The various heat treatment processes are discussed as follows: 1. Annealing 2. Normalising 3. Hardening 4. Tempering 5. Surface Hardening.
1. Annealing:
The objects of annealing are:
(i) To soften the metals.
(ii) To improve machinability.
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(iii) To refine grain size due to phase recrystallisation.
(iv) To increase ductility of metal.
(v) To prepare steel for subsequent treatment.
(vi) To modify electrical and magnetic properties.
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(vii) To relieve internal stresses.
(viii) To remove gases.
(ix) To produce a definite microstructure.
i. Full Annealing:
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Refer to Fig. 5.17. If it is desired to refine the grain structure and produce a lamellar pearlite, a full annealing cycle should be used. This consists of heating the steel to a temperature above the transformation range, holding for one to two hours, and then cooling at a predetermined rate to obtain the desired microstructure.
Grain refinement is accomplished in this instance by the recrystallisation of the steel in passing through the critical range both in heating and in cooling. The microstructure obtained in cooling any steel from above the critical temperature range is dependent both upon the temperature range in which transformation occurs and the time required for completion of transformation in that range.
Thus, it is obvious that the rate at which any steel is cooled determines the final microstructure, since the degree of transformation will depend on the amount of time allowed for it to occur. Therefore, the slower the rate of cooling and the higher the temperature at which complete transformation occurs during full annealing, the coarser the pearlite will be with correspondingly lower hardness.
Such treatment is performed usually on steel of 0.30 to 0.60 percent carbon content which is to be machined.
ii. Isothermal Annealing:
Refer to Fig. 5.18. It is a type of full annealing in which the steel first is cooled to the temperature at which it is desired to have transformation occur, at a rate sufficiently rapid to prevent any structural change above that temperature. The steel then is held at the selected temperature for the time necessary to complete such transformation.
Thus it is possible, with this process, to obtain a more uniform microstructure that could be expected by continuous cooling. However, since it is necessary to drop the temperature rapidly to prevent any transformation above the desired temperature, there are definite limitations as to the mass that can be so treated. It is applicable, therefore, only to small sections and would be suitable for large bars or large load in batch type furnaces since it would be impossible to cool them at a rate sufficiently rapid to prevent some transformation.
A modified application of isothermal annealing is possible however, in which the charge is heated in one furnace and transferred to another, which has been set at a temperature somewhat lower than the desired temperature of transformation in order that the temperature of the change will drop rapidly to that required.
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The selection of the temperature of the second furnace will be governed by the temperature to which the charge first is heated, the mass of the charge and the desired transformation temperature. Suitable handling equipment must be available to transfer the entire charge rapidly since any undue delays might result in portions of the charge being cooled to too low a temperature. Continuous furnaces also are applicable to this type of cycle.
Isothermal annealing process not only improves machinability in general, but also results in a better finish by machining. The process is of great use for alloy steel as these steels have to be cooled slowly.
However, it has the following important limitation:
It is suitable only for small-sized components. Heavy components cannot be subjected to this treatment because it is not possible to cool them rapidly and uniformly to the holding temperature at which transformation occurs. For this reason, structure will not be homogeneous and mechanical properties will vary across the cross-section.
iii. Process or Sub-Critical Annealing:
Refer to Fig. 5.19. Another type of annealing called process or sub-critical annealing consists of heating the steel to a temperature first under lower critical point and holding at this temperature for the proper time (usually 2 to 4 hours) followed by air cooling.
This type of annealing results in softening the steel due to particle coagulation of the carbide to form the spheroids or small globules of carbide. It is not suitable when a close control of hardness or structure is desired, because the prior structure of steel determines to a marked degree the extent of spheroidisation which will occur.
The treatment is quite satisfactory for rendering bars more suitable for cold sawing or shearing and is used to great extent for these purposes. Since the temperature to which the bars are heated is somewhat lower than in full annealing there is less scaling and warping can be controlled.
iv. Spheroidisation:
Refer to Fig. 5.20. It is a type of annealing which causes practically all carbides in the steel to agglomerate in the form of small gobules or spheroids. There may be wide range of hardness with such a structure for any grade of steel since the size of the globules has a direct relation to hardness.
Spheroidising may be accomplished by heating to a temperature just below the lower critical and holding for sufficient period of time. A more desirable and commonly used method for spheroidising is to heat to temperature just above the critical and cool very slowly (about 6°C per hour) through the critical range or to heat to a temperature within the critical range but not above the upper critical and cool slowly.
