What is Quenching Process – Common Quenching mediums

Process of Quenching:
In hardening of steels, the rapid cooling rates may be obtained by bringing into contact, the hot surface of the object with some cooler material, which may he gaseous, liquid, or solid. This operation is called quenching and includes methods of cooling by jets of air, water or other liquids- immersion in liquids, such as brine, water, polymer quenchant, salt baths, cooling between plates.

However, the rate of cooling (the rate of heat transfer from a hot metallic body to the quenching medium) depends on sectional dimensions of the object, its temperature, its thermal properties, the condition of its surface as regards the nature of the oxide film and degree of roughness, initial temperature of the coolant, its boiling point, specific heat of coolant, latent heat of vapourisation, the specific heat of its vapour, its thermal conductivity, its viscosity and its velocity past the immersed object.

Before proceeding to consider the cooling characteristics of commonly used coolants, it may be advantageous to study what happens when a heated steel object (say at 840°C) is plunged into a stationary bath of cold water.

Instead of showing a constant cooling rate throughout the quench, the cooling curve shows three stages as:

Stage A – Vapour-Blanket Stage:

Immediately after the start of the quench, the quenching coolant gets vapourised due to metal being at high temperature, and a continuous vapour blanket envelopes the surface of the object.

Now no liquid comes in contact with the metal surface, and heat escapes from the hot surface very slowly by radiation and conduction through the layer of water vapour to liquid-vapour interface. Since vapour films are poor heat conductors, the cooling rate is relatively slow. Tins stage is undesirable in most quenching operations.

Stage B – Intermittent Contact Stage (Liquid Boiling Stage):

Heat is removed very rapidly in this stage as the heat of vapourisation, as indicated by steep slope of the cooling curve. During this stage, the vapour-blanket is broken intermittently allowing the coolant to come in contact with the hot surface at one instant, but soon being pushed away by violent boiling actions of vapour bubbles. The bubbles are carried away by convection currents and the liquid touches the metal again.

The rapid cooling in this stage soon brings the surface below the boiling point of the quenching medium. The vapourisation then, ceases. The second stage corresponds to temperature range of 100°C to 500°C, in which the steel in the austenitic condition transforms most rapidly (≈ nose of the CCT curve). Thus, the rate of cooling in this stage is of great importance in hardening of steels.

Stage C – Direct Contact Stage (Liquid-Cooling Stage):

The stage begins when the temperature of the surface of object decreases to boiling point, or below of the quenching medium. Vapours do not form. The cooling is due to convection and conduction through the liquid. The cooling rate is lowest in this stage.

Some common quenching medium are:

1. Water:

Probably, the oldest and still the most popular quenching medium, water meets the requirements of low cost, general availability, easy handling and safety. The cooling characteristics change more than oil with the rise of temperature specially there is rapid fall in cooling capacity as the temperature rises above 60°C, because of increased vapour blanket stage. The optimum cooling power is when water is between 20-40°C.

The cooling power of water is between brine and oils. Though, water provides high cooling power near the nose of the curve to avoid transformation to pearlite, or bainite but the greatest drawback of water as illustrated in table 6.11, is that the rate of cooling is high in the temperature range of martensitic formation. At that stage, steel is simultaneously under the influence of structural stresses and thermal stresses, the added effects of which increase the risk of crack formation.

2. Brine:

Sodium chloride aqueous solutions of about 10% (by weight) are widely used industrially, are called brines. They provide cooling rate intermediate between water and 10% NaOH aqueous solution. They are corrosive as regards appliances, but are not hazardous to workmen, as are the caustic solutions.

The greater efficiency of brines, caustic soda solution, or aqueous solutions is explained as- In brine, or caustic, the heating of the solution at the hot steel surface causes the deposition of crystals of sodium chloride/sodium hydroxide on the hot steel surface. This film of solid crystals disrupts with mild explosive violence, and throws off a cloud of crystals.

3. Sodium Hydroxide Solutions:

Normally 10% (by weight) sodium hydroxide is added in water. These solutions extract heat at a rapid rate from the steel, the moment it is immersed in the coolant, and do not show the initial period of (‘A’ stage) comparative ‘inaction’ of water. Hence, these are useful, where cooling rates in excess of those given by water baths, are required.

