Quenching of transmission gear

Quenching is to heat the alloy to the solution solution curve, keep it warm for a period of time, and then rapidly cool it at a speed greater than the critical cooling rate, so as to obtain the heat treatment method of supersaturated solid solution.

The purpose of quenching is to strengthen steel parts and give full play to the potential of steel properties. For example, to improve the mechanical properties of steel parts, such as hardness, wear resistance, elastic limit, fatigue strength, etc., and to improve the physical or chemical properties of some special steel, such as to enhance the ferromagnetism of magnetic steel and improve the corrosion resistance of stainless steel.

After quenching, the properties of different alloys are also different: for aluminum alloy and most non-ferrous alloy, after quenching, different alloys also have different ① σ increase, δ or ψ decrease ② σ decrease, δ or ψ increase ③ σ increase, δ or ψ increase ④ σ, δ, ψ increase ④ σ, ψ basically unchanged after quenching. The reason for the difference is that solution strengthening and the second phase (or excess phase , which is caused by the difference between strengthening and other phases of solid solution equilibrium. If the strengthening effect of the second phase before quenching is less than that of the solution strengthening after quenching, the strength of the alloy increases after quenching; otherwise, if the strengthening effect of the second phase before quenching is more than that of the solution strengthening after quenching, the strength of the alloy decreases after quenching. Quenching of steel: a heat treatment process in which the steel is heated to a certain temperature above the transformation point of AC3 or AC1, held for a period of time, and then cooled at a speed greater than VK to obtain martensite or lower bainite structure. Quenching is to improve the strength and hardness of materials, that is, the ability of materials to resist bending, distortion and scratch. After quenching, there is a very high internal stress in materials, which is easy to reduce the fatigue strength of materials (that is, the ability of materials to resist medium and high frequency slight vibration to produce cracks). Tempering can effectively remove the internal stress of materials, which can be divided into Quenching + high temperature tempering (i.e. quenching and tempering), quenching + medium temperature tempering and quenching + low temperature tempering. The purpose of quenching is to form martensitic structure inside the material, and the crystallization temperature of different materials is also different. It is a very professional discipline to pay attention to the temperature of this material and the heat preservation time of a certain temperature section, so the quenching medium and how many materials are selected There are quite high requirements on how much quenchant solution is used.

Quenching has the following advantages:

  1. The heat source is on the surface of the workpiece, with fast heating speed and high thermal efficiency
  2. Because the workpiece is not heated as a whole, the deformation is small
  3. Short heating time and less surface oxidation and decarburization
  4. The surface hardness of workpiece is high, notch sensitivity is small, impact toughness, fatigue strength and wear resistance are greatly improved. It is beneficial to develop the potential of materials, save material consumption and improve the service life of parts
  5. Compact equipment, convenient use and good working conditions
  6. Convenient for mechanization and automation

Generally, the heating temperature of steel can be selected according to Fe-Fe3C phase diagram. 20CrMnTi steel is hypoeutectoid steel, and the heating temperature of quenching is 30 ℃ ~ 50 ℃ above AC3. According to the distribution of carbon content in the surface layer of carburized gear and practical experience, oil cooling from 930 ℃ to 860 ℃ can achieve good results. The quenching cooling speed is too fast, the transformation from austenite to martensite is violent, the volume shrinkage causes great internal stress, which is easy to cause gear deformation and cracking. As 20CrMnTi is an alloy steel with good hardenability, oil cooling is selected to reduce the cooling speed to prevent gear deformation or cracking caused by quenching. At the same time, the martensite structure can be obtained and the hardness can be high.

The determination of holding time includes two periods: heating time and holding time. Heating time includes transformation recrystallization time. In fact, holding time only considers the time needed for carbide dissolution and austenite composition homogenization.