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The difference between quenching and tempering

I. Introduction to quenching
The quenching of steel is to heat the steel to a temperature above the critical temperature Ac3 (hypegmatized steel) or Ac1 (hyper-eutectoid steel), hold it for a period of time, make it all or part austenitized, and then cool it at a critical cooling rate. A rapid heat treatment process for martensite (or bainite) transformation below Ms (or isothermal near Ms). The solution treatment of materials such as aluminum alloys, copper alloys, titanium alloys, tempered glass, or the heat treatment process with a rapid cooling process is also referred to as quenching.
Second, the introduction of tempering
Reheating the quenched workpiece to a suitable temperature lower than the lower critical temperature Ac1 (the starting temperature of the pearlite to austenite transformation upon heating), and heat-treating the metal in a medium such as air or water or oil for a period of time after being kept for a while. . Or the quenched alloy workpiece is heated to a suitable temperature for a certain period of time and then cooled slowly or rapidly. Generally used to reduce or eliminate internal stress in hardened steel, or reduce its hardness and strength to improve its ductility or toughness. The quenched workpiece should be tempered in time, and the desired mechanical properties can be obtained by the combination of quenching and tempering.

Quenching

The difference between quenching and tempering


3. What are the differences between quenching and tempering?
1. Different process: The quenching of steel is to heat the steel to the temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hyper-eutectoid steel), keep it for a period of time, make it all or part of austenitizing, and then to be larger than The critical cooling rate is rapidly cooled to a temperature below the Ms (or isothermal near Ms) for a martensite (or bainite) transformation heat treatment process.
2. Different material structure changes: After normalizing, the microstructure is: ferrite + pearlite, the eutectoid steel is pearlite, and the hypereutectoid steel is pearlite + secondary cementite, and is discontinuous. . The low temperature tempering is obtained by martensite structure; the structure obtained by medium temperature tempering is tempered torsite; and the high temperature tempering and quenching and tempering treatment is tempered sorbite structure. After annealing, the grains are refined, the structure is adjusted, and the tissue defects are eliminated. Quenching causes martensite or bainite transformation of supercooled austenite to obtain martensite or bainite structure, and finally obtains martensite-based unbalanced structure (also has bainite or retaining order as needed) Austenite).
3, the material properties change results are different: During normalizing, the crystal grain of the steel can be refined in a slightly faster cooling, not only can obtain satisfactory strength, but also can significantly improve the toughness (AKV value) and reduce the cracking tendency of the member. - After low-alloy hot-rolled steel sheets, low-alloy steel forgings and castings are normalized, the overall mechanical properties of the material can be greatly improved, and the cutting performance is also improved. After tempering, it will be quenched and normalized and put back into the medium temperature for immersion (aging) for a period of time, which will promote the precipitation of a part of the carbides, and at the same time, it can eliminate some residual stress caused by rapid cooling, thus improving the toughness of the material. With flexibility. Annealing can reduce hardness, improve machinability, eliminate residual stress, stabilize size, reduce deformation and cracking tendency, refine grains, adjust microstructure, and eliminate tissue defects. Uniform material organization and composition, improve material properties or prepare tissue for future heat treatment. Quenching can greatly improve the rigidity, hardness, wear resistance, fatigue strength and toughness of steel to meet the different requirements of various mechanical parts and tools. It can also meet the special physical and chemical properties such as ferromagnetism and corrosion resistance of some special steels by quenching.
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