Common Heat Treatment Media and Problem Solutions

The medium used for quenching and cooling workpieces is called a quenching coolant (or quenching medium). An ideal quenching medium should enable the workpiece to transform into martensite while minimizing excessive quenching stresses. This requires slow cooling above the "nose" of the C-curve to reduce thermal stress caused by rapid cooling; at the "nose," the cooling rate must exceed the critical cooling rate to prevent non-martensitic transformation of undercooled austenite; below the "nose," especially below the Ms point, the cooling rate should be as low as possible to minimize transformation-induced stresses.

Commonly used quenching media include water, aqueous solutions, mineral oils, molten salts, and molten alkalis.

● Water  
Water is a highly effective quenching medium due to its wide availability, low cost, and stable composition that resists degradation. However, in the "nose" region of the C-curve (approximately 500–600°C), water enters the vapor film stage, where cooling is insufficient, potentially leading to the formation of "soft spots." In the martensite transformation temperature range (300–100°C), water reaches the boiling stage, causing extremely rapid cooling. This can accelerate martensite transformation too quickly, generating high internal stresses that may cause deformation or cracking of the workpiece. The cooling capacity of water significantly decreases when its temperature rises, or when it contains dissolved gases or insoluble impurities such as oil, soap, or mud. Therefore, water is suitable only for small-section, simple-shaped carbon steel workpieces.

● Saltwater and alkaline water  
By adding appropriate amounts of salt or alkali to water, the cooling medium can disrupt the vapor film during the high-temperature phase. When hot workpieces are immersed, crystals of salt or alkali precipitate and immediately burst, breaking the vapor film and fragmenting the oxide scale on the workpiece surface. This enhances cooling efficiency at elevated temperatures. The main drawback is the medium's strong corrosiveness. Typically, saltwater has a concentration of 10%, while sodium hydroxide solution ranges from 10% to 15%. These media are suitable for quenching carbon steels and low-alloy structural steels, with operating temperatures not exceeding 60°C. After quenching, workpieces should be promptly cleaned and treated for rust prevention.

● Oil  
Mineral oils—such as machine oil, transformer oil, and diesel—are commonly used as quenching media. Machine oils typically include grades 10, 20, and 30; higher-grade numbers indicate greater viscosity, higher flash points, lower cooling capacity, and correspondingly higher usable temperatures.

Currently, three types of advanced quenching oils are widely used: high-speed quenching oil, bright quenching oil, and vacuum quenching oil.

High-speed quenching oil achieves enhanced cooling rates at high temperatures. There are two primary methods to produce it: one involves blending different types and viscosities of mineral oils in specific proportions to increase characteristic temperature and thus improve high-temperature cooling performance; the other adds chemical additives to ordinary quenching oil, forming fine, suspended particulates. Common additives include barium, sodium, and calcium sulfonates, phosphates, and stearates. Practical experience shows that high-speed quenching oil exhibits significantly faster cooling rates than conventional oil in the unstable austenite zone, while maintaining similar cooling speeds in the low-temperature martensite transformation zone. This combination provides excellent hardenability and penetration depth while greatly reducing distortion, making it ideal for complex-shaped alloy steel components.

Bright quenching oil maintains a bright surface finish after quenching. By adding various high-molecular-weight additives to mineral oil, bright quenching oils with different cooling rates can be produced. The key component of these additives is a brightener, which suspends oil-insoluble aging products to prevent their accumulation and deposition on the workpiece. Additionally, bright quenching oil formulations often contain antioxidants, surfactants, and accelerators.

Vacuum quenching oil is specifically designed for use in vacuum heat treatment processes. Vacuum quenching oil must have low saturated vapor pressure, high and stable cooling capacity, as well as good brightness and thermal stability; otherwise, it will affect the effectiveness of vacuum heat treatment.

Salt bath and alkaline bath quenching media are generally used in step quenching and isothermal quenching.

● New types of quenchants include polyvinyl alcohol (PVA) aqueous solutions and trinitrate aqueous solutions.

A PVA solution typically has a concentration between 0.1% and 0.3% by mass, with cooling capacity intermediate between water and oil. When a workpiece is quenched into this solution, a vapor film and a gelatinous film form on its surface. These two films facilitate cooling of the heated workpiece. Once boiling begins, the films break down, accelerating cooling. At lower temperatures, the PVA gel film reforms, slowing the cooling rate again. Therefore, this solution exhibits low cooling ability at both high and low temperature ranges but high cooling capacity in the medium range, providing excellent cooling characteristics.

The trinitrate aqueous solution consists of 25% sodium nitrate, 20% sodium nitrite, 20% potassium nitrate, and 35% water. At high temperatures (650–500°C), salt crystals precipitate, disrupting vapor film formation, resulting in cooling performance similar to water. At low temperatures (300–200°C), due to high concentration and poor fluidity, cooling performance approaches that of oil. Thus, it can replace the traditional water-oil dual-medium quenching method.

Quenching methods are widely classified in industrial practice according to different cooling modes, including single-liquid quenching, double-liquid quenching, step quenching, and isothermal quenching.

● Single-liquid quenching involves immersing an austenitized workpiece into a single quenching medium and cooling it continuously until room temperature. Common single-liquid quenchants include water, saltwater, alkaline water, oil, and specially formulated quenchants. Generally, carbon steel is quenched in water, while alloy steel is quenched in oil.

