In order to cool a steel along a suitable Cooling Path, quenchants are needed to remove heat quickly. In order of effectiveness, they are brine, water, oil and air.

The quenching velocity of oil is much less than water. Ferrite and troostite are formed even in small sections. Intermediate rates between water and oil can be obtained with water containing 10-30 % Ucon, a substance with an inverse solubility which therefore deposits on the object to slow rate of cooling. To minimise distortion, long cylindrical objects should be quenched vertically, flat sections edgeways and thick sections should enter the bath first. To prevent steam bubbles forming soft spots, a water quenching bath should be agitated.

Quench cracks are liable to occur:
a) due to presence of non-metallic inclusions, cementite masses, etc.;
b) when Austenite is coarse grained due to high quenching temperature;
c) owing to uneven quenching;
d) in pieces of irregular section and when sharp re-entrant angles are present in the design.

Hardening or Quenching Baths:

The purpose of a quenching bath is to remove heat from the steel being hardened at a rate that is faster than the critical cooling rate. Generally speaking, the more rapid the rate of heat extraction above the cooling rate, the higher will be the resulting hardness. To obtain the different rates of cooling required by different classes of work, baths of various kinds are used. These include plain or fresh water, brine, caustic soda solutions, oils of various classes, oil-water emulsions, baths of molten salt or lead for high-speed steels, and air cooling for some high-speed steel tools when a slow rate of cooling is required. To minimize distortion and cracking where such tendencies are present, without sacrificing depth-of-hardness penetration, a quenching medium should be selected that will cool rapidly at the higher temperatures and more slowly at the lower temperatures, that is below 750 degrees F. Oil quenches in general meet this requirement.

Quenching in Water:

Many carbon tool steels are hardened by immersing them in a bath of fresh water, but water is not an ideal quenching medium. Contact between the water and work and the cooling of the hot steel are impaired by the formation of gas bubbles or an insulating vapor film especially in holes, cavities, or pockets. The result is uneven cooling and sometimes excessive strains which may cause the blade to crack; in fact, there is greater danger of cracking in a fresh-water bath than in one containing salt water or brine.

In order to secure more even cooling and reduce danger of cracking, either rock salt (7 or 8 per cent) or caustic soda (3 to 5 per cent) may be added to the bath to eliminate or prevent the formation of a vapor film or gas pockets, thus promoting rapid early cooling. Brine is commonly used and 3/4 pound of rock salt per gallon of water is equivalent to about 8 per cent of salt. Brine is not inherently a more severe or drastic quenching medium than plain water, although it may seem to be because the brine makes better contact with the heated steel and, consequently, cooling is more effective. In still-bath quenching, a slow up-and-down movement of the tool is preferable to a violent swishing around.

In general, the temperature of water-base quenching baths are usually kept around 70 degrees F, but 70F to 140F is a safe range. The temperature of the hardening bath has a great deal to do with the hardness obtained. The higher the temperature of the quenching water, the more nearly does its effect approach that of oil; and if boiling water is used for quenching, it will have an effect even more gentle than that of oil, in fact, it would leave the steel nearly soft.

The bath should be amply large to dissipate the heat rapidly. Irregularly shaped parts should be immersed so that the heaviest or thickest section enters the bath first. After immersion, the part to be hardened should be agitated in the bath; the agitation reduces the tendency of the formation of a vapor coating on certain surfaces, and a more uniform rate of cooling is obtained. The blade should never be dropped to the bottom of the bath until quite cool.

NaOH and NaCl both increase the surface tension of water and water's surface tension drops as temperature rises. The quenching effect of sodium compounds (salt, sodium hydroxide, etc.) may simply be the result of increases in water's surface tension. Also, the viscosity of water drops as temperature rises--by a factor of six from zero to boiling. And then agitation has a sizable effect on a quenchant's effectiveness as seen below.