This treatment is used for practically all steels containing over 0.6 percent carbon that are to be machined or cold formed.
This process, also known as homogenizing annealing, is employed to remove any structural non-uniformity. Dendrites, columns grains and chemical inhomogeneities are generally observed in the case of ingots, heavy plain carbon steel casting, and high alloy steel castings. These defects promote brittleness and reduce ductility and toughness of steel.
In diffusion annealing treatment, steel is heated sufficiently above the upper critical temperature (say 1000-1200°C), and is held at this temperature for prolonged periods, usually 10-20 hours followed by slow cooling. Segregated zones are eliminated, and a chemically homogeneous steel is obtained by this treatment as a result of diffusion.
Practically all steels, which have been heavily cold worked, are subjected to this treatment.
The process consists of heating steel above the recrystallization temperature, holding at this temperature and cooling thereafter. It results in decrease in hardness or strength and increase in ductility. The process is used both as an intermediate operation and as a final treatment. The treatment is very important and is frequently employed in industries manufacturing steel wires, sheets and strips.
High carbon steels and alloy steels require higher recrystallization temperatures.
2. Normalising:
The objects of normalising are:
(i) To eliminate coarse gain structure obtained during forging, rolling and stamping.
(ii) To increase strength of medium carbon steel.
(iii) To improve machinability of low carbon steel.
(iv) To improve the structure of welds.
(v) To reduce internal stresses.
(vi) To achieve desired results in mechanical and electrical properties.
Refer to Fig. 5.16.
Normalising is the term applied to the process of heating the steel approximately 4°C above the critical temperature followed by cooling below this range in still air. This is one of the simplest treatments.
a. The steel produced by this treatment is harder and stronger but less ductile than annealed steel having the same composition.
b. This treatment is frequently applied to castings, forgings, etc., to refine grain structure and to relieve stresses set up in previous operations.
c. It is commonly applied after cold working, overheating or any other operation resulting in non-uniform heating or cooling.
d. It may be used to efface the effects of previous heat treatments.
Normalising versus Annealing:
1. Normalised steels are harder than annealed ones. Relatively rapid cooling in the case of normalising results in higher degree of super-cooling. Therefore, austenite decomposes at relatively lower temperatures, resulting in better dispension of ferrite-carbide aggregate. Also the amount of pearlite is more.
Both of these factors result in higher strength and hardness. So where these properties are desired, annealing treatment cannot be employed, and normalising should be done. Prolonged heat treatment time and higher energy consumption make the annealing treatment more expensive than normalising.
2. Cooling rates are not critical for normalising as in the case of annealing. They can be increased considerably in order to cut short the total time for treatment. The only point to be considered is that cooling should result only in production of equilibrium micro-constituents. After a particular temperature is attained, which is well below the lower critical temperature, steel may be quenched.
3. Normalised steel has lower impact transition temperature than annealed steel. This is essentially due to the fine grain size of normalized steel.
4. Annealing improves the machinability of medium carbon steels, whereas normalising improves machinability of low carbon steels.
3. Hardening:
Hardening is a process in which steel is heated to a temperature above the critical point, held at this temperature and quenched (rapidly cooled) in water, oil or molten salt baths.
If a piece of steel is heated above its upper critical temperature and plunged into water to cool it an extremely hard, needle-shaped structure known as martensite is formed. In other words, sudden quenching of steel greatly increases its hardness.
After hardening, steels must be tempered to:
(i) Reduce the brittleness,
(ii) Relieve the internal stresses, and
(iii) Obtain predetermined mechanical properties.
The hardening process is based on a very important metallurgical reaction of decomposition of eutectoid.
This reaction is dependent upon the following factors:
(i) Adequate carbon content to produce hardening.
(ii) Austenite decomposition to produce pearlite, bainite and martensite structures.
(iii) Heating rate and time.
(iv) Quenching medium.
(v) Quenching rate.
(vi) Size of the part.
(vii) Surface conditions.
a. The rapidity with which the heat is absorbed by the quenching bath has a considerable effect on the hardness of the metal. Clear, cold water is very often used, while the addition of salt still increases degree of hardness.
