The most commonly used operations of heat treatment for iron and steel are: 1. Annealing 2. Normalising 3. Hardening 4. Tempering 5. Carburising (Case-Hardening) 6. Cyaniding 7. Nitriding 8. Induction Hardening 9. Flame-Hardening.
Operation # 1. Annealing:
It is one of the most widely used operations in heat treatment of iron and steel and is defined (according to American Society of Material Testing) as the softening process in which iron base alloys are heated above the transformation range, held there for a proper time and then cooled slowly (at the rate of 30 to 150°C per hour) below the transformation-range in the furnace itself. Alternately, the steel may be transferred to another furnace at about 650°C, and held there until the austenite has transformed into pearlite (final cooling can be done in still air).
The success of annealing depends on controlling the formation of austenite, and the subsequent transformation of the austenite at high sub-critical temperatures. Heating is carried out (20°C) above the upper critical temperature of steel in case of hypo-eutectoid steel and the same degree above the lower critical point in case of hyper- eutectoid steel.
Since prolonged heating of steel can cause scaling due to carbon coming out, steel work pieces are packed into refractory-lined and sealed boxes to overcome this problem, or heated in a controlled atmosphere furnace to exclude air during the annealing operation.
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The steel on cooling changes into ferrite and pearlite for hypo-eutectoid steel, pearlite for eutectoid steel, and pearlite and cementite for hyper-eutectoid steel.
The time for which an article is held in the furnace is recommended as 1 to 2 hours.
The following important observations regarding annealing operation should be borne in mind:
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(i) The structure of annealed parts depends on the austenising temperature and the degree of homogeneity at that temperature. If the structure of steel at austenising temperature is homogenous then the structure of the annealed steel will be lamellar. However, if the structure of steel at austenising temperature is heterogeneous then the annealed structure will be spheroidal.
It must be remembered that at low austenising temperatures, the carbides dissolve fairly rapidly in the austenite in case of hypo-eutectoid steels. But in hyper-eutectoid steels carbides only agglomerate at low austenising temperature and thus high temperature is required for them. In the case of high alloy steels the structure even at high austenising temperature never becomes homogeneous.
(ii) When steel is heated above its critical temperature, austenite is formed and when it is cooled below the critical temperature (transformation temperature), austenite transforms back to ferrite and carbide. This transformation is a relatively slow process at some temperatures, and may be very rapid at other temperatures.
The final structure depends on the transformation temperature. Softest steel is obtained when steel is austenitised at less than 55°C above the critical, and transformed at less than 55°C below the critical temperature.
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(iii) Since the time for complete transformation at temperatures less than 55°C below the critical is very long, most of the transformation is allowed to take place at the higher temperature to form a soft product, and then transformation is finished at a lower temperature so that time for completion of transformation is short.
(iv) The steel after austenitising should be cooled, as rapidly as possible, to the transformation temperature in order to decrease the total time of the annealing operation and to avoid separation of some ferrite from austenite which is undesirable from point of machinability.
(v) In order to reduce the total time of the annealing operation, the steel after being completely transformed at the desired temperature (the one that produces the desired microstructure and hardness), is cooled as rapidly as feasible.
(vi) In order to establish that the residual carbides in the austenite serve as nuclei for the formation of coarse spheroidal carbides during the subsequent transformation of the austenite, (a) the steel is preheated for several hours at a temperature of about 28°C below the critical before austenitising (this assures a minimum of lamellar pearlite in the structure of annealed 0.7 to 0.9% carbon tool steel and other low-alloy medium-carbon steels), (b) the steel is heated for 10 to 15 hours at the austenitising temperature (this assures minimum hardness in annealed hyper-eutectoid alloy tool steels).
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The objects of annealing are:
(i) To soften the metal so that it can be cold worked.
(ii) To reduce hardness and improve machinability.
(iii) To refine grain size due to phase recrystallisation and produce uniformity.
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(iv) To increase ductility of metal.
(v) To prepare steel for subsequent heat-treatment.
(vi) To obtain desired mechanical, physical, electrical and magnetic properties.
(vii) To relieve internal stresses.
(viii) To produce a desired micro-structure.
