Heat Treatment with 4 Fires: Quenching, Tempering, Normalizing, Annealing

Heat treatment with 4 fires: quenching, tempering, normalizing, annealing

 Quenching

1. What is quenching?

Quenching of steel is a heat treatment process in which the steel is heated to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel), held for a period of time to fully or partially austenitize, and then rapidly cooled to below Ms (or isothermal near Ms) at a cooling rate greater than the critical cooling rate for martensitic (or bainitic) transformation. The solid solution treatment or heat treatment process with rapid cooling process for materials such as aluminum alloy, copper alloy, titanium alloy, and tempered glass is usually referred to as quenching.

2. Purpose of quenching:

1) Improve the mechanical properties of metal products or parts. For example, improving the hardness and wear resistance of tools, bearings, etc., increasing the elastic limit of springs, and improving the comprehensive mechanical performance of shaft parts.

2) Improve the material or chemical properties of certain special steels. Such as improving the corrosion resistance of stainless steel and increasing the permanent magnetism of magnetic steel.

When quenching and cooling, in addition to selecting the appropriate quenching medium, it is also necessary to have the correct quenching method. Common quenching methods include single liquid quenching, double liquid quenching, graded quenching, isothermal quenching, localized quenching, etc.

3. Steel workpieces have the following characteristics after quenching:

① Obtained unbalanced (i.e. unstable) structures such as martensite, bainite, and residual austenite.

② There is significant internal stress present.

③ The mechanical properties cannot meet the requirements. Therefore, steel workpieces generally undergo tempering after quenching

Tempering

1.What is tempering?

Tempering is a heat treatment process that heats quenched metal products or parts to a certain temperature, holds them for a certain period of time, and then cools them in a certain way. Tempering is an operation that is carried out immediately after quenching, and is usually the last step of heat treatment for workpieces. Therefore, the combined process of quenching and tempering is called final treatment.

The main purposes of quenching and tempering are:

1) Reducing internal stress and reducing brittleness, quenched parts have significant stress and brittleness, and failure to temper in a timely manner often leads to deformation and even cracking.

2) Adjust the mechanical properties of the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the different performance requirements of various workpieces, tempering can be used to adjust the hardness, strength, plasticity, and toughness.

3) Stable workpiece size. Tempering can stabilize the metallographic structure to ensure that deformation does not occur during future use.

4) Improve the cutting performance of certain alloy steels.

2. The function of tempering is to:

① Improve organizational stability to prevent structural changes in the workpiece during use, thereby maintaining stable geometric dimensions and performance of the workpiece.

② Eliminate internal stress to improve the performance of the workpiece and stabilize its geometric dimensions.

③ Adjust the mechanical properties of steel to meet usage requirements.

The reason why tempering has these effects is that as the temperature increases, the atomic activity ability is enhanced, and the atoms of iron, carbon, and other alloy elements in steel can diffuse quickly, achieving the rearrangement and combination of atoms, thereby gradually transforming the unstable unbalanced structure into a stable equilibrium structure. The elimination of internal stress is also related to the decrease in metal strength as the temperature increases. When steel is tempered, its hardness and strength decrease while its plasticity increases. The higher the tempering temperature, the greater the change in these mechanical properties. Some alloy steels with high content of alloying elements will precipitate some fine metal compounds during tempering within a certain temperature range, resulting in an increase in strength and hardness. This phenomenon is called secondary hardening.

Tempering requirements: Workpieces with different uses should be tempered at different temperatures to meet the requirements during use.

① Cutting tools, bearings, carburized and quenched parts, and surface quenched parts are usually subjected to low-temperature tempering below 250 ℃. After low-temperature tempering, the hardness does not change much, the internal stress decreases, and the toughness slightly improves.

② Springs can achieve high elasticity and necessary toughness by tempering at a medium temperature of 350-500 ℃.

③ Parts made of medium carbon structural steel are usually subjected to high-temperature tempering at 500-600 ℃ to obtain a suitable combination of strength and toughness.

When steel is tempered at around 300 ℃, its brittleness often increases, and this phenomenon is called the first type of tempering brittleness. Generally, tempering should not be carried out within this temperature range. Some medium carbon alloy structural steels are also prone to embrittlement when slowly cooled to room temperature after high-temperature tempering. This phenomenon is called the second type of tempering brittleness. Adding molybdenum to steel or cooling it in oil or water during tempering can prevent the second type of tempering brittleness. Reheating the second type of tempered brittle steel to the original tempering temperature can eliminate this brittleness.

In production, it is often based on the requirements for the performance of the workpiece. According to different heating temperatures, tempering can be divided into low-temperature tempering, medium temperature tempering, and high-temperature tempering. The heat treatment process combining quenching and subsequent high-temperature tempering is called quenching and tempering, which has both high strength and good plasticity and toughness.

1) Low temperature tempering: 150-250 ℃, M cycle, reducing internal stress and brittleness, improving plasticity and toughness, with high hardness and wear resistance. Used for making measuring tools, cutting tools, and rolling bearings, etc.

2) Medium temperature tempering: 350-500 ℃, T-cycle, with high elasticity, certain plasticity and hardness. Used for making springs, forging dies, etc.

3) High temperature tempering: 500-650 ℃, S-cycle, with good comprehensive mechanical properties. Used for making gears, crankshafts, etc.

Normalization

1.What is normalization?

