Common media for heat treatment and troubleshooting of related issues

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The medium used for quenching and cooling workpieces is called a quenching cooling medium (or quenching medium). The ideal quenching medium should have the property of enabling the workpiece to be quenched into martensite without inducing excessive quenching stress. This requires a gradual cooling of the temperature above the “nose” of the C curve in order to reduce the thermal stresses resulting from rapid cooling ； The cooling rate at the “nose” area must be greater than the critical cooling rate to ensure that the supercooled austenite does not undergo non-Martensitic transformation ； Below the \"nose\", especially when pressing the Ms point with temperature applied, the cooling rate should be as low as possible to reduce the stress associated with tissue transformation. 　　Common quenching media include water, aqueous solutions, mineral oils, molten salts, and molten alkalis. 　　● Water Water is a quenching medium with strong cooling capacity. It has a wide source, low price, and stable ingredients that are not prone to deterioration. The drawback is that in the \"nose\" region of the C curve (around 500–600°C), water is in the vapor film phase; cooling is not fast enough, which results in the formation of \"soft spots\"” ； In the martensite transformation temperature range (300–100°C), water is in a boiling state; rapid cooling can cause the martensite transformation to occur too quickly, resulting in high internal stresses that may lead to deformation or even cracking of the workpiece. When the water temperature rises, an excess of gases in the water or the presence of insoluble impurities (such as oil, soap, mud, etc.) will significantly reduce its cooling capacity. Therefore, water is suitable for the quenching and cooling of carbon steel workpieces with small cross-sectional dimensions and simple shapes. 　　● Saltwater and alkaline water: By adding an appropriate amount of salt and alkali to water, when high-temperature workpieces are immersed in this cooling medium, crystals of salt and alkali precipitate during the steam film stage and burst immediately, thereby breaking apart the steam film. The oxide scale on the surface of the workpieces is also shattered, which enhances the cooling capacity of the medium in high-temperature areas. Its disadvantage is the high corrosivity of the medium. 　　Under normal circumstances, the concentration of saltwater is 10%, while the concentration of sodium hydroxide 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, and the parts should be cleaned promptly after quenching and treated to prevent rust. 　　● Oil: Mineral oil is generally used as the cooling medium. Such as engine oil, transformer oil, and diesel, etc. Engine oils generally come in grades 10, 20, and 30. The higher the grade, the greater the viscosity, the higher the flash point, the lower the cooling capacity, and accordingly, the higher the operating temperature. 　　The main types of new quenching oils currently in use are high-speed quenching oil, brightening quenching oil, and vacuum quenching oil. 　　High-speed quenching oil is a quenching oil that provides an increased cooling rate in high-temperature areas. There are two main ways to obtain high-speed quenching oil. One involves selecting mineral oils of different types and viscosities, mixing them in appropriate proportions, and enhancing their cooling capacity in high-temperature environments by raising their characteristic temperature ； Another method is to add additives to ordinary quenching oil, thereby forming powder-like suspended particles in the oil. Barium salts, sodium salts, calcium salts of the additive thiosulfonic acid, as well as phosphates, stearates, etc. Production practice shows that high-speed quenching oil has a significantly higher cooling rate in the unstable region of supercooled austenite compared to ordinary quenching oil, while its cooling rate in the low-temperature martensite transformation region is similar to that of ordinary quenching oil. This approach not only yields higher hardenability and strength but also **reduces deformation, making it suitable for quenching alloy steel workpieces with complex shapes. 　　Bright quenching oil enables the workpiece to maintain a bright surface after quenching. By adding polymers with different properties to mineral oil, bright quenching oils with varying cooling rates can be obtained. The main component of these additives is a brightener, whose function is to suspend the aging products that are insoluble in oil, thereby preventing them from accumulating and precipitating on the workpiece. In addition, bright quenching oil additives also contain antioxidants, surfactants, and accelerators, among others. 　　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, as well as good luster and thermal stability; otherwise, it will affect the effectiveness of vacuum heat treatment. 　　