Chapter no-03

Heat TreatmentHeat Treatment:- (two marks for explanation)

The heat treatment is very broad term and includes any heating and cooling operation or any sequence of two or more such operation- applied to any material in order to modify its nternal structure or to a ter its physical, mechanical or chemical properties.

Usually it consists of heating the material to some specific temperature, holding at this temperature for a definite period and cooling to room temperature or below room temperature with a definite rate.

Application :- ( two marks for any four application)

1. It is used for the hardening of a component like, shaft, crankshaft, etc.

2. It is used to increase the wear and abrasion resistance of cutting tools.

3. It is used for removing internal stresses formed during cold working operations.

4. It is used to increase corrosion resistance of steel

Two Objectives of Heat Treatment.- ( any Four x ½ mark each )

1. To refine grain structure.2. To improve machinability.3. To relieve internal stresses.4. To increase strength and wear resistance.5. To increase corrosion resistance.6. To increase Hardness and toughness of

metal surface.7. Formation of stable phase.

Needs of Heat Treatment-1. To refine grain structure.2. To improve machanibility.3. To relive internal stresses.

4. To increase strength and wear resistance.

5.To increase corrosion resistance. 6. To increase hardness and

toughness of metal surfaces

Figure 1. Heat-treatment temperature Ranges of Classes of Carbon Steels in Relation to the Equilibrium Diagram

ANNEALING -

ANNEALING -Explain any two methods of annealing.

1)Full annealing or Conventional Annealing :-Full annealing implies annealing a ferrous Alloy by austenitizing and then cooling slowly in the furnace itself through the transformation range. The austenitizing temperature range for hypoeutectoid steels is usually between 723ºC to 910 ºC and for hypereutectoid steels, temperature is 723ºC to 1130 ºC full annealing thus involves.-Heating steel to proper annealing temperature in the austenitic zone

Holding the steel object at that temperature for a definite period of time depending upon its thickness or diameter so that it becomes completely austenitic and then.-Cooling very slowly the steel object through the transformation range preferably in the furnace upto room temperature. The purpose of full annealing is to reduce hardness, to refine grain size, to make material homogeneous

Isothermal Annealing :--In this process transformation occurs at constant temperature. Steel is heated up to austenitic range then fast cooled to a constant temperature below AC1, and held at this temperature for sufficient period for the completion of transformation and then cooled to room temperature in air.It reduces the annealing time as compared to full annealing. Because of equalization of temperature, transformation occurs at the same time throughout the cross-section. This leads to more homogeneity in structure

3)SpheroidiseAnnealing :-(2M)This heat treatment is given to high carbon and air

hardening alloy steels to soften them and to increase machinability. Following methods produce spheroidised structures.

i)Hardening and high temperature tempering: -Due to tempering of hardened steelsat 650ºC

-700 ºC for a long time, cementite globules are formed in the matrix of ferrite.

ii)Holding at just below AC1 :-Due to holding for a long time at just below the lower critical temperature, cementite from pearlite globularises. The process is very slow

b) Describe spheroidise annealing in brief? ( 04 marks for appropriate answer)

This heat treatment is given to high carbon and air hardening alloy steels to soften them and to increase mach inability. The microstructure, typical of this heat treatment shows globules of cementite or carbides in the matrix of ferrite. Any heat treatment that produces a structure of the above is called spheroidises annealing.

Following methods produces spheroidised structure:i) Hardening and high temperature temperingDue to tempering of hardened steels at 650-700°c for a

long time, cementite globules are formed in the matrix of ferrite from martensite.

Martensite → cementite (in globular form) + ferrite.

ii)Thermal Cycling Around Ac1 :-Due to thermal cycling in a narrow temperature interval around AC 1, cementite lamellae from pearlite becomes spheroidal. During heating above A1, Cementite or carbides try to dissolve and during cooling they try to form. This repeated action spherodises the carbide particles.

4)Process Annealing :-Process annealing is usually subcritical

annealing and is applied to remove the effects of cold work, to soften and permit further cold work as in sheet and wire industries.

Ferrous alloys are heated to a temperature close to but below the lower limit of the transformation range (550ºC to 650 ºC) are held at that temperature and then cooled usually in air in order to soften the alloy for further cold working as in wire drawing.

5)Bright Annealing :-Annealing of steel components is carried out using some

protective medium to prevent oxidation and surface discoloration. Such type of annealing keeps the surface bright and

Hence it is called bright annealing. The surface protection is obtained by the use of an inert gas such as argon or nitrogen or by using reducing atmospheres.

6)Box annealing: -Here annealing is carried out in a sealed container under

conditions that minimize oxidation. The components are packed with cast Iron chips, charcoal or clean sand and annealed in a way similar to full annealing(Any two methods from above.)