Oil, however, gives the best balance between hardness, toughness and distortion for standard steels. Special soluble oils are used in many plants instead of ordinary fish oil, linseed oil or cotton seed oil.
b. In order to increase the cooling rate the parts may be moved around the quenching bath, either by hand, or by passing them through the tank in a basket attached to mechanical conveyer. Large parts may be lowered into the tank by a crane and kept moving while cooling. It is often cheaper and more efficient, however, to circulate the cooling liquid around the hot part.
Modern quenching equipment is often highly mechanised, a rapid conveyor taking the load from the furnace to the quenching tank at 7.5 metres/min., lowering the charge into the tank then moving it in the liquid and withdrawing it when cool, the whole operation being controlled by push-buttons on an automatic cycle.
c. The heating rate and heating time depend on the composition of the steel, its structure, residual stresses, the form and size of the part to be hardened. The more the intricate and large the part being hardened, the slower it should be heated to avoid stresses due to temperature differences between the internal and external layers of the metal, warping, and even cracking.
The practically attainable heating rate depends upon the thermal capacity of the furnace, the bulk of the charged parts, their arrangement in the furnace, and other factors. The heating rate is usually reduced, not by reducing the furnace temperature but by preheating the articles.
d. The heating time for carbon tool steels and medium-alloy structural steels should be from 25 to 30% more than for carbon structural steels. The heating time for high-alloy structural and tool steels should be from 50 to 100% higher.
The quenching media in general use are:
(i) Water
(ii) Brine
(iii) Oils
(iv) Air
(v) Molten salt.
(i) Water:
It is probably the most widely used as it is simple and effective, it cools at the rate of 982°C per second. It tends, however, to form bubbles on the surface of the metal being quenched and causes soft spots, so a brine solution is often used to prevent this trouble.
(ii) Brine:
It is a very rapid cooling agent and may tend to cause distortion of the parts, as will water.
(iii) Oil:
It is used when there is any risk of distortion although it is more suitable for alloy steels than plain carbon steels.
(iv) Air Blast:
When the risk of distortion is great, quenching must be carried out in an air blast. Since the rate of cooling is then lower, more hardening elements must be added to the steel, forming an air-hardening alloy. The air blast must be dry, since any moisture in the air will crack the steel.
(v) Molten Salts:
High speed steels are often quenched in molten salt to harden them.
Note:
Hypoeutectoid steel containing very little carbon, say less than 0.25%, cannot be easily hardened by sudden quenching because of large amount of soft ferrite which it contains and all of which cannot be retained in solution even on very quick cooling. The hardening capacity of steel increases with carbon content.
Mass Effect:
Mass effect is the variation in hardness across a section of the components having higher thickness, through heat treatment.
Hardness of plain carbon steel depends upon its carbon content and the rate of cooling from the hardening temperature. A part having less thickness will cool more quickly than a part having higher thickness if both are cooled in the same quenching bath.
In a thicker component, outer layer will cool faster than the core and heat will get trapped at the centre. This leads to a variation in hardness across a section of the component. The result can be another layer of martensite and inner core of particle, this variation in hardness is referred to as mass effect.
Owing to “mass effect” plain carbon steels having large sections are said to have a poor hardenability, as they cannot be fully hardened throughout.
4. Tempering:
Martensitic structures formed by direct quenching of high-carbon steel are hard and strong, but are also brittle. They cannot be plastically deformed and have very little toughness, and although strong they are unable to resist impact loads and are extremely sensitive to stress concentrations. Some of the hardness and strength must be sacrificed to obtain suitable ductility and toughness. This is done by tempering the martensitic steel.
Thus tempering process is carried out to:
(i) Increase toughness,
(ii) Decrease hardness,
(iii) Stabilise structure,
(iv) Relieve stresses, and
(v) Change volume.
The process of tempering consists of heating quenched, hardend steel, steel in martensitic condition, to some pre-determined temperature between room temperature and the critical temperature of the steel for a certain length of time, followed by air cooling.
Method of Tempering:
Tempering of steel may be carried out in liquid baths such as oil, salt or lead, the bath being heated to the correct temperature and steel immersed in the bath for the determined length of time after which it (steel) is removed and allowed to cool to room temperature. Air tempering furnances are very successfully employed for tempering. These furances are fully automatic and the hot air (heated by gas or electric means) is circulated around the parts to be tempered.