Normalization is a heat treatment that improves the toughness of steel. After heating the steel components to a temperature above Ac3 of 30-50 ℃, keep them warm for a period of time and cool them out of the furnace. The main characteristic is that the cooling rate is faster than annealing and lower than quenching. During normalizing, the crystal grains of the steel can be refined in slightly faster cooling, which not only achieves satisfactory strength, but also significantly improves toughness (AKV value) and reduces the tendency of components to crack. After normalizing treatment, the comprehensive mechanical properties of some low alloy hot-rolled steel plates, low alloy steel forgings, and castings can be greatly improved, and the cutting performance can also be improved.

2. Normalization has the following purposes and uses:

① For hypoeutectoid steel, normalizing is used to eliminate the overheated coarse grain structure and Weinstein structure in castings, forgings, and welded parts, as well as the banded structure in rolled materials; Refine grain size; And it can be used as a pre heat treatment before quenching.

② For eutectoid steel, normalizing can eliminate the secondary cementite network and refine the pearlite, which not only improves mechanical properties but also facilitates subsequent spheroidization annealing.

③ For low-carbon deep drawing thin steel plates, normalizing can eliminate free carbides at grain boundaries to improve their deep drawing performance.

④ For low-carbon steel and low-carbon low alloy steel, normalizing can obtain more fine-grained pearlite structure, increase the hardness to HB140-190, avoid tool sticking during cutting, and improve cutting performance. For medium carbon steel, normalizing is more economical and convenient in situations where both normalizing and annealing can be used.

⑤ For ordinary medium carbon structural steel, in situations where mechanical performance requirements are not high, normalizing can be used instead of quenching and high-temperature tempering. This not only simplifies the operation, but also stabilizes the structure and size of the steel.

⑥ High temperature normalization (150-200 ℃ above Ac3) can reduce component segregation in castings and forgings due to the higher diffusion rate at high temperatures. The coarse grains after high-temperature normalization can be refined by a subsequent second lower temperature normalization.

⑦ For some low and medium carbon alloy steels used in steam turbines and boilers, normalizing is often used to obtain bainitic structure, which is then high-temperature tempered to have good creep resistance when used at 400-550 ℃.

⑧ In addition to steel parts and steel, normalizing is also widely used for heat treatment of ductile iron to obtain a pearlite matrix and improve the strength of ductile iron.

Due to the characteristic of air cooling in normalizing, environmental temperature, stacking method, airflow, and workpiece size all have an impact on the microstructure and properties after normalizing. Normalized microstructure can also be used as a classification method for alloy steels. Alloy steel is usually divided into pearlite steel, bainitic steel, martensitic steel, and austenitic steel based on the microstructure obtained by heating a sample with a diameter of 25 millimeters to 900 ℃ and air cooling.

Annealing

1. What is annealing?

Annealing is a metal heat treatment process that slowly heats a metal to a certain temperature, maintains it for sufficient time, and then cools it at an appropriate rate. Annealing heat treatment can be divided into complete annealing, incomplete annealing, and stress relief annealing. The mechanical properties of annealed materials can be tested using tensile tests or hardness tests. Many steels are supplied in a annealed heat treated state. Rockwell hardness tester can be used to test HRB hardness for steel hardness testing. For thinner steel plates, steel strips, and thin-walled steel pipes, surface Rockwell hardness tester can be used to test HRT hardness.

The purpose of annealing is to:

① Improve or eliminate various structural defects and residual stresses caused by steel during casting, forging, rolling, and welding processes, and prevent workpiece deformation and cracking.

② Soften the workpiece for cutting processing.

③ Refine the grain size and improve the microstructure to enhance the mechanical properties of the workpiece.

④ Prepare the microstructure for the final heat treatment (quenching, tempering).

2. Common annealing processes include:

① Complete annealing.

Used to refine the coarse overheated structure of medium and low carbon steel with poor mechanical properties after casting, forging, and welding. Heat the workpiece to 30-50 ℃ above the temperature at which all ferrite transforms into austenite, hold it for a period of time, and then slowly cool with the furnace. During the cooling process, austenite undergoes another transformation, which can refine the structure of the steel.

② Spheroidization annealing.

Used to reduce the excessive hardness of tool steel and bearing steel after forging. Heat the workpiece to a temperature 20-40 ℃ above the temperature at which austenite begins to form in the steel. After insulation, cool slowly. During the cooling process, the layered carbides in the pearlite transform into spherical shapes, thereby reducing hardness.

③ Isothermal annealing.

Used to reduce the high hardness of certain alloy structural steels with high nickel and chromium content for cutting processing. Generally, the austenite is cooled at a relatively fast rate to the most unstable temperature of the austenite, and held for an appropriate period of time. The austenite transforms into martensite or sorbite, and the hardness can be reduced.

④ Recrystallization annealing.

Used to eliminate the hardening phenomenon of metal wires and sheets during cold drawing and cold rolling processes (increasing hardness and decreasing plasticity). The heating temperature is generally 50-150 ℃ below the temperature at which steel begins to form austenite. Only in this way can the work hardening effect be eliminated and the metal soften.

⑤ Graphitization annealing.

Used to transform cast iron containing a large amount of carbides into malleable cast iron with good plasticity. The process operation is to heat the casting to around 950 ℃, keep it warm for a certain period of time, and then cool it appropriately to decompose the cementite into flocculent graphite.

⑥ Diffusion annealing.

Used to homogenize the chemical composition of alloy castings and improve their performance. The method is to heat the casting to the highest possible temperature without melting, keep it warm for a long time, and wait for the diffusion of various elements in the alloy to reach a uniform distribution before slowly cooling.

⑦ Stress annealing.

Used to eliminate internal stress in steel castings and welded components. For steel products that begin to form austenite after heating at a temperature below 100-200 ℃, internal stress can be eliminated by cooling them in air after insulation.