New development of quenching media for the hottest

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New progress of heat treatment quenching medium

over the years, China has done a lot of work in the research and application of quenching medium, and achieved certain results, which basically meets the needs of heat treatment production, but there is a big gap compared with the advanced level of foreign countries. At the same time, due to the long-term tendency of "emphasizing heat and neglecting cooling" (that is, attaching importance to the heating technology of workpiece and neglecting 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, quenching and cooling technology lags behind the development of other technical fields of heat treatment, which is a weak link in the heat treatment industry. In order to shorten the gap between China's quenching medium and foreign countries, develop synchronously with other technical fields of heat treatment, and 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, promotion and application of quenching medium

1 classification and characteristics of quenching media

there are many kinds of quenching media actually used, which can generally be divided into three categories: 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.). Among them, water, inorganic aqueous solution, organic polymer aqueous solution, various quenching oils, etc. will change their state during quenching, while gas, molten metal, molten salt, molten alkali, metal plate, etc. will not change their state during quenching [1]

the ideal effect of workpiece quenching 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 martensite structure, the cooling rate should be greater than the critical cooling rate in the temperature range with low stability of supercooled austenite, that is, near the nose temperature of C curve, so that the workpiece can quickly pass through the pearlite and bainite transformation zone, so as to ensure that the workpiece can obtain sufficient hardness after quenching; At the temperature slightly below MS point and A1 point, the cooling speed of the workpiece is expected to be as slow as possible, so as to reduce the thermal stress and structural stress caused by the temperature difference inside and outside the workpiece, so as to effectively prevent the deformation and cracking of the workpiece. That is, the ideal cooling of "fast cooling in high temperature stage and slow cooling in low temperature stage" is usually realized. But such quenching medium is difficult to find in practice

generally, the requirements for the characteristics of quenching medium are to meet the cooling rate requirements of the austenite cooling transformation curve of steel and to avoid workpiece deformation and cracking; After quenching, the surface of the workpiece should be kept clean, even if there is adhesion, it is easy to clean and does not corrode the workpiece; In the process of use, it has stable performance, does not decompose, does not deteriorate, does not age, and is easy to control; The workpiece will not produce a lot of smoke and harmful gases when immersed, so as to maintain good working conditions; Easy to prepare, transport and store, safe to use; Raw materials are easy to obtain and low cost

the cooling capacity of quenching medium mainly depends on the composition and physical and chemical properties of the medium. In actual production, we should pay attention to the influence of the cooling characteristics of the quenching medium on the quality of the quenched workpiece, and select the appropriate quenching medium according to the carbon content of the workpiece, the 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 can be adjusted appropriately, the best quenching effect can be obtained. For example, when the quenching medium is circulated, stirred or applied with a certain pressure through the workpiece surface, the cooling capacity of the quenching medium and the uniformity of workpiece cooling can be improved, which plays a good role in avoiding the formation of quenching soft spots and reducing deformation and cracking

2 advantages and disadvantages of common quenching media

2.1 water

water is the earliest, most widely used and most economical quenching medium. It is cheap and easy to obtain, non-toxic, non combustible, stable physical and chemical properties, and strong cooling capacity. By controlling the temperature of water, increasing the number of high value-added products and refined and deep processed products, providing strong support, increasing the pressure, increasing the flow rate, using circulating water, using the role of magnetic field, etc., the cooling characteristics of water can be improved, deformation and cracking can be reduced, and a relatively ideal quenching effect can be obtained. However, because these methods require additional special equipment and the performance of the workpiece after quenching is not very stable, they have not been popularized and applied. Therefore, pure water is only suitable for quenching a few steel parts with low carbon content, low hardenability and simple shape

2.2 quenching oil

mineral oil used for quenching usually takes neutral paraffin base oil with high refining degree as the base oil. It has the advantages of high flash point, low viscosity, less oil smoke, less oil dirt, good oxidation resistance and thermal stability, long service life and so on. It is suitable for use as quenching oil. Quenching oil is only suitable for workpieces with good hardenability, small wall thickness, complex shape and small quenching deformation. Quenching oil pollutes the surrounding environment greatly, and it is easy to cause fire during quenching. Necessary cleaning, ventilation and fire safety facilities should be equipped

the main factor affecting the cooling capacity of quenching oil is its viscosity. At room temperature, the cooling capacity of low viscosity oil is greater than that of high viscosity oil. With the increase of temperature, the fluidity of oil increases and the cooling capacity improves. Properly raising the service temperature of quenching oil can also improve the cooling capacity of oil. The service temperature of ordinary engine oil is generally controlled at 60 ~ 80 ℃, and the maximum is not more than 120 ℃, so as to ensure the safety of use. In addition, during the use of quenching oil, the carbon black and residue formed will increase the viscosity and flash point, reduce its cooling capacity, and cause the aging and failure of quenching oil. The changes of flash point, viscosity, acid value and saponification value of quenching oil are important data for its approaching aging. Therefore, regular detection and maintenance, regular sedimentation and filtration, and timely replenishment of new oil must be carried out, which is very important for prolonging the service life of quenching oil

