Understanding the Applications of Quenching, Tempering, Normalizing, and Annealing
1. Quenching
1. What is quenching?
Quenching is a heat treatment process used for steel. In this process, the steel is heated to a temperature above the critical temperature Ac3 (for hypereutectoid steel) or Ac1 (for hypereutectoid steel). It is then kept at this temperature for a period of time to fully or partially austenitize the steel, and then quickly cooled to below Ms (or held isothermally near Ms) at a cooling rate higher than the critical cooling rate to transform it into martensite (or bainite). Quenching is also used for solid solution treatment and rapid cooling of materials such as aluminum alloys, copper alloys, titanium alloys, and tempered glass.
2. The purpose of quenching:
1) Improve the mechanical properties of metal products or parts. For example, it enhances the hardness and wear resistance of tools, bearings, etc., increases the elastic limit of springs, improves the overall mechanical properties of shaft parts, etc.
2) To enhance the material or chemical properties of specific types of steel, such as improving the corrosion resistance of stainless steel or increasing the permanent magnetism of magnetic steel, it is important to carefully select the quenching media and use the correct quenching method during the quenching and cooling process. Commonly used quenching methods include single-liquid quenching, double-liquid quenching, graded quenching, isothermal quenching, and local quenching. Each method has its specific applications and benefits.
3. After quenching, steel workpieces exhibit the following characteristics:
- Unstable structures such as martensite, bainite, and residual austenite are present.
- There is high internal stress.
- The mechanical properties do not meet the requirements. Consequently, steel workpieces usually undergo tempering after quenching.
2. Tempering
1. What is tempering?
Tempering is a heat treatment process that involves heating quenched metal materials or parts to a specific temperature, maintaining the temperature for a certain period, and then cooling them in a specific manner. Tempering is performed immediately after quenching and is typically the final step in the heat treatment of the workpiece. The combined process of quenching and tempering is referred to as the final treatment.
2. The main purposes of quenching and tempering are:
- Tempering is essential to reduce internal stress and brittleness in quenched parts. If not tempered in a timely manner, these parts may deform or crack due to the high stress and brittleness caused by quenching.
- Tempering can also be used to adjust the mechanical properties of the workpiece, such as hardness, strength, plasticity, and toughness, to meet different performance requirements.
- Additionally, tempering helps stabilize the size of the workpiece by ensuring that no deformation occurs during subsequent use, as it stabilizes the metallographic structure.
- Tempering can also improve the cutting performance of certain alloy steels.
3. The role of tempering is:
In order to ensure that the workpiece remains stable and undergoes no structural transformation during use, it is important to improve the stability of the structure. This involves eliminating internal stress, which in turn helps stabilize the geometric dimensions and improve the performance of the workpiece. Additionally, tempering can help adjust the mechanical properties of steel to meet specific use requirements.
Tempering has these effects because when the temperature rises, the atomic activity is enhanced, allowing the atoms of iron, carbon, and other alloy elements in steel to diffuse faster. This enables the rearrangement of atoms, transforming the unstable, unbalanced structure into a stable, balanced structure.
When steel is tempered, the hardness and strength decrease while the plasticity increases. The extent of these changes in mechanical properties depends on the tempering temperature, with higher temperatures leading to greater changes. In some alloy steels with a high content of alloying elements, tempering in a certain temperature range can lead to the precipitation of fine metal compounds. This increases strength and hardness, a phenomenon known as secondary hardening.
Tempering requirements: Different machined parts require tempering at different temperatures to meet specific usage requirements. Here are the recommended tempering temperatures for different types of workpieces:
1. Cutting tools, bearings, carburized and quenched parts, and surface quenched parts are usually tempered at low temperatures below 250°C. This process results in minimal change in hardness, reduced internal stress, and a slight improvement in toughness.
2. Springs are tempered at medium temperatures ranging from 350-500°C to achieve higher elasticity and necessary toughness.
3. Parts made of medium-carbon structural steel are typically tempered at high temperatures of 500-600°C to attain an optimal combination of strength and toughness.
When steel is tempered at around 300°C, it can become more brittle, a phenomenon known as the first type of temper brittleness. Generally, tempering should not be done in this temperature range. Some medium-carbon alloy structural steels are also prone to brittleness if they are slowly cooled to room temperature after high-temperature tempering, known as the second type of temper brittleness. Adding molybdenum to steel or cooling in oil or water during tempering can prevent the second type of temper brittleness. Reheating the second type of tempered brittle steel to the original tempering temperature can eliminate this brittleness.
In production, the choice of tempering temperature depends on the performance requirements of the workpiece. Tempering is categorized based on the different heating temperatures into low-temperature tempering, medium-temperature tempering, and high-temperature tempering. The heat treatment process that involves quenching followed by high-temperature tempering is referred to as tempering, resulting in high strength, good plasticity, and toughness.
- Low-temperature tempering: 150-250°C, M tempering. This process reduces internal stress and brittleness, improves plasticity and toughness, and results in higher hardness and wear resistance. It’s typically used to make measuring tools, cutting tools, rolling bearings, etc.
- Medium-temperature tempering: 350-500°C, T tempering. This tempering process results in higher elasticity, certain plasticity, and hardness. It’s commonly used to manufacture springs, forging dies, etc.
- High-temperature tempering: 500-650°C, S tempering. This process results in good comprehensive mechanical properties and is often used to make gears, crankshafts, etc.
