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What is quenching, tempering, normalizing and annealing

What is quenching, tempering, normalizing and annealing

The differences of normalizing, tempering, annealing and quenching lie in different processes, different changes of material structure and different results of material properties.

What is quenching?

Quenching of Steel is to heat the steel to the critical temperature above Ac3 (hypoperformances steel) or Ac1 (hyperperformances steel), and keep it warm for a period of time to refine all or part of it, then conduct the heat treatment process of martensite (or baine) transformation with the cooling speed which is higher than the critical cooling speed to be cooled below Ms (or the isothermal around Ms). Generally, solid solution treatment of aluminum alloy, copper alloy, titanium alloy, steel glass and other materials or thermal treatment process with rapid cooling process are also called quenching.
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, improve the comprehensive mechanical properties of shaft parts, etc.
  • 2) improve the material properties or chemical properties of some special steel. Such as improving the corrosion resistance of stainless steel, increasing the permanent magnetism of magnetic steel, etc.
  • When quenching and cooling, in addition to reasonable selection of quenching medium, there must be correct quenching methods. The common quenching methods mainly include single-Liquid Quenching, double-Liquid Quenching, graded quenching and isothermal quenching, local quenching, etc.

The steel workpiece has the following characteristics after quenching:
  • ① unbalanced (I .e. unstable) structures such as martensite, bainitic and retained austenite are obtained.
  • ② there is a large internal stress.
  • ③ mechanical properties cannot meet the requirements. Therefore, steel workpieces generally have to be tempered after quenching.

What is tempering?

Tempering is a heat treatment process that heats the quenched metal materials or parts to a certain temperature and cools them down in a certain way after holding for a certain period of time, tempering is an operation that is followed after quenching. It is usually the last process for thermal treatment of the workpiece. Therefore, the combined process of quenching and tempering is called the final process. The main purpose of quenching and tempering is:
  • 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 size of the workpiece. Tempering can stabilize the metallographic structure to ensure that no deformation will occur in the future use process.
  • 4) improve the cutting performance of some alloy steel.

The role of tempering is:
  • ① improve the stability of the tissue, so that the workpiece will not be changed in the process of use, so that the geometric size and performance of the workpiece remain stable.
  • ② eliminate internal stress in order to improve the performance of the workpiece and stabilize the geometric size of the workpiece.
  • ③ adjust the mechanical properties of steel to meet the use requirements.
The reason why tempering has these effects is that with the temperature rising, the atomic mobility will be enhanced, and the atoms of iron, carbon and other alloying elements in steel can be diffused faster, thus realizing the rearrangement and combination of atoms, so as to gradually transform the unstable and unbalanced organization into a stable and balanced organization. The elimination of internal stress is also related to the decrease of metal strength when the temperature rises. Generally, when steel is tempered, the hardness and strength decrease and the plasticity increases. The higher the tempering temperature, the greater the change of these mechanical properties. For the alloy steel with high alloying elements, when tempering at a certain temperature range, some metallic compounds with fine particles will be precipitated out to 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 in use.
  • ① tools, bearings, carburizing and quenching parts, surface quenching parts are usually tempered at a low temperature below 250℃. After tempering at low temperature, the hardness changes little, the internal stress decreases, and the toughness increases slightly.
  • ② the spring can obtain higher elasticity and necessary toughness by tempering at medium temperature of 350 ~ 500℃.
  • ③ parts made of medium carbon structural steel are usually tempered at 500 ~ 600℃ to obtain a good coordination of appropriate strength and toughness.
When steel is tempered at about 300℃, its brittleness is often increased. This phenomenon is called the first class of tempering brittleness. Generally should not temper in this temperature range. Some medium carbon alloy structural steel after high temperature tempering, if slowly cooled to room temperature, also easy to become brittle. This phenomenon is called the second class of tempering brittleness. Adding molybdenum to steel, or cooling in oil or water when tempering, can prevent the second class of tempering brittleness. Reheating the steel of the second type of tempering brittleness to the original tempering temperature can eliminate this brittleness.
In production, it is often based on the requirements of workpiece performance. According to different heating temperatures, 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 ℃ ,M loop, reduce internal stress and brittleness, improve plastic toughness, and have higher hardness and wear resistance. It is used to make measuring tools, cutting tools and rolling bearings.
  • 2. Medium temperature tempering: 350-500 ℃ ,T back, with high elasticity and certain plasticity and hardness. Used to make springs, forging dies, etc.
  • 3. High temperature tempering: 500-650℃ ,S back, with good comprehensive mechanical properties. Used to make gears, crankshaft, etc.

What is normal fire?

