Heat Treatment
$\displaystyle \small \bullet$ Heat treatment is an operation involving heating and cooling of a metal or alloy so as to obtain certain desirable properties.
$\displaystyle \small \bullet$ Purposes/objectives of heat treatment
$\displaystyle \small \circ$ Improving machinability by softening them
$\displaystyle \small \circ$ To prevent wear and tear of the materials
$\displaystyle \small \circ$ To improve mechanical properties (hardness, brittleness, tensile strength)
$\displaystyle \small \circ$ To improve electric and magnetic properties
$\displaystyle \small \circ$ To remove internal stress
$\displaystyle \small \circ$ To increase resistance to heat and corrosion
Critical Temperature
$\displaystyle \small \bullet$ When steel is heated, its temperature rises.
$\displaystyle \small \bullet$ The heat supplied is utilized in order to increase its temperature.
$\displaystyle \small \bullet$ After attaining 723$\displaystyle \small ^{0}C$, the temperature does not increase because the supplied heat is utilized for re-arrangement of molecules.
$\displaystyle \small \bullet$ This process results in the formation of a dense fluid called austenite.
$\displaystyle \small \bullet$ Critical temperature is temperature at which the formation of austenite is completed.
$\displaystyle \small \bullet$ Its magnitude depends upon the presence of existing carbon content.
Three Stages of Heat Treatment
Heating
$\displaystyle \small \bullet$ Heating depends on following factors – type of furnace, type of fuel, time interval, temperature control in achieving the temperature in the job, heat rate, time, structure and formation, shape and dimensions of the job.
$\displaystyle \small \bullet$ Preheating – steel should be preheated gradually up till 600$\displaystyle \small ^{0}C$
Soaking
$\displaystyle \small \bullet$ The steel is held for some time when it has reached the required temperature. During this the entire job gets consistently heated, called soaking.
$\displaystyle \small \bullet$ Soaking time depends on following factors – cross-sectional area, chemical structure, the amount and arrangement of charge in the furnace.
$\displaystyle \small \bullet$ Ex: An alloy of carbon and steel of 10mm thickness needs 5 minutes.
$\displaystyle \small \bullet$ A high steel alloy piece of 10mm thickness needs 10 minutes.
Quenching
$\displaystyle \small \bullet$ Depending on the cooling requirement, top quenching media are – water, oil, brine mixture (salt solution), air.
$\displaystyle \small \bullet$ The salt solution cools fastest while air does it slowest.
$\displaystyle \small \bullet$ For plain carbon steel, water is used.
$\displaystyle \small \bullet$ When water is used for quenching, the rate of cooling can be increased by constantly shaking the job in the water.
$\displaystyle \small \bullet$ When oil is used, viscosity of used oil should be less, hence, it’s utilized where there is less smoke, less fire expectations.
Methods of Heat Treatment
1. Normalizing
2. Annealing
3. Hardening
4. Tempering
5. Case hardening
Normalizing
$\displaystyle \small \bullet$ Purpose of normalising –
$\displaystyle \small \circ$ To relieve the internal stresses due to hot working, cold working and machining.
$\displaystyle \small \circ$ To bring about uniformity of structure.
$\displaystyle \small \circ$ To improve mechanical properties.
$\displaystyle \small \bullet$ The process involves heating of steel to a temperature above critical temperature and then cooling to room temperature in still air.
$\displaystyle \small \bullet$ As the cooling is done in air, the grain structure of normalized material is greatly affected by its size or mass.
$\displaystyle \small \bullet$ Normalizing is usually done on low-carbon steel, medium-carbon steel, alloy steel.
Annealing
$\displaystyle \small \bullet$ Purpose of annealing –
$\displaystyle \small \circ$ To remove stresses
$\displaystyle \small \circ$ To increase ductility
$\displaystyle \small \circ$ To produce uniformity
$\displaystyle \small \circ$ To induce softness
$\displaystyle \small \circ$ To refine the grain size for machining
$\displaystyle \small \bullet$ The process involves heating the steel to a suitable temperature depending upon the percentage of carbon in the steel, holding it at that temperature for some time and then cooling it slowly.
$\displaystyle \small \bullet$ Cooling may be done in same furnace or by transferring to another furnace or by burying in hot ashes and then left to cool.
$\displaystyle \small \bullet$ The rate of cooling of steel is 100$\displaystyle \small ^{0}C$ to 150$\displaystyle \small ^{0}C$ per hour.
