HTE - Quenchants

2 Quenchants:

In heat treatment of steels, quenching is a process of rapid cooling of steel from austenitizing temperature. That results in the transformation of austenite to martensite. During cooling, heat must be extracted at a very fast rate from the steel piece. This is possible only when a steel piece is allowed to come in contact with some medium, which can absorb heat from the steel within a short period and the heat absorbed should be rejected to the surroundings immediately. A medium that is used for quenching is known as a quenchant. Effectiveness of a quenching process depends on the characteristics of the quenchant used, chemical composition of the steel, design of steel component, and surface conditions of steel component. Once a quenching medium is selected, the properties of quenched steel are largely determined by the manner in which the medium is used. Most of the quenchants commonly used are liquids. Air and gases are used in special cases.

A comparison can be made between different quenching media with water. Such a comparison is shown in the fallowing table.

2.1 Removal of heat during quenching:

The mechanism of removal of heat from the work-piece as a result of quenching is not as simple as that associated with annealing or normalizing. The removal of heat during quenching takes place in three stages. As soon as a work-piece comes into contact with a liquid coolant (quenchant), the surrounding quenchant layer is instanta­neously heated up to the boiling point of the quenchant and gets vaporized due to the high temperature of the work-piece. This vapour forms an envelope around the work-piece and thus checks further cooling of the work-piece. This is so because the vapour film is a poor conductor of heat. The work piece is cooled at this stage by conduction and radiation through the vapour film. Only the surface of the work-piece is cooled considerably prior to the formation of this vapour envelope. This is referred to as the first stage of cooling and is named vapour blanket stage. The fallowing figure shows different stages of cooling in immersion quenching.

The first stage is followed by the second stage known as vapour transport cooling stage or liquid boiling stage. The temperature of the work-piece comes down, though very slowly, in the first stage and, consequently, the vapour film is no longer stable below a particular temperature. This is the start of the second stage. As soon as the vapour film is broken, the quen­chant comes in contact with the surface of the work-piece and is immediately pushed away from it in the form of bubbles. Fresh coolant now comes in contact with the work-piece surface and the process is repeated. It continues till the temperature of the surface of the work-piece comes down to below the boiling point of the liquid. Very rapid cooling takes place at this stage as the quenchant is always in contact with the surface of the work-piece.

The third stage is known as liquid cooling stage or convection stage. It starts when the temperature of the surface of the work-piece becomes equal to the boiling point of the quenchant. Cooling at this 'stage takes place by both conduction and convection processes. The rate of cooling is the slowest at this stage.

1. Vapour film around the probe, 2. Boiling commences at corner, 3. Boiling front moves along the probe, 4. Showing vapour. Boiling and convection phases. 5. Convection.

The mechanism of quenching can be summarized as fallows.

Quenching occurs in three stages.

1)        Vapour Phase

a)    Formation of vapour film around the part.

b)    Heat transfer is slow.

c)    Heat transfer occurs primarily through radiation and conduction through vapour.

2)        Nucleate Boiling Phase

a)    High heat extraction rates.

b)    Heat removal by bubble formation and contact of cool quenchant on part surface.

3)        Convection Phase

a)    Start at below boiling temperature of quenchant.

b)    Heat transfer is slow.

2.2 Characteristics of quenchants:

The effectiveness of a quenchant depends largely on its characteristics. Some of the factors, which control quenching characteristics, are as follows:

1.    Temperature of the quenchant

2.    Latent heat of vaporization

3.    Specific heat of the quenchant

4.    Thermal conductivity of the quenchant

5.    Viscosity of the quenchant

6.    Degree of agitation of the quenching bath.

2.3 Quenching media:

Some of the widely employed quenching media are water, aqueous solu­tions, oils, molten salts and air. In addition to these, mediums such as polymer solutions, molten metals and gases are also used but to a lesser extent.

A comparison of different quenching medias with respect to water is given in the fallowing table.

a)  Water:

Water is the most popular quenching medium because of its low cost, availability in abundance and easy handling. No pollution problem is asso­ciated with the use of water and it can be easily disposed of. Water has maxi­mum cooling rate amongst all common quenchants except aqueous solutions. Water can therefore be successfully used for carbon steels, alloy steels and non-ferrous alloys. Most of the non-ferrous alloys are water quenched. The layer of scale formed on the surface during heating is also broken by water quenching, thus eliminating an additional process of surface cleaning.

