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What is quenching, tempering, normalizing, annealing

What is quenching?

The quenching of steel is to heat the steel to a temperature above the critical temperature Ac3 (hypegmatized steel) or Ac1 (hyper-eutectoid steel), hold it for a period of time, make it all or part austenitized, and then cool it at a critical cooling rate. A rapid heat treatment process for martensite (or bainite) transformation below Ms (or isothermal near Ms). The solution treatment of materials such as aluminum alloys, copper alloys, titanium alloys, tempered glass, or the heat treatment process with a rapid cooling process is also referred to as quenching.

The purpose of quenching:

1) Improve the mechanical properties of metal materials or parts. For example, improve the hardness and wear resistance of tools, bearings, etc., improve the elastic limit of the spring, and improve the comprehensive mechanical properties of the shaft parts.

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

When quenching and cooling, in addition to the reasonable selection of quenching medium, there must be a correct quenching method. The commonly used quenching methods mainly include single-liquid quenching, two-liquid quenching, fractional quenching, austempering, and local quenching.
What is quenching, tempering, normalizing, annealing

Steel workpieces have the following characteristics after quenching:

1 Unbalanced (ie, unstable) structures such as martensite, bainite, and retained austenite are obtained.

2 There is a large internal stress.

3 Mechanical properties can not meet the requirements. Therefore, steel workpieces are generally tempered after quenching

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What is tempering?

Tempering is a heat treatment process in which a quenched metal material or part is heated to a certain temperature and then cooled in a certain manner after being kept for a certain period of time. Tempering is an operation performed immediately after quenching, and is usually the last heat treatment of the workpiece. A process, so the combined process of quenching and tempering is called final treatment. The main purposes of quenching and tempering are:

1) Reduce internal stress and reduce brittleness. Quenching parts have great stress and brittleness. If they are not tempered in time, they will often deform or even crack.

2) Adjust the mechanical properties of the workpiece. After the workpiece is quenched, the hardness is high and the brittleness is large. In order to meet the different performance requirements of various workpieces, it can be adjusted by tempering, hardness, strength, plasticity and toughness.

3) Stabilize the workpiece size. By tempering, the metallographic structure can be stabilized to ensure that deformation does not occur during subsequent use.

4) Improve the cutting performance of certain alloy steels.
What is quenching, tempering, normalizing, annealing

The role of tempering is:

1 Improve the stability of the structure, so that the workpiece no longer undergoes tissue transformation during use, so that the workpiece geometry and performance are stable.

2 Eliminate internal stresses to improve the performance of the workpiece and stabilize the geometry of the workpiece.

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

The reason why tempering has these effects is because the atomic activity is enhanced when the temperature is raised, and the atoms of iron, carbon and other alloying elements in steel can be diffused faster, realizing the rearrangement and combination of atoms, thereby making the instability The imbalanced organization has gradually turned into a stable and balanced organization. The elimination of internal stress is also related to the decrease in metal strength at elevated temperatures. In general, when steel is tempered, the hardness and strength are lowered, and the plasticity is improved. The higher the tempering temperature, the greater the change in these mechanical properties. Some alloy steels with a high content of alloying elements will precipitate some fine metal compounds when tempered in a certain temperature range, which will increase the 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 of use.

1 Tools, bearings, carburized and quenched parts, and surface hardened parts are usually tempered at a low temperature below 250 °C. After low temperature tempering, the hardness does not change much, the internal stress decreases, and the toughness is slightly improved.

2 The spring is tempered at 350-500 ° C at medium temperature to obtain high elasticity and necessary toughness.

3 Parts made of medium carbon structural steel are usually tempered at a high temperature of 500 to 600 ° C to obtain a good fit of strength and toughness.

When steel is tempered at around 300 °C, its brittleness is often increased. This phenomenon is called the first type of temper brittleness. Generally should not temper in this temperature range. Some medium carbon alloy structural steels tend to become brittle if they are slowly cooled to room temperature after tempering at high temperatures. This phenomenon is called the second type of temper brittleness. The addition of molybdenum to steel or cooling in oil or water during tempering prevents the second type of temper brittleness. This brittleness can be eliminated by reheating the second type of tempered brittle steel to the original tempering temperature.

In production, often based on the requirements of the performance of the workpiece. According to the heating temperature, the tempering is divided into low temperature tempering, medium temperature tempering, and high temperature tempering. The heat treatment process combined with quenching and subsequent high temperature tempering is called quenching and tempering, that is, it has high strength and good plastic toughness.

1. Low temperature tempering: 150-250 ° C, M back, reduce internal stress and brittleness, improve plastic toughness, and have high hardness and wear resistance. Used in the production of measuring tools, tools and rolling bearings.

2, medium temperature tempering: 350-500 ° C, T back, with high elasticity, a certain degree of plasticity and hardness. Used to make springs, forging dies, etc.

3, high temperature tempering: 500-650 ° C, S back, with good comprehensive mechanical properties. Used to make gears, crankshafts, etc.

What is quenching, tempering, normalizing, annealing
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What is normalizing?

