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.
Commonly used quenching media include water, aqueous solution, mineral oil, molten salt, molten alkali, etc.
Water is a quenching medium with strong cooling capacity.
Advantages: wide source, low price, stable ingredients and not easy to deteriorate.
Disadvantages: The cooling capacity is unstable and can easily deform or crack the workpiece. In the “nose” area of the C curve (around 500-600°C), water is in the vapor film stage and is not cooled fast enough, which will form a “soft spot”; while in the martensitic transformation temperature area (300-100°C), water is in the vapor film stage. During the boiling stage, if the cooling is too fast, the martensite transformation speed will be too fast and a large internal stress will be generated, causing 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.), it will significantly reduce its cooling capacity.
Application: Suitable for quenching and cooling of carbon steel workpieces with small cross-section size and simple shape.
● Salt water and alkaline water
Add an appropriate amount of salt and alkali to the water, and after the high-temperature workpiece is immersed in the cooling medium, salt and alkali crystals will precipitate in the steam film stage and burst immediately, destroying the steam film, and the oxide scale on the surface of the workpiece will also be blown into pieces, which can improve the performance of the workpiece. The disadvantage of the cooling ability of the medium in the high temperature area is that the medium is highly corrosive.
Application: Generally, the concentration of brine is 10%, and the concentration of caustic soda solution is 10% to 15%. It can be used as a quenching medium for carbon steel and low-alloy structural steel workpieces. The operating temperature should not exceed 60°C. After quenching, it should be cleaned in time and treated with anti-rust treatment.
The cooling medium generally uses mineral oil (mineral oil). Such as engine oil, transformer oil and diesel oil, etc. Engine oil generally uses 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 operating temperature increases accordingly.