Quenching media during the heat treatment

quenching media is very important to hardening because it is a very effective of hardness of the material
quenching medias:

Water:
water is fairly good quenching medium.it is cheap,readily available, easily stored nontoxic nonflammable smokeless and easy to filer and pump but with water quench the formation of bubbles may cause soft spots in the metal.Agitation is recommended with use of water quench.Still other problems with waterquench inclued its oxidizing nature,its corrosivity and the tendency to excessive distortion and cracking although this bad properties for plain carbon steels.

Brine (salt water) : Brine is a more severe quench medium than water.Unfortunately it tends to accelerate corrosion problems unless completely removed.Sodium or potasium hydroxide can be used when very severe quenching is desiredand one wishes to obtain good hardness in low carbon steels

Oil: When slower cooling rate is desired oil quenches can be employed.The slower cooling throug the ms to mf temperature range leads to a milder temperature gradient and a reduced likelyhood of cracking.Problems associated with quenchants include water contamination,smoke and fire hazards.in addition quench oils tend to be somewhat expensive.

Air: Low alloy steels in light sections and high alloy steels may be succesfully hardened by means of compressed air or still air.The advantages of using air are that distortion is negligible and that the steel can easily be straigtened during cooling precess.One drawback here is that the surface may be oxidized the cooling.

Quenching

the rapid cooling necessary to harden a steel is known as quenchingand the liquid in to which the steel is immersed to promote this rapid cooling is called the quenching bath. heat tratment process consists of heating the steel to a temperature just above its critical point holding the metal at this temperature for suffcient time then quenching it by using a suitable quenching bath.
some quenching methods are;
direct quenching to martensite:
the first quenching method was direct quenching.according to this method the steel is cooled or quenched directly from hardening temperature to the room temperature the cooling medium is usually water sometimes oil and sometimes air.this method has some big disadvantages a most important disadvantage is distortions because of temperature differance in center of material with surface of the material.
martempering:
in this treatment the cooling takes place in two stages fist tool quenched in a molten salt bath,kept at a temperature somewhat above that at which the austenitic structure of the steel starts to transform to martenzite.the steel is held there until temperature equalization is complate than it is allowed to cool freely in air.the advantage of this method is less residual stresses and distortions than direct quenching but it has a big disadvantage this method can not be usable for all kind of materials.after the this process you have to do tempering
austempering:
it is an isothermal heat treatment which produces a bainite structure in some plain carbon steels.it is an alternative procedure to quenching and tempering for increasing the thoughness and ductility of some steels it likes martempering but it has some differences; first material quenched in salt bath to the temperature which is kept above the matersite formation.keeping time has to sufficient for austenine to completely transformed to bainite.the materials which cabaple being martempered also be austempered.because of long time for complate transformation the high alloy steels are not capable for being austempered.austempering usually improves ductilitiy and impact resistance of certain steels over those values obtained by direct quenching and tempering.it also causes decreased distortion of the quenched material.