Normalizing is a heat treatment that improves the toughness of steel. After heating the steel members to the temperature of Ac3 above 30~50 ℃, heat the temperature for a period of time to cool the air. The main feature is that the cooling rate is faster than annealing but lower than quenching. When normalizing can refine the crystalline grains of the steel in the faster cooling, not only can obtain satisfactory strength, moreover, the toughness (AKV value) can be obviously improved and the cracking tendency of components can be reduced. The comprehensive mechanical properties of materials can be greatly improved, and the cutting properties can also be improved after normalized treatment for low alloy hot rolled steel plates, low alloy steel forgings and castings.
Normalizing has the following purposes and uses:
  • ① for hypoeutectic steel, normalizing is used to eliminate the over-heated coarse crystal structure and widthhouse structure of casting, forging and welding parts, and the band structure in rolled material; Refine the crystal grain; and can be used as pre-heat treatment before quenching.
  • ② as for hyperdialysis steel, normalizing can eliminate remullated carbonized body and refine pearlite, which not only improves mechanical property, but also is beneficial to later spheroidization annealing.
  • ③ for low carbon deep drawing thin steel plates, normalizing can eliminate free cementite at grain boundary to improve their deep drawing performance.
  • ④ for low carbon steel and low carbon and low alloy steel, using normalizing can get more fine flake pearlite structure, increasing the hardness to HB140-190, and avoiding the “sticking” phenomenon during cutting, improve machinability. For medium carbon steel, it is more economical and convenient to use normalizing in the situation where normalizing and annealing are available.
  • ⑤ for ordinary medium carbon structural steel, under the occasion of low requirement of mechanical property, use normalizing instead of quenching and tempering high temperature, which is not only easy to operate, but also stablize the structure and size of the steel.
  • ⑥ high temperature normalizing (150 ~ 200℃ above Ac3) due to the high diffusion velocity at high temperature, composition segregation of castings and forgings can be reduced. Coarse and large grains after high temperature normalizing can be refined by the following second lower temperature normalizing.
  • ⑦ as for some low-carbon and middle-carbon alloy steels used for steam turbine and boiler, normalizing is usually used to obtain bainitic structure, and then after tempering at high temperature, they have good creep resistance when temperature is between 400℃ and 550℃.
  • ⑧ in addition to steel and steel, normalizing is also widely used in the heat treatment of ductile iron to obtain pearlite matrix and improve the strength of ductile iron.

Because the characteristic of normalizing is air cooling, the environment temperature, stacking mode, air flow and workpiece size have influence on the organization and performance after normalizing. Normalizing structure can also be used as a classification method for alloy steel. Usually, the microstructure obtained by air cooling after heating the sample with a diameter of 25mm is divided into pearlite steel, balanced steel, martensite steel and austenitic steel.

What is annealing?

Annealing is a metal heat treatment process that slowly heats the metal to a certain temperature, maintains enough time, and then cools at an appropriate rate. Annealing heat treatment is divided into complete annealing, incomplete annealing and stress relief annealing. The mechanical properties of annealed materials can be tested by tensile test or hardness test. Many steels are supplied in the state of annealing heat treatment. For steel hardness test, Rockwell hardness meter can be used to test HRB hardness. For thinner steel plate, steel strip and thin steel tube, surface Rockwell hardness meter can be used, HRT hardness was detected.
The purpose of annealing is:
  • ① improve or eliminate all kinds of microstructure defects and residual stress caused by steel in the process of casting, forging, rolling and welding, and prevent deformation and cracking of workpieces.
  • ② soften the workpiece for cutting.
  • ③ refine the grain and improve the structure to improve the mechanical properties of the workpiece.
  • ④ prepare for the final heat treatment (quenching and tempering).
Commonly used annealing processes are:
  • ① complete annealing. Used to refine medium and low carbon steel with poor mechanical properties after casting, forging and welding. The workpiece is heated to a temperature above 30 ~ 50℃ that ferrite will all transform into austenite, kept warm for a period of time, and then cooled slowly with the furnace. During the cooling process, austenite changes again, the structure of the steel can be thinned.
  • ② spheroidization annealing. Used to reduce the hardness of tool steel and bearing steel after forging. The workpiece is heated to a temperature of 20 ~ 40℃ When austenite begins to form in steel, and then cooled slowly after heat preservation. During the cooling process, the lamellar cementite in pearlite becomes spherical, thus reducing the hardness.
  • ③ isothermal annealing. It is used to reduce the high hardness of some alloy structural steel with high content of nickel and chromium to be cut. Generally, it is cooled to the most unstable temperature of austenite at a relatively fast speed first, and when it is kept at a suitable time, austenite is converted into torchian or Sorbite, and the hardness can be reduced.
  • ④ recrystallization annealing. It is used to eliminate the hardening phenomenon (increase in hardness and decrease in plasticity) of metal wire and sheet in the process of cold drawing and cold rolling. The heating temperature is generally the steel began to form austenite temperature below 50~150 ℃, only in this way can eliminate work hardening effect to soften the metal.
  • ⑤ graphitization annealing. It is used to turn cast iron containing a large amount of cementite into malleable cast iron with good plasticity. The process operation is to heat the casting to about 950℃ and cool down after holding for a certain period of time, so that the cementite can decompose into bulk graphite.
  • ⑥ diffusion annealing. It is used to homogenize the chemical composition of alloy castings and improve their performance. The method is, on the premise of no melting, to heat the casting to the highest temperature as much as possible, and keep the heat for a long time, and then after the various elements in the alloy diffusion tend to uniform distribution, slow cooling.
  • ⑦ stress relief annealing. It is used to eliminate internal stress of steel castings and welding parts. For steel products after heating began to form austenite temperature below 100~200 ℃, after the heat insulation in the air cooling, can eliminate internal stress.

Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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