In practice, the applicability of water as quenching medium is restricted only to plain carbon-steels and a few grades of low alloy steels, which have low hardenability values. The higher cooling rat involved in water quenching may lead to development of internal stresses, which can cause distortion and formation of cracks. This is because the cooling rate is much higher than the critical cooling rate. High cooling rate obtained by water quenching also puts limitation on the shape and size of the object to be heat-treated. Thus, only articles with simple geometrical shapes can be water quenched.

Another major disadvantage associated with water quenching is that vapour blanket stage is quite stable for prolonged periods. The duration of the period increases with the complexity of shape of work-piece as com­plicated shapes favor vapour entrapment. This problem can be minimized or eliminated by agitating the quenching bath.

Best results are obtained by using water at room temperature, i.e. in the temperature range 20-40°C. Water quenched objects are prone to get rusted. So they are either dressed or coated with some rust preventive chemical.

b)  Aqueous solutions:

Addition of highly ionized salts to water decreases the viscosity of water and reduces the duration of vapour blanket stage. Both these parameters help in improving the cooling power of water and minimize the necessity of agitating the quenching bath. By increasing the degree of agitation, the cooling rate may be increased further. In this way, the possibility of formation of soft spots arising from the presence of steam pockets is reduced to a great extent.

Aqueous solutions of salts such as sodium chloride and calcium chloride are referred to as brine solutions, whereas solution of sodium hydroxide is referred to as caustic solution. Such solutions are of great use, especially forshallow hardenable grades of steels. Though the cooling rates are high in these solutions, distortion is less severe than in water quenching. Just as in the case of water, these solutions too are commonly used at room tempe­rature. However, maximum cooling power is obtained when the temperature of aqueous solution is maintained at about 20°C. Aqueous solutions with about 5-10 percent concentration are in general use.

These solutions have certain disadvantages over water. They are costlier than water due to the extra cost of the salts. The corrosive nature of these solutions is a major limitation. Due to this, the cost of handling equipments such as quench tanks and pumps increases.

Special attention should be paid to the periodical checking of handling equipments and to solution, which in turn raises the labor cost. Corrosive fumes produced pollute the atmosphere and affects the human skin. It also affects the service life of nearby equipments.

c)  Oil:

Most of the oils used as quenchants are mineral oils. These are, in general, paraffin based and do not possess any fatty oils. Quenching in oils provides slower cooling rates as compared to those achieved by water quenching. The slow cooling rate developed during oil quenching reduces hardening defects. The temperature difference between the case and core of the work ­piece is less for oil quenching than for water quenching.

Quenching oils are graded according to their viscosity values. Com­monly used quenching oils have the viscosity values of about 100 SUS (Saybolt Universal Seconds) at 40ºC. For these oils, the duration of the first stage is longer than the corresponding value achieved by water quenching. The cooling rate in the second stage is also considerably lower, and the duration of this stage is shorter as well. On account of all these factors, these oils are not considered well suited as quenchants where severe quenching is desired. However, they offer less distortion. For majority of applications, these oils are used at temperatures varying from room temperature to 65°C. However, in certain cases, especially where slow cooling rates are required, oils are maintained in the temperature range 65-95°C.

For obtaining faster cooling rates, oils with viscosity values as low as 50 SUS at 40°C are employed. These oils are referred to as fast quenching oils. The duration of the first stage is considerably less for these oils than for common oils. The initial cooling rate associated with these oils approach the value developed by water quenching.

Hot quenching oils generally possess viscosity values in the range 250­-3000 SUS at 40°C. These include plain and inhibited mineral oils, which are generally used in the temperature, range 100-150°C. Use of these stable oils results in low distortion and cracking. These oils are very well suited to quenching intricately shaped objects.

Marquenching oils have viscosity values more than 2000 SUS at 40°C. These oils are inhibited to provide excellent oxidation and thermal stability. Marquenching oils are generally used at temperatures higher than 150°C. These oils have specific advantages such as uniform cooling rate, minimum possible distortion ad cracking.