Figures are not essential but if draw it would be considered

NormalizingExplain the procedure in normalizing. (4m)Normalizing is similar to annealing. The process consist of heating to above the upper critical

temperature AC3 for hypoeutectoid steels and above Acm for hypereutectoid steel by 30 to 50ºC, holding long enough at this temperature for homogeneous austenization and cooling to room temperature in still air.

Due to air cooling which is slightly fast as compared to furnace cooling employed in full annealing, normalized components show slightly different structure and properties than annealed components.

Hypereutectoid steels are usually normalized from above Acm temperature. Normalising produces microstructures consisting of ferrite and pearlite for hypoeutectoid steels. For eutectoid steels, the microstructure is only pearlite and it is pearlite and cementite for hypereutectoid steels

Purpose Of Normalising (Any Four) ½ Mark each)1. To increase strength and hardness.2. To obtain more refined grains than the annealing.3. To remove the internal stresses induced by heat treating, welding, casting etc.4. To improve machinability of low carbon steel

ANNEALING NORMALISING

Main purpose of annealing is to

relieve internal stresses

Main purpose of normalizing

is to improve mechanical

properties of steel.

Less hardness, more T.S. & toughness

Slightly more hardness, less T.S. andtoughness

Pearlite is coarse and usually gets

Pearlite is fine and usually appearsunresolved with optical microscope

Grain size distribution is more uniform.

Grain size distribution is slightly lessuniform

•Distinguish between annealing &Normalising. ( At least 4 points 1 Mark for each point – 4 Marks)

Internal stresses are least. Internal stresses are slightly more.

Furnace cooling is employed Air cooling is employed.

Cooling rate is slow Cooling rate is fast.

Temp range :Hypoeutectiod Steel – AC1 + 50o CEutectiod steel – AC1 + 50o C

Hypereutectoid Steel – AC1 + 50o

C

Temp range :Hypoeutectiod Steel – AC1 + 50o CEutectiod steel – AC1 + 50o C

Hypereutectoid Steel – ACm+

50o C

Costly and inconvenient Economical and more

convinient.

Flame hardening :- (four marks for explanation )

Flame hardening is process of heating the surface layer of a hardenable steel ( or cast Iron ) to above its upper critical temperature by means of oxyacetylene flame followed by water spray quenching or immersion quenching to transform austenite to martensite. Flame hardening can be done in different ways such as by spot or local area of the component is heated by one or more flames followed by quenching in water

Flame hardening is process of heating the surface layer of a hardenable steel ( or cast Iron ) to above its upper critical temperature by means of oxyacetylene flame followed by water spray quenching or immersion quenching to transform austenite to martensite. Flame hardening can be done in different ways such as by spot or local area of the component is heated by one or more flames followed by quenching in water.

. In progressive method, heating and quenching devices are moved over the component surface at a controlled rate. Spinning method is used for parts having a rotational summetry in which the flames are held against a rotating workpiece and when heating is complete the parts is quenched by water spray or by complete immersion in water. In combination method the work is rotated and the flames are transferred for heating followed by quenching in water or by water spray.

The depth of hardened layer depends on the following parameters :1. Distance between the gas flames and the component surface.2. Gas pressure and ratio.3. Rate of travel of flame head or component.4. Type, volume and application of quench.Flame hardening causes less distortion than conventional hardening

and due to high heating rate, oxidation and decarburization are minimum.

Advantages of Flame Hardening. (Any Four) ½ Mark each

1. It is a fastest process.2. There is less distortion of surface.3. It is economical and useful method.4. Large part can be surface hardened economically5. The hardened zone is generally much deeper than that obtained by carburizing. its range from 3 to 6 mm depth.6.Thinner case ( 1. mm ) can be obtained by increasing

the speed of heating and quenching

These processes are similar in principle to flame-hardening, except that the component is held stationary- whilst the whole of its surface is heated simultaneously by electro-magnetic induction, as shown in figure , The component is surrounded by an inductor block through which a high-frequency current in the region of 2ooo Hz, passes. This raises the temperature of the surface layer to above its upper critical in a few seconds. The surface is then quenched by pressure jets of water which pass through holes in the inductor block.Thus, as in flame-hardening, the induction processes make use of the existing carbon content (which must be above 0-4%), whilst in both case-hardening and nitriding an alteration in the composition of the surges layer of the steel takes place.

