After quenching, the steel is heated to the austenitizing temperature for a certain period of time and then cooled at a temperature greater than the critical cooling rate to obtain a heat treatment process of non-diffusing transformed structures such as martensite, bainite and austenite. Quenching and quenching, heating the steel to austenitizing temperature for a certain period of time, and then cooling at a temperature greater than the critical cooling rate to obtain a heat treatment process for non-diffusing transformed structures such as martensite, bainite and austenite .
Quenching of steel is to heat the steel to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hyper-eutectoid steel), keep it for a period of time, make all or part of austenite, and then cool it by more than critical The speed of the cold is rapidly cooled to below the Ms (or isothermal near Ms) for a martensite (or bainite) transformation heat treatment process. The solution treatment of materials such as aluminum alloys, copper alloys, titanium alloys, tempered glass, or heat treatment processes with rapid cooling processes is also referred to as quenching.
The purpose of quenching for quenching purposes is to transform the supercooled austenite into martensite or bainite to obtain martensite or bainite structure, and then mix with tempering at different temperatures to greatly increase the strength and hardness of the steel. Wear resistance, fatigue strength and toughness to meet the different requirements of various mechanical parts and tools. Special physical and chemical properties such as ferromagnetism and corrosion resistance of certain special steels can also be met by quenching.
The quenching process heats the metal workpiece to a suitable temperature for a period of time and then immerses it in a rapidly cooled metal heat treatment process in the quenching medium. Commonly used quenching media are brine, water, mineral oil, air, and the like. 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 surface wear resistance (such as gears, rolls, carburized parts, etc.). Through quenching and tempering at different temperatures, the strength, toughness and fatigue strength of the metal can be greatly improved, and the combination of these properties (comprehensive mechanical properties) can be obtained to meet different application requirements. In addition, quenching can also obtain certain physical and chemical properties of some special properties of steel, such as quenching to make permanent magnet steel strengthen its ferromagnetism, stainless steel "target=_blank> stainless steel to improve its corrosion resistance, etc. Quenching process is mainly used for steel parts. When the commonly used steel is heated above the critical temperature, the original or at room temperature will transform all or most of the microstructure into austenite. Then the steel is immersed in water or oil to cool rapidly, and the austenite is transformed into martensite. Compared with other microstructures in steel, martensite hardness is the highest. Rapid cooling during quenching will cause internal stress inside the workpiece. When it is large to some extent, the workpiece will be distorted or even cracked. For this, proper cooling must be selected. According to the cooling method, the quenching process is divided into four types: single liquid quenching, double medium quenching, martensite grading quenching and bainite austempering.
Hardness of the quenched workpiece The hardness of the quenched workpiece affects the quenching effect. The quenched workpiece is generally tested by Rockwell hardness tester for HRC hardness. Hardened thin hard steel plates and surface hardened workpieces can be used to test the hardness of HRA. For quenched steel sheets with a thickness of less than 0.8 mm, shallow surface hardened workpieces and quenched steel rods with a diameter of less than 5 mm, the surface hardness tester may be used to test the HRN hardness.
When welding medium carbon steel and some alloy steels, quenching may occur in the heat-affected zone and become hard, which is easy to form cold cracks, which is to be prevented during the welding process.
Since the metal is hard and brittle after quenching, the residual surface stress will cause cold cracking, and tempering can be used as one of the means to eliminate cold cracks without affecting the hardness.
Quenching is suitable for parts with small thickness and diameter. For oversized parts, the quenching depth is not enough, and carburizing has the same problem. In this case, alloys such as chromium should be added to the steel to increase the strength.
Quenching is one of the basic means of strengthening steel materials. Martensite in steel is the hardest phase in the iron-based solid solution structure (Table 1), so steel parts can be quenched to obtain high hardness and high strength. However, the brittleness of martensite is very large, and after quenching, there is a large quenching internal stress inside the steel piece, so it is not suitable for direct application and must be tempered.
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