What is heat treatment ?

 What is Heat Treatment?

Heat  treatment is  defined as an operation  involving the heating and cooling of a  metal or an alloy in the solid- state to  obtain  certain  desirable  properties  without  change composition. The  process of heat  treatment is  carried out to  change the grain size, to  modify the  structure of the  material and to relive the stresses  set up the  material after  hot or  cold working.

 The  heat  treatment is  done  to improve the machinability. 

 To  improve magnetic and  electrical  properties.

 To  increase resistance to wear,  heat and corrosion  and much  more reason. 

Heat  treatment  consists of heating the  metal  near or above its  critical temperature, held for a particular time at that  finally cooling the  metal in  some medium which  may be air, water, brine or molten salts. The  heat  treatment  process  includes annealing, case hardening, tempering, normalizing and quenching, nitriding, cyaniding etc.

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Types of Heat Treatment Processes

Eight  different  types of  heat  treatment  processes are as follows:

1. Annealing

2. Normalizing

3. hardening

4. Tempering

5. Nitriding

6. Cyaniding

7. Induction Hardening

8. Flame Hardening

Types of Heat Treatment

1. Annealing

Annealing is one of the most important processes of heat treatment. It is one of the most widely used operations in the heat treatment of iron and steel and is defined as the softening process.Heating of from 30 - 50°C above the upper critical temperature and cooling it at the very slow rate by seeking it the furnace. The main aim of annealing is to make steel more ductile and malleable and to remove internal stresses. This process makes the steel soft so that it can be easily machined.

1.1 Purpose of Annealing

 It softens steel and to improve its machinability.

 To refine grain size and remove gases.   

  It  removes the  internal stresses  developed  during the  previous process.

 To  obtain  desired ductility, malleability and toughness.

 It modifies  the electrical and magnetic properties.

1.2 Procedure for annealing

Depending  on the carbon content, the  steel is heated to a temperature of  about 50° to 55°C above its  critical temperature range. It is held at this temperature for a  definite  period of time depending  on the  type of furnace and nature of work. The  steel is then allowed  to cool  inside the furnace 

2. Normalizing

Normalizing: The  main  aim of normalizing is to  remove the  internal stresses  developed after the  cold working  process. In this,  steel is heated 30 - 50°C above its  upper  critical temperature and cooling it  in the air.It improves mechanical and  electrical  properties, machinability & tensile strength. Normalizing is the process  of heat  treatment  carried out to  restore the  structure of  normal condition.

2.1 Purpose of Normalizing

 Promote uniformity of  structure.

 To  secure grain refinement.

 To  bring about  desirable  changes  in the  properties of  steel.

2.2 Procedure for Normalizing

The  steel is heated to a temperature of  about 40° to 50°C above its  upper  critical temperature. It is held at this temperature for a  short duration. The  steel is then allowed cool in  still air at room temperature,  which is  known as air quenching.

2. 3 Application of normalizing

1. It is  applied castings and forgings to refine grain  structure and  to relieve stresses.

2. It is  applied after  cold  working  such as rolling, stamping and hammering. 

3. Hardening

Hardening: The  main  aim of the hardening  process is to make  steel  hard tough. In this  process,  steel is heated 30° - 40°C above the  upper  critical temperature  and then  followed  by  continues cooling to room temperature  by quenching in water or oil. It is  the opposite  process of annealing. 

 3.1 Purpose of hardening

 By hardening, it  increases the hardness of  steel.

 To  resist to wear

 Allows the  steel to  cut  other metals

 3.2 Procedure for hardening

The  steel is heated above its  critical temperature range. It is held at that temperature for a definite period of time. The  steel is then  rapidly cooled in a medium of quenching. The quenching medium  is selected  according to the  degree of hardness desired. The air, water, bring, oils and molten salts are used as quenching mediums. A  thin  section such knife blades are cooled in air. Water is  widely used medium  but it  results  in the formation of bubbles  on the surface of the metal. Hence brine  solution is used to  prevent this. Oil is used  when  there is a  risk of distortion on cracks and is  suitable for alloy steels. The molten salts are used  to cool  thin  section to  obtain crack- free and impact-resistant products.

 3. 3 Application of hardening

It is  applied for chisels, sledgehammer, hand hammer, centre punches, taps, dies, milling cutters, knife blades and gears. 

4. Tempering

Tempering: When the hardening  process hardens a  steel specimen, it  becomes brittle and has high residual stress. It is an operation used to  modify the  properties of  steel hardened  by quenching for the  purpose of  increasing its usefulness.Tempering or draw  results in  a reduction of brittleness and  removal of  internal  strains  caused during 

hardening. Steel  must be tempered after the hardening  process.The tempering  is divided into  three  categories  according to the usefulness of  steel required. 

