What is quenching?
Quenching is a heat treatment process that heats steel to above the critical temperature, keeps it warm for a certain period of time, and then cools it at a rate greater than the critical cooling rate to obtain an unbalanced structure dominated by martensite (bainite can also be obtained or single-phase austenite can be maintained as required). Quenching is the most widely used process in steel heat treatment.
There are four basic processes for steel heat treatment: annealing, normalizing, quenching, and tempering. The purpose of quenching is to transform the supercooled austenite into martensite or bainite to obtain martensite or bainite structure, and then temper it at different temperatures to greatly improve the rigidity, hardness, wear resistance, fatigue strength, and toughness of the steel, so as to meet the different use requirements of various mechanical parts and tools. Quenching can also be used to meet the special physical and chemical properties of certain special steels, such as ferromagnetism and corrosion resistance. A metal heat treatment process in which a metal workpiece is heated to a certain appropriate temperature and maintained for a period of time, and then immersed in a quenching medium for rapid cooling. Commonly used quenching media include 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, gauges and parts that require surface wear resistance (such as gears, rollers, carburized parts, etc.). By combining quenching with 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 usage requirements. In addition, quenching can also enable some steels with special properties to obtain certain physical and chemical properties, such as quenching to enhance the ferromagnetism of permanent magnet steel and improve the corrosion resistance of stainless steel. The quenching process is mainly used for steel parts. When commonly used steel is heated above the critical temperature, the original structure at room temperature will be completely or mostly transformed into austenite. Then the steel is immersed in water or oil for rapid cooling, and the austenite is transformed into martensite. Compared with other structures in steel, martensite has the highest hardness. Rapid cooling during quenching will cause internal stress inside the workpiece. When it reaches a certain level, the workpiece will be distorted and even cracked. For this reason, a suitable cooling method must be selected. According to the cooling method, the quenching process is divided into four categories: single-liquid quenching, dual-medium quenching, martensite graded quenching and bainite isothermal quenching.
Quenching method Single-medium quenching The workpiece is cooled in one medium, such as water quenching and oil quenching. The advantages are simple operation, easy mechanization and wide application. The disadvantages are that the quenching stress in water is large, and the workpiece is easy to deform and crack; quenching in oil, the cooling rate is low, the quenching diameter is small, and large workpieces are not easy to quench. The dual-medium quenching workpiece is first cooled to about 300°C in a medium with a stronger cooling capacity, and then cooled in a medium with a weaker cooling capacity, such as: water quenching first and then oil quenching, which can effectively reduce the internal stress of martensite transformation and reduce the tendency of workpiece deformation and cracking. It can be used for quenching workpieces with complex shapes and uneven cross-sections. The disadvantage of dual-liquid quenching is that it is difficult to grasp the moment of dual-liquid conversion. If the conversion is too early, it is easy to quench without hardening, and if the conversion is too late, it is easy to quench and crack. In order to overcome this disadvantage, the graded quenching method was developed. The workpiece is quenched in a low-temperature salt bath or alkali bath furnace. The temperature of the salt bath or alkali bath is near the Ms point. The workpiece stays at this temperature for 2min to 5min, and then taken out for air cooling. This cooling method is called graded quenching. The purpose of graded cooling is to make the temperature inside and outside the workpiece more uniform, and at the same time to carry out martensite transformation, which can greatly reduce quenching stress and prevent deformation and cracking. The grading temperature was previously set slightly higher than the Ms point, and the workpiece entered the martensite zone after the temperature inside and outside the workpiece was uniform. It has been improved to grade at a temperature slightly lower than the Ms point. Practice shows that the effect of grading below the Ms point is better. For example, high-carbon steel molds are graded quenched in an alkali bath at 160°C, which can be hardened and deformed less, so it is widely used. Isothermal quenching The workpiece is quenched in an isothermal salt bath. The salt bath temperature is in the lower part of the bainite zone (slightly higher than Ms). The workpiece stays isothermally for a long time until the bainite transformation is completed, and then taken out for air cooling. Isothermal quenching is used for steel with medium carbon or above, the purpose is to obtain lower bainite to improve strength, hardness, toughness and wear resistance. Isothermal quenching is generally not used for low carbon steel. Surface quenching Surface quenching is a local quenching method in which the surface layer of a steel part is quenched to a certain depth, while the core part remains unquenched. During surface quenching, the surface of the steel part is quickly heated to the quenching temperature, and the heat is immediately cooled before it can penetrate the core of the workpiece, thereby achieving local quenching. Induction quenching Induction heating is to use electromagnetic induction to generate eddy currents in the workpiece to heat the workpiece. Cold quenching is quenching cooling by immersing in a cold water solution with strong cooling capacity as a cooling medium. Local quenching is quenching only the part of the workpiece that needs to be hardened. Air-cooled quenching refers specifically to heating in a vacuum and quenching and cooling in a high-speed circulating negative pressure, normal pressure or high pressure neutral and inert gas. Surface quenching is quenching only the surface layer of the workpiece, including induction quenching, contact resistance heating quenching, flame quenching, laser quenching, electron beam quenching, etc. Air cooling is the quenching and cooling with forced air or compressed air as the cooling medium. Brine quenching is the quenching and cooling with an aqueous solution of salt as the cooling medium. Organic solution quenching is the quenching and cooling with an aqueous solution of organic polymer as the cooling medium. Spray quenching is the quenching and cooling with a jet of liquid as the cooling medium. Spray cooling is the quenching and cooling of the workpiece in a mist of a mixture of water and air. Hot bath cooling is the quenching and cooling of the workpiece in a hot bath such as molten salt, molten alkali, molten metal or high-temperature oil, such as salt bath quenching, lead bath quenching, alkali bath quenching, etc. Double liquid quenching is the quenching and cooling of the workpiece after heating and austenitizing, and immediately transferring to a medium with weak cooling ability for cooling when the structure is about to undergo martensitic transformation. Pressurized quenching is the quenching and cooling of the workpiece after heating and austenitizing under a specific fixture, the purpose of which is to reduce quenching and cooling distortion. Through quenching is the quenching of the workpiece from the surface to the core. Isothermal quenching is a process in which the workpiece is heated and austenitized, then quickly cooled to the bainite transformation temperature range and is isothermally maintained to convert austenite into bainite. Graded quenching is a process in which the workpiece is heated and austenitized, then immersed in an alkali bath or salt bath with a temperature slightly higher or slightly lower than the M1 point for an appropriate period of time, and then taken out and air-cooled after the workpiece reaches the medium temperature to obtain martensite quenching. Sub-temperature quenching is a process in which the hypoeutectoid steel workpiece is austenitized in the Ac1-Ac3 temperature range, then quenched and cooled to obtain martensite and ferrite structures. Direct quenching is a process in which the workpiece is directly quenched and cooled after carbon is infiltrated. After the workpiece is carburized and cooled twice, it is first austenitized at a temperature higher than Ac3 and quenched to refine the core structure, and then austenitized at a temperature slightly higher than Ac3 to refine the infiltration layer structure. Self-cooling quenching is a process in which the heat of the heated area is automatically transferred to the unheated area after the local or surface of the workpiece is quickly heated and austenitized, so that the austenitized area is quickly cooled.
