In the forging process, quenching is a critically important step, and the quenching media is a key factor that affects the success of the entire quenching process. Quenching media not only influence the mechanical properties and metallographic structure of forged parts, but may also cause problems such as deformation and cracking. Therefore, selecting the appropriate quenching medium and properly maintaining it is crucial for producing high-quality forged components.
For carbon steel and low-alloy steel above 650°C, austenite is in a relatively stable state. In this temperature range, we aim to cool at a relatively moderate rate to reduce thermal stress caused by temperature differences between the interior and exterior of the workpiece. When the temperature drops to the range of 650°C–450°C, a sufficiently fast cooling rate is required, exceeding the critical cooling rate, to effectively suppress the formation of non-martensitic structures. When the temperature falls below 400°C, especially below the martensite start point (M point), the cooling rate should slow down. This is because slower cooling can reduce structural stress, thereby preventing excessive deformation and quench cracking.
During quenching, to achieve martensite or bainite structures, rapid cooling is necessary. However, even for achieving martensite, different types of steel have different requirements for the critical cooling rate. If the cooling rate is too fast, it will cause excessive internal stress in the workpiece, leading to deformation or even cracking. Therefore, while meeting the conditions to achieve martensite or bainite, the actual cooling rate of the forged part should be kept as low as possible to effectively reduce the risk of deformation or cracking.
In the forging process, the choice of quenching medium is crucial because it directly affects the final performance of the workpiece. Different cooling media have different cooling characteristics and are suitable for different materials and process requirements. Below is a detailed overview of commonly used quenching media and their applications.
Water is a quenching medium with rapid cooling, and it is also the cheapest, safest, and non-polluting medium. However, water is not an ideal quenching medium. In the temperature range of 700°C–800°C, a vapor film forms around the workpiece, causing a relatively slow cooling rate. In areas requiring rapid cooling, the rate does not exceed 200°C/s. However, in areas where rapid cooling is not required, such as below 400°C, the vapor film collapses, and the cooling rate rapidly increases, reaching 700°C/s in the martensite transformation region. Such abrupt changes in cooling rate generate significant stress and may even cause the workpiece to crack.
The formation of the vapor film also causes locally slow cooling, preventing the formation of martensite and resulting in soft spots. Moreover, as the water temperature rises, its cooling capacity drops sharply. When water reaches 40°C, the cooling rate in critical areas is only 100°C/s. Therefore, to ensure uniform quenching quality, water temperature must be maintained at 15°C–25°C, requiring continuous addition of fresh water. To reduce soft spots, the water should be circulated so that water on the workpiece surface is continuously moving.
When water contains impurities such as soil, oil, or soap, vapor film formation accelerates, greatly reducing the cooling capacity and affecting cooling uniformity, leading to soft spots. Maintaining clean cooling water is therefore very important. Using compressed air to stir the water is not reasonable, as it introduces large amounts of dissolved gas. To eliminate the adverse effects caused by the vapor film, substances such as salt or alkali can be added to form saline or alkaline solutions.
During quenching, salt crystallizes on the hot surface of the workpiece and may explode. This explosion not only destroys the vapor film but also removes scale from the workpiece. Therefore, after quenching in saline, the workpiece surface becomes very clean. Adding 5%–10% NaCl to water can achieve a cooling rate of 1000°C/s at critical areas (measured using a φ20mm silver probe). At low temperatures, the cooling rate is similar to that of clean water. This makes it easier for the workpiece to achieve uniform high hardness and a deeper hardening layer. Therefore, in industrial production, pure water quenching is rarely used; NaCl aqueous solutions are more common.
Adding 10%–15% NaOH to water results in a higher cooling rate than saline at 650°C–550°C in critical areas, but a lower rate at low temperatures. Using this alkaline solution, the workpiece can achieve high hardness without soft spots and reduce deformation and cracking, making it an ideal cooling medium for carbon steel. In addition, alkali reacts with metal, releasing hydrogen and removing scale, producing a bright silver surface. However, NaOH has some disadvantages, including inconvenience in use, corrosiveness, skin irritation, absorption of CO₂ from air reducing cooling capacity, and susceptibility to deterioration. For these reasons, alkaline solutions are less widely used than saline.
