Introduction
Quenching is a heat treatment process that involves heating steel to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hyper-eutectoid steel), preserving the heat for a period of time to allow for complete or partial austenitization of the steel, then cooling it off rapidly at a rate above the critical cooling rate, to a temperature below Ms for martensite (or bainite) transformation (or isothermal near Ms). From time to time, quenching also entails solution treatment of materials such as aluminum alloys, copper alloys, titanium alloys, tempered glass; or heat treatment involving rapid cooling processes.
Purpose of quenching
The purpose of quenching is to transform the undercooled austenite into martensite or bainite to obtain martensite or bainite structure; quenching associated with tempering at different temperatures can significantly improve the steel rigidity, hardness, wear resistance, fatigue strength, toughness etc., enabling the steel to meet various requirements of different mechanical parts and tools. Quenching can also give some special steels special physical and chemical properties such as ferromagnetism and corrosion resistance.
Quenching is a metal heat treatment technology in which a metal workpiece is heated to a suitable temperature for a period of time and then immersed in a quenching medium for rapid cooling. Brine, water, mineral oil, air, and the like are the commonly used quenching media. As quenching can improve the hardness and wear resistance of metal workpieces, various tools, molds, measuring tools and parts that require surface wear resistance, such as gears, rolls, carburized parts, etc. will resort to this process. Through quenching and tempering at different temperatures, the strength, toughness and fatigue strength of the metal can be greatly improved. With these combined properties or enhanced comprehensive mechanical properties, the metal can meet different application requirements. In addition, through quenching, some special properties of steel can obtain certain physical and chemical properties, such as strengthened ferromagnetism for permanent magnet steel, and improved corrosion resistance for stainless steel.
The quenching process is mainly used for steel parts. When the commonly used steel is heated above the critical temperature, the original structure at room temperature will undergo transformation that changes all or most of its microstructure into austenite. Then put the steel into the water or oil for rapid cooling and the austenite will be transformed into martensite. Martensite features the highest hardness compared to other steel structures. Keep noted that rapid cooling during quenching can cause internal stresses in the workpiece. If the stress is large enough, the workpiece will be distorted or even cracked. To avoid it, we must choose the right cooling method.
Quenching application
The quenching technology is widely used in the modern machinery manufacturing industry. Important parts in machinery, especially those used in automobiles, airplanes, and rockets, are all almost quenched. In order to meet the technical requirements of various parts, various quenching processes have been developed. For example, in light of the parts to be treated, there are integral quenching, partial quenching and surface quenching. Based on whether the phase transformation is complete under heating, there are complete quenching and incomplete quenching (for hypoeutectic steel, the method is also called subcritical quenching). According to the contents of phase change, there are step quenching, isothermal quenching and slack quenching.
Related
- What is Annealing? Why Steel Needs to be Annealed?
- Introduction of Metal Heat Treatment Process (Ⅱ)
- Introduction of Metal Heat Treatment Process (Ⅰ)
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