Quenching is a heat treatment process. 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 for a period of time, make it all or part of austenitizing, and then cool it at a temperature greater than the critical cooling rate. The heat treatment process of rapid cooling to below Ms (or isothermal near Ms) for martensite (or bainite) transformation. The solution treatment of aluminum alloy, copper alloy, titanium alloy, tempered glass and other materials or the heat treatment process with rapid cooling process is also called quenching. Quenching is the most widely used method in steel heat treatment.
1. The purpose of quenching
Quenching is to transform supercooled austenite into martensite or bainite to obtain martensite or bainite structure, and then tempering at different temperatures to greatly improve the rigidity, hardness, wear resistance, Fatigue strength and toughness, etc., to meet the different requirements of various mechanical parts and tools. It can also meet the special physical and chemical properties of some special steels such as ferromagnetism and corrosion resistance through quenching.
Commonly used quenching media are brine, water, mineral oil, air, etc. Quenching can improve the hardness and wear resistance of metal workpieces, so it is widely used in various tools, molds, measuring tools and parts that require wear-resistant surfaces (such as gears, rolls, carburized parts, etc.). Through the combination of quenching and tempering at different temperatures, the strength, toughness reduction and fatigue strength of the metal can be greatly improved, and the coordination between these properties (comprehensive mechanical properties) can be obtained to meet different application requirements. In addition, quenching can also make some special properties of steel obtain certain physical and chemical properties, such as quenching to enhance the ferromagnetism of permanent magnet steel, and stainless steel to improve its corrosion resistance.
The quenching process is mainly used for steel parts. When the commonly used steel is heated above the critical temperature, all or most of the original structure at room temperature will be transformed into austenite. The austenite is then transformed into martensite by immersing the steel in water or oil for rapid cooling. Compared with other structures in steel, martensite has the highest hardness. The rapid cooling during quenching will generate internal stress inside the workpiece, and when it is too large, the workpiece will be twisted and deformed or even cracked. A suitable cooling method must be selected for this purpose.
According to the cooling method, the quenching process is divided into four types: single-liquid quenching, double-medium quenching, martensitic graded quenching and bainite isothermal quenching.
2. Application of quenching
The quenching process is widely used in the modern machinery manufacturing industry. Important parts in machinery, especially steel parts used in automobiles, airplanes, and rockets, are almost all quenched. In order to meet the various technical requirements of various parts, various quenching processes have been developed. For example, according to the parts to be treated, there are overall, local quenching and surface quenching; according to whether the phase transformation is complete during heating, there are complete quenching and incomplete quenching (for hypoeutectoid steel, this method is also called subcritical quenching); according to cooling The content of the phase transition includes graded quenching, isothermal quenching and underspeed quenching.
In addition, due to the characteristics and limitations of the inferior methods, they are all applied under certain conditions. The most common applications are induction heating surface quenching and flame quenching. Laser beam heating and electron beam heating are rapidly developing high-energy-density heating and quenching methods. They are attracting attention because they have some characteristics that other heating methods do not have.
Surface hardening is widely used for machine parts made of medium carbon quenched and tempered steel or ductile iron. Because the medium carbon quenched and tempered steel is pre-treated (quenched and tempered or normalized) and then surface quenched, it can not only maintain high comprehensive mechanical properties of the core, but also make the surface have high hardness (>HRC 50) and wear resistance. Such as machine tool spindles, gears, diesel engine crankshafts, camshafts, etc. In principle, grey cast iron, ductile iron, malleable cast iron, alloy cast iron, etc., whose matrix is equivalent to pearlitic ferrite based on
medium carbon steel, can be surface quenched. The comprehensive mechanical properties, so the most widely used.
After surface quenching of high carbon steel, although the surface hardness and wear resistance are improved, the plasticity and toughness of the core are low, so the surface quenching of high carbon steel is mainly used for tools, Measuring tools and high chilled rolls.
Since the strengthening effect of low carbon steel surface quenching is not significant, it is rarely used.
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