Temperature has little effect on the cooling capacity of saline and alkaline solutions. Saline can be used up to 50°C, and alkaline solution up to 60°C. Workpieces must be cleaned after quenching in saline or alkaline solutions to prevent corrosion.
Mineral oil has a relatively slow cooling rate, about 100°C/s in critical areas, and very slow cooling below 300°C, reducing deformation and cracking. Due to its low cooling capacity, it is suitable only for steels with C-curves on the right or carbon steel parts with low hardness requirements.
As oil temperature rises, viscosity decreases, fluidity improves, and cooling capacity increases. However, excessively high temperatures can easily cause fire hazards, requiring caution. Generally, oil should be used below 80°C. Commonly used mineral oil media include 10#–30# engine oils, with approximately 90% being N22 or N32 mechanical lubricating oil or equivalent spindle oil. The higher the grade, the higher the viscosity, and the lower the cooling capacity.
Contact with hot workpieces causes oil decomposition and formation of oil residues. Oxidation also reduces cooling capacity over time, leading to oil aging. The cooling capacities of water, aqueous solutions, and oil can be understood from standard tables (measured using φ20mm silver probes).
Adding efficient accelerators to oil produces fast-quenching oil. High-polymer quenching media, especially water-based polymer solutions with added anti-corrosion, defoaming, and rust inhibitors, can be diluted to various concentrations according to requirements, producing cooling capacities between water and oil or slower than oil. They are non-flammable, smoke-free, and considered the most promising oil substitutes. When using water-based polymer quenching media, a polymer film often forms on the workpiece surface. Higher concentration slows cooling, higher liquid temperature slows cooling, and agitation accelerates cooling.
There are many types of high-polymer quenching media, with polyalkylene glycol (PAG) aqForging ueous solutions being widely used. Their cooling capacity spans the range between water and oil and can be adjusted by controlling concentration and agitation. They have good wetting properties, uniform cooling, and stable long-term performance, and have been widely applied.
Besides selecting the appropriate medium, proper quenching methods are essential. Common methods include single-medium quenching, double-medium quenching, step quenching, isothermal quenching, and localized quenching.
Single-Medium Quenching: Single-medium quenching is the simplest method, where the heated workpiece is directly immersed in a single quenching medium. This method is easy to operate but requires high-quality media, as the entire cooling process depends on a single medium. Improper selection may easily cause deformation or cracking.
Double-Medium Quenching: Double-medium quenching immerses the workpiece first in a fast-cooling medium, and once the temperature drops to a certain level, it is quickly transferred to a slower medium to continue cooling. This method effectively controls the cooling rate, reducing deformation and cracking risks. However, the process is complex and requires precise timing and temperature control.
Step Quenching: Step quenching immerses the heated workpiece first in a medium at higher temperature to equalize internal and external temperatures, then transfers it to a cooler medium for rapid cooling. This further reduces deformation and cracking, but requires precise temperature control and is more difficult to operate.
Isothermal Quenching: Isothermal quenching involves cooling the workpiece to a specific temperature in a medium and holding it for isothermal transformation. This method achieves good overall properties but requires special equipment and is costly.
Localized Quenching: Localized quenching treats only specific regions of the workpiece, while other areas remain unquenched. This method allows targeted treatment based on part requirements but requires precise control and high operator skill.
Steel workpieces after quenching have features such as martensite, bainite, or retained austenite—unstable microstructures with high internal stresses, and mechanical properties that do not meet requirements. Therefore, tempered treatment is generally required. The main purpose of tempering is to reduce internal stress and decrease brittleness. Quenched parts have high stress and brittleness, and without timely tempering, deformation or cracking often occurs.
Tempering also adjusts mechanical properties. Quenched parts are hard but brittle; to meet different performance requirements, tempering adjusts hardness, strength, plasticity, and toughness. Tempering also stabilizes dimensions, making the microstructure stable and preventing further deformation during use. Additionally, it improves machinability for certain alloy steels.
In the forging process, selecting and using the appropriate quenching medium is critical. Only by considering the material, shape, and performance requirements of the workpiece, properly choosing quenching media, adopting correct quenching methods, and performing subsequent tempering can the quality and performance of workpieces be ensured. With continuous technological advancement, the types and performance of quenching media are constantly improving, and it is expected that more efficient and environmentally friendly quenching media will be developed in the future to better support the forging industry.