Theory of Tempering:
The steel which has been hardened by quenching is considered to be in an unstable condition. And perhaps in most quenching operations some austenite is retained at room temperature. The martensitic structure of hardened steel is much different from the structure of normal pearlite formed in steels that have been slow cooled from the austenitic state.
The martensite which is fresh born has a tetragonal atomic arrangement and is known an alpha martensite. In this condition, martensite is eager to change to a more stable structure (more nearly pearlitic) and undergoes this change when offered an opportunity (such as when temperature is raised during the tempering operation).
When alpha martensite is heated to app. 99°C, beta martensite (with body centered cubic lattice) is formed. If beta martensite is further heated it precipitates carbon in the form of cementite which has been held in supersaturated solution in the martensite.
The size of the precipitated cementite particles is very small and their size is not revealed by microscopic examination when the tempering temperature is low and around 200°C; but if the tempering temperature is raised beyond 200°C the size of minute cementite particles continue to grow in size and finally becomes microscopic.
Upon reheating to a temperature of 260°C any retained austenite found in hardened carbon or low alloy steels may be transferred to martensite or a tempered form of martensite. When austenite changes to martensite upon tempering it is accompanied by an expansion (which may be very marked) and such a change induces internal stresses and may be removed as partially by increasing the temperature. Changing the retained austensite to martensite by reheating to 260°C will effect a change in original martensite which results in a loss of maximum hardness.
Any retained austenite at room temperature may be transformed to martensite by cold treating (i.e., cooling to sub-normal temperatures 21 to 33°C). Cold treating does not cause any loss of hardness of the original martensite as that which occurs during tempting and yields maximum hardness. The cold treating of hardened steel may be carried out after tempering treatment.
The results from tempering depend on the time of treatment. The longer the time of treatment (at a given tempering temperature) better are the results. It is recommended that for getting satisfactory results at least one hour be allowed at any tempering temperature. Some tempering operations consume several hours.
Temper Colours:
Tempering can be judged by the temper colours which appear on the bright red surface, and experienced eyes are generally guided by those colours while heating steel materials for tempering.
The following are the colours formed on steel in the process of tempering:
Effects of tempering temperature on the mechanical properties of steel:
Tempering process decomposes the martensite into a ferrite-cementite mixture and thus the properties of steel are strongly affected.
(i) At low tempering temperature (upto 200° to 250°C),
(a) Bending and true tensile strength are increased, and
(b) Hardness changes to a small extent.
(ii) If the temperature is further increased the following results:
(a) Hardness, true tensile strength, proportional limit and yield point are reduced.
(b) Reduction of area and relative elongation are increased.
(iii) Tempering at 250° to 400°C reduces the impact strength of steel. Therefore, the temperature range 250°C to 400°C should be avoided in assigning tempering temperatures.
(iv) The properties after structural improvement, i.e., hardening followed by high tempering are always higher than those of annealed steel. This is due to the difference in structure of the ferrite-cementite mixture.
Temper Brittleness (Embrittlement):
Alloy steels containing nickel, manganese and chromium when cooled slowly from tempering temperature of about 350° to 550°C become brittle in impact. However, they usually show normal ductility in the standard tension test. If these steels are quenched in oil or water from the above temperatures, they remain tough in impact.
The embrittlement produced during slow cooling may be due to the separation of some brittle phase. This phase may be soluble above 350°C and hence its separation suppressed during rapid cooling, eliminating the embrittling effect.
Addition of about 0.5 percent molybdenum also eliminates temper embrittlement.
Steels produced at about 350°C appear blue in colour and hence the brittleness observed at 350°C is called as blue brittleness.
Surface of component, during quenching, cools rapidly and centre cools slowly; therefore, phases appearing at the surface and centre are likely to be different. This results in non-uniform volume changes. The overall effect of non-uniform cooling and non-uniform volume changes is to cause heavy distortion and cracking of the components. The cracking may result during quenching or sometimes after quenching, if tempering is delayed or in the early stages of tempering.
Quenching cracks are liable to occur due to following reasons:
a. Improper selection of steel.
b. Time delays between hardening and tempering operations.
c. Excessive amount of non-metallic inclusions in steel.
d. Improper selection of quenching medium.
e. Improper entry of the component into the quenching medium with respect to the shape of the component. This leads to non-uniform and eccentric loading.
f. Improper design of keyways, holes, sharp changes in cross-section, mass-distribution and non-uniform sections.
g. Quenching from higher temperature. Higher austenitizing temperatures lead to grain coarsening of austenite resulting in coarse grained martensite which is more prone to cracking.