Types of Annealing:
Various types of annealing treatments are:
1. Process Annealing:
It is a process that provides the recrystallization of ferrite at sub-critical temperature. But in this process hardness of steel is decreased as the ductility is increased. When the effects of cold working become so pronounced as to make further cold working impracticable, the process of sub-or process-annealing is carried out as an intermediate operation and steel thus becomes soft for further cold working.
So it is generally used on rolled products such as flats, sheets, etc. The steel is heated to about 650°C (10 to 20°C below AC1), soaked, and cooled freely in still air.
The free cooling can be employed because the heating and soaking do not produce any changes in the micro- constituents.
2. Patenting:
It is patent process and is similar to sub-annealing with the only difference that it is applied to steel wires.
3. Full Annealing:
It is described in the definition of annealing, i.e., annealing ferrous alloy by austenitising and then cooling slowly through the transformation range.
4. Spheroidising:
It is another process of annealing in which high carbon steels, tool steels containing a large amount of free cementite, which makes them brittle, are heated 20 to 40°C below the lower critical temperature, held there for a considerable period of time e.g. for 2.5 cm diameter piece the time recommended is four hours. It is then allowed to cool very slowly at room temperature in the furnace itself.
During this period, the cementite of steel which is in combined form of carbon becomes globular or spheroidal and these set in ferrite matrix, thus imparting softness and ductility to steel.
After normalising of steels, the hardness is of the order of 229 H.B. and as such machining becomes difficult and hence to improve machining, these are spheroidised first and then machined. This treatment is carried out on medium and carbon steels (0.6 to 1.4% carbon).
The objects of spheroidising are:
(i) To reduce tensile strength and to increase ductility.
(ii) To ease machining (minimum-hardness and maximum ductility). Spheroidising is seldom used on low- carbon steels because it becomes extremely soft. On low carbon steels, it is used to permit severe deformation.
(iii) To give a basic tool structure for subsequent hardening process.
5. Isothermal Annealing:
This process is used for plain and alloy high carbon steels. The time required to produce the desired microstructure is much less in this process than in conventional annealing. In this process a ferrous alloy is austenitised, and then cooled and held at transformation temperature at which austenite transforms to a relatively soft ferrite carbide aggregate.
For lamellar micro structure higher austenitising temperature is used, and for spheroidal structure lower austenitising temperature is employed. For softest structure, minimum austenitising temperatures and maximum transformation temperatures are used. Furnace time is saved by cooling rapidly from the austenitising temperature to transformation temperature and cooling rapidly by removing part from the furnace after the steel has completely transformed.
For high-carbon high chromium bearing steels, 100% spheroidised microstructure (ideal for machining) is obtained by austenitising at 775°C and transforming at 727°C for 4 hours and at 705°C for 1 hour and then cooling rapidly.
6. Box Annealing:
It is also known as closed annealing or pot annealing. In box annealing, the metal or alloy is annealed in a sealed container under conditions that minimise oxidation. The charge is heated slowly and then cooled slowly.
7. Black Annealing:
It is box annealing for ferrous alloy sheet, strip or wire.
8. Blue Annealing:
In this process, the hot rolled ferrous sheet is heated in an open furnace to a temperature within the transformation range and then cooled in air, to soften it, and a bluish oxide is formed on the surface.
9. Bright Annealing:
In this process, the steel is annealed in a protective medium to prevent surface discolouration.
10. Flame Annealing:
In this process the heat is applied directly by a flame.
11. Intermediate Annealing:
It refers to annealing wrought metals at one or more stages during manufacture and before final treatment.
12. Recrystallization Annealing:
In this process the cold worked metal is annealed to produce a new grain structure without phase change.
13. Soft (Subcritical) Annealing:
In this process the cold worked steel is annealed at temperatures of about 704 to 732°C to achieve nearly the maximum ductility obtainable in full annealing but with less risk of distortion. It is used before moderate to severe cold forming operations. Metals after this process are usually not suitable for general machining.
14. Stress Relieving:
Any process of annealing used with a sole purpose of reducing stresses.
15. Finish Annealing:
It is a low temperature annealing treatment (at about 510°C) applied to cold worked steels with low or medium carbon content. As compromise treatment, it lowers the level of residual stresses so as to lessen the risk of distortion in machining, and at the same time also retains most of the benefits to machinability contributed by cold working.
A lamellar structure is usually preferred for most machining operations on annealed medium-carbon alloy steels. However in some applications, spheroidal structure (globular carbides in a ferrite matrix) is desired. Austenitising temperature is the main key factor for this purpose.
