New Development of Heat Treatment Quenching Medium

Over the years, my country has done a lot of work in the research and application of quenching media, and achieved certain results, basically meeting the needs of heat treatment production, but it is far behind the advanced level of foreign countries. At the same time, due to the long-term tendency of “focusing on heat and light on cold” (that is, focusing on the heating technology of the workpiece and despising the quenching and cooling technology) in the heat treatment industry, and the production practice of quenching and cooling lags behind the theoretical research of cooling transformation, resulting in quenching Cooling technology lags behind the development of other technical fields of heat treatment and is a weak link in the heat treatment industry. In order to shorten the gap between our country’s quenching media and foreign countries, and to be able to develop simultaneously with other technical fields of heat treatment, to meet the progress of heat treatment technology and higher requirements for the quality of heat-treated workpieces, we must pay attention to and vigorously strengthen the research and promotion of quenching media.

1   Classification and characteristics of quenching media There are many types of quenching media actually used, which can generally be divided into liquid (water, inorganic aqueous solution, organic polymer aqueous solution, quenching oil, molten metal, molten salt, molten alkali, etc.), gas (air, Compressed air, liquefied gas, etc.), solid (fluid bed, metal plate, etc.) 3 categories. Among them, water, aqueous solutions of inorganic substances, aqueous solutions of organic polymers, various quenching oils, etc., undergo physical state changes during quenching, while gases, molten metals, molten salts, molten alkalis, metal plates, etc., do not occur during quenching. Changes in physical state [1].

The desired effect of quenching the workpiece is to obtain high and uniform surface hardness and sufficient quenching depth, eliminate quenching cracks and reduce quenching deformation. Therefore, the ideal quenching medium should be that when the quenched workpiece is immersed in the quenching medium, in order to obtain the martensite structure, the cooling rate should be greater than the critical temperature in the low temperature range of the undercooled austenite stability, that is, near the nose temperature of the C curve The cooling rate allows the workpiece to quickly pass through the pearlite and bainite transformation zone to ensure that the workpiece obtains sufficient hardness after quenching; and at the temperature slightly below the Ms point and A1 point, the cooling rate of the workpiece is expected to be as slow as possible to reduce the internal and external Thermal stress and tissue stress caused by temperature difference can effectively prevent deformation and cracking of the workpiece. That is to say, the ideal cooling of “fast cooling at high temperature stage and slow cooling at low temperature stage” is commonly referred to. But such quenching medium is difficult to find in practice.

Generally, the requirements for the characteristics of the quenching medium are to meet the cooling rate requirements of the austenite cooling transformation curve of the steel to avoid deformation and cracking of the workpiece; the surface of the workpiece should be kept clean after quenching, and it is easy to clean even if there are adherents, and does not corrode the workpiece The performance is stable during use, does not decompose, deteriorate, does not age, and is easy to control; when the workpiece is immersed, it does not produce a lot of smoke and harmful gases to maintain good working conditions; it is easy to prepare, transport and store, and is safe to use; raw materials are easy Yes, the cost is low.

The cooling capacity of the quenching medium mainly depends on the composition of the medium and its physical and chemical properties. In actual production, attention should be paid to the influence of quenching medium cooling characteristics on the quality of quenched workpieces, and the appropriate quenching medium should be selected according to factors such as the carbon content of the workpiece, the level of hardenability, the effective thickness and the complexity of the shape. Using the same quenching medium, if the cooling method can be improved and the process parameters adjusted appropriately, the best quenching effect can be obtained. For example, when the quenching medium is circulated, stirred or applied with a certain pressure to pass through the surface of the workpiece, the cooling capacity of the quenching medium and the uniformity of the workpiece cooling can be improved, which has a good effect on avoiding the formation of quenching soft spots, reducing deformation and cracking .

2   Advantages and disadvantages of commonly used quenching media 2.1  Water is the earliest, most widely used, and most economical quenching medium. It is cheap and easy to obtain, non-toxic, non-combustible, stable in physical and chemical properties, and strong in cooling capacity. By controlling the temperature of the water, increasing the pressure, increasing the flow rate, using circulating water, using the action of a magnetic field, etc., the cooling characteristics of the water can be improved, deformation and cracking can be reduced, and a more ideal quenching effect can be obtained. However, because these methods require the addition of specialized equipment, and the performance of the workpiece after quenching is not stable, they have not been promoted and applied. Therefore, pure water is only suitable for the quenching of a few steel parts with low carbon content, low hardenability and simple shapes.

