of different properties to mineral oil. The main component of these additives is the brightener, and its function is to suspend the aging products that are not dissolved in the oil to prevent accumulation and precipitation on the workpiece. In addition, bright quenching oil additives also contain antioxidants, surfactants and coolants. & Nbsp;

Vacuum quenching oil is a cooling medium used 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. & nbsp;

Salt bath and alkali bath quenching media are generally used in step quenching and austempering. & nbsp;

New quenchant 

There are polyvinyl alcohol aqueous solution and 3 nitrate aqueous solution. The mass fraction of polyvinyl alcohol is commonly used in aqueous solution 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 is broken, and the cooling of the workpiece is accelerated. When the low temperature is reached, the polyvinyl alcohol gel film is formed again, and the cooling speed of the workpiece is decreased. Therefore, this solution has low cooling capacity in high and low temperature zones, high cooling capacity in medium temperature zones, and good cooling characteristics. & nbsp;

The 3 nitrate solution consists 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 formed, and the cooling capacity is close to that of water. At low temperature (300~200 ℃) due to the high concentration, poor fluidity, cooling capacity close to the oil, so it can replace the water-oil double medium quenching.