The sudden cooling of heated metal by immersion in oil, water, or some other liquid medium (e.g., glycol or liquid nitrogen), a molten salt, or by spraying with a jet of water or compressed air. The purpose of quenching is to produce desired weld strength properties in hardenable steel.

Ferrous alloys (e.g., especially, plain carbon, high-strength low-alloy, and tool steels) which can undergo transformation hardening, or non-ferrous alloys which can be precipitation hardened, are generally quenched to either produce or retain a particular microstructure.

In non-ferrous alloys (for example age-hardenable aluminum alloys with copper, magnesium-silicon, lithium, or other additions) quenching is usually applied after the alloy is rendered single-phase by heating, i.e., is solution-treated or solutionized, in order to retain that single phase in a supersaturated state relative to a key solute element. Heating under controlled temperature-time cycles allows a second-phase to precipitate and induce hardening in what is called aging.

The rate of cooling through the critical range determines the form in which the steel will be retained. In annealing, the heated steel may be furnace-cooled to about 595°C (1100″F), then it may be air cooled to room temperature. Slow cooling to 595°C (1000°F), which is below the critical range, provides sufficient time for complete transition from austenite to pearlite, which is the stabilized condition of steel at atmospheric temperature. In normalizing, the heated steel is removed from the furnace and allowed to cool slowly in the air. Such cooling is more rapid than in annealing and complete transition to pearlite is not obtained. In this instance, air cooling is a mild form of quenching.

To harden steels it is necessary to use a more rapid quenching medium. The three common mediums used are brine, water, and oil. Brine produces the fastest temperature change; water is next, while oil produces the least drastic change. Although oil does not cool the heated steel through the critical range as rapidly as water or brine, it cools the steel rapidly enough to develop sufficient hardness for practical purposes.

A drastic quench is required for relatively low-carbon steels in order to develop the required hardness. However, this type of quench is likely to cause the steel to warp and crack, and may set up internal stresses. When the structure changes from austenite to martensite, the volume of the steel is increased. If the change is too sudden cracking will occur. Cracking occurs particularly in the lower temperature ranges, when the steel is no longer plastic enough to adjust itself to expansion and contraction.

The shape and thickness of the workpiece influences warping and cracking. Thin flanges on heavy sections are especially susceptible to warping. When tubular parts are quenched they should be immersed with the long axis vertical to reduce warping. Because of the less drastic action of the oil quench, many of these difficulties are avoided, and for this reason oil is preferred over brine or water if sufficient hardness can be obtained.

The quenching medium is normally maintained at about 20°C (70°F), and provision should be incorporated to prevent temperature change of more than +/-10°C (+/- 20°F). This involves a large reservoir of liquid and a method of providing circulation and cooling. It is important to note that the rate of cooling throughout the critical range is governed by the temperature maintained in the quenching medium. Since a slight variation in the temperature of the quenching medium will have an appreciable effect on the rate of cooling, the quenching medium temperature must be held within narrow limits to obtain consistent results.  After steel is reheated and prepared for tempering, it is quenched in either air or oil. Chrome-nickel steels, because of their tendency toward temper brittleness, should always be quenched in oil.

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