What are the types of heat treatment quenching process?

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The types of quenching are divided according to the different cooling forms, mainly including single-liquid quenching, double-liquid quenching, graded quenching and isothermal quenching.

  The types of quenching are divided according to the different cooling forms, mainly including single-liquid quenching, double-liquid quenching, graded quenching and isothermal quenching.
  (1) Liquid quenching
  Single-liquid quenching is a quenching operation method in which austenitic workpieces are quickly immersed in a certain quenching medium and cooled to room temperature. The basis for selecting the cooling medium for single-liquid quenching is: the cooling rate of the workpiece in the medium must be greater than the critical cooling rate of the steel type of the workpiece, and the workpiece should not be quenched and cracked. Single-liquid quenching media include water, brine, alkaline water, oil, and some specially formulated water-based quenching agents.
  (2) Double-liquid quenching
  In order to overcome the shortcomings of single-liquid quenching and make the quenching and cooling of the workpiece as close as possible to the ideal situation, two media with different cooling capabilities can be used together, that is, the heated workpiece is first quenched into the first cooling In the medium with large cooling capacity, wait until it is cooled to slightly higher than Ms temperature (about 300 ° C), and then immediately transfer to the second medium with smaller cooling capacity to cool to room temperature. This quenching and cooling method is called double liquid quenching.
  For some workpieces, in order to further slow down the cooling rate below Ms, water quenching air cooling or oil quenching air cooling can also be used, and air can also be treated as a cooling medium.
  (3) Staged quenching (martensitic staged quenching)
  This cooling method is characterized by first immersing the workpiece in a bath with a temperature slightly higher than Ms, keeping it warm in the bath until the surface and center of the workpiece are cooled to the temperature of the bath, and then taking it out for air cooling . The bath temperature is generally Ms+(10~20)℃. The composition of the medium in the bath adopts nitrate salt bath, alkali bath and neutral salt bath.
  (4) Pre-cooling
  and quenching After quenching and heating, the workpiece is not immediately immersed in the cooling medium, but is cooled in the air for a short time, and then immersed in the cooling medium when the workpiece drops to a certain temperature. This quenching method is called Pre-cooling quenching or delayed quenching.
  The key to pre-cooling and quenching is to control the pre-cooling time. If the pre-cooling time is short, it will be poor; if the time is long, the quenching hardness of the workpiece may be reduced (non-martensitic transformation occurs). Since the material of the workpiece is different, the shape and size vary widely, and it is also affected by the furnace temperature and ambient temperature, it is difficult to accurately calculate the pre-cooling time, which mainly depends on the operator's technology and experience.
  (5) Partial quenching
  Some workpieces only have a high hardness in a certain part, and other parts have no hardness requirements or require lower hardness. In this case, the local quenching method can generally be used, that is, the method of quenching only a certain part of the workpiece. There are two main forms of local quenching, local heating and local cooling and overall heating and local cooling. The former is mainly suitable for workpieces heated by salt bath furnaces, while the latter can be used in box furnaces and salt bath furnaces.
  (6) Cold treatment
  Cold treatment is a post-quenching operation that continues to cool the quenched steel to a temperature below room temperature, so that the retained austenite that has not been transformed at room temperature continues to transform into martensite.