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The difference between quenching, tempering, normalizing and annealing

 In order to make the metal workpiece have the required working performance, the heat treatment process is often indispensable.

The heat treatment process generally includes three processes: heating, heat preservation, and cooling, which are divided into quenching, tempering, normalizing, annealing, etc. due to different processes. Can you tell the difference?

01What is quenching?

The quenching of steel is to heat the steel to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel), hold it for a period of time to make it fully or partially austenitized, and then cool the steel at a rate greater than the critical cooling rate. Fast cooling to below Ms (or isothermal near Ms) is a heat treatment process for martensite (or bainite) transformation. The solution treatment of aluminum alloy, copper alloy, titanium alloy, tempered glass and other materials or the heat treatment process with rapid cooling process is usually called 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 springs, and improve the comprehensive mechanical properties of shaft parts.
  • 2) Improve the material properties or chemical properties of some 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 the reasonable selection of quenching media, there must be a correct quenching method. Commonly used quenching methods include single-liquid quenching, two-liquid quenching, graded quenching, austempering, and partial quenching.

The steel workpiece has 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 The mechanical properties cannot meet the requirements. Therefore, steel workpieces are generally tempered after quenching

02What is tempering?

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

The main purpose of quenching and tempering is:

  • 1) Reduce internal stress and reduce brittleness. Quenched parts have great stress and brittleness. If they are not tempered in time, they will tend to deform or even crack.
  • 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, it can be adjusted by tempering, hardness, strength, plasticity and toughness.
  • 3) Stable workpiece size. The metallographic structure can be stabilized by tempering to ensure that no deformation occurs in the future use process.
  • 4) Improve the cutting performance of certain alloy steels.

The effect of tempering is:

  • ① Improve the stability of the organization, so that the structure of the workpiece no longer changes during 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 service requirements.

The reason why tempering has these effects is that when the temperature rises, the atomic activity increases, and the atoms of iron, carbon and other alloying elements in the steel can diffuse faster to realize the rearrangement and combination of atoms, which makes it unstable The unbalanced organization gradually transformed into a stable balanced organization. The elimination of internal stress is also related to the decrease in metal strength when the temperature rises. When general steel is tempered, its hardness and strength decrease and its plasticity increases. The higher the tempering temperature, the greater the change in these mechanical properties. Some alloy steels with higher content of alloying elements, when tempered in a certain temperature range, will precipitate some fine particles of metal compounds, which will increase the strength and hardness. This phenomenon is called secondary hardening.

Tempering requirements: workpieces with different purposes should be tempered at different temperatures to meet the requirements in use.

  • ① Tools, bearings, carburized and hardened parts, and surface hardened parts are usually tempered at low temperature below 250°C. The hardness changes little after low temperature tempering, the internal stress is reduced, and the toughness is slightly improved.
  • ② The spring is tempered at medium temperature at 350~500℃ to obtain higher elasticity and necessary toughness.
  • ③ Parts made of medium carbon structural steel are usually tempered at high temperature at 500~600℃ to obtain a good match of suitable strength and toughness.

When steel is tempered at around 300°C, it often increases its brittleness. This phenomenon is called the first type of temper brittleness. Generally, it should not be tempered in this temperature range. Certain medium-carbon alloy structural steels are also prone to become brittle if they are slowly cooled to room temperature after high-temperature tempering. This phenomenon is called 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. This kind of brittleness can be eliminated by reheating the second type of tempered brittle steel to the original tempering temperature.

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

  • 1) Low temperature tempering: 150-250°C, M cycles, reduce internal stress and brittleness, improve plastic toughness, and have higher hardness and wear resistance. Used to make measuring tools, cutting tools and rolling bearings, etc.
  • 2) Intermediate temperature tempering: 350-500℃, T cycle, high elasticity, certain plasticity and hardness. Used to make springs, forging dies, etc.
  • 3) High temperature tempering: 500-650℃, S time, with good comprehensive mechanical properties. Used to make gears, crankshafts, etc.

03What is normalizing?