3. Normalizing
1. What is normalizing?
The cnc process of normalizing is a heat treatment used to enhance the toughness of steel. The steel component is heated to a temperature between 30 to 50°C above the Ac3 temperature, held at that temperature for a period of time, and then air cooled outside of the furnace. Normalizing involves faster cooling than annealing but slower cooling than quenching. This process results in refined crystal grains in the steel, improving strength, toughness (as indicated by the AKV value), and reducing the component’s tendency to crack. Normalizing can significantly enhance the comprehensive mechanical properties of low-alloy hot-rolled steel plates, low-alloy steel forgings, and castings, as well as improve cutting performance.
2. Normalizing has the following purposes and uses:
1. Hypereutectoid steel: Normalizing is used to eliminate overheated coarse-grained and Widmanstatten structures in castings, forgings, and weldments, as well as banded structures in rolled materials. It refines the grains and can be used as a pre-heat treatment before quenching.
2. Hypereutectoid steel: Normalizing can eliminate network secondary cementite and refine pearlite, improving mechanical properties and facilitating subsequent spheroidizing annealing.
3. Low-carbon, deep-drawn thin steel plates: Normalizing can eliminate free cementite at the grain boundary, improving deep-drawing performance.
4. Low-carbon steel and low-carbon low-alloy steel: Normalizing can obtain finer, flaky pearlite structures, increasing hardness to HB140-190, avoiding the “sticking knife” phenomenon during cutting, and improving machinability. In situations where both normalizing and annealing can be used for medium-carbon steel, normalizing is more economical and convenient.
5. Ordinary medium-carbon structural steel: Normalizing can be used instead of quenching and high-temperature tempering when high mechanical properties are not required, making the process simple and ensuring stable steel structure and size.
6. High-temperature normalizing (150-200°C above Ac3): Reducing component segregation of castings and forgings due to high diffusion rate at high temperatures. Coarse grains can be refined by subsequent second normalizing at a lower temperature.
7. Low- and medium-carbon alloy steels used in steam turbines and boilers: Normalizing is used to obtain a bainite structure, followed by high-temperature tempering for good creep resistance at 400-550°C.
8. In addition to steel parts and steel materials, normalizing is also widely used in heat treatment of ductile iron to obtain a pearlite matrix and improve the strength of ductile iron. The characteristics of normalizing involve air cooling, so the ambient temperature, stacking method, airflow, and workpiece size all have an impact on the structure and performance after normalizing. The normalizing structure can also be used as a classification method for alloy steel. Typically, alloy steel is categorized into pearlite steel, bainite steel, martensite steel, and austenite steel, depending on the structure obtained by air cooling after heating a sample with a diameter of 25 mm to 900°C.
4. Annealing
1. What is annealing?
Annealing is a heat treatment process for metal. It involves slowly heating the metal to a specific temperature, maintaining it at that temperature for a certain duration, and then cooling it at an appropriate rate. Annealing can be categorized into complete annealing, incomplete annealing, and stress relief annealing. The mechanical properties of annealed materials can be assessed through tensile tests or hardness tests. Many steels are supplied in the annealed state. Steel hardness can be evaluated using a Rockwell hardness tester, which measures HRB hardness. For thinner steel plates, steel strips, and thin-walled steel pipes, a surface Rockwell hardness tester can be used to measure HRT hardness.
2. The purpose of annealing is:
- Improve or eliminate various structural defects and residual stresses caused by steel in the casting, forging, rolling, and welding processes to prevent deformation and cracking of die casting parts.
- Soften the workpiece for cutting.
- Refine the grains and improve the structure to enhance the mechanical properties of the workpiece.
- Prepare the structure for the final heat treatment (quenching and tempering).
3. Common annealing processes are:
① Complete annealing.
To improve the mechanical properties of medium and low carbon steel after casting, forging, and welding, it is necessary to refine the coarse overheated structure. The process involves heating the workpiece to a temperature 30-50℃ above the point at which all ferrite is transformed into austenite, maintaining this temperature for a period of time, and then gradually cooling the workpiece in a furnace. As the workpiece cools, the austenite will transform once again, resulting in a finer steel structure.
② Spheroidizing annealing.
To reduce the high hardness of tool steel and bearing steel after forging, you need to heat the workpiece to a temperature that is 20-40℃ above the point at which steel starts to form austenite, keep it warm, and then cool it slowly. As the workpiece cools, the lamellar cementite in the pearlite turns into a spherical shape, which reduces the hardness of the steel.
③ Isothermal annealing.
This process is used to reduce the high hardness of certain alloy structural steels with high nickel and chromium content for cutting processing. Typically, the steel is rapidly cooled to the most unstable temperature of austenite and then held at a warm temperature for a specific period of time. This causes the austenite to transform into troostite or sorbite, resulting in a reduction of hardness.
④ Recrystallization annealing.
The process is used to reduce the hardening of metal wires and thin plates that occurs during cold drawing and cold rolling. The metal is heated to a temperature that is generally 50-150℃ below the point at which steel begins to form austenite. This allows the elimination of work-hardening effects and softens the metal.
⑤ Graphitization annealing.
In order to transform cast iron with a high cementite content into forgeable cast iron with good plasticity, the process involves heating the casting to around 950°C, maintaining this temperature for a specific period, and then cooling it appropriately to break down the cementite and generate flocculent graphite.
⑥ Diffusion annealing.
The process is used to even out the chemical composition of alloy castings and enhance their performance. The method involves heating the casting to the highest possible temperature without melting, maintaining this temperature for an extended period, and then slowly cooling it. This allows the various elements in the alloy to diffuse and become uniformly distributed.
⑦ Stress relief annealing.
This process is used to reduce the internal stress in steel castings and welded parts. For steel products that start forming austenite after heating at a temperature 100-200℃ below, they should be kept warm and then cooled in the air in order to eliminate the internal stress.
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