Write the advantages and disadvantages of Induction hardening.Advantages :-1) Fast heating and no holding time leads to increase in production rates.2) It can be applied to both external and internal surfaces.3) No scaling and decarburization.4) Less distortion because of heating of only surface.5) Through proper design of the heating coils, the shape of the hardened portion can be controlled very closely.6) Depth of hardening can be controlled by selecting current of appropriate frequency7) This process is automatic so it can be carried out with unskilled labourDisadvantages :-1) Irregular shaped parts are not suitable for Induction hardening. 2) Cost of equipment is high.3) Steels having less than 0.4 % carbon cannot be induction hardened.4) It is beneficial in mass production only.5) Associates high maintenance cost

CASE HARDENING -What do you mean by case hardening / state its advantages. Various machine components like cams, gears,shafts, etc require a hard wear resistant surface and a relatively soft,tough and resistant inside ,called a core. Both these requirements may be achieved by employing a low carbon steel having soft , tough core and then adding carbon, nitrogen or both to the surface of the component to provide a hard case. This treatment is known as case hardening.case hardening is a technique whereby both surface hardness and

fatigue life are enhanced for steel alloys. This is accomplished by a carburizing or nitriding process whereby a component is exposed to a carbonaceous or nitrogenous atmosphere at a elevated temperature.

( 02 Marks)Advantages : ( any four advantages, ½ mark each)1.Hard wear resistant case and soft,tough core is obtained .2.Most of the methods suitable for mass production .3.Good corrosion & wear resistance.4.Good fatigue resistance.5.Uniform case depth can be obtained.6.Negligible change in dimension after case hardening

Describe Case Hardening.Numerous industrial applications such as cams, gears, etc. require a

hard wear resistant surface called the case and a relatively soft, tough and shock resistant inside called the core. No plain carbon steel can possess both these requirements at the same time, because a low carbon steel, containing about 0.1 % carbon will be tough, while a high carbon steel of 0.9 % or more carbon will possess adequate hardness when suitable heat treated.

However both these requirements may be met by employing a low carbon steel with suitable core properties and then adding ( or penetrating ) carbon, Nitrogen or both to the surface of the steel part in order to provide a hardened case ( or layer ) of a definite depth. These treatmentsare known as case hardening.

The processes used to createhardened cases are

i)Carburizing :-Increasing the carbon on the surface of a low carbon ( o.1 –0.2 % C) and subsequently heat treating the component in a specific manner to produce hard and wear resistant surface and tough center.

ii)Nitriding:-Introducing nitrogen in the surface ofa tough steel so as to produce hard nitrided

case with no subsequent heat treatment.iii)Carbonitriding :-Introducing carbon and nitrogenin the

surface of a tough steel and produce hard and wear resistant case.

iv)Flame Hardening.

Tempering :-

Tempering process consists of heating the hardened components to a temp.between 100 0 C & 700 0 C. ( Below A1) holding at this temp. for specific period ( 1 to 2 hrs.) & cooling to room temp. usually in air.Tempering is classified as 1) Low temp. tempering (100 0 C & 200 0 C)2) Medium temp. tempering (200 0 C & 500 0 C)3) High temp. tempering (500 0 C & 700 0 C)

Tempering is done because of following purposes :-1) To relieve the internal stresses developed due to rapid cooling of steels duringhardening process & volume changes occurring in above transformation 2) To reduce brittleness of material .3) The high internal stresses produced due to hardening are likely to cause cracking ofcomponents , if tempering operation is delayed.4) To reduce hardness & to increase ductility & toughness.

EXPLAINTEMPERING ROCESS (4M)

A quench-hardened plain carbon steel is hard, brittle and hardening stresses are present. In such a condition it is of little practical use and it has to be reheated, or tempered, to relieve the stresses and reduce the brittleness.

This temperature will remove internal stress setup during quenching, remove some, or all, of the hardness, and increase the toughness of the material.

Tempering causes the transformation of martensite into less brittle structures. Unfortunately, any increase in toughness is accompanied by some decrease in hardness. Tempering always tends to transform the unstable martensite back into the stable pearlite of the equilibrium transformations.

Tempering temperatures below 200 °C only relieve the hardening stresses, but above 220 � C the hard, brittle martensite starts to transform into a fine pearlitic structure called secondary troostite (or just'troostite'). Troostite is much tougher although somewhat less hard than martensite and is the structure to be found in most carbon-steel cutting tools.

Tempering above 400 � C causes any cementite particles present to "ball-up" giving a structure called sorbite.

This is tougher and more ductile than troostite and is the structure used in components subjected to shock loads and where a lower order of hardness can be tolerated, for example springs. It is normal to quench the steel once the tempering

For most steels, cooling form the tempering temperature may be either cooling in air, or quenching in oil or water. Some alloy steels, however, may be become embrittled if slowly cooled temperature has been reached. from the tempering temperature, and these steels have to be quenched

Martemparing-: (Four marks for explanation)

In this process the austenitized steel is cooled rapidly avoiding the nose of the TTT. diagram to a temperature between the nose and MS soaked at this temperature for a sufficient time for the equalization of temperature but not long enough to permit the formation of bainite and then cooled to room temperature in air or oil. Since the component has to be held for some time for equilization of temperature, the process will be applicable to steels of slightly high hardenability such as high carbon steels and low alloy steels. The process produces martensitic structures with the following advantages .

i) It results in less distortion and warping, since the matensite formation occurs at the same time throughout the cross section of the component.ii) There is less possibility of quenching cracks appearing in the component.This is a hardening process and therefore the name martempering is a misonomer for thetreatment

Martempering Austempering.

