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What are the water-soluble quenching fluids? Its advantages, disadvantages, uses and precautions

　　water-soluble quenching fluid includes water, sodium chloride aqueous solution, sodium hydroxide aqueous solution, calcium chloride aqueous solution, nitrate aqueous solution, zinc chloride-sodium hydroxide aqueous solution and sodium silicate aqueous solution, etc. its advantages, disadvantages, uses and precautions are as follows:
　　1, water:
　　advantages: high heat of vaporization, high heat transfer coefficient, good chemical stability, very cheap, easy to use.
　　Disadvantages: The cooling rate changes significantly with the change of water temperature. The cooling rate in the 650-550 ℃ range is less than that in the 300-200 ℃ range. Because the cooling rate is low in the austenite unstable region, the phenomenon of non-hardening occurs. Quenched parts will also produce huge stress during quenching, resulting in cracking and deformation. The steam film stage is long and prone to bubbles. In the quenching parts of the groove and hole steam is not easy to escape, resulting in uneven cooling, so prone to soft spots.
　　Purpose: Only used for wrong quenching of carbon steel parts with small cross-section and simple shape, with smooth working surface.
　　Precautions: It is best to use stirring or forced circulation to improve the uniformity of cooling and prevent soft spots and deformation. Water should not be mixed with dust, oil and other impurities. The working temperature should not exceed 40 ℃.
　　2, inorganic aqueous solution:
　　1) sodium chloride (common salt) aqueous solution:
　　medium composition: NaCl concentration can be 5% or 10%.
　　Advantages: NaCl can adhere to the surface of hot quenching parts and explode violently into mist (film collapse), which destroys the steam film and greatly shortens the steam film stage, thus obviously improving the cooling speed of water and cooling evenly. The price is cheap, the quenching part can reach higher hardness, and the hardness is uniform.
　　Disadvantages: The cooling rate changes with the solution temperature, and the quenched parts are easy to rust after quenching.
　　application: used for quenched parts with low hardenability, not easy to crack and low requirement for deformation prevention, such as carbon steel parts (effective thickness 30-100mm, quenched by brine and oil), alloy structural steel (40Cr,40CrMoV, effective thickness 30-150mm;38CrMoAl effective thickness & gt;80mm.)
　　Precautions: The temperature of the solution should be controlled below 60°C during use. After quenching, it should be cleaned and anti-rust treatment should be carried out.
　　2) sodium hydroxide aqueous solution:
　　advantages: the cooling curve is basically the same as that of sodium chloride solution. NaOH can interact with the oxide scale on the surface of the quenched part to generate hydrogen, which can rapidly peel off the oxide scale and make the surface of the quenched part appear bright silvery white. The cooling capacity is greater than that of sodium chloride.
　　Disadvantages: corrosive, poor working conditions, easy to absorb carbon dioxide in the air during use and gradually change the composition. As with the previous two, the cooling rate also varies significantly with the solution temperature.
　　Application: For carbon steel.
　　Note: Change the solution regularly.
　　3) saturated calcium chloride aqueous solution:
　　advantages: in the austenite unstable region (650-550 ℃), it has a high cooling rate. in the martensite transformation region, because its boiling point is higher than that of water, the convection start temperature is also higher, and at the same time, its viscosity is higher than that of water and its heat transfer is poor, so the cooling rate is slower, thus reducing the quenching stress, prevent deformation and cracking. It is convenient to prepare, easy to purchase, low price and long service life.
　　Disadvantages: When the temperature is too low, calcium chloride crystals will precipitate and block the quenching tank pipeline. Quenched parts are easy to rust when placed.
　　Precautions: After quenching, the quenched parts should be cleaned in time for anti-rust treatment.
　　4) supersaturated nitrate aqueous solution:
　　medium composition:(1) 3 nitrate solution sodium nitrate 25%, sodium nitrite 20%, potassium nitrate 20%, water 35%, the relative density should be controlled between 1.4 and 1.5;(2) 2 nitrate solution sodium nitrate 31.2, sodium nitrite 20.8, the relative density should be controlled between 0.36 and 1.41.
　　Advantages: In the high temperature zone (650-550°C), the presence of a large amount of nitrate will destroy the formation and stability of the steam film, making the cooling speed close to water. In the low temperature area (300-200 ℃), due to the high concentration of the solution, high viscosity, poor fluidity, and slow convection, the cooling rate is close to the oil. 3 nitrate solution is similar to saturated calcium chloride solution, high quenching hardness, deep hardening layer, small deformation, not easy to crack. The quenching parts have good rust resistance, and the cooling characteristics are between water and oil. At high temperature, the cooling speed is 3 times faster than that of oil, and at low temperature, it is 1 times that of oil, but it is still slower than water.
　　Disadvantages: The price is much higher than calcium chloride, sodium nitrite is toxic and easy to produce carcinogens.
　　Application: Used for quenching of medium carbon steel, high carbon steel, low alloy and ductile graphite castings, and can also replace water-oil two-liquid quenching of carbon steel. The effective thickness of 45 steel is less than or equal to 40mm.Some simple-shaped alloy steel [40Cr,65Mn,20Cr (after carburizing),GCr15] workpieces can also be used.
　　Precautions: Note that sodium nitrite is toxic and carcinogenic, and non-toxic rust inhibitors should be used instead. The use temperature is 30-60 ℃. When the temperature rises to 100 ° C, the cooling capacity will decrease and soft spots will appear.
　　5) zinc chloride-sodium hydroxide aqueous solution (bright quenching agent):
　　medium composition: zinc chloride 12.25, sodium hydroxide 12.25, soap 0.5, water 75%.
　　Performance: Zinc chloride reacts with sodium hydroxide to generate zinc hydroxide, which is Milky White. The cooling rate in the high temperature zone is faster than that in water, and the cooling rate in the low temperature zone is slower than that in water.
　　Advantages: The quenched parts have high hardness, small deformation, not easy to crack, and bright surface.
　　Purpose: Used for quenching of workpieces with complex shapes such as carbon steel and carbon tool steel, which can replace water-oil two-liquid quenching.
　　Precautions: Pay attention to the uniform stirring before quenching. It is better to stir with compressed air. The working temperature should be controlled between 20-60 ℃.
　　6) aqueous solution of water glass (sodium silicate):
　　medium composition: single water glass solution, modulus M = SiO2/Na20 = 2.4, relative density 1.091-1.125(12-16 Bc) (sodium silicate can be replaced by potassium silicate).

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