Age hardening or precipitation hardening, as it is often called, is the most important method of improving the physical properties of some of the non-ferrous alloys by solid state reaction. This method is most applicable to the alloys of aluminium, magnesium and nickel and occasionally, used for alloys of copper and iron.
The process consists of two stages:
a. In first stage an unstable condition is produced by the formation of a supersaturated solid solution. In this state, there is no appreciable change in physical properties and the alloy remains soft and ductile. The second stage consists of precipitation of the supersaturated phase which increases the hardness and strength of the alloy.
The precipitation will take more time at lower temperatures and may take several days at room temperatures, at higher temperatures the process is quickened and when done in furnace, it is called “precipitation heat treatment”.
b. Steel is age-hardened by keeping it at a low temperature for many hours during which the carbon slowly diffuses and renders the steel hard. It is found that if kept at 20°C the steel reaches its maximum hardness in 500 hours, while if maintained at 30°C the steel reaches maximum hardness in 10 hours.
5. Surface Hardening:
The various methods of surface hardening are discussed below:
The objects of case hardening are:
1. To obtain a hard and wear resistance surface on machine parts with enrichment of the surface layer with carbon to concentration of 0.75 to 1.2%.
2. To obtain a tough core.
3. To obtain close tolerances in machining parts.
4. To obtain a higher fatigue limit and high mechanical properties in the core.
Case hardening consists in heating a steel in the presence of a solid, liquid or gas, rich in carbon in order to enable the surface to be hardened, while retaining a tough ductile core.
There are three methods of adding carbon to the surface of the metal:
1. Pack hardening.
2. Liquid carburizing.
3. Gas carburizing.
ii. Nitriding:
The nature of the nitriding process used to obtain a case hardened product is very different from that of the carburizing process. Nitrogen, instead of carbon, is added to the surface of the steel. Carbon does not play any part in the nitriding operation but influences the machinability of steel. The temperatures used in nitriding are much lower than those used in carburizing and below the critical temperature of the steel.
Simple carbon steels, which are often used for carburizing are not used for nitriding. Steels used in the process are special alloy steels. With the nitriding developing rather thin cases, a high core hardness is required to withstand any high crushing loads. High tempering temperatures call for a steel with a higher carbon content in order to develop this increase in core hardness.
In addition to higher carbon content, various alloying elements are called for in the steel to bring about an increase in the formation of these nitrides. Aluminium seems to display the strongest tendency in the formation of these nitrides. Chromium, molybdenum, vanadium and tungsten, all being nitrides formers, are also used in nitriding steels. Nickel in nitriding steels hardens and strengthens the core and toughens the case but with slight loss in its hardness.
Nitriding Operation:
In nitriding process, nitrogen in introduced to the steel by passing ammonia gas through a muffle furnace containing the steel to be nitrided. The ammonia is purchased in tanks as a liquid and introduced into the furnace as a gas at slightly greater than atmospheric pressure. With the nitriding furnace operating at a temperature of 480° to 540°C, the ammonia gas partially dissociates into nitrogen and hydrogen gas mixture.
The dissociation of ammonia is shown by the following equation:
2NH3 = 2N + 3H2
The operation of the nitriding cycle is usually controlled so that the dissociation of the ammonia gas is held to approximately 30% but may be varied from 15% to 95%, depending upon operating conditions.
The gas mixture leaving the furnace consists of hydrogen, nitrogen and undissociated ammonia. The undissociated ammonia, which is soluble in water, is usually discharged into water and disposed of in this manner.
The free nitrogen formed by this dissociation is very active, uniting with the iron and other elements in the steel to form nitrides. These nitrides are more or less soluble in the iron and form a solid solution, or more likely, are in a fine state of dispersion, imparting hardness to the surface of the steel.
From the surface the nitrides diffuse slowly, and the hardness decreases inwardly until the unaffected core is reached. The depth of penetration depends largely upon the length of time spent at the nitriding temperature. Diffusion of these nitrides is much slower than diffusion of carbon in the carburizing operations so a much longer time is required to develop similar penetration.