2.2  Quenching oil The mineral oil used for quenching is usually based on a highly refined neutral paraffin base oil. It has high flash point, low viscosity, less oily smoke, less grease, good oxidation resistance and thermal stability. Long service life and other advantages, suitable for use as quenching oil. Quenching oil is only suitable for workpieces with good hardenability, small wall thickness, complex shapes, and small quenching deformation. Quenching oil pollutes the surrounding environment, and it is easy to cause fire during quenching. Necessary cleaning, ventilation and fire safety facilities are required.  The main factor that affects the cooling capacity of quenching oil is its viscosity value. At room temperature, low-viscosity oil has a greater cooling capacity than high-viscosity oil.

As the temperature rises, the fluidity of the oil increases, and the cooling capacity increases. Appropriately increasing the use temperature of the quenching oil can also increase the cooling capacity of the oil. The operating temperature of ordinary engine oil is generally controlled at 60-80°C, and the maximum temperature does not exceed 120°C to ensure safe use. In addition, during the use of quenching oil, the formation of carbon black and residues will increase the viscosity, increase the flash point, and reduce its cooling capacity, resulting in aging and failure of the quenching oil. The changes in the flash point, viscosity, acid value, and saponification value of quenching oil are important data for its approaching aging. Therefore, it is necessary to carry out regular inspection and maintenance, regular sedimentation and filtration, and timely replenishment of new oil. This is essential for prolonging the service life of quenching oil. very important.

Due to the different composition of various quenching oils, their density, viscosity and flash point are also different, so they have different types and ranges of use. Adding various additives (such as catalysts, brighteners, antioxidants, etc.) to the oil, combined with stirring, spraying, ultrasonic strengthening and improving quenching equipment, can greatly increase the cooling rate of quenching oil and improve the uniformity of cooling Performance, or make the surface of the workpiece bright and clean, or extend the service life of the quenching oil. With the development of heat treatment technology, various quenching oils (such as ordinary quenching oil, rapid quenching oil, bright quenching oil, vacuum quenching oil, isothermal, graded quenching oil, etc.) have also been rapidly developed and widely used. Some advanced countries and my country Has formed a complete series of quenching oil products for users to choose.

For example, Jinyu Y15 rapid bright quenching oil, Y15D rapid quenching oil for thick and large parts, Y35 isothermal rapid quenching oil, YG bright quenching oil produced by Beijing Huali Fine Chemical Co., Ltd., its cooling characteristics, thermal stability and effective life Has reached the level of imported quenching oil, and the price is only half of the imported quenching oil; also produces quenching oil additives such as Y15T, Y15DT, Y35T, YGT, etc., adding them to the appropriate oil at a ratio of 10%, which can cool the oil The capacity is greatly improved, and the corresponding quenching oil can be obtained. 2.3  The characteristics of quenching media such as molten salt and molten alkali are that they do not change their state during the cooling process. The quenching of workpieces mainly depends on convection cooling.

Usually, the cooling rate is fast in the high temperature area, and the cooling rate is slow in the low temperature area. The quenching performance is excellent. Strong penetrating power, small quenching deformation, basically no cracks, but large environmental pollution, poor working conditions, high energy consumption, high cost, often used for the quenching of workpieces and molds with complex shapes and large cross-sectional dimensions. Molten salts include chloride, nitrate, nitrite, etc. The workpiece can be quenched in a salt bath to obtain high hardness, and the deformation is very small, and it is not easy to crack. It is usually used for austempering or graded quenching.

The disadvantage is that molten salt is easy to age, and has oxidation and corrosion effects on the workpiece. Molten alkali includes sodium hydroxide, potassium hydroxide, etc., which has a large cooling capacity. If the workpiece is not oxidized when heated, a silver-gray clean surface can be obtained after quenching, which also has certain applications. However, the molten alkali vapor is corrosive and irritating to the skin, so ventilation and protective measures should be taken when using it.