For obtaining lamellar structure high austenitising temperature (110—220°C above the critical) is used, and for spheroidal structure low austenitising temperature (5 to 28°C above the critical) is used. In high- alloy steels in which many residual carbides exist even at very high austenitising temperatures, a lamellar structure can’t be produced. Similarly on low-alloy medium-carbon steels, the production of a completely spheroidal structure requires special care.
It may be remembered that preheating (heating the low-alloy with 0.4 to 0.8% carbon steel at a temperature of about 15 to 55°C below the critical) before austenising helps in obtaining spheroidal structure. Preheating agglomerates the carbides in the steel and these carbides are more resistant to solution in the austenite during the subsequent heating, and thus help in formation of spheroidal structure.
It must be remembered that time and temperature affect austenitisation and thus influence the number of un-dissolved carbides from which nucleation and coalescence of the spheroids occur. The temperature control is therefore very essential.
Operation # 2. Normalising:
According to American Society of Material Testing, it is defined as the process in which iron base alloys are heated 40 to 50°C above the upper transformation range and held there for a specified period (to ensure that a fully austenitic structure is produced) and followed by cooling in still air at room temperature.
The heating of hypo-eutectoid as well as hyper- eutectoid steel is done above the upper critical temperature. The normalised steel consists of ferrite and pearlite for hypo- eutectoid, and pearlite and cementite for hyper-eutectoid steel.
Normalising operation of steel is carried out to improve the machining characteristics, refine grain size and homogenise microstructure, modify and refine cast dendritic- structures, and provide desired mechanical properties.
The parts on which normalising treatment is carried out are:
(a) Normalising is generally carried out on large castings and forgings to put steel in the best condition for machining or hardening. It is also applied to low and medium carbon steel parts.
(b) It is frequently applied as the final heat-treatment process on items which are subjected to relatively high stresses.
The objects of normalising are:
(i) To eliminate coarse grain structure obtained during forging, rolling and stamping and produce fine grains.
(ii) To increase the strength of medium carbon steel.
(iii) To improve the machinability of low carbon steel.
(iv) To improve the structure of welds (uniformity of structure).
(v) To reduce internal stresses.
(vi) To achieve desired results in mechanical and electrical properties.
The parts subjected to normalising treatment have higher yield strength, ultimate tensile strength and impact strength but ductility is somewhat reduced.
Operation # 3. Hardening:
According to American Society of Material Testing, it is defined as the heat-treating process in which steel is heated at 20°C above the transformation range, soaking at this temperature for a considerable period to ensure thorough penetration of the temperature inside the component, followed by continuous cooling to room temperature by quenching in water, oil or brine solution.
Heating and cooling of steel is carried out as follows:
Heating is carried out (20°C) above the upper critical temperature of steel in case of hypo-eutectoid steel and same degree above the lower critical point of steel in case of hyper- eutectoid steel.
The former consists of ferrite and pearlite while the latter consists of pearlite and cementite. On heating above the critical temperature, these get changed into a single structure known as austenite structure containing a considerable part of cementite.
Upon cooling, which in this case, is a critical cooling (200°C/minute), the austenite is changed into a fine, needle-like microstructure known as martensite. Martensite is a super-saturated solution of carbon in a-iron. Hardness in steel is due to this very microstructure.
The hardness produced by hardening treatment depends on the carbon content of steel as shown in Fig. 2.2. It may be noted that steel containing less than 0.15% carbon does not respond to hardening treatment. It may be stressed again that martensite microstructure is obtained from austenite when quenching produces cooling at a critical rate. If the cooling rate is slightly less than the critical cooling rate, then austenite will be transformed into a fine form of pearlite called troostitic pearlite.
At a still slightly lower rate (but higher than at which pearlite is formed), austenite is transformed into another fine form of pearlite called sorbitic pearlite. Both these forms are hard, strong and brittle but not as hard and brittle as martensitic steel.
The rate of cooling is controlled by the quenching medium.
The medium are:
i. Solution of salt or caustic soda:
Very rapid and violent quench and thus not suitable for most of the applications.
ii. High flash point oil/clean water free from grease or soap:
Intermediate quenches. Most common quenching mediums. Oil quenched steel does not crack, while water quenched can.
iii. Blast of dry air:
Slow quench suitable only for certain special alloy steels.