The presence of water as an impurity in quenching oils is most undesir­able. It may lead to development of non-uniform hardness distribution, distortion and cracks. Water can be removed by heating oils to 130°C for about 4 hours.

d)  Air:

Many alloy steels are capable of getting hardened by cooling either in still air or blast of air. Such steels are popularly known as air hardening steels. These steels are almost free from distortion problem. However, the problem of oxidation during cooling (quenching) may be encountered in practice. Many grades of tool steels are subjected to air hardening. Cooling rates may be improved upon by using air-water mixtures.

e)  Gases:

The use of gases as quench ant results in development of intermediate cooling rates which are faster than those associated with still air and slower than those developed by oil quenching. A fast moving stream of gas removes the heat from the work-piece at a much faster rate than by still air quench­ing. Different gases hydrogen, helium, nitrogen and argon are used.  Hydrogen possesses maximum cool­ing efficiency, followed by helium, nitrogen and argon in decreasing order of cooling powers. In general, only nitrogen is used as quenchant. The reasons for this are Hydrogen is not safe to use helium is quite expensive, and argon possesses the lowest cooling efficiency.

The cooling efficiency of gaseous quenchants can be raised to a signi­ficant extent by using a fast moving stream of gas mixed with droplets of water. Here, fine droplets of water provide additional source for the heat extraction from the work-piece surface. These droplets will always be in contact with surface of work-piece, and very efficient and effective cooling of work-piece will take place. Due to continuous flow of gas stream, there is no possibility of formation of vapour envelope around the work-piece.

f)  Salt baths:

Since long, salt baths have been used as quenchants in commercial practice, especially for tool steels. Salt bath is the best-suited quenching medium for steel with good hardenability and thin sections. Some of the advantages offered by salt bath quenchants are as follows:

·         Temperature is uniform throughout the bath.

·         There is uniformity in the rate of heat transfer from the work-piece.

·         There is no danger of oxidation, carburization or decarburization during cooling.

·         Selective hardening can be performed.

Some common salt baths are, NaN0₃, 50% NaN03 + 50% KN0₃, 50% NaN0₃ + 50% KN0₂ and 20% NaOH + 80% KOH.

g)  Synthetic quenchants:

Synthetic quenchants are relatively a new introduction in the field of quenchants. Commonly available synthetic quench ants are generally oxyalky­lene polyglycol based, polyalkylene glycol based or polyvinyl pyrolidone based organic materials. Polyalkylene glycol based materials are more com­monly used as quenchants. These organic materials are water-soluble and, therefore, quenchants with widely differing characteristics can be developed by varying the concentration of the solution. Attainment of desired cooling rates, better, transfer characteristics, and inverse solubility are the salient features of these quenchants. Inverse solubility is a unique feature of these quenchants, and it enables a thin film of glycol to wet the hot work piece as it is quenched. This helps in two ways: Firstly, it results in the suppression of formation of vapour envelope around the work-piece. Secondly, it provides uniform rate of heat transfer, resulting in minimum distortion in the work-piece. As the temperature of the work-piece comes down, the thin film of glycol dissolves and thereby permits fast removal of heat from the work-piece. These quenchants are very safe as there is no danger of fire hazards or pollution. They are used at room

temperature. For varying cooling rates, concentration of the solution is changed. Slower cooling rates can be achieved by increasing the concentration. Agitation of the bath may be avoided. The consistency of the results is one of the important factors enhancing the popularity of quenchants. These are well suited for mass production since rise in bath temperature is much less than in oil baths because of the high specific heats of these quenchants.

2.3 Temperature measurement and control in heat treatment:

Since temperatures applied during the heat treatment are very important for the end results obtained, accurate measurement and control of temperature are the two impor­tant operations involved in heat-treating of metals and alloys. Any variation from the desired temperature may lead to development of inferior proper­ties in metals and alloys after heat treatment. Though most of the heat treatment processes involve high temperatures, low temperatures as well as sub-zero temperatures are also involved in some heat treatment processes. Therefore, it is essential to construct a suitable scale of temperature. The results obtained from this temperature scale should be reproducible in nature with a high degree of precision and accuracy. The temperatures involved in heat-treating of metals and alloys, in general, varies from - 100°C to+ 1400°C.

