Quenching, tempering, normalizing, annealing, silly and unclear?


Release time:

2022-06-17

The quenching of steel is to heat the steel to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel), keep it warm for a period of time to make it fully or partially austenitized, and then cool it with a cooling rate greater than the critical cooling rate.

  What 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), keep it warm for a period of time to make it fully or partially austenitized, and then cool it with a cooling rate greater than the critical cooling rate. A heat treatment process for rapid and rapid cooling below Ms (or isothermal near Ms) for martensite (or bainite) transformation. Usually, the solid solution treatment of aluminum alloy, copper alloy, titanium alloy, tempered glass and other materials or the heat treatment process with rapid cooling process is called quenching.
  The 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, improving the comprehensive mechanical properties of shaft parts, etc.
  2) Improve the material properties or chemical properties of some special steels. Such as improving the corrosion resistance of stainless steel, increasing the permanent magnetism of magnetic steel, etc.
  When quenching and cooling, in addition to the reasonable selection of quenching medium, correct quenching methods are also required. The commonly used quenching methods mainly include single-liquid quenching, double-liquid quenching, graded quenching, isothermal quenching, and partial quenching.
  Iron and steel workpieces have the following characteristics after quenching:
  ① Unbalanced (that is, unstable) structures such as martensite, bainite, and retained austenite have been obtained.
  ② There is a large internal stress.
  ③ The 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 quenched metal products or parts to a certain temperature, and then cools them in a certain way after holding 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 joint process of quenching and tempering is called final treatment. The main purpose of quenching and tempering is:
  1) To reduce internal stress and reduce brittleness, quenched parts have great stress and brittleness, if not tempered in time, deformation or even cracking will often occur.
  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 will occur during future use.
  4) Improve the cutting performance of some alloy steels.
  The role of tempering is:
  ① Improve the stability of the structure, so that the workpiece will no longer undergo organizational transformation during use, so that the geometric size and performance of the workpiece will remain stable.
  ② Eliminate internal stress in order to improve the performance of the workpiece and stabilize the geometric dimensions of the workpiece.
  ③ Adjust the mechanical properties of steel to meet the requirements of use.
  The reason why tempering has these effects is that when the temperature rises, the activity of atoms increases, and the atoms of iron, carbon and other alloying elements in steel can diffuse quickly to realize the rearrangement of atoms, thus making them unstable. The unbalanced organization gradually transforms into a stable balanced organization. The relief of internal stress is also related to the decrease in metal strength as the temperature increases. Generally, when steel is tempered, the hardness and strength decrease, and the 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-grained 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 in use.
  ① Cutting tools, bearings, carburized and quenched parts, and surface quenched parts are usually tempered at a temperature below 250°C. After low-temperature tempering, the hardness does not change much, the internal stress decreases, and the toughness improves slightly.
  ② The spring is tempered at a medium temperature at 350-500°C to obtain high elasticity and necessary toughness.
  ③ Parts made of medium carbon structural steel are usually tempered at a high temperature of 500-600 ° C to obtain a good combination of strength and toughness.
  When steel is tempered at around 300°C, its brittleness often increases. This phenomenon is called the first type of temper brittleness. Generally, it should not be tempered in this temperature range. Some 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. The addition of molybdenum to the steel, or cooling in oil or water during tempering, can prevent the second type of temper brittleness. This brittleness can be eliminated by reheating the second type of temper brittle steel to the original tempering temperature.
  In production, it is often based on the requirements for the performance of the workpiece. According to different heating temperatures, tempering is 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, that is, it has good plasticity and toughness while having high strength.
  1. Low temperature tempering: 150-250℃, M times, reduce internal stress and brittleness, improve plastic toughness, have higher hardness and wear resistance. Used to make measuring tools, knives and rolling bearings, etc.
  2. Tempering at medium temperature: 350-500°C, T time, with 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.
  What is normalizing?