Fig. 2.3 shows the effect of cooling rate on micro- structure produced.
It may be mentioned that severe quenching can lead to formation of cracks and as such quenching medium must be just severe enough to produce the desired hardness and strength. High-flash point oil is best for this application.
Ordinary lubricating oil is not suitable because it is subject to change, and certain lighter portions are evaporated by the hot metal and a sludge consisting of carbonised oil, and scale from quenched steel are formed, thus increasing the viscosity, and lowering the cooling properties.
The articles to be hardened should be introduced in quenching bath in vertical position to avoid distortion, and places where bubbles of vapour are likely to cling should be sealed. If possible, oil should be kept in turbulent condition by directing jets of oil on component, or by steady flow of oil by pumps or rotating paddles, etc.
The shape of parts for heat treatment deserves some attention. Sharp corners and asymmetrical or abruptly changing shapes are likely to develop cracks and should be avoided. Thin sections which will readily overheat in the furnace should also be avoided. Similarly blind hole, long thin section can also be problematic.
For best results, attempts should be made to approach a shape in which every point of any section or surface receives and gives back the same amount of heat with the same speed. Workpiece body should be simple, uniform and symmetrical changes in cross- section must be made gradually to minimise stress- concentration. Holes should be correctly located.
The parts on which hardening process is applied:
(a) As the hardness in steel is due to carbon content only, the hardening process is carried out only on high carbon steels.
(b) It is also applied on tool and structural steels.
Purpose of Hardening:
(i) To harden the steel to resist wear.
(ii) To enable steel to cut other metals.
The parts which are subjected to hardening are brittle, poor in ductility and toughness, good in tensile strength, and poor in impact strength, relative elongation and reduction in area.
Operation # 4. Tempering:
According to American Society of Material Testing, it is defined as the reheat process, reheating being carried out under sub-critical temperatures. Such a reheating permits the trapped martensite to transform into troostite or sorbite depending on the tempering temperature and relieve the internal stresses. Toughness and ductility are improved at the expense of hardness and strength.
It is an operation used to modify the properties of steel hardened by quenching for the purpose of increasing its usefulness.
The mechanical properties produced by hardening and tempering depend upon the carbon content of the steel, the rate at which it is cooled during the hardening process, and the tempering temperature. The troostite or sorbite structure consists of ferrite and finely divided cementite, and is different from that produced by mild quenching which is of a laminated form.
Tempering is divided into three classes according to the usefulness of steel required:
(i) Low temperature tempering.
(ii) Medium temperature tempering,
(iii) High temperature tempering.
(i) Low temperature tempering:
This process is used in order to retain hard microstructure of martensite, but to give up brittleness. The temperature upto which hardened steel is reheated is nearly 200°C. This type of tempering operation is applied on cutting and measuring tools of carbon and low alloy steel, and parts which are surface hardened and case carburised.
(ii) Medium temperature tempering:
It is employed for coil and laminated spring as it provides the highest attainable elastic limit in conjunction with ample toughness. The temperature upto which the steel is heated varies from 250 to 350°C. The steel on tempering develops troostite structure.
(iii) High temperature tempering:
This process is employed for structural steel as it provides the most favourable ratio of strength to toughness. It also eliminates completely internal stresses which are produced by quenching. The temperature up to which the steel is heated varies from 350 to 550°C. The steel on tempering develops sorbite structure.
The duration of heating depends upon the thickness of the workpiece. In order to avoid distortion of components, cooling is usually carried out in dry air. Fig. 2.11 shows the effect of tempering temperature on various properties.
Austempering of Steel: (T-T-T. Curve or S-Curve or Isothermal curve):
The time-temperature-transformation (T-T-T) diagram (Refer Fig. 2.12) is a plot between temperature and time showing how transformations take place at constant temperature for a carbon steel as the austenite is cooled. Curves C1 and C2 represent the beginning and ending of transformation from austenite, to different structures depending on the temperature at which transformation takes place.
Thus, when austenite is suddenly cooled to 575°C and then kept at this temperature (Refer curve ABCD in Fig. 2.12), then transformation from austenite to pearlite starts at C and completes at D. It may be seen that at temperatures above 600°C, transformation period goes on decreasing and coarse pearlite forms due to less time for formation of new nuclei.
