Quenching
In materials science, quenching is the rapid cooling of a workpiece to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such asphase transformations, from occurring. It does this by reducing the window of time during which these undesired reactions are both thermodynamically favorable, and kinetically accessible; For instance, quenching can reduce the crystal grain size of both metallic and plastic materials, increasing their hardness.
In metallurgy,it is most commonly used to harden steel by introducing martensite, in which case the steel must be rapidly cooled through its eutectoid point, the temperature at which austenite becomes unstable. In steel alloyed with metals such as nickel and manganese, the eutectoid temperature becomes much lower, but the kinetic barriers to phase transformation remain the same. This allows quenching to start at a lower temperature, making the process much easier. High speed steel also has added tungsten, which serves to raise kinetic barriers and give the illusion that the material has been cooled more rapidly than it really has. Even cooling such alloys slowly in air has most of the desired effects of quenching.
Extremely rapid cooling can prevent the formation of all crystal structure, resulting in amorphous metal or "metallic glass".
If the percentage of carbon is less than 0.4 percent, quenching is not possible.
Quench hardening
Quench hardening is a mechanical process in which steel and cast iron alloys are strengthened and hardened. These metals consist of ferrous metals and alloys. This is done by heating the material to a certain temperature, depending on the material. This produces a harder material by either surface hardening or through-hardening varying on the rate at which the material is cooled. The material is then often tempered to reduce the brittleness that may increase from the quench hardening process. Items that may be quenched include gears, shafts, and wear blocks.
Process
The process of quenching is a progression, beginning with heating the sample. Most materials are heated from anywhere to 815 to 900 °C (1,500 to 1,650 °F), with careful attention paid to keeping temperatures throughout the workpiece uniform. Minimizing uneven heating and overheating is key to imparting desired material properties.
The second step in the quenching process is soaking. Workpieces can be soaked in air (air furnace), a liquid bath, or a vacuum. (give figure for average soak time). The recommended time allotment in salt or lead baths is 0 to 6 minutes. Soaking times can range a little higher within a vacuum and, soak is generally similar to in air. Like in the heating step, it is important that the temperature throughout the sample remains as uniform as possible during soaking. Once the part has finished soaking, it moves on to the cooling step. During this step, the part is submerged into some kind of quenching fluid; different quenching fluids can have a significant effect on the final characteristics of a quenched part. Water is one of the most efficient quenching media where maximum hardness is desired, but there is a small chance that it may cause distortion and tiny cracking. When hardness can be sacrificed, whale oil, cottonseed oil and mineral oils are often used. These oil based fluids often oxidize and form a sludge during quenching, which consequently lowers the efficiency of the process. The quenching velocity (cooling rate) of oil is much less than water. Intermediate rates between water and oil can be obtained with water containing 10-30% UCON from DOW, a substance with an inverse solubility which therefore deposits on the object to slow the rate of cooling.
Quenching can also be accomplished using inert gases, such as nitrogen and noble gasses. commonly, nitrogen is used at pressures greater than atmospheric pressure ranging up to 20 bar absolute. Helium is also used because of its greater thermal capacity than nitrogen. Alternatively argon can be used however its density requires significantly more energy to move, and its thermal capacity is less than the alternatives. To minimize distortion in the part, long cylindrical work pieces are quenched vertically; flat work pieces are quenched on edge; and thick sections should enter the bath first. To prevent steam bubbles the bath is agitated
Purpose
Before hardening, cast steels and iron are of a uniform and lammelar (or layered) Pearlite grain structure. This is a mixture of ferrite and cementite formed when steel or cast iron are manufactured and cooled at a slow rate. Pearlite is not an ideal material for many common applications of steel alloys, as it is quite soft. By heating pearlite past its eutectoid transition temperature of 727 °C and then rapidly cooling, some of the material’s crystal structure can be transformed into a much harder structure known as Martinsite. Steels with this Martinsitic structure are often used in applications when the workpiece must be highly resistant to deformation, such as the cutting edge of blades.
Often, an iron or steel alloy will be excessively hard and brittle due to an overabundance of Martinsite after quenching. In these cases, another heat treatment technique known as tempering is performed on the quenched material in order to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air.
History
Although there is evidence of the use of quenching processes by blacksmiths stretching back into the middle of the Iron Age, little detailed information exists related to the development of these techniques and the procedures employed by early smiths. It is suspected that this lack of written records was due to the tendency of blacksmiths and their guilds to fiercely protect their intellectual property from their rivals.[1]
Research indicates that heat treatment techniques most likely originated with the Hittites, in the Anatolia region of what is now Turkey. One of the earliest known references to quenching was this line from Homer’s Odyssey [1]
“As when a man who works as a blacksmith plunges a screaming great axe blade or adze into cold water, treating it for temper, since this is the way steel is made strong, even so Cyclops' eye sizzled about the beam of the olive…”
By 600 B.C., metalworkers in what is now southern India developed a more refined quenching process as a step in the production of extremely high quality, crucible forged steel known as Wootz. Wootz steel was highly valued as a trade commodity among empires of the time, and by 500 BC, large quantities were being exported to Romans, Egyptians, Chinese and Arabs. For roughly the next 2000 years, steels produced by these Indian metallurgists were regarded as the best in the world. The techniques developed by these indian smiths were studied by European scientists during enlightenment-era trips to the region, and were used as the foundation for much of the metallurgical research undertaken by the British Royal Society during the 19th century. This research, in turn, forms the basis for much of modern day metallurgy.[1]
See also
References
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Look up quenching in Wiktionary, the free dictionary. |
- 1 2 3 Mackenzie, Scott (2009). "The history of quenching: this article describes the early history of quenching from the dawn of civilization to the early industrial age. The focus will be primarily on the contributions of European, Indian, Chinese, and Japanese peoples". Advanced Materials & Processes 164.9.