  Normalizing is a heat treatment to improve the toughness of steel. After the steel member is heated to 30-50°C above the Ac3 temperature, it is kept for a period of time and then air-cooled. The main feature is that the cooling rate is faster than annealing and lower than quenching. During normalizing, the crystal grains of steel can be refined in a slightly faster cooling, not only can obtain satisfactory strength, but also can significantly improve toughness (AKV value), reduce Cracking tendency of components. After normalizing some low-alloy hot-rolled steel plates, low-alloy steel forgings and castings, 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 hypoeutectoid steel, normalizing is used to eliminate overheated coarse-grained structure and Widmanstatten structure in castings, forgings, weldments, banded structure in rolled products; refine grains; and It can be used as pre-heat treatment before quenching.
  ② For hypereutectoid steel, normalizing can eliminate reticular secondary cementite and refine pearlite, which not only improves mechanical properties, but also facilitates subsequent spheroidizing annealing.
  ③ For low-carbon deep-drawing thin steel plates, normalizing can eliminate free cementite at grain boundaries to improve their deep-drawing properties.
  ④ For low-carbon steel and low-carbon low-alloy steel, use normalizing to obtain more fine-flaky pearlite structure, increase the hardness to HB140-190, avoid the phenomenon of "sticking knife" during cutting, and improve machinability . For medium carbon steel, when both normalizing and annealing can be used, it is more economical and convenient to use normalizing.
  ⑤ For ordinary medium-carbon structural steel, normalizing can be used instead of quenching and high-temperature tempering when the mechanical properties are not high, which is not only easy to operate, but also stabilizes the structure and size of the steel.
  ⑥ Normalizing at high temperature (150-200°C above Ac3) can reduce the composition segregation of castings and forgings due to the high diffusion rate at high temperature. Coarse grains after normalizing at high temperature can be refined by subsequent normalizing at a second lower temperature.
  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 tempered at high temperature. It has good creep resistance when used at 400-550 °C.
  ⑧ In addition to steel parts and steel products, 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, the ambient temperature, stacking method, airflow and workpiece size all have an impact on the structure and performance after normalizing. The normalized structure can also be used as a classification method of alloy steel. Generally, alloy steels are divided into pearlite steel, bainite steel, martensitic steel and austenitic steel according to the microstructure obtained by heating a sample with a diameter of 25 mm to 900 °C and air cooling.
  What is annealing?
  Annealing is a metal heat treatment process in which the metal is slowly heated to a certain temperature, kept for a sufficient time, and then cooled 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 detected by tensile test or hardness test. Many steel products are supplied in the state of annealing and heat treatment. Rockwell hardness tester can be used to test the hardness of steel. For thinner steel plates, steel strips and thin-walled steel pipes, surface Rockwell hardness testers 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 deformation and cracking of workpieces.
  ② Soften the workpiece for cutting.
  ③ Refining the grains and improving the structure to improve the mechanical properties of the workpiece.
  ④ Make organizational preparations for final heat treatment (quenching, tempering).
  Commonly used annealing processes are:
  ① Complete annealing. 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 ferrite is completely transformed into austenite, keep it warm for a period of time, and then cool slowly with the furnace. During the cooling process, the austenite will transform again to make the steel structure thinner .
  ② Spheroidizing annealing. It is 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 then slowly cooled after heat preservation. 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 high nickel and chromium content for cutting. Generally, it is first cooled to the most unstable temperature of austenite at a faster rate, and the austenite is transformed into troostite or sorbite for an appropriate time, 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 thin plate in the process of 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 softened.
  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°C, keep it warm for a certain period of time and then cool it properly to decompose the cementite 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 keep it warm for a long time, and then cool slowly after the diffusion of various elements in the alloy tends to be evenly distributed.
  ⑦ Stress relief annealing. Used to eliminate the internal stress of steel castings and weldments. For iron and steel products heated to 100-200°C below the temperature at which austenite begins to form, cooling in air after heat preservation can eliminate internal stress.