2.3.1 Concept of temperature:

Thermal phenomena cannot be described in terms of the three funda­mental quantities, i.e. length, mass, and time. A fourth fundamental quan­tity is needed to explain thermal phenomena. This quantity is referred to as temperature. Till 1843, the quantity of heat was regarded as a fundamental quantity. It was the work of Rumford and Joule (1843-78), which showed that heat is a form of energy and that the quantity of heat can be expressed in terms of three fundamental units.

Temperature is a physical quantity entirely different from quantity of heat. A hot body is considered to be at a higher temperature than a cold body. Therefore, temperature can be used to measure the degree of hotness or coldness. In other words, temperature determines the direction of flow of heat. If two bodies are brought in physical contact with each other, heat will flow from the body at higher temperature to the body at lower temperature. This flow will continue and after some time there will be no further flow of heat. Such a state is explained by stating that both the bodies are in thermal equilibrium. If a number of bodies (or systems) are in thermal equilibrium, the common property of all the bodies can be assigned a single numerical value referred to as temperature. Bodies, which are not in thermal equilibrium, have different temperatures. Therefore, the temperature of a system can be defined as the property that determines whether or not the system is in thermal equilibrium with the neighboring system(s).

2.3.2 Measurement of temperature:

The physical properties of materials change with application of heat. These properties are known as thermometric properties. Anyone of these properties can be used for the measurement of temperature. The science and technology dealing with the measurement of temperatures below 500°C is referred to as thermometry, and the instruments used for this purpose are known as thermometers. On the other hand, pyrometry deals with measure­ment of temperatures above 500ºC, and the instruments used for this pur­pose are known as pyrometers. For the quantitative measurement of tempe­rature, it is essential to select (at least two) fixed temperatures, which must be reproducible in nature. The most important fixed points are the ice point and the steam point. The ice point is defined as the temperature at which ice and air saturated water at normal atmospheric pressure exist in equilibrium. Similarly, steam point is the equilibrium temperature between liquid water and water vapour at one atmospheric pressure. The most popular tempera­ture scale is due to Celsius. In 1742, Celsius suggested the centigrade system of temperature measurement. The two fixed points corresponding to ice and steam points were marked as zero and 100 on the lower and upper ends of the scale, respectively. The interval between the two fixed points was divided into hundred equal parts. Each of these parts was called one degree centigrade or one degree Celsius and was represented as 1ºC.

2.3.3 Thermometers:

Thermometers can be grouped into several classes. The classification of thermometers is based on the thermometric property used for the measure­ment of temperature. Examples of various thermometers are liquid thermo­meters, gas thermometers, resistance thermometers, vapour pressure thermometers and magnetic thermometers.

a) Liquid thermometers:

Liquid thermometers are based on the principle that the volume of a liquid changes with variation in temperature. Mercury and alcohol thermo­meters are examples of commercial thermometers belonging to this class. The temperature of general-purpose mercury thermometer ranges from -39°C to +357°C. Alcohol thermometer is used to measure temperatures near the ice point. Some of the advantages offered by mercury for use in liquid-in-glass thermometers are due to its low specific heat, good thermal conductivity and uniform coefficient of expansion. Mercury thermometers are generally used for rough and quick work. However, after several correc­tions, mercury thermometer can be used for precision work. Some of these corrections are due to change of zero point, rapid heating and cooling, exposed stem, inequality of the bore, and thermal capacity and thermal conductivity of the bulb.

b) Gas thermometers:

The principle that the volume or the pressure of a gas changes with variation in temperature forms the basis of gas thermometers. Callendar's constant pressure air thermometer, Jolly's constant volume air thermometer and constant volume hydrogen thermometer belong to this class. In compa­rison to liquid thermometers, gas thermometers offer certain advantages. These advantages are essentially due to large coefficient of expansion, uniform and regular expansion over a wide range of temperature, and low thermal capacity. Gas thermometers can be used over a wide range of tem­perature and are suitable for both low and high temperature measurement. These thermometers are not used for routine work, as they are bulky and cumbersome.

c) Bimetallic thermometers:

Bimetallic thermometers are based on the principle of differential expan­sion of solids. The bimetallic strip consists of two bonded strips. One strip is made of a high expansion metal, and the other of a low expansion metal. When heat is applied, such an arrangement will result in non-uniform expansion along the two strips. This non-uniform expansion is utilized for the measurement of temperature. An industrial bimetallic strip is taken in the form of a spiral. One end of it is fixed and the other end is attached to a pointer, which moves over a calibrated scale. In general, invar is selected as a low expansion metal. The metal for the other strip depends on the temperature range of the interest. Yellow brass strip is generally used for low temperatures and nickel or nickel alloys for higher temperatures. Bimetallic thermometers can be used to measure temperatures which vary from -73°C to + 537°C.

d) Vapour pressure thermometer:

Vapour pressure thermometers are based on the principle that vapour pressure changes with temperature. For a given temperature, a fixed vapour pressure can be assigned. These thermometers are used for low temperature measurement. The greatest drawback associated with these thermometers is that their useful temperature range is very narrow. The useful temperature ranges are as follows:

                              Oxygen        –150ºC to –210ºC

                              Neon            –246ºC to –249ºC

                              Hydrogen    –253ºC to –260ºC

                              Helium         –268ºC to –272ºC

e) Resistance thermometer:

Electrical resistance of a metal changes with temperature. This principle forms the basis of resistance thermometers. Platinum resistance thermometer is capable of measuring temperatures in the range - 200°C to + 1200°C, with high degree of accuracy. The temperature vs. resistance curve for platinum resistance thermometer is almost a straight line. The relationship between temperature and the electrical resistance can be expressed by the equation

R_t= R_o(1 +α.t)

Where R, is the resistance (in ohms) of the platinum at temperature t^OC; Ro is the resistance of platinum (in ohms) at ice point; α is the temperature coefficient, and t is the temperature in °C.

f) Magnetic thermometer:

The magnetic susceptibility of a substance varies with temperature. Based on this principle, magnetic thermometers are used for the measure­ment of temperature. These

thermometers can primarily be used for measurement of extremely low temperature, generally in the vicinity of -273°C.

g) Thermo-couples:

Thermo-couples are based on the thermo-electric effect. If two dissimilar metals are joined together at both the ends, and these junctions are main­tained at different temperatures by heating one junction and keeping the other cold, an electromotive force (emf) is induced in the circuit. This is the well-established Seebeck effect. In general, the cold junction is maintained at room temperature or at 0^oC and the hot junction is made to touch the object whose temperature has to be measured. The electromotive force induced will be proportional to the difference in temperature between the two junctions.

A thermo-couple assembly consists of three units, namely, thermo-couple lead wires, and indicator. A thermo-couple is composed of two homogene­ous but dissimilar metal wires. The ends of these wires are joined together by soldering, welding or fusing together. Thus, a closed circuit is formed. The free ends of the thermo-couple are connected to lead wires. Other ends of lead wires are connected to a suitable indicator. The indicator measures the electromotive force developed in the circuit. The extension lead wires are generally made of the same materials as the material of thermo-couple wire. This practice is generally adopted for base metal thermo-couples. For noble metal thermo-couples, lead wires are made of copper and/or copper nickel alloys.

2.4 Choice of thermo-couple materials:

According to the Seebeck effect, any two metals can be selected for forming the thermo-couple. However, in practice, only limited combinations of metals are in use. The choice is limited because of several factors. A few of these factors are as follows:

1.    The metals should be homogeneous.

2. The materials under consideration should be resistant to oxidation and corrosion in medium/media and at temperature(s) at which they are supposed to be used.

3.    The metals should be good conductors, i.e. their electrical resistance should be low.

4.    The formability of metals to desired shapes, particularly into wires, should be good.

5.    The melting points of thermo-couple materials should be higher than the highest temperature encountered during the service.

6.    The electromotive force (emf) induced should increase with rise in the temperature or the temperature difference between hot and cold junctions.

7. There should be a linear or an almost linear relationship between electromotive force induced and temperature.
8. The induced electromotive force should be sufficiently large. This will enable precise and accurate measurement of electromotive force and temperature.
9. The induced electromotive force should be reproducible and stable. It should not vary because of physical and chemical changes or con­taminations from the surroundings.
10. The material should not be very expensive.