HEAT TREATMENT

Heat treatment is a heating and cooling process of a metal or an alloy in the solid state with the purpose of changing their properties. Rapid cooling the metal from above the critical range, results in hard structure. Whereas very slow cooling produces the opposite affect i.e. soft structure. In any heat treatment operation, the rate of heating and cooling is important. A hard material is difficult to shape by cutting, forming, etc.

HEAT TREATMENT FURNACES

Hearth Furnaces
These furnaces are heated by fuel which may be coke, coal, gas (town, blast or natural) and
fuel oil. They can also be operated electrically. They are generally of two types.
(a) Stationary type
      It consists of four types
      (1) Direct fuel fired furnace
      (2) Indirect fuel fired furnace
      (3) Multiple furnace
      (4) Re-circulation furnace

(b) Movable type
      It consists of two types
     (1) The car bottom type
     (2) The rotary type

Bath Furnaces

In bath type furnaces, heating may be done using by gas, oil or electricity. These furnaces are further classified as:

(1) Liquid bath type
(2) Salt bath type
(3) Lead bath type
(4) Oil bath type

Structures in Fe-C-diagram
The main microscopic constituents of iron and steel are as follows:
1. Austenite
2. Ferrite
3. Cementite
4. Pearlite

Figure 1. Fe-C equilibrium diagram

Austenite
Austenite is a solid solution of free carbon (ferrite) and iron in gamma iron. On heating the steel, after upper critical temperature, the formation of structure completes into austenite which is hard, ductile and non-magnetic. It is able to dissolve large amount of carbon. It is in between the critical or transfer ranges during heating and cooling of steel. It is formed when steel contains carbon up to 1.8% at 1130°C. On cooling below 723°C, it starts transforming into pearlite and ferrite. Austenitic steels cannot be hardened by usual heat treatment methods and are non-magnetic.

Ferrite
Ferrite contains very little or no carbon in iron. It is the name given to pure iron crystals which are soft and ductile. The slow cooling of low carbon steel below the critical temperature produces ferrite structure. Ferrite does not harden when cooled rapidly. It is very soft and highly magnetic.

Cementite
Cementite is a chemical compound of carbon with iron and is known as iron carbide (Fe3C). Cast iron having 6.67% carbon is possessing complete structure of cementite. Free cementite is found in all steel containing more than 0.83% carbon. It increases with increase in carbon % as reflected in Fe-C Equilibrium diagram. It is extremely hard. The hardness and brittleness of cast iron is believed to be due to the presence of the cementite. It decreases tensile strength. This is formed when the carbon forms definite combinations with iron in form of iron carbides which are extremely hard in nature. The brittleness and hardness of cast iron is mainly controlled by the presence of cementite in it. It is magnetic below 200°C.

Pearlite
Pearlite is a eutectoid alloy of ferrite and cementite. It occurs particularly in medium and low carbon steels in the form of mechanical mixture of ferrite and cementite in the ratio of 87:13. Its hardness increases with the proportional of pearlite in ferrous material. Pearlite is relatively strong, hard and ductile, whilst ferrite is weak, soft and ductile. It is built up of alternate light and dark plates. These layers are alternately ferrite and cementite. When seen with the help of a microscope, the surface has appearance like pearl, hence it is called pearlite. Hard steels are mixtures of pearlite and cementite while soft steels are mixtures of ferrite and pearlite.
As the carbon content increases beyond 0.2% in the temperature at which the ferrite is first rejected from austenite drop until, at or above 0.8% carbon, no free ferrite is rejected from the austenite. This steel is called eutectoid steel, and it is the pearlite structure in composition.
As iron having various % of carbon (up to 6%) is heated and cooled, the following phases representing the lines will tell the about the structure of iron, how it charges.

Objectives of Heat Treatment

The major objectives of heat treatment are given as under :
1. It relieves internal stresses induced during hot or cold working.
2. It changes or refines grain size.
3. It increases resistance to heat and corrosion.
4. It improves mechanical properties such as ductility, strength, hardness, toughness, etc.
5. It helps to improve machinability.
6. It increases wear resistance
7. It removes gases.
8. It improves electrical and magnetic properties.
9. It changes the chemical composition.
10. It helps to improve shock resistance.
11. It improves weldability.

The above objectives of heat treatment may be served by one or more of the following heat treatment processes:

1. Normalizing
2. Anne aling.
3. Hardening.
4. Tempering
5. Case hardening
    (a) Carburizing
    (b) Cyaniding
    (c) Nitriding
6. Surface hardening
    (a) Induction hardening,
    (b) Flame hardening.

NORMALIZING
Normalizing is a defined as softening process in which iron base alloys are heated 40 to 50°C above the upper-critical limit for both hypo and hyper eutectoid steels and held there for a specified period and followed by cooling in still air up to room temperature.