The transformation time is more if temperature is below 600°C and produces fine pearlite. Such a transformation, taking place at constant temperature, is known as isothermal transformation. For curve ABEFG, austenite transforms into bainite (intimate mixture of ferrite and cementite in the form of tiny spheroids). It may be noted that around 200°C, curve CFC1 is flat and thus cooling below 220°C results in instantaneous transforming of austenite into martensite.
Austempering is a kind of tempering process which consists of holding the steel in a molten salt bath having temperature of 250 to 500°C above the critical temperature when the structure consists purely of Austenite. The part is then quenched at sufficient rate to avoid transformation to ferrite and pearlite and is held at the quenching temperature for a time sufficient to give complete transformation to an intermediate structure referred to as bainite. It is then cooled to room temperature.
It works best on work pieces of small and delicate cross-section and appears to be most beneficial in steels with carbon content ranging from 0.5 to 1% carbon.
Advantages:
(i) Quenching cracks are avoided.
(ii) Distortion and warping are avoided.
(iii) A more uniform micro-structure is obtained.
(iv) Mechanical properties of bainite are superior to conventional hardening micro-structure.
Limitations:
The process is very costly and time-consuming and it is mainly used for aero craft engine parts.
Martempering:
It is a type of the tempering process in which iron and its base alloys are heated above the transformation range and then suddenly quenched in a molten salt bath at a temperature of 180 to 300°C. The piece is held at that temperature until the core and outside temperature are equalised. The part is then removed and cooled at moderate rate.
Martempering produces martensite steel but with minimum distortion and residual stresses.
Spheroidising:
When hardened steel is tempered at a temperature (just below lower critical or A1 line), the cementite converts to small spheroids surrounded by ferrite when heated at this temperature for about 16 to 70 hours. The process can be accelerated by alternatively heating and cooling slightly above and below A1 line.
Spheroidising results in greater ductility, which improves the forming qualities and machinability.
Operation # 5. Case Hardening:
This process is used to produce a high surface hardness for wear-resistance supported by a tough, shock-resisting core.
It is the process of carburisation, i.e. saturating the surface layer of steel with carbon to about 0.9%, or some other process by which, case is hardened and core remains soft. The carburised steel is then heated and quenched, so that only the surface layers will respond, and the core remaining soft and tough since, its carbon content is low.
The parts to which it is applied are:
(a) Low carbon steel containing 0.1 to 0.18% carbon, which otherwise will not respond to direct hardening.
(b) Steel with 0.2 to 0.3% carbon for large components.
(c) Recently it has become the tendency to case harden steel with higher carbon content for medium and small machine parts.
(d) Alloy steel such as chromium steel, Cr-Ni-steel, Cr- Mn-Ti-steel.
(e) It is possible to keep some areas soft by insulating them during carburisation.
The objects of case hardening are:
(i) To obtain a hard and wear resistant surface on machine parts with enrichment of the surface layer with carbon to a concentration of 0.75 to 1.2%.
(ii) To obtain a tough core.
(iii) To obtain close tolerances in machining parts.
(iv) To obtain a higher fatigue limit and high mechanical properties in the core.
Various Processes of Case Hardening:
Carburisation:
In this process, the carbonaceous medium is a solid carburiser. The chief carburisers for pack carburising are activated charcoal with grain size varying from 3.5 to 10 mm in diameter, semi-coke and peat coke. Carbonates are added to the charcoal to accelerate the carburising process. Usually barium carbonate (BaCO3) and soda-ash (Na2CO3) are added in an amount varying from 10 to 40% of the total weight of charcoal.
A specific composition of a carburising medium is given below:
80—85% of charcoal with grain size varying from 3.5 to 10 mm diameter.
10 — 12% of BaCO3
1— 3% of CaCO3
1% of Na2CO3
A working mixture consists of 25 to 35% of fresh carburiser and 65 to 75% of used material.
The components which are to be pack carburised are first cleaned from dirt, scale and rust. These are then packed into the box. Welded steel boxes usually of rectangular or cylindrical form are employed. Packing of work is accomplished by first covering the bottom of the box with 40 to 45 mm layer of carburiser, then the component is placed with 20 to 25 mm space between the component and box walls. The remaining space is then covered with dense layer of carburiser 20 to 25 mm thick and rammed (Refer Fig. 2.15).