Hardening
$\displaystyle \small \bullet$ Purpose of hardening – to considerably increase the hardness of the workpiece, to increase abrasion resistivity.
$\displaystyle \small \bullet$ Proper hardening temperature is determined. It is usually 10$\displaystyle \small ^{0}C$ – 38$\displaystyle \small ^{0}C$ above the critical temperature, at which structural changes take place.
$\displaystyle \small \bullet$ The part is heated to that temperature uniformly in a furnace.
$\displaystyle \small \bullet$ It is then permitted to soak at that temperature for specified time.
$\displaystyle \small \bullet$ The part is then removed and quenched in an appropriate quenching media (water/oil/brine).
$\displaystyle \small \bullet$ Hardening of steel depends on –
$\displaystyle \small \circ$ Carbon content in the steel
$\displaystyle \small \circ$ Internal structure of steel
$\displaystyle \small \circ$ Cooling medium- air, water, oil etc.
$\displaystyle \small \circ$ Rate of cooling
$\displaystyle \small \circ$ Process of heating and cooling the steel
Tempering
$\displaystyle \small \bullet$ Purpose of tempering – after hardening, the steel is brittle and may break with the slightest tap. To overcome this brittleness and to strengthen, steel is tempered.
$\displaystyle \small \bullet$ The part where tempering is required is heated at the tempering temperature and then quenched.
$\displaystyle \small \bullet$ Tempering toughens the steel and makes it less brittle.
$\displaystyle \small \bullet$ As steel is heated it changes colour, and these colours indicate various tempering temperatures.
$\displaystyle \small \bullet$ Tempering temperature for high speed steel is 220$\displaystyle \small ^{0}C$ to 330$\displaystyle \small ^{0}C$.
$\displaystyle \small \bullet$ Tempering temperature for hardened steel is 400$\displaystyle \small ^{0}C$.
$\displaystyle \small \bullet$ Tempering temperature for alloy steel is 560$\displaystyle \small ^{0}C$.
Case Hardening
$\displaystyle \small \bullet$ Purpose of case hardening – to harden the outer surface of low-carbon steel while leaving the centre or core soft and ductile.
$\displaystyle \small \bullet$ Heat to just above the upper critical temperature and quench in oil. Reheat to just above the upper critical temperature and quench in water.
$\displaystyle \small \bullet$ Temper at low temperature of 100$\displaystyle \small ^{0}C$ – 150$\displaystyle \small ^{0}C$ to relieve stresses.
Methods of case hardening
Carburizing
$\displaystyle \small \bullet$ Steel components having 0.2% carbon are packed in cast iron boxes, which consists of a mixture (mainly charcoal).
$\displaystyle \small \bullet$ It is heated to 900$\displaystyle \small ^{0}C$ for a certain period.
$\displaystyle \small \bullet$ The carbon content at the surface of the component increases.
$\displaystyle \small \bullet$ The depth to which carbon penetrates depends on the temperature and heating time.
Nitriding
$\displaystyle \small \bullet$ In this method, steel is case hardened by nitrogen carburizing.
$\displaystyle \small \bullet$ The steel part is kept at a temperature of 500$\displaystyle \small ^{0}C$ in ammonia gas or close to any substance containing nitrogen.
Cyaniding
$\displaystyle \small \bullet$ In this method, steel component is heated in molten cyanide.
$\displaystyle \small \bullet$ This method is usually followed by quenching for deeper hardening.
Induction hardening
$\displaystyle \small \bullet$ In this method, the component is kept in an electrically regulated coil, where the coil is made up of copper.
$\displaystyle \small \bullet$ Heat is generated by supplying AC voltage, maintaining a temperature of 750$\displaystyle \small ^{0}C$ to 820$\displaystyle \small ^{0}C$.
$\displaystyle \small \bullet$ The component is then passed through a quenching jet.
Flame hardening
$\displaystyle \small \bullet$ In this method, the component is heated by a continuously moving oxy-acetylene flame, followed by a quenching jet of water, air or nitrogen.
$\displaystyle \small \bullet$ Heat treatment is an operation involving heating and cooling of a metal or alloy so as to obtain certain desirable properties.
$\displaystyle \small \bullet$ Purposes/objectives of heat treatment
$\displaystyle \small \circ$ Improving machinability by softening them
$\displaystyle \small \circ$ To prevent wear and tear of the materials
$\displaystyle \small \circ$ To improve mechanical properties (hardness, brittleness, tensile strength)
$\displaystyle \small \circ$ To improve electric and magnetic properties
$\displaystyle \small \circ$ To remove internal stress
$\displaystyle \small \circ$ To increase resistance to heat and corrosion
Critical Temperature
$\displaystyle \small \bullet$ When steel is heated, its temperature rises.