due to the different composition of various quenching oils, their density, viscosity and flash point are also different, so they have different types and application ranges. Adding various additives (such as catalyst, brightener, antioxidant, etc.) to the oil, combined with stirring, spraying, ultrasonic strengthening and improving quenching equipment, can greatly improve the cooling speed of quenching oil, improve the uniformity of cooling, or make the workpiece surface bright and clean, or prolong the service life of quenching oil. With the development of heat treatment technology, various quenching oils (such as ordinary quenching oil, fast quenching oil, bright quenching oil, vacuum quenching oil, isothermal, graded quenching oil, etc.) have also been rapidly developed and widely used. Some advanced countries abroad and China have formed a complete series of quenching oil products for users to choose. For example, the cooling characteristics, thermal stability and effective life of today's Yu Y15 fast bright quenching oil, y15d thick and large pieces fast quenching oil, Y35 isothermal fast quenching oil and YG bright quenching oil produced by Beijing Huali fine chemical company have reached the level of imported quenching oil, and the price is only half of that of imported quenching oil; It also produces Y15T, y15dt, y35t, ygt and other quenching oil additives. Adding them to the appropriate oil at the proportion of 10% can greatly improve the cooling capacity of the oil and obtain the quenching oil of the corresponding brand

2.3 molten salt, molten alkali

this kind of quenching medium is characterized by no physical state change during the cooling process. Workpiece quenching mainly depends on convective cooling. Usually, the cooling speed is fast in high-temperature areas, slow in low-temperature areas, excellent quenching performance, strong hardenability, small quenching deformation, basically no cracks, but it has great environmental pollution, poor labor conditions, high energy consumption, high cost, and is commonly used in complex shapes Quenching of workpieces and tools and dies with wide variation of cross-section size. Molten salts include chloride, nitrate, nitrite, etc. workpiece quenching in salt bath can obtain high hardness, with minimal deformation and not easy to crack. It is usually used for isothermal quenching. Based on two force columns, measure fire with dial indicator or step quenching. Its disadvantage is that the molten salt is easy to age and has oxidation and corrosion effects on the workpiece. Molten alkali includes sodium hydroxide, potassium hydroxide, etc. it has great cooling capacity. If the workpiece is not oxidized during heating, a silver gray clean surface can be obtained after quenching, and it also has certain applications. However, molten alkali vapor is corrosive and irritating to the skin. When using it, pay attention to ventilation and take protective measures

3 new quenchants and their applications

3.1 Organic Polymer Quenchants

in recent years, the most remarkable progress of new quenchants is the research and application of Organic Polymer Quenchants. This kind of quenching medium dissolves the organic polymer in water, and adjusts the concentration and temperature of the solution according to the needs to prepare an aqueous solution with cooling performance that meets the requirements. Its cooling speed is close to water at high temperature stage and oil at low temperature stage. Its advantages are non-toxic, smoke-free, odorless, corrosion-free, non combustible, anti-aging, safe and reliable use, good cooling performance, adjustable cooling speed, wide application range, uniform hardening of workpieces, which can significantly reduce the tendency of deformation and cracking. Therefore, it can improve the quality of workpieces, improve the working environment and working conditions, and bring energy conservation, environmental protection, technical and economic benefits to the factory. At present, Organic Polymer Quenchants are widely used in large quantities and single varieties of heat treatment, especially for workpieces that are cracked by water quenching, have large deformation, and cannot be hardened by oil quenching, using Organic Polymer Quenchants is a more successful choice. Using organic polymer quenchant is more economical, efficient and energy-saving than using quenching oil. From the perspective of improving workpiece quality, improving working conditions, avoiding fire and saving energy, organic polymer quenchant has the trend of gradually replacing quenching oil (the circulating cooling system originally used for quenching oil does not need to be modified and replaced when using organic polymer quenchant), which is the main development direction of quenching medium

the cooling rate of organic polymer quenchant is affected by three basic parameters: concentration, service temperature and stirring degree. Generally speaking, the higher the concentration, the slower the cooling speed; The higher the temperature, the slower the cooling speed; The more intense the stirring, the faster the cooling speed. The function of stirring is very important: ① make the solution concentration uniform; ② Strengthen the thermal conductivity of the solution, so as to ensure that the hardness of the workpiece after quenching is high and evenly distributed, and reduce the tendency of quenching soft spots, deformation and cracking. By controlling these factors, the cooling rate of organic polymer quenchant can be adjusted, so as to achieve the ideal quenching effect. Generally speaking, the concentration can be lower in summer and higher in winter, and it should be fully stirred. Organic Polymer Quenchants are mostly made into aqueous concentrates for sale. When in use, they can be diluted into different concentrations by adding water according to the characteristics and technical requirements of the workpiece, and then quenchants with various quenching intensities can be obtained to meet different quenching needs. In addition, pay attention to careful maintenance, prevent pollution, and try to maintain the good cleanliness of the quenchant. For long-term stability, it is very important to focus on the problem of capacity reduction in these two difficult industries and make good use of Organic Polymer Quenchants

different kinds of Organic Polymer Quenchants have significantly different cooling characteristics and stability, which can meet the needs of different quenching processes [2]. At present, the most stable and widely used Organic Polymer Quenchants in the world are polyalkylene glycol (PAG) quenchants

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