Objectives :
1. To soften metals
2. Refine grain structure
3. Improve machinability after forging and rolling
4. improve grain size
5. Improve structure of weld
6. Prepare steel for sub heat treatment

Figure 2. Structure of normalized medium carbon steel

ANNEALING
It is a softening process in which iron base alloys are heated above the transformation range held there for proper time and then cool slowly (at the of rate of 30 to 150°C per hour) below the transformation range in the furnace itself. Heating is carried out 20°C above upper critical temperature point of steel in case of hypo eutectoid steel and the same degree above the lower critical temperature point in case of type eutectoid steel.

Objectives of Annealing
The purpose of annealing is to achieve the following
1. Soften the steel.
2. Relieve internal stresses
3. Reduce or eliminate structural in-homogeneity.
4. Refine grain size.
5. Improve machinability.
6. Increase or restore ductility and toughness.

Annealing is of two types
(a) Process annealing
(b) Full annealing.
In process annealing, ductility is increased with somewhat decrease in internal stresses. In this, metal is heated to temperature some below or close to the lower critical temperature generally it is heated 550°C to 650°C holding at this temperature and it is slowly cooled. This causes completely recrystallisation in steel.
The main purpose of full annealing of steel is to soften it and to refine its grain structure. In this, the hypo-eutectoid steel is heated to a temperature approximately 20° to 30°C above the higher critical temperature and for hypereutectoid steel and tool steel is heated to a temperature 20 to 30°C above the lower critical temperature and this temperature is maintained for a definite time and then slowly cooed very slow1y in the furnace itself.

Figure 3 Structure of annealed medium carbon steel

SPHEROIDIZATION
It is lowest temperature range of annealing process in which iron base alloys are heated 20 to 40°C below the lower critical temperature, held therefore 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.

After normalizing of steels, the hardness of the order of 229 BHN and as such machining becomes difficult and hence to improve machining, these are spheroidised first and then machined. This treatment is carried out on steels having 0.6 to 1.4% carbon. The objectives of spheroidising are given as under.

1. To reduce tensile strength
2. To increase ductility
3. To ease machining
4. To impart structure for subsequent hardening process

HARDENING
Hardening is a hardness inducing kind of heat treatment process in which steel is heated to a temperature above the critical point and held at that temperature for a definite time and then quenched rapidly in water, oil or molten salt bath. It is some time said as rapid quenching also. Steel is hardened by heating 20-30°C above the upper critical point for hypo eutectoid steel and 20-30°C above the lower critical point for hyper eutectoid steel and held at this temperature for some time and then quenched in water or oil or molten salt bath 

Figure 4 (a) shows the structure obtained on water quenching on hardening of medium carbon steel. Figure 4 (b) shows the structure obtained on oil quenching on hardening of medium carbon steel. Figure 4 (c) shows the structure obtained on water quenching on hardening of medium carbon steel and followed by tempering.

Figure 4. Structure of hardened carbon steel

 

Metal is heated up to austenite formation and is followed by fast and continuous cooling of austenite to temperature 205° to 315°C or even lower than that. Due to such rapid cooling, austenitic structure changes to new structure known as martensite. Martensite is a body centered phase produced by entrapping carbon on decomposition of austenite when cooled rapidly. It is extremely hard and brittle.

TEMPERING
If high carbon steel is quenched for hardening in a bath, it becomes extra hard, extra brittle and has unequal distribution internal stresses and strain and hence unequal harness and toughness in structure. These extra hardness, brittleness and unwanted induced stress and strain in hardened metal reduce the usability the metal. Therefore, these undesired needs must be reduced for by reheating and cooling at constant bath temperature. In tempering, steel after hardening, is reheated to a temperature below the lower critical temperature and then followed by a desired rate of cooling.

Depending upon the temperature of reheat, the tempering process is generally classified in to three main categories. Which are discussed as under.

Low Temperature Tempering
Hardened steel parts requiring tempering are heated up to 200°C and then quenched in oil. Tempering is used to retain hard micro-structure of martensite which increases brittleness. Figure 5a represents the microstructure of martensite.

Medium Temperature Tempering
Hardened steel parts requiring tempering are heated in the temperature range of 200-350°C. This process gives troosite structure. Troosite structure is another constituent of steel obtained by quenching tempering martensite. It is composed of the cementite phase in a ferrite matrix that cannot be resolved by light microscope. It is less hard and brittle than martensite. It is also produced by cooling the metal slowly until transformation begins and then cooling rapidly to prevent its completion. It has a dark appearance on etching. It is weaker than martensite. Figure 5b represents the microstructure of troosite 

High Temperature Tempering
Hardened steel parts requiring tempering are heated in the temperature range of 350-550°C. This process gives sorbite structure. Sorbite structure is produced by the, transformation of tempered martensite. It is produced when steel is heated at a fairly rapid rate from the temperature of the solid solution to normal room temperature. It has good strength and is practically pearlite. Its properties are intermediate between those of pearlite and troosite.
Parts requiring tempering are heated in the temperature range of 550-750°C. This process gives spheriodite structure. Figure 5(c) represents the microstructure of sorbite. However there are other special kinds of tempering also which are discussed as under. 