Fig. 2.15. Packing of work for Carburisation.
After this the top is covered with a cover and the edges are luted with fire clay. This is done so that carbon gas may not escape and air may not enter the box to cause decarburisation. The packed box is placed in the furnace and is heated to temperature of 900 to 950°C and held at that temperature for a sufficient length of period depending upon the hard surface depth required.
Normal practice shows that for obtaining hardness upto a depth of 0.1 mm. about 1 hour heating is necessary. Initially the rate of carburising is about 0.3 mm per hour and subsequently it takes four hours to produce depth of about 1.3 mm of high carbon steel at the surface.
The carburising of steel is due to atomic carbon and in pack carburising the atomic carbon is liberated by the decomposition of CO.
Air as always present in the carburising box, even when filled with carburiser. At higher temperatures oxygen in the air reacts with carbon in the carburiser to produce CO.
2C + O2 ——– > 2CO
BaCO3 + Heat ——– > BaO + CO2
CO2 + C ——- > CO
2CO + Heat ——- > CO2 + C (atomic)
2CO + Low carbon austenite ——— > CO2 + High carbon austenite, which makes the case hard.
The portion of the surface to be kept soft is insulated against carburising, by coating it with a suitable paste, or by plating it with about 0.1 mm of copper.
Carburising may also be carried out in a heated sodium carbonate and sodium cyanide salt-bath. Cases upto 0.3 mm depth can be produced, and at very fast rate. It is however observed that case is likely to flake because of abrupt change in carbon from surface to core. Gas-carburising is also used to carburise to a depth of about 1 mm in 4 hours, the carbon content being controlled by the composition of gas fired in a furnace.
The prolonged heating during carburisation makes the core coarse. It needs to be refined in order to make it tough. The core is refined by reheating to about 870oC and holding at that temperature long enough to produce uniformity of structure and then cooling rapidly.
Since outer surface contains lot of carbon, it gets transformed into extremely brittle martensite. The case is then refined by reheating steel to about 760oC and then quenching. Finally, the case is tempered at about 200oC, to relieve the quenching stresses. Core refining operation is carried out when it is required to resist shock load.
Operation # 6. Nitriding:
It is the process of saturating the surface of steel with N2 by holding it for a prolonged period (upto 100 hours) at a temperature ranging from 480 to 650oC in an atmosphere of NH3. A case of 0.7 mm takes about 100 hours.
For a steel specimen to respond to nitriding treatment, it must contain a small amount of chromium, or both chromium and aluminium. Steel with 3% Cr develops about 850 HV hardness and steel with 1.5% Cr and 1.5% aluminium about 1100 HV. Carbon content is between 0.2 to 0.5%, according to the core strength required. The required core properties are attained prior to nitriding treatment. Since no quenching is done after nitriding, finish machining operation is done before hand.
The parts on which nitriding process is applied are:
(i) Alloy steels containing Cr, Ni, Al, Mo, V. These elements may be present one at a time or all of them be present in the steel.
(ii) Best steel to be nitrated is Nitre-alloy. (A trade name given to nickel-steel).
(iii) Plain carbon steels are seldom nitrided.
Objects of nitriding are:
(a) To increase the hardness of surface to a very high degree.
(b) To increase the wear-resistance of surface.
(c) To increase the fatigue limit.
(d) To increase resistance to corrosion in media (atmosphere, water, steam).
Process:
It consists of heating the steel in atmosphere of NH3 at temperature ranging from 600 to 650°C without any further treatment. Anhydrous ammonia is dissociated into nascent nitrogen. The atomic nitrogen thus formed diffuses into iron to form nitrides of chromium, molybdenum and vanadium.
These nitrides give extreme hardness to surface. The hardness is of the order of 1000 to 1100 V.P.N. The grain growth is very less in this process. Local surface areas can be kept soft by insulating.
Advantages:
(i) No heat treatment is required after nitriding operation.
(ii) No possibility of any damage to the job such as scale on surface, crack and distortion.
(iii) Very high hardness is obtained (1000 to 1100 V.P.N.) which cannot be obtained by any other process.
(iv) No machining is required.
Disadvantages:
(a) Only a few alloys respond to this treatment.
(b) It takes a very long time to case harden the job and at the same time, it is a costly process.