• Martempering is a hardening

treatment

• Austempering is not a

hardening treatment

• Martempering gives

martensite product.

• Austempering gives bainite

product.

• More distortion and

quenching cracks.

• Less distortion and quenching

cracks.

e)Differentiate between Martempering And Austempering.

• Tempering is needed after

martempering

• Tempering is not needed after

austempering.

• It requires less time. • It requires more time.

• Low ductility and toughness

obtained

• Greater ductility and toughness

obtained

CARBURIZING:-

Define and explain the carburizing. State its applications.Defn-The method of increasing the carbon on the surface of a steel is called carburizingORCarburising is a method of introducing, carbon into solid iron basealloyssuch as low carbon steels in order to produce a hard case (surface).

CARBURIZING:-Define and explain the carburizing. State its

applications.Defn-The method of increasing the carbon on the

surface of a steel is called carburizingORCarburising is a method of introducing, carbon into

solid iron basealloyssuch as low carbon steels in order to produce a

hard case (surface)...

Explanation :-Carburising increases the carbon content

of the steel surface by a process of absorption and diffusion. It consists of heating the steel in the austenitic region in contact with a carburizing medium, holding at this temperature for a sufficient period and cooling to room temperature

In the austenitic region, the solubility of carbon is more and hence the carbon from medium diffuses into the steel i.e.in the austenite. High carbon content on surface does not mean high hardness of the surface, unless the carbon is present in the martensitic form. Hence after carburizing hardening treatment is necessary to bring the carbon is the martensitic form. Depending on the medium used for carburizing it is classified as

•solid carburizing•Gas carburizing •liquid carburizing

Application :-Gears, Camshaft, bearings, crank shaft sin these

components hard and wear surface is required and tough core to withstand impact loads

orThe method of increasing the carbon on the surface of a

steel is called carburizing. It consists of heating the steel in the austenitic region in

contact with a carburizing medium , holding at this temp for a sufficient period and cooling to room temperature.

In the austenitic region the solubility of carbon is more and hence the carbon from medium diffuses in to the steel in the austenite.

High carbon content on the surface does not mean high hardness of the surface ,unless carbon is present in the martensitic form. hence after carburizing ,hardening treatment is necessary to bring the carbon in the martensitic form.

Therefore the hardening heat treatment that follows the carburizing operation is as important as the carburizing itself. It is also known as case hardening .depending on medium used for carburizing it is classified as solid carburizing, gas carburizing, liquid carburizing.

Advantages of Carburising (two marks for any two advantages )

1. With this process carbon is deposited on the surface of lower or medium carbon steel material and with proper heat treatment it can be hardened.

2. Certain depth of hardening can be provided on the surface of metals. Hence it is called as case hardening.

3. distortion and cracking during hardening is less.4. It can be done in the solid, gas and liquid

carburizing medium.5 Depth of hardness can be controlled easily

Demerits :- ( Any two ½ M each ) –1 Mark

i) Some times uniform case depth is difficult to obtain.

ii) Skilled personnel are required.iii) In case of liquid carburizing necessary

care is to be taken. iv) In case of solid carburizing more time

required for process

Nitriding process-Describe Nitriding process. ( Suitable description.04 Marks)Nitriding accompanies the introduction of nitrogen

into the surface of certain types of steel ( e.g. containing A1 and Cr) by heating it and holding it at a suitable temperature in contact ith partially dissociated ammonia or other suitable medium. This process produces a hard case without quenching or any further heat treatment.

Nitriding is accomplished by heating the steel in contact with a source of atomic nitrogen at a temperature of about 550 ºC. The atomic nitrogen diffuses into the steel and combines with iron and certain alloying elements present in the steel and forms respective nitrides. These nitrides increase the hardness and wear resistance of steels. The atomic nitrogen source can be a molten salt bath containing NacN.

In gas nitriding, the components are placed in a heat resistant metal container which is then filled with ammonia. When it is completely purged, it is sealed, placed in a furnace and raised to a temperature of approximately 500 ºC .At this temperature the ammonia dissociates.

NH3gives3H + N and N is absorbed in the surface layer of steel. Parts are maintained at 500 ºC for between 40 to 100 Hours depending upon the depth of case required, afterwhich parts are allowed to cool in the container