 Low-temperature tempering. 

 Medium temperature tempering. 

 High-temperature tempering.

4.1 Purpose of Tempering

 To relieve internally  stressed  caused by hardening.

 To  reduce brittleness.

 Improve ductility,  strength and toughness.

 To  increase  wear resistance.

 To  obtain  desired mechanical  properties. 

4.2 Procedure for tempering

The  steel after being quenched  in the hardening  process is reheated to a temperature  slightly above the temperature  range at which  it is to be used,  but  below the  lower  critical temperature. The temperature  here varies from 100°C to 700°C.The reheating is  done in a  bath of oil or molten lead or molten salt. The specimen is held  in the  bath 

for a  period of time  till attains the temperature evenly, the time  depends  on the composition and desired  quality of  steel. Now the specimen is  removed from  the bath and  allow  to cool slowly in still air.

4. 3 Application of Tempering

It is  applied to  cutting tools,  tool and gears,  which are hardened  by the hardening  process. 

5. Nitriding

Nitriding is the  process of the case or  surface hardening  in which nitrogen  gas is  employed to obtain hard  skin of the metal. In this  process,  steel is heated  in the presence of ammonia environment.Due to this, a nitrogen atom is deposited and makes  material  hard. Induction hardening and Flame hardening  objects are heated  by an oxy-acetylene flame.

5.1 Purpose of Nitriding

 To harden the  surface of the  steel to a  certain  depth.

 Increase resistance to  wear and fatigue.

 To  increase corrosion resistance.

5.2 Procedure for Nitriding

It is  done  in the  electric furnace  where temperature  varying  between 450° and 510°C is maintained. The  part is  well machined and  finished and  placed in an  airtight  container  provided with outlet and inlet tubes  through which ammonia  gas is circulated. The  container with the  part is  placed  in the furnace and ammonia  gas is  passed  through it  while the furnace is heated.During the  process of heating nitrogen  gas is  released from ammonia  in the  form of atomic nitrogen, 

which reacts with the  surface of the  part, and  forms iron nitrate.The  depth of  entrance  depends upon the  length of time spent  at the nitriding temperature. The  part is taken out and it does  not require any quenching or  further  heat treatment.

5. 3 Application of Nitriding

 It is  applied for hardening the  surface of medium carbon alloy steels. 

6. Cyaniding

Cyaniding: In this process,  steel is heated  in the presence of sodium cyanide environment. Due to this, carbon and nitrogen atoms are deposited  on the  surface of  steel and make it  hard.

6.1 Purpose of Cyaniding

 This  method is  effective for  increasing the fatigue  limit of medium and small-sized  parts such as gears, shafts, wrist pins etc.

 To  increase  surface hardness.

  increase  wear resistance.

 To  give the clean,  bright and  pleasing  appearance to the hardened  surface.

6.2 Produce for Cyaniding

The  parts to be  treated is dipped in a molten cyanide salt  bath maintained at a temperature of 950°C. The molten salts used are sodium chloride, sodium carbonate, sodium cyanide and soda ash.The immersed article is left  in the molten cyanide salt at a temperature of 950°C for  about 15 to 20 minutes. The decomposition of sodium cyanide yield nitrogen and carbon from carbon monoxide,  which is  diffused into the  surface  resulting in hardening the  surface. The  part is then taken out of the bath and quenched in water or oil.

6. 3 Application of Cyaniding

 It is  applied to small articles like gears, bushing, screws, pins and small hand tools, which require  a thin and  hard  wear-resisting  surface. 

7. Carburising

Carburising: In this  process,  steel is heated  in the presence of carbon environment. Due to this carbon atoms are deposited  on the  metal  surface and make it  hard.

8. Case hardening or  surface hardening

The  main  aim of this  process is to make the  only  external  surface of  steel  hard and  inner  core soft. It is the  process of carburisation i.e., saturating the  surface layer of  steel with carbon, or  some other process  by which case is hardened and the  core  remains soft.

8.1 Purpose of Case hardening

 To  obtain a  hard and  wear resistance to  machine parts.

 By case hardening, it obtains a  tough  core.

 To  obtain a  higher fatigue  limit and  high mechanical  properties  in the  core. Purpose of Heat Treatment of Steel The following are the  purposes  of heat treatment.

1. To  improve mechanical  properties  such as tensile strength, hardness, ductility, shock 

resistance and resistance to corrosion.

2. Improve machinability.

3. To relieve the  internal stresses of the metal- induced  during  cold or  hot working.

4. To  change or refine grain size.

5. Improve magnetic and  electric  properties.

6. Increase resistance to wear, and corrosion.

7. To  increase the  surface hardness.