Quenching: Process and Mediums | Steel | Metallurgy

In this article we will discuss about:- 1. Process of Quenching 2. Characteristics of Quenching Media 3. Important Quenching Mediums  4. Quenching Stage.

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.

Characteristics of Quenching Media:

The effectiveness of a quenching medium to provide desired cooling rate depends on its characteristics such as:

1. Temperature of the Coolant:

In water and brine, the cooling rate decreases as the temperature of the coolant increases, as it increases stage ‘A’, i.e., helps in persistence of the vapour-blanket stage. The increased temperature brings it close to its boiling point, and thus, requires less heat to form vapour- blanket, specially above 60°C. Normally water is used in temperature range of 20-40°C. Oils show increased cooling rate with the rise of temperature. Two opposing factors are to be considered in oils.

The rise of temperature of the oil, increases persistence of vapour-blanket stage, and thereby tries to decrease the cooling rate. The rise of temperature of the oil makes it more fluid, i.e., decreases its viscosity, which increases the’ rate of heat conduction through the oil. The cooling rate is thus dependent on dominance of one over the other factor. Optimum cooling rates in conventional oils are obtained between 50- 80°C.

Sufficient volume of coolant should be in the tank to prevent rise of temperatures. Cooling coils, or even heating coils, depending on the need, are used in the tank. Hot, or cold water flows through the heat exchangers fitted in the bath. If the required temperature of oils is higher, the cheapest method is to cool some hot pieces of steel in it before the actual first quenching treatment is done in oil.

2. Boiling Point:

Lesser is the boiling point of a coolant, more easily the vapours form to increase the ‘A’ stage of cooling, which provides slower rate of cooling. Coolants of higher boiling point should provide better cooling rate.

3. Specific Heat of Coolant:

As, it is the amount of heat required to raise the temperature of unit mass of the coolant through one degree, a coolant with low specific heat will get heated up at a faster rate than the one with higher specific heat. A coolant with low specific heat shall require proper cooling arrangement specially when continuous mass quenching is being done, otherwise the cooling power of coolant may be badly effected.

4. Latent Heat of Vapourisation:

As it is the amount of heat required to change unit mass of liquid coolant to vapour at a constant temperature, a coolant with low latent heat of vapourisation changes into vapour easily, and thus shall promote the ‘A’ stage of cooling, i.e. provides slower cooling rate. The profound fumes, not only create pollution problems, but the consumption of the coolant shall be high. The rise of temperature of the coolant is high in a coolant with high latent heat of vapourisation.

5. Thermal Conductivity of the Coolant:

A coolant with high thermal conductivity transfers the heat rapidly from the component to its entire mass increasing thereby cooling rate of component.

6. Viscosity:

A more viscous coolant conducts the heat slowly from hot component to its entire mass. Oils as coolants are generally heated to 50-80°C to increase its cooling power by lowering its viscosity. Thus, coolant with low viscosity not only provides faster cooling rate, but decreases the vapour- blanket stage.

7. Velocity Past the Immersed Object (Circulation of the Coolant), or Agitation of the Component:

Both these factors effectively wiped off the vapour film as quickly as it forms, eliminating, or reducing the length of the vapour-blanket stage, and the piece more, or less starts cooling under ‘B’ stage of cooling, i.e., the component gets cooled at a faster rate. Both these factors help in maintaining uniform temperature of the cooling bath.

Important Quenching Mediums:

Hardening aims in getting martensite, at least in the surface layers of the steel. Thus, the rate of cooling should be controlled to avoid high temperature transformations of austenite to pearlite, or bainite. Once the size of a component has been decided, then the required depth of hardening depends on the choice of steel and the quenching medium. Plain carbon steels invariably require cooling in water.

The higher the alloy content in steel, milder is the cooling medium required. Also, milder the cooling medium, lesser the internal stresses developed, and thus, lesser the danger of distortion, or cracks.

From the residual stress development point of view, an ideal quenching medium should be able to provide very high cooling rate (i.e., faster than critical cooling rate of the steel) to avoid transformations at the nose region of the CCT diagram (≈ 650-550°C), i.e., prevent change of austenite to pearlite or bainite.

At the same time, it is desirable to cool a component at a considerably slower rate within the range of martensitic transformation (300 – 200°C) to minimise internal stresses, i.e. to minimise distortion and cracks. Unfortunately, the commercially available quenching mediums do not confirm to atleast one of above given conditions as illustrated in table 6.11, i.e., there is no quenching medium that exhibits these two ideal properties. 

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.

This danger may be reduced, if the steel is rapidly transferred from water to oil-bath (which has slower cooling rate near 300°-200°C) when it has cooled to around 400-200°C. This method “through water to oil” incidentally increases the depth of hardening of oil-hardening low alloy steels.