Salt bath and alkali bath quenching media are generally used in step quenching and isothermal quenching. 　　● New types of quenching agents include polyvinyl alcohol aqueous solutions and trinitrate aqueous solutions, among others. 　　Polyvinyl alcohol is commonly used in aqueous solutions with a mass fraction of 0.1% to 0.3%, and its total cooling capacity lies between that of water and oil. When the workpiece is immersed in this solution, a vapor film and a gel film form on its surface; these two films help to cool the heated workpiece. Once the boiling stage is reached, the film ruptures and the cooling rate of the workpiece increases; when a low temperature is attained, the polyvinyl alcohol gel film forms again and the cooling rate of the workpiece decreases. Therefore, this solution has a low cooling capacity in high and low temperature ranges, but a high cooling capacity in the medium temperature range, exhibiting good cooling properties. 　　The trinitrate aqueous solution is composed of 25% sodium nitrate + 20% sodium nitrite + 20% potassium nitrate + 35% water. At high temperatures (650–500°C), the precipitation of salt crystals leads to the breakdown of the steam film, resulting in a cooling capacity similar to that of water. At low temperatures (300–200°C), due to the extremely high concentration and poor fluidity, its cooling capacity is similar to that of oil; therefore, it can replace water-oil dual-medium quenching. Cooling method The most widely used classification of quenching in production practice is based on the differences in cooling methods. The main methods include single-fluid quenching, dual-fluid quenching, staged quenching, and isothermal quenching. 　　● Single-fluid quenching is a quenching method in which the austenitized workpiece is immersed in a certain quenching medium and cooled down to room temperature. Single-fluid quenching media include water, saltwater, alkaline water, oil, and specially formulated quenchants. Generally, carbon steel is quenched in water, while alloy steel is quenched in oil. 　　One-fluid quenching is simple to operate, which facilitates mechanization and automation. Its drawback is that the cooling rate is limited by the cooling characteristics of the medium, which affects the quality of quenching. Single-fluid quenching is only suitable for carbon steel workpieces with relatively simple shapes. 　　● Two-fluid quenching involves first immersing the austenitized workpiece in a medium with high cooling capacity; before the steel reaches the temperature of this quenching medium, it is removed and immediately immersed in another medium with lower cooling capacity for further cooling, such as using water followed by oil, or water followed by air. Two-fluid quenching reduces the tendency for deformation and cracking, but it is difficult to master properly and has certain limitations in terms of application. 　　● Martensitic staged quenching is a quenching process in which the austenitized workpiece is first immersed in a liquid medium (salt bath or alkali bath) at a temperature slightly higher or lower than the martensite transformation point of the steel. It is held there for an appropriate period of time; once both the inner and outer layers of the steel piece reach the temperature of the medium, it is taken out and cooled air-dry to obtain a martensitic structure. This process is also known as staged quenching. 　　Due to the air cooling after staying at the grading temperature until the temperature inside and outside the workpiece is uniform, gradient quenching can effectively reduce phase transformation stress and thermal stress, thereby minimizing the tendency for quenching deformation and cracking. Gradient quenching is suitable for alloy steel and high-alloy steel workpieces with high requirements for deformation, as well as for carbon steel workpieces with small cross-sectional dimensions and complex shapes. 　　● Isothermal quenching of bainite is a quenching process in which the steel part is austenitized and then rapidly cooled to the bainite transformation temperature range (260–400°C), where it is held at that temperature to allow the austenite to transform into bainite; it is sometimes also referred to as isothermal treatment. The typical insulation time is 30–60 minutes. 　　● Compound quenching: The workpiece is rapidly cooled below the Ms temperature to obtain 10%–20% martensite, followed by isothermal treatment in the lower bainite temperature range. This cooling method enables workpieces with larger cross-sections to obtain an M+B microstructure. The martensite formed during pre-quenching can promote bainitic transformation, and during isothermal treatment it causes the martensite to temper. Compound quenching is used for alloy tool steel workpieces to avoid type 1 temper brittleness and reduce the amount of residual austenite, thereby decreasing the tendency to deformation and cracking. 