$\displaystyle \small \bullet$ The heat supplied is utilized in order to increase its temperature.
$\displaystyle \small \bullet$ After attaining 723$\displaystyle \small ^{0}C$, the temperature does not increase because the supplied heat is utilized for re-arrangement of molecules.
$\displaystyle \small \bullet$ This process results in the formation of a dense fluid called austenite.
$\displaystyle \small \bullet$ Critical temperature is temperature at which the formation of austenite is completed.
$\displaystyle \small \bullet$ Its magnitude depends upon the presence of existing carbon content.
Three Stages of Heat Treatment
Heating
$\displaystyle \small \bullet$ Heating depends on following factors – type of furnace, type of fuel, time interval, temperature control in achieving the temperature in the job, heat rate, time, structure and formation, shape and dimensions of the job.
$\displaystyle \small \bullet$ Preheating – steel should be preheated gradually up till 600$\displaystyle \small ^{0}C$
Soaking
$\displaystyle \small \bullet$ The steel is held for some time when it has reached the required temperature. During this the entire job gets consistently heated, called soaking.
$\displaystyle \small \bullet$ Soaking time depends on following factors – cross-sectional area, chemical structure, the amount and arrangement of charge in the furnace.
$\displaystyle \small \bullet$ Ex: An alloy of carbon and steel of 10mm thickness needs 5 minutes.
$\displaystyle \small \bullet$ A high steel alloy piece of 10mm thickness needs 10 minutes.
Quenching
$\displaystyle \small \bullet$ Depending on the cooling requirement, top quenching media are – water, oil, brine mixture (salt solution), air.
$\displaystyle \small \bullet$ The salt solution cools fastest while air does it slowest.
$\displaystyle \small \bullet$ For plain carbon steel, water is used.
$\displaystyle \small \bullet$ When water is used for quenching, the rate of cooling can be increased by constantly shaking the job in the water.
$\displaystyle \small \bullet$ When oil is used, viscosity of used oil should be less, hence, it’s utilized where there is less smoke, less fire expectations.
Methods of Heat Treatment
1. Normalizing
2. Annealing
3. Hardening
4. Tempering
5. Case hardening
Normalizing
$\displaystyle \small \bullet$ Purpose of normalising –
$\displaystyle \small \circ$ To relieve the internal stresses due to hot working, cold working and machining.
$\displaystyle \small \circ$ To bring about uniformity of structure.
$\displaystyle \small \circ$ To improve mechanical properties.
$\displaystyle \small \bullet$ The process involves heating of steel to a temperature above critical temperature and then cooling to room temperature in still air.
$\displaystyle \small \bullet$ As the cooling is done in air, the grain structure of normalized material is greatly affected by its size or mass.
$\displaystyle \small \bullet$ Normalizing is usually done on low-carbon steel, medium-carbon steel, alloy steel.
Annealing
$\displaystyle \small \bullet$ Purpose of annealing –
$\displaystyle \small \circ$ To remove stresses
$\displaystyle \small \circ$ To increase ductility
$\displaystyle \small \circ$ To produce uniformity
$\displaystyle \small \circ$ To induce softness
$\displaystyle \small \circ$ To refine the grain size for machining
$\displaystyle \small \bullet$ The process involves heating the steel to a suitable temperature depending upon the percentage of carbon in the steel, holding it at that temperature for some time and then cooling it slowly.
$\displaystyle \small \bullet$ Cooling may be done in same furnace or by transferring to another furnace or by burying in hot ashes and then left to cool.
$\displaystyle \small \bullet$ The rate of cooling of steel is 100$\displaystyle \small ^{0}C$ to 150$\displaystyle \small ^{0}C$ per hour.
| Carbon content(%) | Annealing temperature |
|---|---|
| < 0.12 | 875-925 |
| 0.12 to 0.25 | 840-970 |
| 0.25 to 0.50 | 815-840 |
| 0.50 to 0.90 | 780-810 |
| 0.90 to 1.30 | 760-780 |
Hardening
$\displaystyle \small \bullet$ Purpose of hardening – to considerably increase the hardness of the workpiece, to increase abrasion resistivity.
$\displaystyle \small \bullet$ Proper hardening temperature is determined. It is usually 10$\displaystyle \small ^{0}C$ – 38$\displaystyle \small ^{0}C$ above the critical temperature, at which structural changes take place.