In industry, water as a coolant is used to harden plain carbon steels and some low alloy steels, i.e., the shallow-hardening steels. Deep-hardening steels can be cooled at a rate faster than critical cooling in oils with no danger of cracks. Water, the more drastic coolant would have caused distortion and cracks. Even the intricate shaped parts are avoided being cooled in water for similar reasons.

A still-water-quench may lead to soft spots in the hardened object due to prolonged stage of stable vapour-blanket. The problem becomes more complex if the shape of the component is complex.

This can be avoided if either the water is circulated in tank, or the component is agitated in the tank. Spray-quenching by water under pressure, can provide cooling rates more than still brine, or caustic soda solution. The high cooling power of water is mainly clue to high specific heat and high latent heal of vapourisation. Water quenching also breaks the scale formed during heating.

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.

This action destroys the vapour film-blanket from the surface, and therefore, permits actual contacts of the coolant and the steel surface with an accompanying rapid removal of heal. This violent action also tears off the scale from the surface. This also lakes care of lack of coolant circulation/or agitation of the part. The ‘A’ stage is almost missing.

These are used where faster cooling than provided by water is needed, but the fast cooling is maintained even in temperature ranges, when steel transforms to martensite, and thus, makes the steels more prone to warping and cracking.

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. (Table 6.11).

The effectiveness of cooling of these solutions is maintained even at lower temperatures when austenite transforms to martensite, and this is a disadvantage, as it increases the danger to warping and cracking. Here also, through caustic solution to oil (or air) method may be used.

The reason of high cooling power of this solution is similar as explained for brines. These solutions compensate well for lack of circulation, or agitation, and are less effected by the rise of temperatures. These are able to remove the scale in a better way than water.

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.

Also, the oils should be used at 50°-80°C, when these are more fluid, i.e. less viscous, which increases the cooling capacity. This moderate increase of temperature has little effect on the duration of vapour-blanket stage, because of high boiling points of oils. However, the cooling rate of oil in stage ‘B’ is increased in hot oil as compared to cold oil, which is desired. Slower cooling rate in stage ‘C’ in oils reduces danger to cracking of steel components.

In general, oils with lower viscosity are more volatile. Higher volatility prolongs the vapour-blanket stage. Moreover, this loss of volatile matter (due to initial heating while in use) increases the viscosity of the oils, thus reducing the cooling power after a short period of use. Whereas, highly viscous oils with lower volatility reduces the vapour blanket stage, i.e., increases the cooling rate. That is why, probably, quenching oils are graded generally according to their viscosity.

Quenching oils, normally used are mineral oils and have viscosity around 100 SUS (Saybolt Universal Seconds) at 40°C. These oils cannot be used for shallow- hardening steels. Fast quenching oils have viscosity around 50 SUS at 40°C and are blended mineral oils and approach water-quenching power only in the initial stage of cooling.

Hot-quenching oils-used generally in the temperature range of 100-150°C, have viscosity in the range of 250- 3000 SUS at 40°C. There is least danger of distortion and cracking and thus, are suited to intricate parts. Martempering oils are used in temperature range of 100- 200°C, and are solvent refined paraffin type mineral oils, having viscosity of 2000 SUS at 40°C.

Alloy steels as a rule have high hardenability, are oil quenched with least danger of distortion of cracks. Such oils are mineral oils (usually of petroleum type). Dies or special fixtures are used in oil quenching to reduce warping of intricate shaped parts.

The disadvantages of oil-quenching is their high inflammability (flash point is 165-300°C), insufficient stability, low cooling power near the nose of the curve and higher cost.

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.

Water, if added to normal quenching oils was found to cause cracking specially in deep hardening steels as martensite forms in the centre much later, when surface has already transformed to brittle martensite. These conditions induce large internal stresses.

Emulsions invariably form layers with water at the bottom of the tank. Steam generated may produce explosions.

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.

Inverse solubility is a unique property of such quenchants. For example, as a hot steel piece is immersed in quenchant, and as the temperature of the solution rises above 77°C, the organic polymer becomes insoluble, but when the solution is cooled, the polymer goes back into solution below 77°C and is fully soluble.

When a hot steel part is quenched in the quenchant, the solution in immediate vicinity of part gets heated up to above 77°C. The polymer becomes insoluble in water and generally forms a thin deposit on the surface of the part.

This suppresses the formation of vapour-blanket around the part, and does not slow down the cooling rate in the temperature range when martensite forms. As the temperature of the part falls below the inversion temperature (here 77°C), the thin film of polymer dissolves and thus, permits fast removal of heat from the part.

The cooling rate of the solution depends on the amount of polymer added in water as illustrated in Fig. 6.14, i.e., higher the content of the polymer, slower is the cooling rate of the solution. The cooling rate, thus, can be adjusted to get ideal cooling rate for a steel component. For example- A solution with 3-4% of polymer compares well with brine. 12-15% additions are good for shallow hardening steels; 15-30% for case hardening steels.

