Best Mechanical engineering tutorials HEAT TREATMENT PROCESS

HEAT TREATMENT PROCESS

Heat treatment may be defined as operation or combination of operations involving heating and cooling of metal / alloy in solid steels to obtain desired conditions ( Relieve stress ) and properties ( like Machinability, ductility etc. ). 

Purpose of Heat Treatment:

1. To relieve stress created during cold working, welding, casting etc.
2. Improve Machinability.
3. Change grain size.
4. Improve ductility and
5. Homogenous structure.

           Cooling rates also plays a important role.  Slow cooling produces pearlitic structure and rapid cooling produces a Martensitic ( hard ) structure.

Different heat treatment process:

Annealing:

           It is the process of heating the metal which is in a metal stable of distorted structural state to a temperature which will remove the distortion and then cooling it, so that room temperature is stable.

Normalizing:

           It is also called as air quenching, where the steel is heated to about 40^o - 50^o above the upper critical temperature and if necessary, hold at that temperature for a certain length of time, after which it is cooled in air at room temperature.

Hardening:

           This process increases the hardness and it requires the formation of martensite.  For hardening, the steel should contain at least 0.3 % of carbon.  Following is the procedure.

1. Steel will contain sufficient carbon ( 0.35 % - 0.70 % )
2. Heated to 30^o to 50^o above the A3 line.
3. Held at that temperature for 15 - 20 Mins / 25 mm of Cross section.
4. Cooled rapidly and quenched in a suitable medium.

Tempering:

           Hardening process produces martensite and retained austentite.  It is extremely brittle and hence should not be used as such.  Further the austentite may be in a unstable state.  Hence to return to equilibrium, tempering is done after quench hardening by heating to a temperature below the lower critical temperature. Tempering has the following process

1. Heating hardened steel below the lower critical temperature.
2. Holding at that temperature for 3 - 5 Mins / 1 mm Thickness.
3. Cooling the steel slowly.

           There are three types of tempering.  They are

• Low temperature tempering ( 150^o C - 250^o C )
• Medium temperature tempering ( 350^o C - 450^oC )
• High temperature tempering ( 500^o C - 600^o C)

      Martempering, Austempering and Maraging are the other types of heat treatment process.

Case Hardening:

           For many applications, there is a need for a hard case and a soft tough core, which is shock resistant.  No carbon can possess both these properties at the same time.  Hence low carbon steel with desired core properties are chosen and Carbon / Nitrogen is added to the surface to provide a hardened case to a specified depth by using the following process

• Carburising.
• Nitriding,
• Cyaniding and
• Carbon Nitriding.

           Also medium Carbon steel could be taken in normalized condition and case hardened by Induction and Flame Hardening.
Carburizing:

            This process is also called as cementation.  Low carbon steel ( 0.2 % C ) is heated to 870 - 925 C in contact with gases or carbon for several hours.  There are three types.  They are Pack Carburizing, Gas Carburizing and Liquid Carburizing.  This method is used for case hardening Gears, Camshafts and Bearing.

Nitriding:

           It involves the addition of Nitrogen on certain types of steels and heating them and holding at a suitable temperature, in contact with ammonia or any other suitable medium.  The steel should contain Aluminum or chromium to form hard nitrides.

       In this the component to be case hardened is heat resistant container along with ammonia.  It is then heated to a temperature of about 500^o C.

Cyaniding:

           Both Carbon and nitrogen are introduced on the surface of steel by heating to a suitable temperature and holding the component in  molten cyanide.  Sodium cyanide is mostly used.  This results in the formation of hardened Carbide - Nitride case.  In this process Nitrogen provides hardening, but carbon responds to quenching process.

Carbo - Nitriding:

           Both Carbon and Nitrogen are added to the surface of steel by using Gas atmosphere and not Molten Cyanide .  The gaseous atmosphere contains the following

1. Carrier gas ( H₂, N₂ or CO )
2. Enriching gas ( Natural Gas )
3. Ammonia.

Flame Hardening:
           The material is heated on the surface with flame.  This is followed by quenching.  Thus creating a hardened case and a soft core.  Oxy acetylene flame is used and the steel should contain 0.3 % to 0.6 % of carbon.

Induction Hardening:

           The material is heated in a alternative magnetic field followed by quenching.

Effect of Alloying elements in Steel:

Aluminum :

1. Its content varies from 1 to 5 %.
2. It is the alloying element of nitriding steels.
3. It deoxidizes efficiently, restricts grain growth.

Boron :

1. Its content does not exceed 0.003 %
2. It is the alloying element of low and medium carbon steel.
3. It effectively increases hardenability.

Chromium :

1. The amount of chromium may be from a fraction to 30%.
2. It resists abrasion, wear, corrosion and oxidation.
3. The addition of chromium results in the formation of various carbides of chromium which are hard, yet ductile.
4. Chromium changes the grain structure, thus increasing the toughness and hardness.

Cobalt :

1. Its content varies from 5 - 12 %
2. It is used to increase the hot hardness of cutting tools so that it retains its hardness and cutting edge even at high temperature.

Copper :

1. Its content varies from 0.15 - 0.30 %
2. It lowers the critical temperature and improves resistance to atmospheric corrosion.

Manganese :

1. It content varies from 0.4 - 2  % and 11 - 14 %
2. It lowers the critical range of temperature.
3. It acts as a deoxidizing and desulphurising agent.
4. It increase the time required for transformation, so that oil quenching becomes practicable.

Molybdenum :

1. It content varies from 0.2 - 0.7 %.
2. It acts very much like chromium but is more powerful in action.
3. It increase the critical range of temperature.

Nickel :

1. It content ranges upto  50 %
2. It also creases the critical range of temperature.
3. It is soluble in ferrite and doe snot form carbides or oxides, and thus increase the strength without decreasing the ductility.

Sulphur :

1. The content varies from 0.06 - 0.30 %
2. Its presence is undesirable because it forms iron sulphides and leads to cracking.
3. In the presence of manganese from manganese sulphide and thus improves the machinability of steels.

Silicon :

1. Its content is upto 0.8 %
2. Silicon is added with other alloying element like manganese, chromium and vanadium to stabilize the carbides.

Tungsten :

1. The amount of tungsten varies from 0.4 - 22 %
2. Its function is similar to molybdenum,  except for the fact that large quantities must be added.
3. It is widely used in tools to maintain the hardness in red heat.
4. It produces a fine dense structure and adds both toughness and hardness.

Vanadium :

1. Vanadium has a very strong tendency to form carbides, hence it is used in small amounts in the order of 0.2 - 0.5 %
2. It acts as a cleaner and degasifier.
3. It reduces the grain size and toughness and strengthen the steel.
4. It has a desirable property of increasing the life of tools, springs and other members subjected to high temperatures.

IRON - CARBON Diagram:

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