$\displaystyle \small \bullet$ The part is heated to that temperature uniformly in a furnace.
$\displaystyle \small \bullet$ It is then permitted to soak at that temperature for specified time.
$\displaystyle \small \bullet$ The part is then removed and quenched in an appropriate quenching media (water/oil/brine).
$\displaystyle \small \bullet$ Hardening of steel depends on –
$\displaystyle \small \circ$ Carbon content in the steel
$\displaystyle \small \circ$ Internal structure of steel
$\displaystyle \small \circ$ Cooling medium- air, water, oil etc.
$\displaystyle \small \circ$ Rate of cooling
$\displaystyle \small \circ$ Process of heating and cooling the steel
Tempering
$\displaystyle \small \bullet$ Purpose of tempering – after hardening, the steel is brittle and may break with the slightest tap. To overcome this brittleness and to strengthen, steel is tempered.
$\displaystyle \small \bullet$ The part where tempering is required is heated at the tempering temperature and then quenched.
$\displaystyle \small \bullet$ Tempering toughens the steel and makes it less brittle.
$\displaystyle \small \bullet$ As steel is heated it changes colour, and these colours indicate various tempering temperatures.
$\displaystyle \small \bullet$ Tempering temperature for high speed steel is 220$\displaystyle \small ^{0}C$ to 330$\displaystyle \small ^{0}C$.
$\displaystyle \small \bullet$ Tempering temperature for hardened steel is 400$\displaystyle \small ^{0}C$.
$\displaystyle \small \bullet$ Tempering temperature for alloy steel is 560$\displaystyle \small ^{0}C$.
| Tools | $\displaystyle \small ^{0}C$ | Colour |
|---|---|---|
| Toolkits, drills, taps | 220 | Faint straw |
| Punches and dies, milling cutters | 240 | Medium straw |
| Shear blades, hammer faces | 255 | Dark straw |
| Axes, wood chisels and tools | 270 | Purple |
| Knives, steel chisels | 300 | Dark blue |
| Screwdrivers, springs | 320 | Light blue |
Case Hardening
$\displaystyle \small \bullet$ Purpose of case hardening – to harden the outer surface of low-carbon steel while leaving the centre or core soft and ductile.
$\displaystyle \small \bullet$ Heat to just above the upper critical temperature and quench in oil. Reheat to just above the upper critical temperature and quench in water.
$\displaystyle \small \bullet$ Temper at low temperature of 100$\displaystyle \small ^{0}C$ – 150$\displaystyle \small ^{0}C$ to relieve stresses.
Methods of case hardening
Carburizing
$\displaystyle \small \bullet$ Steel components having 0.2% carbon are packed in cast iron boxes, which consists of a mixture (mainly charcoal).
$\displaystyle \small \bullet$ It is heated to 900$\displaystyle \small ^{0}C$ for a certain period.
$\displaystyle \small \bullet$ The carbon content at the surface of the component increases.
$\displaystyle \small \bullet$ The depth to which carbon penetrates depends on the temperature and heating time.
| Case hardening (mm) | Time (hrs) |
|---|---|
| 0.5 – 0.9 mm | 4 – 6 hrs |
| 1 – 1.5 mm | 8 – 12 hrs |
| 1.5 – 2 mm | 12 – 18 hrs |
| 2 – 2.5 mm | 18 – 24 hrs |
Nitriding
$\displaystyle \small \bullet$ In this method, steel is case hardened by nitrogen carburizing.
$\displaystyle \small \bullet$ The steel part is kept at a temperature of 500$\displaystyle \small ^{0}C$ in ammonia gas or close to any substance containing nitrogen.
Cyaniding
$\displaystyle \small \bullet$ In this method, steel component is heated in molten cyanide.
$\displaystyle \small \bullet$ This method is usually followed by quenching for deeper hardening.
Induction hardening
$\displaystyle \small \bullet$ In this method, the component is kept in an electrically regulated coil, where the coil is made up of copper.
$\displaystyle \small \bullet$ Heat is generated by supplying AC voltage, maintaining a temperature of 750$\displaystyle \small ^{0}C$ to 820$\displaystyle \small ^{0}C$.
$\displaystyle \small \bullet$ The component is then passed through a quenching jet.
$\displaystyle \small \bullet$ In this method, the component is heated by a continuously moving oxy-acetylene flame, followed by a quenching jet of water, air or nitrogen.
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