Single-liquid quenching is simple to operate and conducive to mechanization and automation. However, its drawback lies in the limitation of cooling rate by the medium's cooling characteristics, which may affect quenching quality. For carbon steel, this method is suitable only for relatively simple-shaped workpieces.

● Double-liquid quenching involves first immersing an austenitized workpiece into a medium with strong cooling capacity, removing it before the workpiece reaches the medium’s temperature, and immediately transferring it into another medium with weaker cooling capacity—such as from water to oil or from water to air. This method reduces deformation and cracking tendency, but requires precise control during operation, limiting its application.

● Martensitic step quenching involves initially immersing an austenitized workpiece into a liquid medium (salt bath or alkaline bath) at a temperature slightly above or below the steel’s martensite start point (Ms), holding it for a suitable period until the internal and external layers of the workpiece reach the medium’s temperature, then removing it for air cooling to obtain a martensitic microstructure. This process is also known as step quenching.

Step quenching effectively reduces phase transformation stress and thermal stress by allowing the workpiece to equilibrate in temperature at the step temperature before air cooling, thereby minimizing quenching distortion and cracking. It is suitable for alloy steels and high-alloy steels requiring minimal deformation, as well as for complex-shaped carbon steel workpieces with small cross-sections.

● Bainite isothermal quenching involves austenitizing the steel, rapidly cooling it to the bainite transformation temperature range (260–400°C), and holding it isothermally to allow austenite to transform into bainite. This process is sometimes referred to simply as isothermal quenching. Typical holding times range from 30 to 60 minutes.

● Composite quenching involves rapidly cooling the workpiece below the Ms temperature to obtain 10%–20% martensite, followed by isothermal holding within the lower bainite temperature range. This cooling method enables larger cross-sectional workpieces to achieve a mixed microstructure of martensite and bainite. The martensite formed during pre-quenching promotes bainite transformation, while during isothermal holding, the martensite undergoes tempering. Composite quenching is applied to alloy tool steels to avoid type I temper brittleness and reduce residual austenite content, thus minimizing deformation and cracking tendencies.

1. What should be considered when using new oil in a full tank? Before adding new oil, the quenching oil tank and cooling system must be thoroughly inspected and cleaned. Any residual water, sludge, or other debris should be completely removed. When replacing old oil in an existing tank system, oil stains on the tank walls above the oil level and on various frames should also be scraped and cleaned off. If original oil residues and sludge mix into the new oil, they may alter the oil's cooling characteristics. Therefore, cleaning should be more thorough than when using a brand-new tank.

After filling a new tank with fresh oil, it should not be immediately used for quenching operations. During production, transportation, and pouring, quenching oil inevitably absorbs a small amount of air. The presence of gas reduces the cooling rate during the high-temperature phase of quenching and should therefore be removed. Gas solubility in oil decreases as temperature increases. Raising the oil temperature reduces viscosity, facilitating bubble rise. Thus, increasing the oil temperature is an effective method to remove gases from new oil.

2. Why should quenching oil be circulated and agitated? Proper circulation prevents localized overheating of the oil, ensuring uniform temperature throughout the tank. Circulation enhances the relative flow velocity between workpieces and the quenching oil, thereby improving cooling efficiency and preventing soft spots on the workpiece surface.

When the oil temperature is too high, dropping a workpiece into the oil can cause a sudden spike in local temperature, posing a fire hazard.

3. How can oil contamination be minimized? Contamination sources for quenching oil include external pollutants and self-generated contaminants.

External pollution: Oxide scale brought in during quenching, water leakage from the cooling system, and other foreign substances.

Self-generated pollution: Oxidation byproducts that remain in the oil without being naturally expelled during use; plus reaction byproducts formed between external contaminants and the quenching oil or its own impurities. Accumulation of both internal and external contaminants gradually alters the oil’s color, clarity, viscosity, flash point, residual carbon content, and acid value. This process represents the deterioration of quenching oil. Among the effects caused by degradation, changes in cooling performance and reduced brightness after quenching have the greatest impact on heat treatment results. Changes in cooling characteristics often affect the hardness, case depth, and deformation of workpieces. Source: Heat Treatment Ecosystem.

Preventing and minimizing external contamination, proper usage and management of quenching oil, and regular cleaning can slow down oil deterioration and extend its service life.

4. What are the criteria for changing quenching oil? Oil replacement depends on specific aging conditions rather than time elapsed since installation. Regular sampling and analysis during use are essential. The degree of oil aging mainly depends on base oil quality, additives, and actual operating conditions.

General criteria for oil replacement: (1) A drop in characteristic temperature exceeding 40°C; (2) Water content exceeding 0.1% (by volume) under normal operation; (3) Viscosity increase exceeding 15%; (4) Significant reduction in brightness.

5. What should be done if water enters the quenching oil? Water contamination affects cooling performance, especially at low temperatures, potentially causing excessive deformation or cracking of workpieces after quenching.

Quenching oil contaminated with water should undergo high-temperature dehydration treatment—heat the oil to 80–130°C, hold at this temperature, and stir for a certain period, depending on the amount of water introduced. After dehydration, cooling performance should be tested. Only when it meets factory specifications can the oil be used normally; otherwise, additives should be added to adjust its properties.

6. How should water-based quenchants be used? Water-based quenchants are diluted with water, with the dilution ratio determined by the type and material of the workpiece. Operating temperature must not exceed 50°C, as higher temperatures severely reduce cooling capacity, resulting in insufficient hardness of the workpiece.