4. Oils:

Oils, as a group, are intermediate in cooling velocity between water at 40°C and water at 90°C. In an oil-quench, a considerable variation is possible by the use of animal, vegetable, or mineral oil, or blends of two, or more of these varieties. The vapour pressure of the oil is particularly important as this determines the thickness of oil-vapour film produced on the surface of the hot steel, which limits the rate of heat removal. However, the oils, used generally, have high boiling points.

Oils in contrast to water, or brine are much lower in their quenching power (having greatest cooling rate at about 600°C), and are relatively slow in the range of martensitic formation, the latter minimises the danger to crack formation. The cooling power near the nose of the CCT curve of the steels can be increased by agitating vigorously the bath, or the part.

5. Emulsions (Water and Oil):

The temptation to get fast cooling rates of water (near the nose of the CCT curves) and the slow cooling rate of oils at later stage (in M_s – M_f temperature range) led to development of emulsions-water and ‘water soluble’ oil mixture of different proportions. Emulsion of 90% oil and 10% water resulted in having properties-cooling rate-inferior to oil. Emulsion of 90% water and 10% oil is also inferior to oil as it has faster cooling than oil at around 300°C when martensite forms-which thus increases danger to distortion and cracking.

6. Polymer Quenchants:

These are new entrants in the field of coolant which approach the characteristics of an ideal quenching medium (6.3) i.e., cool the steel rapidly to M_s temperature, and then rather slowly when martensite is forming.

These synthetic quenchants are organic chemicals of high molecular weight and are generally polyalkylene glycol based, or polyvinyl pyrolidene based, but generally the former are more commonly used as quenchants. These are water soluble materials, and thus, quenchants with widely different cooling rates can be obtained by varying concentration of the organic additive. With 5% addition, the quenchant can give similar surface hardness as water at 60°C, with least danger of cracking, while quenching unalloyed steels. Quenchant with 15% additive has same cooling properties as an oil with no hazards of fire.

7. Salt Baths:

A salt bath is the ideal quenching medium for a steel of not too large section with good hardenability. Table 6.12 gives some composition of salts and the useful temperature range for each mixture. The recommended holding time in the salt bath is 2-4 min/cm of section thickness, the shorter time for lighter sections. A bath like 100% NaNO₃ is for 400-600°C. The cooling capacity to about 400°C is high, and then decreases as the temperature of the steel continues to drop.

Thus, lower the temperature of bath, and greater the agitation, the better the cooling capacity. The cooling efficiency of a bath gets decreased, if it is contaminated. The stirring of the bath puts the impurities in suspension, which get attached to the part being cooled, and decrease the heat transfer. Addition of 0.3- 0.5% water to the salt baths, which leaves the surface of bath continuously as steam, almost doubles the cooling capacity.

8. Air:

Compressed air or still air is also possible to be used if the steels have high hardenability, i.e., high alloy steels such as air hardening steels; or light sections of low alloy steels. As air cooling is slower and more uniform, the danger of distortion is negligible. Steels invariably get oxidised on surface during cooling.

9. Gases:

Of the gases, hydrogen and helium though have higher cooling efficiency, but nitrogen is used commonly for hot-work steels and high speed steels because of possible explosions while using hydrogen and helium is expensive.

Gas quenching results in more uniform cooling in heavy sections, intricate shapes and varying section thickness parts which, results in more uniform mechanical properties. There is least danger to crack, or distortion. The fast moving stream of gas meets directly the austenitised steel part in gas chamber, to remove the heat rapidly.

10. Fluidized Bed:

It consists of aluminium oxide particles in a retort, fluidized by a continuous stream of gas blown upwards through the base of the retort. The particles move like a fluid. Use of nitrogen provides an inert atmosphere.

It is mainly used for quenching highly alloyed cold-work steels, hot-work-steels, high speed steels, air hardening steels, etc. The fluidized bed cooling is slower than water, or oil, and 10% slower than quenching in molten salts, but significantly faster than air. Fluidized bed can operate at any low temperature. There are no residues left on parts and they require no post treatment. There are no fumes and no hazards of pollution.