Steel Quenching Process and Classification Applications

Quenching is a heat treatment process in which the workpiece is heated to a certain critical temperature, then selectively cooled to obtain high-hardness martensite or tough lower bainite, followed by tempering.

Quenching improves the wear resistance and strength of parts, as well as their fatigue life.

 

 

Quenching Principles and Basic Process Flow

 

Steel and other materials undergo phase transformations when heated, and the austenitization phase transformation is the prerequisite for quenching.

Generally, hypoeutectoid steel is quenched to a temperature 20 to 30°C above Ac3, while hypereutectoid steel is quenched to a temperature 20 to 30°C above Ac1.

When quenching, it is crucial to avoid excessive heating. The holding time should be determined based on the workpiece size and furnace model to ensure uniform microstructural transformation.

The key process in quenching is cooling. The ideal cooling curve avoids the pearlite nose region, primarily to prevent the formation of non-martensitic structures. The cooling rate must also be controlled to reduce internal stress and deformation in the part.

Note: Ac1 and Ac3 represent the temperature at which austenite begins to form and the temperature at which hypoeutectoid steel fully austenitizes when heated, respectively. These are critical reference points for determining the annealing process.

 

Main Quenching Process Methods and Key Operational Guidelines

 

1. Single-Liquid Quenching

Single-liquid quenching is the most basic and simplest quenching method. It is primarily used for parts with simple, symmetrical shapes and minimal cross-sectional variation, such as shafts and pins.

For single-liquid quenching, the appropriate coolant must be selected based on the material and workpiece thickness. Low-carbon and low-alloy steels are generally quenched with water or brine, while high-alloy steels and small, complex-shaped parts can be quenched with oil or a polymer solution.

During quenching, ensure that the workpiece is evenly immersed in the medium and that the medium is properly agitated to prevent a vapor film from hindering cooling.

Single-liquid quenching offers high-quality stability and is suitable for large-scale production, but caution is advised regarding the risk of deformation and cracking caused by overly rapid cooling.

 

2. Two-liquid quenching

Two-liquid quenching combines the advantages of rapid and slow cooling. First, a first medium, such as water or salt water, is rapidly passed through the material's most unstable austenite region. Once the material's temperature approaches the martensite transformation temperature, the material is quickly transferred to a second medium, such as oil, to complete the martensite transformation.

Two-liquid quenching is particularly common for medium-to-large workpieces made of high-carbon steel and medium-to-high alloy steels, such as molds and gear blanks.

The key to this operation is precisely controlling the workpiece's dwell time in the first medium. Typically, a quenching time of 1 second per 3-5 mm of effective workpiece thickness is used, though this can also be determined by observing the disappearance of bubbles on the workpiece surface.

The transfer from the first medium to the second medium must be rapid and continuous to avoid a temperature rise that could cause decomposition of the material structure.

Two-liquid quenching significantly reduces thermal and structural stresses, making it an effective method for controlling deformation and preventing cracking.

 

3. Step Quenching

Step quenching requires heating the workpiece to austenitization, then quenching it into a hot bath slightly above the Ms point, such as a nitrate bath or alkaline bath, for holding. Once the temperature is uniform across the workpiece's cross-section, the workpiece is removed and air-cooled.

Step quenching is often used on tools and parts with complex shapes and high precision requirements, such as taps, small gears, and precision shafts.

During operation, bath temperature fluctuations must be strictly controlled, typically within a range of plus or minus ten degrees Celsius. The holding time is calculated based on the largest cross-section, generally estimated at five millimeters per minute.

Step quenching effectively reduces thermal stresses, allowing the martensitic transformation to proceed gradually during air cooling. This results in low microstructure stress and minimal deformation. However, due to the limited cooling capacity of molten salts, it is only suitable for workpieces with an effective thickness of no more than 20 mm.

 

4. Isothermal Quenching

Isothermal quenching involves completely quenching a workpiece heated to the austenitizing temperature into a hot bath at the bainite transformation zone. A prolonged holding period allows the austenite to completely transform to lower bainite, followed by air cooling. Austempering is primarily used for parts requiring high strength, toughness, and dimensional stability, such as bearings and precision molds. The isothermal temperature and duration are determined based on the steel's TTT curve, with temperatures typically between 250 and 400°C.

After austempering, the material becomes bainite, exhibiting excellent strength and toughness, with little tendency to deformation or cracking. However, austempering has a long production cycle, high costs, and is limited to workpieces with smaller cross-sections.

 

Common Problems and Quality Control

 

Quenching often results in defects such as insufficient hardness, soft spots, deformation, cracking, oxidation, and decarburization.

Insufficient hardness is generally caused by heating at too low a temperature or insufficient cooling. Another cause is surface decarburization of the workpiece.

Soft spots are generally caused by contaminated quenching media or the presence of oxide scale on the workpiece surface. Deformation and cracking can be caused by excessive thermal or structural stress.

Therefore, quality control during quenching requires comprehensive process monitoring, especially precise temperature control during the heating phase. During the cooling phase, the temperature, concentration, and circulation rate of the medium must be kept stable. Tempering is necessary immediately after quenching to eliminate residual stresses in the parts.

During mass production, regular spot checks of the metallographic structure and hardness gradient should be performed. Nondestructive testing can be used on critical parts to detect cracks.

 

Material Compatibility and Process Selection

 

Differences in composition between materials result in varying hardenability, martensitic transformation start temperatures, and critical cooling rates. Therefore, a specific quenching process must be selected based on the material's characteristics.

Low-carbon steel and low-alloy structural steel have relatively low hardenability and are generally quenched with water or two-liquid quenching to improve the surface hardness and wear resistance of the parts.

Medium-carbon quenched and tempered steel requires ensuring a thorough hardening of the core, so oil quenching or rapid oil quenching is generally used to avoid deformation and cracking.

High-carbon tool steel is prone to overheating, which increases the likelihood of deformation. Therefore, step quenching or low-temperature alkaline bath quenching can be used.

High-alloy die steels such as H13 have excellent hardenability, so they can be gas quenched or hardened in a high-temperature salt bath to minimize distortion.

Carburized workpieces require particularly high core toughness, so they require relatively low temperatures, either direct quenching or hardening in a salt bath.

Stainless steel and high-speed steel require preheating and salt bath quenching to prevent cracking and carbide precipitation.

Quenching is a complex and critical process, so the appropriate quenching method must be selected based on the material properties, part shape, and operating conditions.

Quenching requires strict control of the heating and cooling processes to achieve the desired microstructure and properties, while ensuring consistent and reliable product quality.