Normalizing is a heat treatment that improves the toughness of steel. After the steel components are heated to 30-50°C above the Ac3 temperature, they are kept for a period of time and then air-cooled. The main feature is that the cooling rate is faster than annealing but lower than quenching. During normalizing, the crystal grains of the steel can be refined in a slightly faster cooling. Not only can satisfactory strength be obtained, but also the toughness (AKV value) can be significantly improved and reduced The tendency of the component to crack. After normalizing some low-alloy hot-rolled steel plates, low-alloy steel forgings and castings, the comprehensive mechanical properties of the materials can be greatly improved, and the cutting performance is also improved.

Normalizing has the following purposes and uses:

  • ① For hypoeutectoid steel, normalizing is used to eliminate the overheated coarse-grained structure and Widmanstatten structure of the cast, forging, and weldment, the band structure in the rolled material; refine the grain; and can be used as a pre-heat treatment before quenching.
  • ② For hypereutectoid steels, normalizing can eliminate the reticulated 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-drawing 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, normalizing can be used to obtain more flake pearlite structure, increase the hardness to HB140-190, avoid the "sticky knife" phenomenon during cutting, and improve the machinability . For medium carbon steel, it is more economical and convenient to use normalizing when both normalizing and annealing are available.
  • ⑤ For ordinary medium carbon structural steels, 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 stable in structure and size.
  • ⑥ High temperature normalizing (150~200℃ above Ac3) can reduce the composition segregation of castings and forgings due to the high diffusion rate at high temperature. The coarse grains after high temperature normalization can be refined by a 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 bainite structure, and then after high temperature tempering, it has good creep resistance when used at 400-550℃.
  • ⑧ In addition to steel parts and steel, normalizing is also widely used in the heat treatment of ductile iron to obtain a pearlite matrix and increase the strength of ductile iron.

Since normalizing is characterized by air cooling, the ambient temperature, stacking method, airflow and workpiece size all affect the organization and performance after normalizing. The normalizing structure can also be used as a classification method for alloy steel. Generally, alloy steels are divided into pearlite steel, bainite steel, martensitic steel and austenitic steel based on the structure obtained by heating the sample with a diameter of 25 mm to 900 ℃ and air cooling.

04What is annealing?

Annealing is a metal heat treatment process in which the metal is slowly heated to a certain temperature, kept for sufficient time, and then cooled at an appropriate speed. 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 annealed heat treatment state. The hardness of steel can be tested by Rockwell hardness tester to test HRB hardness. For thinner steel plates, steel strips and thin-walled steel pipes, the 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 casting, forging, rolling and welding, and prevent workpiece deformation and cracking.
  • 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).

Common annealing processes are:

  • ① Fully annealed. It is used to refine the coarse superheated structure with poor mechanical properties after casting, forging and welding of medium and low carbon steel. Heat the workpiece to 30-50°C above the temperature at which all ferrite is transformed into austenite, hold it for a period of time, and then slowly cool down with the furnace. During the cooling process, the austenite transforms again to make the steel structure finer .
  • ② Spheroidizing annealing. Used to reduce the high hardness of tool steel and bearing steel after forging. The workpiece is heated to 20-40°C above the temperature at which the steel begins to form austenite, and slowly cooled after holding it. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thereby reducing the hardness.
  • ③ Isothermal annealing. It is used to reduce the high hardness of some alloy structural steels with higher nickel and chromium content for cutting. Generally, it is first cooled to the most unstable temperature of austenite at a faster rate, and after holding for a proper time, the austenite is transformed into troostite 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 during cold drawing and cold rolling. The heating temperature is generally 50-150°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 can be softened.
  • ⑤ Graphitization annealing. It is used to make 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°C, keep it for a certain period of time and then cool it appropriately to decompose the cementite to form 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, and keep it for a long time, and then slowly cool down after the diffusion of various elements in the alloy tends to be evenly distributed.
  • ⑦ Stress relief annealing. It is used to eliminate the internal stress of steel castings and welding parts. For iron and steel products, the temperature at which austenite begins to form after heating is 100 to 200°C, and the internal stress can be eliminated by cooling in air after heat preservation.

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