Quenching
Quenching - is the process of rapidly cooling the metal from the solution or austenitizing treating temperature, typically from within the range of 815 to 1100 °C (1500 to 2012 °F) for steel. High-alloy and stainless steel may be quenched to minimize grain boundary carbides or to improve the ferrite distribution but most steel including carbon, low-alloy, and tool steel, are quenched to produce controlled amounts of martensite in the microstructure. Successful hardening usually means achieving the required microstructure, hardness, strength, or toughness while minimizing residual stress, distortion, and the possibility of cracking.
The a quench medium is usually a liquid such as water and depends on the harden-abilityof the particular alloy, the section thickness and shape involved, and the cooling rates needed to achieve the desired microstructure. The most common quenchmedia are either liquids or gases.
The liquid quenchcommonly used include:
Oil that may contain a variety of additives
Leceted liquid polymers
Water that may contain salt or caustic additives
Gas type quench medians may also be used, such as inert gases including helium, argon, and nitrogen. These quench gasses are sometimes used after austenitizing in a vacuum.
The ability of a quenchto harden steel depends on the cooling characteristics of the quenching medium validity and quantity. Quenching effectiveness is dependent on the steel chemical composition, type of quench or the quenchuse conditions. The design of the quenching system and the thoroughness with which the system is maintained also contribute to the success of the process.
The reason of the quenching process is to cool steel from the austenitizing temperature quickly enough to form the desired micro-structural phases, sometimes bainite but more often martensite. The basic quenchfunction is to control the rate of heat transfer from the surface of the part being quenched.
Quenching Process
The rate of heat extraction by a quenching medium and the way it is used substantially affects quenchperformance. Variations in quenching practices have resulted in the assignment of specific names to some quenching techniques:
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Direct quenching
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Time quenching
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Selective quenching
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Spray quenching
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Fog quenching
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Interrupted quenching
Quench Equipment:
There are three types of furnaces that are commonly used in quench hardening: salt bath furnace, continuous furnace, and box furnace. Each is used depending on what other processes or types of quench hardening are being done on the different materials.
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