Manufacturing processes may be classified as:
1. Processing (a) Heavy processing (b) Light processing.
2. Treatment (a) Heat treatment (b) Surface treatment.
3. Fabrication (a) Heavy fabrication (b) Medium fabrication (c) Light fabri0cation.
Heavy processing industries for industrial metals such as steel and copper and also heavy chemical industries manufacturing Sulphuric acid and heavy alkalis fall into this category. These are continuous production industries.
Light processing include processing industries that produce chemical compounds, drugs and some metals for example boron and titanium. It also includes industries manufacturing plastics.
Treatment: Heat treatment of steel alloys is needed to obtain desired mechanical properties such as hardness, toughness etc.
Surface treatment refers to the chemical or mechanical processes which are used to alter the surface characteristics of a metal.
Fabrication: Heavy fabrication includes fabrication of large and heavy parts or assemblies made of iron and steel, incorporating castings, forgings, welded structures and parts made of thick metal plates, e.g., manufacture of locomotives, earth moving equipments, electric generators, etc.
Medium fabrication includes forgings and castings and assembly work for manufacturing products such as automobiles, aircrafts, missiles, etc.
Light fabrication involves manufacture of products such as utensils, bicycles and furniture involving conventional manufacturing methods and mass production assembly lines.
Since heat treatment and surface treatment are more commonly used processes, they are described in greater detail below:-
Heat Treatment Processes: Heat treatment is the process of heating and cooling of metals or alloys in the solid state in order to obtain certain desired properties.
By heat treatment processes, it is possible to alter the properties of a metal or alloy, to make it more suitable for a particular application. Steels are especially suitable for heat treatment, because of the polymorphic change of iron as austenite, which transforms during cooling. The crystalline structure and therefore the mechanical properties of steel can be greatly affected by suitable heat treatment processes. The important heat-treating processes for steels are hardening, tempering, annealing, normalizing and case hardening.
Purposes of Heat treatment:
1. To improve machining ability of a metal or alloy.
2. To relieve internal stresses.
3. To improve mechanical properties such as hardness, toughness, strength and ductility.
4. To improve electrical and magnetic properties.
5. To increase resistance to heat and corrosion.
6. To change the chemical composition.
7. To refine the grain size and grain structure.
For example, hardness and toughness of alloy steels must be increased when they are used in the manufacture of cutting tools.
Gear teeth be case hardened to reduce wear and tear.
Heat treatment is used to obtain optimum combination of hardness and ductility for carbon steels and alloy steels for particular application.
Heat treatment generally involves heating of metals and alloys to a certain temperature and holding at that temperature for certain length of time and then cooling slowly or rapidly (at different cooling rates) in order to change the microstructure of the metals to get the desired mechanical properties.