The medium used for quenching of forgings is called quenching cooling medium (or quenching medium). The ideal quenching medium should have the condition that the forging 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.
Commonly used quenching media are water, aqueous solution, mineral oil, molten salt, molten alkali and the like.
Water is a quenching medium with a 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 a boiling stage, the cooling is too fast, and the martensite transformation speed is too fast to generate a large internal stress, which causes the forging to be deformed or even cracked. When the water temperature rises, the water contains more gas or mixed with insoluble impurities (such as oil, soap, mud, etc.), which will significantly reduce its cooling capacity. Therefore, the water is suitable for quenching and cooling of carbon steel forgings having a small cross-sectional size and a simple shape.
● brine and alkaline water
After adding a proper amount of salt and alkali to the water, the high-temperature forgings are immersed in the cooling medium, and the crystals of the salt and the alkali are precipitated at the vapor film stage and immediately burst, the vapor film is destroyed, and the scale on the surface of the forging is also broken, which can be improved. 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. It should be cleaned and rust-proofed after quenching.
The cooling medium is generally a mineral oil (mineral oil). Such as oil, transformer oil and diesel. The engine oil is generally made of No. 10, No. 20, 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.
High-speed quenching oil is a quenching oil that has an increased cooling rate in a high temperature zone. 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 forgings with complicated shapes.
Bright quenching oil allows the forging to maintain 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 acts to suspend the aged product which is insoluble in the oil to prevent accumulation and precipitation on the forging. 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 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 fractional quenching and isothermal quenching.
● New quenching agent
There are aqueous solutions of polyvinyl alcohol and aqueous solutions of trinitrogen.
Polyvinyl alcohol is usually used in an aqueous solution with a mass fraction of between 0.1% and 0.3%, and the co-cooling capacity is between water and oil. When the forging is quenched into the solution, a vapor film and a gel film are formed on the surface of the forging, and the two films cool the heating forging. After entering the boiling stage, the film breaks and the forging cools faster. When the low temperature is reached, the polyvinyl alcohol gel film is formed again, and the cooling speed of the forging is decreased. Therefore, the solution has low cooling capacity in the high and low temperature zones and is cooled in the middle temperature zone. 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 temperature (650~500 °C), due to the precipitation of salt crystals, the vapor film is formed and the cooling capacity is close to that of water. At low temperature (300~200 °C), due to the extremely high concentration, poor fluidity and cooling capacity close to oil, it can replace water-oil double medium quenching.