In general, the surface hardness is much greater after nitriding than it is after carburizing and hardening. The maximum hardness obtained from a carburized and hardened case runs around 67 Rockwell C; whereas, it is possible to obtain surface hardness value in excess of 74 Rockwell C by nitriding.
The surface hardness of the nitrided case cannot be measured by the case; it is usually measured by the Rockwell superficial scale such as 15 N or 30 N scale. The hardness value on the Rockwell C scale may be estimated from a hardness conversion table.
In order to obtain localized nitriding of parts or to stop nitriding on some surface areas, the best method is to use tin as a protecting agent against the nitriding action. Tin, in the form of a paste or paint made from tin powder, or tin oxide mixed with glycerine or shellac, may be applied as a satisfactory stop off method.
Although tin melts at a lower temperature than used in nitriding, ample protection is provided by thin layer of tin that is held to the surface by surface tension. Care should be exercised to apply the tin paint or electroplate to a clean surface and to avoid a thick layer of tin which may run or dip into surface where protection is not desired.
All heat treatments, such as the quench hardening of steel in the nitriding process, are carried out before the nitriding operation. After a rough machining operations, the steel is heated to about 950°C, held for the necessary length of time at this temperature in order to have the alloying elements go into solution in the austenite, thereby imparting core strength and toughness after quenching and finish machining, to produce a sorbitic structure which has a rough case and eliminates any brittleness resulting from any free territe.
This hardening, thus reduces distortion during nitriding. After tempering all oxide film and traces of decarburization are removed. Any decarburisation left on the surface of the steel to be nitrided will usually result in failure of the nitrided surfaces by peeling or spalling off. The steel is then nitrided and allowed to cool slowly to room temperature in the nitriding box or chamber. No quenching is required; the steel develops its maximum hardness without necessitating a further quenching operation.
Merits and Demerits of Nitriding:
Merits:
Nitriding (a hard surfacing operation) is associated with the following merits:
1. Greater resistance to wear and corrosion.
2. Less warping or distortion of parts treated.
3. Greater surface hardness.
4. Greater fatigue strength under corrosive conditions.
5. Higher endurance limit under bending stresses.
6. Better retention of hardness at elevated temperatures.
Demerits:
The demerits of ‘Nitriding’ are as follows:
1. Medium used is expensive.
2. High furnace costs due to the longtime of treatment.
3. Necessity of using special alloy steels.
4. Necessity of using high alloy containers to resist the nitriding.
iii. Cyaniding:
Cyaniding is a process of superficial case hardening in which the steel is heated in a molten cyanide salt at about 850°C followed by quenching. Both carbon and nitrogen are absorbed in this process.
In carbon nitriding steel is heated in a gaseous mixture of ammonia and hydrocarbons whereby both carbon and nitrogen are absorbed.
iv. Flame Hardening:
It is a process of surface hardening by which steel or cast iron is raised to high temperature by a flame and then almost immediately quenched. This process of hardening is used for local hardening of such components as wheel teeth.
Fig. 5.25 shows a flame hardening of gear teeth. Aflame from an oxy-acetylene or similar burner is played on to the teeth so as to raise temperature rapidly above the hardening temperature. Hardening results when the austenised surface is quenched by spray (usually) that follows the flame.
The advantage of this process is that there is much less distortion than in ordinary methods. It is quick, and the hardening is restricted to parts which are affected by wear.
v. Induction Hardening:
This process of surface heating is based upon inductive heating in which a high-frequency current is first transformed from high to low voltage, and the heavy low voltage current is
passed through the inductor blocks which surround the bearing journal to be hardened without actually touching it.
The inductor block current induces current in the surface of the metal which the block surrounds and it is this induced current which heats the surface to be hardened. When the area in question has been thus subjected to an accurately controlled high-frequency current for proper length of time, the electrical circuit is opened and simultaneously the heated surface is quenched by a spray from a water jacket built into the inductor block. Fig. 5.26 shows the scheme of induction hardening.
The induction hardening is at present extensively used for producing hard surface on crankshaft, camshaft, axels and gears.
The principle advantages are listed below:
1. The time required for this heat-treatment operation is less thereby increasing the labour productivity.
2. Deformation due to heat-treatment is considerably reduced.
3. The articles which are induction heated have no scale effect.
4. The hardening of the surface can be easily controlled by controlling the current.
5. The depth of hardness can be easily controlled by varying frequency of supply voltage.