Operation # 7. Cyaniding:
It is the process in which both carbon and nitrogen in the form of cyaniding salt are added to the surface of low and medium carbon steel to increase its hardness and wear resistance. This method is also effective for increasing the fatigue limit of medium and small sized parts such as gears, shafts, wrist-pins etc.
It involves the heating of parts in the molten cyanide salt bath maintained at temperature of 800 to 850°C and then quenching the steel in oil or water bath. The salt bath contains sodium cyanide (NaCN), soda ash (Na2CO3) and the sodium chloride (NaCl) in the proportion varying from 25 to 40%.
Cyaniding time is determined by the depth of the hardened case required and varies from 5 to 20 minutes. The resulting case contains 0.6 to 0.8 per cent C and 0.4 to 0.5 per cent N. The hardness obtained on the case by this process varies from 56 to 60 RC and the depth varies from 0.075 to 2.5 mm.
The principal advantages of this process are that bright finish can be obtained, distortion can be easily avoided, fatigue limit can be increased, decarburising can be reduced and time taken to complete the process is less. The limitations of cyaniding process are high cost and the toxicity of cyaniding salts.
Operation # 8. Induction Hardening:
The purpose of induction or flame hardening is to obtain hard and wear resistant surface whilst the core remains soft. The processes of induction hardening and flame hardening differ from each other in the way of heating.
In process of induction hardening a high frequency current of about 1000 to 10,000 cycles per second is passed through a copper inductor block which acts as a primary coil of the transformer. Heating by high frequency current is accomplished by the thermal effect of the current induced in the article being heated. The latter is placed in an alternating magnetic field set by the high frequency current. The part to be heated is placed in inductor coil which comprises of one or several turns of copper tube or busbar.
When alternating current is passed through the inductor, it sets up a magnetic field, the intensity of which varies periodically in magnitude and direction. The alternating magnetic lines pass through the surface of the articles being heated in the inductor and induce in the surface an alternating current of the same frequency but reversed in direction. This alternating current produces heating effect on the surface and temperature produced is of the order of 750 to 800°C for plain carbon and alloy steel.
The heating areas are then quenched immediately by sprays of water delivered through numerous small holes in the inductor block. The depth of hardness obtained varies from 0.1 to 0.8 mm. The part should have carbon content of about 0.45% for this method.
The induction hardening is at present extensively used for producing hard-surfaces on crank-shafts, cam shafts, axles and gears.
The principle advantages are listed below:
(i) The time required for this heat-treatment operation is less thereby increasing the labour productivity.
(ii) Deformation due to heat-treatment is considerably reduced.
(iii) The articles which are induction heated have no scale effect.
(iv) The hardening of the surface can be easily controlled by controlling the current.
(v) The depth of hardness can be easily controlled by varying the frequency of supply voltage.
Operation # 9. Flame Hardening:
This process is just like induction hardening with the difference that the heating of the specimen is carried out by flame instead of by induction effect. It is also based on rapid heating and quenching. The rapid quenching is achieved by sprays of water, connections of which are integral with the heating device.
The heating is generally accomplished by means of oxy-acetylene flame. Heating is carried out for sufficient time so as to raise the temperature of the surface of the specimen above the critical temperature. As soon as this temperature is reached, spraying is started.
Generally, in mass production work, progressive surface hardening is carried out, in which the flame and spraying equipment arranged suitably move together with respect to the work piece. As the flame progresses, it keeps on heating the surface to critical temperature and simultaneously, the surface is quenched behind the flame.
Thus the operation becomes continuous one. The jets of water and flame remain fixed in position and the work keeps on moving at the calculated rate. In such cases, there are usually more than one flame arranged radially. This is best suited to small works where heating time is short. The method is suitable for cases of upto 0.1 mm depth.
Other method is stationary type is which both the heating torch and work are stationary and it is suitable for large works.
In flame hardening, by proper control the interior surface is not at all affected and the depth of hardened case can be easily controlled by controlling heating time and flame temperature. Also, the surface treated by this process is free of scale and the equipment used is portable.
In comparison to induction hardening, it is cheaper method as initial investment is less. Same equipment can be used for all the sizes of specimen unlike induction hardening process. But by induction hardening process the hardness depth can be controlled very accurately by using different frequencies and also the method is very clean and quick. After flame hardening, steel is usually tempered.