As wide variety of cooling rates are possible with least danger of distortion and cracks, substitution of high alloy steels can be done with cheaper low alloy steels. This aspect of organic quenchant is of great importance, as much cheaper components can be produced. Agitation of the quenchant provides the needed flexibility for different size, mass, geometry, etc.

Organic polymers are increasingly being used because of attainment of desired cooling rates, better heat transfer characteristics, high specific heat and consistency of the results.

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.

Water is normally added into the vortex created by the stirrer impeller. Sizzling sound (due to evolution of steam) indicates the presence of water in the bath. Steam also causes stirring action at the surface of the bath. Parts heated in a cyanide bath having more than 10% cyanide should not be quenched in nitrite- nitrate bath due to danger of explosion.

Also, the oil and wood should not be brought in contact with liquid salt. Salt mixture 5 is used for quenching hot work steels, high speed steels and for tempering of high speed steels. Salt bath austenitised parts show a clean surface. The increased cooling speed of these parts is due to elimination of scale on heating and to the solution of adhering salt.

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.

Some main conclusions are:

1. Increase in the temperature of the coolant lowers the cooling rates in water, brine and caustic solutions, while in oil, the cooling rate increases slightly in stage ‘B’. Warm oil having greater fluidity enables more rapid convection currents to be set up and to carry off the heat. Temperature of oil should not be so high that it flashes, or burns, when object is immersed in it. Preheating of water is not advisable.

The increased temperature of water and aqueous solutions increases rapidly the duration of vapour-blanket stage due to their lower boiling points. Increased temperature of oil has little effect on this duration, because of high boiling points of oils which are used. ‘B’ stage is not effected in water and the aqueous solutions, while it increases in oil. The slower cooling rate in ‘C’ stage in oil is an advantage to reduce danger of cracking.

2. Presence of soap in water prolongs ‘A’ stage, similar to increase of temperature of the water. Gases absorbed in water also drastically reduce the cooling rate, and that is why, fresh water (having Cl₂ etc.) has lower cooling rate than already stored water.

3. The cooling rate of oil is insufficient to avoid transformations to pearlite in plain carbon steels The slower cooling rates of oils in the martensitic transformation range is an advantage.

Quenching Stage:

Quenching stage of hardening heat treatment is a difficult and complex step.

There are two main requirements of contradictory nature:

1. To obtain martensite, austenitised steel must be cooled at a rate faster than the critical cooling rate. Plain carbon steels have very high critical cooling rates, and the high cooling rates have to be attained in the centre of the part in through-hardened steels. Consequently, the actual cooling rate required (also dependent on thickness of the part) may be very rapid. Water has to be used as coolants.

2. Rapid cooling is the main cause of development of large amount of internal stresses more rapid the cooling, larger the stresses. These stresses result in distortion and in worst to the formation of cracks. Development of these stresses is especially dangerous, when steel is being cooled within the range of martensitic transformation, where these reach their optimum value, when the steel has practically no plasticity.

Thus, methods of cooling should be so designed that steel cools fast around the temperature range of the nose of the ‘CCT’ curve, to avoid pearlite formation, but cools very slowly when it is transforming to martensite-particularly when the centre of the part is transforming to martensite.

In many cases, the dangerous level of non-uniformity of the internal stresses is as a result of poor design of a component being heat treated, such as a combination of thick and thin sections; sudden changes in the cross-section; sharp projections; small holes in massive parts, etc.

These and other such designs should be avoided. A more regular and simple shape develop more uniform stresses. But, when it is not possible to change the design to simple shapes, and the component is quite irregular in shape and size, then steps should be taken during quenching to obtain as uniform a cooling as possible.

The following measures should be adopted while immersing a heated component in a quenchant:

1. A component having thick and thin sections should be immersed in the cooling bath with its thicker section first.

2. Long and slender components like a tool bit, screw taps, springs should be immersed strictly in vertical position,  even when such slender components are fixed in a fixture, otherwise these shall distort.

3. The axis of components like thin rings should be normal to the surface of coolant.

4. Thin and flat parts, such as discs and milling cutters are immersed edge wise in the coolant.

5. Components with concave surface should be immersed in cooling bath with this surface downwards, otherwise vapour-blanket forming there remains sticking there, preventing the hardening of that surface of the component.

6. Small holes are stuffed with wet asbestos to prevent the quenching liquid from penetrating into them. As quenching cracks start at sharp edges of components, these are put in fixtures to slow down the cooling rate at the edges, particularly for thin walled conical parts.

7. Very thin flat component like saw discs, with all measures observed, develop distortions. Special fixtures are used for them. Heated component is inserted in a fixture and quickly clamped and plunged as a whole in the cooling tank. The resultant hardness might be a bit low, but distortions are completely prevented.