Normalizing is a heat treatment that improves the toughness of steel. After heating the steel member to 30~50 °C above the Ac3 temperature, the steel is cooled for a while. The main feature is that the cooling rate is faster than annealing and less than quenching. During normalizing, the crystal grains of the steel can be refined in a slightly faster cooling, which not only can obtain satisfactory strength, but also can significantly improve the toughness (AKV value) and reduce The tendency of the member to crack. - After low-alloy hot-rolled steel sheets, low-alloy steel forgings and castings are normalized, the comprehensive mechanical properties of the material can be greatly improved, and the cutting performance is also improved.

Normalizing has the following purposes and uses:

1 For the sub-eutectic steel, normalizing is used to eliminate the superheated coarse-grained structure and Wei's structure of the cast, forged and welded parts, the banded structure in the rolled material, the grain refinement, and the pre-heat treatment before quenching.

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

3 For low-carbon deep-drawn thin steel sheets, normalizing can eliminate the free cementite at the grain boundary to improve its deep-drawing performance.

4 For low-carbon steel and low-carbon low-alloy steel, normalized, more fine-grained pearlite structure can be obtained, so that the hardness is increased to HB140-190, avoiding the "sticking knife" phenomenon during cutting and improving the machinability. . For medium carbon steel, it is more economical and convenient to use normalizing fire in the case where both normalizing and annealing can be used.

5 For ordinary medium carbon structural steel, in the case where the mechanical properties are not high, normalizing can be used instead of quenching and high temperature tempering, which is not only easy to operate, but also makes the steel structure and size stable.

6 High-temperature normalizing (Ac-3 or more 150 to 200 °C) Due to the high diffusion rate at high temperatures, the composition segregation of castings and forgings can be reduced. The coarse grains after high temperature normalizing can be refined by a second lower temperature normalizing.

7 For some low and medium carbon alloy steels used in steam turbines and boilers, normalized fire is often used to obtain bainite structure, and then tempered at high temperature for good creep resistance at 400-550 °C.

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

Since normalizing is characterized by air cooling, ambient air temperature, stacking mode, airflow, and workpiece size all have an impact on the organization and performance after normalizing. Normalizing tissue can also be used as a classification method for alloy steel. Usually, after the sample is heated to 900 ° C according to a diameter of 25 mm, the obtained structure is air-cooled into pearlitic steel, bainitic steel, martensitic steel and austenitic steel.
What is quenching, tempering, normalizing, annealing
 

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What is annealing?

Annealing is a metal heat treatment process in which the metal is slowly heated to a certain temperature for a sufficient period of time and then cooled at a suitable rate. Annealing heat treatment is divided into complete annealing, incomplete annealing and stress relief annealing. The mechanical properties of the annealed material can be tested by tensile testing or by hardness testing. Many steels are supplied in an annealed heat treatment state. The hardness test of steel can be tested by Rockwell hardness tester. 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, and prevent deformation and cracking of workpieces.

2 Soften the workpiece for cutting.

3 Refine the grains and improve the structure to improve the mechanical properties of the workpiece.

4 Prepare the organization for the final heat treatment (quenching, tempering).
What is quenching, tempering, normalizing, annealing
Common annealing processes are:

1 Fully annealed. It is used to refine the coarse superheated structure of medium and low carbon steel which has poor mechanical properties after casting, forging and welding. The workpiece is heated to a temperature of 30 to 50 ° C above the temperature at which all of the ferrite is transformed into austenite, and is kept for a period of time, and then slowly cooled with the furnace, and the austenite is again transformed during the cooling process, so that the microstructure of the steel is thinned. .

2 spheroidizing annealing. It is used to reduce the high hardness of tool steel and bearing steel after forging. The workpiece is heated to a temperature of 20 to 40 ° C above the temperature at which the steel begins to form austenite, and is slowly cooled after the heat preservation. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thereby lowering the hardness.

3 isothermal annealing. It is used to reduce the high hardness of certain alloy structural steels with high nickel and chromium content for cutting. Generally, it is cooled to the most unstable temperature of austenite at a relatively rapid rate, and the austenite is transformed into torsite or sorbite at a suitable temperature for a suitable period of time, and the hardness can be lowered.

4 Recrystallization annealing. It is used to eliminate the hardening phenomenon (hardness increase, plasticity drop) of metal wire and sheet during cold drawing and cold rolling. The heating temperature is generally 50 to 150 ° C below the temperature at which the steel begins to form austenite. Only in this way can the work hardening effect be eliminated and the metal softened.

5 Graphitization annealing. It is used to make cast iron containing a large amount of cementite into a malleable cast iron with good plasticity. The process operation is to heat the casting to about 950 ° C, and after proper cooling for a certain period of time, the cementite is decomposed to form a group of flocculent graphite.

6 diffusion annealing. It is 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, and to keep the steel for a long time, and the various elements in the alloy tend to be uniformly distributed and then slowly cooled.

7 Stress relief annealing. Used to eliminate the internal stress of steel castings and welded parts. After the steel product is heated, the austenite is formed at a temperature of 100 to 200 ° C below, and after cooling, it is cooled in the air to eliminate the internal stress.
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