1. What issues should be considered when using new oil in the entire tank? Before pouring in new oil, it is necessary to carefully inspect and clean the quenching oil tank and cooling system. Any remaining water, sludge, and other residues should be cleaned up completely. When replacing the oil in an old oil tank system with new oil, it is also necessary to remove any oil stains from the walls of the tank above the oil level as well as from various frames. If the original oil residues and sludge mix into the new oil, it may alter the oil’s cooling properties. Therefore, the cleaning work should be done more thoroughly than when using a new oil tank. After the new groove is filled with new oil, it should not be used for quenching operations right away. During production, transportation, and dumping, quenching oil always picks up a small amount of air. Journal of Hot Working in Metal Processing. The presence of gas will reduce the cooling rate during the high-temperature stage of the quenching oil, and it should be removed. The solubility of gas in oil decreases as the oil temperature rises. Raising the oil temperature can reduce the viscosity of the oil, which facilitates the rise of bubbles. Therefore, the gas in new oil can be removed by raising the oil temperature. 2. Why must quenching oil be circulated and stirred? A good cooling cycle can prevent excessive local oil temperatures, helping to keep the oil temperature uniform throughout the tank. The oil cooling cycle can increase the relative flow rate between the workpiece and the quenching oil, thereby enhancing the oil’s cooling capacity and preventing soft spots from forming on the workpiece surface. When the oil temperature is too high, the oil flowing onto the workpiece at that moment can cause the local oil temperature to rise further, posing a risk of fire. 3. How to reduce oil pollution? The sources of contamination in quenching oil include external contamination and internal contamination. External contamination: oxide scale introduced during the quenching process of the workpiece, water leaking from the cooling system, and other substances coming from outside. Self-contamination: Oxidized and degraded products that remain in the oil and cannot be automatically removed during use ； Plus the residues resulting from reactions between foreign contaminants and quenching oil and its contaminants. The accumulation of internal and external contaminants can gradually cause changes in the oil’s color, transparency, viscosity, flash point, residual carbon, and acid value. This process of change is the deterioration process of quenching oil. Among the effects caused by transformation, those that have the greatest impact on the heat treatment results of the workpiece are the changes in the oil cooling properties and the deterioration in the brightness of the workpiece after quenching. Changes in cooling characteristics often affect the quenching hardness, depth of the hardened layer, and deformation of the workpiece. Excerpted from the Heat Treatment Ecosystem. Preventing and reducing external contamination, using quenching oil properly and managing it well, as well as carrying out regular cleaning, can all help to slow down the deterioration of quenching oil and extend its service life. 4. What are the criteria for changing quenching oil? The replacement of quenching oil is determined by the specific degree of aging, rather than the length of time it has been used. During use, regular sampling and analysis are required; the degree of aging of quenching oil depends mainly on the base oil, additives, and operating conditions at the site. General oil change criteria for quenching oil: (1) A decrease in characteristic temperature of more than 40℃ ； (2) The moisture content exceeds 0.1% (by volume) under normal use conditions. (3) Viscosity increase of more than 15% ； (4) Luster is significantly reduced. 5. What to do after water enters the quenching oil? The presence of water in the quenching oil affects the cooling performance, particularly impacting the cooling rate at low temperatures; as a result, the workpiece may experience excessive deformation or cracking after quenching. The quenching oil after water ingress must undergo high-temperature dehydration treatment. The oil should be heated to 80–130°C and stirred for a certain period of time; the exact duration depends on the amount of water that has entered. After dehydration, the cooling performance should be tested; it can be used normally only if it meets the factory requirements, otherwise additives should be added for adjustment. 6. How to use water-based quenching fluids? Water-based quenching fluid is used by diluting it with water; the dilution ratio depends on the specific workpiece and material. The operating temperature must not exceed 50°C, as higher temperatures will significantly reduce the cooling efficiency, resulting in the workpiece not achieving the desired hardness level.