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Quenching is a stage in materials processing in which the material is subjected to very rapid cooling. Metals are classically quenched as they are processed, and polymers may also be quenched, depending on the circumstances in which they are being used. The addition of a dip in a quench bath to the stages of materials processing is quite ancient, as many ancient societies learned that rapid cooling of metals could radically alter their performance. They also learned that different media could be used for quenching and cause different results.
When quenching is performed, the material can be rapidly cooled with air, liquid polymers, oil, or water. Sometimes multiple methods are used. For example, metal may be air cooled and then dipped in a water bath to complete the quenching process. To ensure that the material is evenly quenched, the material may need to be agitated. Conversely, agitators can be used to move the quenching medium around. This prevents temperature differentials which can damage the material by weakening it, allowing some contents to precipitate out, or warping one area.
The purpose of this step of processing is to prevent phase changes which happen during slow cooling. When materials cool slowly, prime opportunities for several different phase changes arise, and the material remains in the right temperature range for an extended period of time. During quenching, the material reaches these temperatures, but does not remain in that temperature zone long enough for a phase change to occur. Quenching also prevents alloyed materials from precipitating out and separating, which could weaken or compromise the material.
This process is not without its problems. Quenching can cause warping, cracking, and other issues with the material, even when it is performed properly. Using water as a quench material, for example, can cause the material to warp as it cools. It is important to control the environment in which the quenching is performed to minimize the risk of damage to the material. When it goes right, the material is harder and more durable, making it suitable for a wide range of uses.
The quenching process can be dangerous, as well. The materials to be quenched are extremely hot, and when dipped into a quench bath, they can throw off a great deal of steam, potentially causing burns. It is important to wear protective garments during this phase of materials processing, and to ensure that everyone in the vicinity is aware of the dangers.
@Mor - There are still traditional blacksmiths who use old fashioned water to temper the objects they make. Although tempering is not just cooling the object, it also involves heating it up, sometimes repeatedly, in order to get certain chemical reactions going in the alloy.
It does usually involve cooling the metal quickly in water at some stage though.
The thing I think of when I think of quenching, is old style blacksmiths quenching a piece of iron in a bucket of water.
I think they used to call it tempering the metal. It must have seemed very dramatic when they thrust the sword or whatever it was they had just made into the bucket. A lot of steam would have exploded up.
I guess if it was quite hot and they wanted to cool it very rapidly, they'd need more than one bucket of water, although I wonder if they tried out different temperatures and things in order to get the metal to just the right kind of hardness?
Nowadays it is a very technical process I'd imagine, with temperature controls and all.