 Figure 5. Structures obtained tempering of hardened steel

Aus-Tempering
It is a special type of tempering process in which and steel is heated above the transformation range then suddenly quenched in a molten salt bath at a temperature 200 to 450°C. The piece is held at that temperature until the and outside temperature are equalized. The part is then reheated and cooled at moderate rate. Aus-tempering produces fine bainite structure in steel but with minimum distortion and residual stresses.

Advantages of Aus-Tempering :
1. Quenching cracks are avoided.
2. Distortion and warping are avoided.
3. A more uniform microstructure is obtained.
4. Mechanical properties of bainite are superior to conventional hardening microstructure.
 

Limitations of Aus-Tempering
1. The process is very costly.
2. The process is time consuming.

Mar Tempering
It is a type of tempering process in which and its base alloys are heated above the transformation range then suddenly quenched in a molten salt bath at a temperature 80 to 300°C. The piece is held at that temperature until the and outside temperature are equalized. The part is then reheated and cooled at moderate rate. Mar-tempering produces martensite in steel but with minimum distortion and residual stresses.

Carburizing
Carburizing can be of three types
1. Pack carburizing
2. Liquid carburizing and
3. Gas carburizing
The above carburizing processes are discussed as under.

Pack Carburizing
Metals to be carburized such as low carbon steel is placed in cast iron or steel boxes containing a rich material in carbon like charcoal, crushed bones, potassium Ferro-cyanide or charred leather. Such boxes are made of heat resisting steel which are then closed and sealed with clay. Long parts to be carburized are kept vertical in -boxes. The boxes are heated to a temperature 900°C to 950°C according to type of steel for absorbing carbon on the outer surface. The carbon enters the on the metal to form a solid solution with iron and converts the outer surface into high carbon steel. Consequently pack hardened steel pieces have carbon content up to 0.85% in their outer case. After this treatment, the carburized parts are cooled in boxes. Only plane carbon steel is carburized in this process for hardening the outer skin and refining the structure of the core to make it soft and tough. Small gears are case hardened by this process for which they are enclosed in the cast iron or steel box containing a material rich in carbon, such as small piece of charcoal and then heat to a temperature slightly above the critical range. Depth of hardness from 0.8-1.6 mm is attained in three to four hours. The gears are then allowed to cool slowly with-in the box and then removed. The second stage consists of reheating the gears (so obtained) to about 900°C and then quenched in oil so that its structure is refined, brittleness removed and the core becomes soft and tough. The metal is then reheated to about 700°C and quenched in water so that outer surface of gear, which had been rendered soft during the preceding operation, is again hardened. 

Liquid Carburizing
Liquid carburizing is carried out in a container filled with a molten salt, such as sodium cyanide. This bath is heated by electrical immersion elements or by a gas burner and stirring is done to ensure uniform temperature. This process gives a thin hardened layer up to 0.08 mm thickness. Parts which are to be case-hardened are dipped into liquid bath solution containing calcium cyanide and polymerized hydro-cyanide acid or sodium or potassium cyanide along-with some salt. Bath temperature is kept from 815°C to 900°C. The furnace is usually carbon steel case pot which may be by fired by oil, gas or electrically. If only selected portions of the components are to be carburized, then the remaining portions are covered by copper plating. There are some advantages of the liquid bath carburizing which are given as under. 

Advantages
1. Greater depth of penetration possible in this process.
2. Selective carburizing is possible if needed.
3. Uniform heating will occur in this process.
4. Little deformation or distortion of articles occur in this process.
5. Ease of carburizing for a wider range of products.
6. It is time saving process.
7. Parts leave the bath with a clean and bright finish.
8. There is no scale in this process as occur in pack hardening.

Gas Carburising
In gas carburizing method, the parts to be gas carburized are surrounded by a hydrocarbon gas in the furnace. The common carburizing gases are methane, ethane, propane, butane and carbon monoxide are used in this process. Carbon containing gas such as carbon monoxide (CO), methane (CH4), ethane (C2H6) or town gas is introduced in the furnace where low carbon steel is placed. The furnace is either gas fired or electrically heated. Average gas carburizing temperature usually varies from 870° to 950°C. Thickness of case hardened portion up to 11 mm can be easily obtained in 6 hours. The carburized parts can heat treated after carburizing. Steel components are quenched in oil after carburizing and then heated again to form fine grain sized austenite and then quenched in water to form martensite in surface layers. This gives maximum toughness of the core and hardness of the surface of product.

