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Quenching refers to the rapid cooling of a machined piece. The aim of quenching is to obtain certain required material characteristics by desired phase transformations. Quenching is dependent upon rapid cooling, which reduces the time available for undesired reactions.
Another application of quenching is in metallurgy when it is used to introduce martensite in steel to harden it. Pre-hardening, cast steels have a uniform pearlitic layered grain structure. This makes them quite soft and unsuitable for most common applications. After quenching is carried out, the pearlitic crystal structure is partially converted to a different microstructure called martensite. This makes it much harder. Thus quenching is extremely useful in making workpieces such as cutting edges that must resist deformation.
The rate of cooling is a crucial factor in quenching since this avoids cracking and warping.
Two techniques are used for liquid quenching, namely, still bath and flush quenching. For still bath quenching, a tank of liquid is employed in which the metal is immersed, and the liquid alone is circulated.
In flush quenching, the whole surface including all irregularities (cavities or recesses) is sprayed using quenching liquid, at different rates to ensure a uniform rate of cooling over the whole object.
Disadvantages of Conventional Liquid Quenchants
Traditional liquid quenching media include water, brine, caustic soda, and mineral oil.
Water is a good quenching agent with some types of steel. However, when it comes to quenching hot tool steel or other steel alloys, the absorbed gases within the water tend to bubble out. These bubbles result in softening of the steel with subsequent cracking or warping.
Brine consists of water in which rock salt is dissolved to reduce gas absorption and thereby prevent bubbling. This improves the surface wetting and cooling rate, promoting uniform rapid cooling. On the other hand, high carbon steels or low alloy steels may be uneven in cross-section, and this may lead to stress or cracking.
Moreover, brine quenching is not suitable for non-ferrous metals because of the potential for corrosion.
Overall, water and brine are used to quench only workpieces with relatively simple shapes, and steels of low hardness, because in other situations they cause warping and cracking.
Oil is a third traditional quenching agent, suitable for high-speed steels and oil-hardened steels, and in fact for any steel for which the required degree of hardness is achievable. Oil has a slower rate of cooling compared to either water or brine, but faster than air, making it an intermediate quench.
However, water may build up at the bottom of the oil tanks, which could affect the quenching process if it was present in large enough amounts that the workpiece extends into it. Moreover, nonferrous metals are usually quenched with other media.
Mineral oils show the best cooling capacity for most alloy steels, but they are more costly, and non-biodegradable. Moreover, oxidation of mineral oils occurs at high temperatures leading to the buildup of toxic polycyclic aromatic hydrocarbons (PAH). The inhalation of oil mists from such processes is associated with potentially serious health effects. The accumulation of such degradation products upon repeated use also affects the quenching performance of the oils significantly.
Oils also have reduced convection properties because of lower heat capacity and higher viscosity compared to water. In many cases the vapor blanket is prolonged, the nucleate boiling stage is short and the cooling rate is significantly less, while the convective stage is long.
Caustic soda in water is a quenchant with a higher cooling rate than water alone and is suitable for some types of steel which require very rapid cooling rates. It is never used to quench nonferrous metals.
The disadvantages associated with most traditional quenchants has made it necessary to look for better media, that can produce the right type of mechanical property by bringing about the required microstructure while avoiding warping and cracking due to quenching.
Polymers used for quenching have a low rate of cooling and are not compatible with some additives and antioxidants in common use. The polymer requires continuous monitoring, and cannot be used for steels which require quenching at high temperatures.
Advantages of Newer Quenchants
Vegetable oils used for cooking are relatively inexpensive, completely biodegradable and non-toxic. They are also derived from renewable sources.
This has led to increasing attention to their use as quenching media.
Tensile strength, impact strength, hardness, percentage elongation and yield strength of the material quenched with such oils (compared with water as a standard) is found to vary with the type of oil used.
With olive oil and palm kernel oil, for instance, the hardness values are lower. On the other hand, the toughness is improved because these oils have better impact energy values. The use of appropriate antioxidants can prevent increases in viscosity following its use in quenching due to the formation of oxidation products at these high temperatures.
Depending on the quenching temperature, different vegetable oils may be selected which perform best at yielding the required mechanical characteristics at a given temperature.
Nanofluids are fluids with suspended nanoparticles, that is, materials with crystallites that have average sizes below 50 nm. Such nanoparticles have unique characteristics, such as:
- Physical properties that vary with size
- Large surface area
- High number density
- Surface structure
Common nanoparticles include oxide ceramics, metal carbides such as silicon carbide, nitrides such as aluminum and silicon nitrides, metals and nonmetals like graphite and carbon nanotubes. These are suspended in fluids like water, ethylene or tri-ethylene glycols, oil, polymer solutions, and bio-fluids.
Advantages of nanoquenchants include:
- Increased thermal conductivity
- Increased stability
- Microchannel cooling without clogging
- Low erosion
- Low pumping power
Nanofluids affect the boiling behavior at the interface, thus influencing the critical heat flux. They change the wetting kinetics as well as heat transfer properties of the conventional fluids. Thus they are expected to be the next generation of heat transfer fluids used for hardening of steels via quenching.
By changing the nanoparticle concentration, size, material, shape, and type of base fluid, the right quenching media can be designed for different alloys of steel. This will change the thermal and physical characteristics of the base fluid. This ability to produce customized quenching media for different severities of quenching is of great potential benefit in industrial heat treatment.
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