Cyaniding
Cyanide may also be used to case harden the steel. It is used to give a very thin but hard outer case. Cyaniding is a case hardening process in which both C and N2 in form of cyaniding salt are added to surface of low and medium carbon steel. Sodium cyanide or potassium cyanide may be used as the hardening medium. It is a process of superficial case hardening which combines the absorption of carbon and nitrogen to obtain surface hardness. The components to be case hardened are immersed in a bath having fused sodium cyanide salts kept at 800-850°C. The component is then quenched in bath or water. This method is very much effective for increasing the fatigue limit of medium and small sized parts such as gears, spindle, shaft etc. Cyanide hardening has some advantages and disadvantage over carburizing and nitriding method. Cyaniding process gives bright finishing on the product. In it, distortion can be easily avoided and fatigue limit can be increased. Decarburizing can be reduced and time taken to complete the process is less. But the main disadvantage of this process is that it is costly and highly toxic process in comparison to other process of case hardening. There are some common applications of cyaniding process which are given as under.
Application
Cyaniding is generally applied to the low carbon steel parts of automobiles (sleeves, brake cam, speed box gears, drive worm screws, oil pump gears etc), motor cycle parts (gears, shaft, pins etc.) and agriculture machinery. 

Nitriding
Nitriding is a special case hardening process of saturating the surface of steel with nitrogen by holding it for prolonged period generally in electric furnace at temperature from 480°C to 650°C in atmosphere of Ammonia gas (NH3). The nitrogen from the ammonia gas enters into on the surface of the steel and forms nitrides and that impart extreme hardness to surface of the metal. Nitriding is a case hardening process in which nitrogen instead of carbon is added to the outer skin of the steel. This process is used for those alloys which are susceptible to the formation a chemical nitrides. The article to be nitride is placed in a container (made of high nickel chromium steel). Container is having inlet and outlet tubes through which ammonia gas is circulated. Ammonia gas is used as the nitrogen producing material. The alloy steel containing Cr, Ni, Al, Mo, V and Nitre-alloy are widely used for this process. Plain carbon steels are seldom nitirided. There are some common applications of this process which are given as under.
Application
Many automobile, diesel engines parts, pumps, shafts, gears, clutches, etc. are treated with the nitriding process. This process is used for the parts which require high wear resistance at elevated temperatures such as automobile and air plane valve’s and valve parts, piston pins, crankshafts, cylinder liners etc. It is also used in ball and roller bearing parts die casting dies, wire drawing dies etc.

Flame Hardening
It consists of moving an oxyacetylene flame, over the part where hardening is required. Immediately after this, the heated portion is quenched by means of water spray or air passing over it. Temperature attained by the surface is controlled and the rate of cooling is controlled by selecting a suitable medium. Flame hardening is suitable for large sized articles where only some portions of the surface requiring hardening and hence there is no need to heat the whole article in the furnace. Metal is heated by means of oxy-acetylene flame for a sufficient time unto hardening range and than quenched by spray of water on it. The hardened depth can be easily controlled by adjusting and regulating the heating time, temperature, flame and water spray. The main advantages of the process is that a portion of metal can be hardened by this process, leaving rest surface unaffected by confining the flame at relevant part only where hardening is required. This process is best suited to smal1 numbers of jobs which requiring short heating time. This method is highly suitable for stationary type of larger and bulky jobs.

Induction Hardening
Induction hardening is accomplished by placing the part in a high frequency alternating magnetic field. It differs from surface hardening in the way that hardness of surface is not due to the increase in carbon content but due to rapid heating followed by controlled quenching. In this process, a high frequency current is introduced in the metal surface and its temperature is raised up to hardening range. As this temperature is attained, the current supply is cut off instantaneously water is sprayed on the surface. Heat is generated by the rapid reversals of polarity. The primary current is carried by a water cooled copper tube and is induced into the surface layers of the work piece. Thin walled sections require high frequencies and thicker sections must require low frequencies for adequate penetration of the electrical energy. The heating effect is due to induced eddy currents and hysteresis losses in the surface material. Some portion of the metal part is heated above the hardening temperature and is then quenched to obtain martensite on the metal surface. There are some advantages of this process which are given as under.
Advantages
Induction hardening is comparatively quicker. A minimum distortion or oxidation is encountered because of the short cycle time. The operation is very fast and comparatively large parts can be processed in a minimum time. There are some applications of this process which are given as under.
Application
Induction hardening is widely used for hardening surfaces of crankshafts, cam shafts, gear automobile components, spline shafts, spindles, brake drums etc. It is also used for producing hard surfaces on cam, axles, shafts and gears.