Heat treatment is an operation or combination of operations, involving heating and cooling of a metal or alloy in the solid state for the purpose of changing the properties of the metal. Heat treatment consists of three phases.
- Heating of the metal.
- Soaking of the heat into metal.
- Cooling of the metal.
Purpose of Heat Treatment
The purpose of heat treatment is as follows:
- To improve machinability.
- To improve mechanical properties, e.g., tensile strength, ductility, hardness and shock resistance.
- To relieve the stress induced during hot or cold working.
- To change or refine grain size.
- To improve magnetic and electrical properties.
- To improve wear resistance.
- To improve weldability.
Structure Of Iron And Steel
Various structures of iron and steel are as follows:
It is the name given to pure iron crystals which are soft and ductile. The slow cooling of low carbon steel below the critical temperature produces ferrite structure. Ferrite does not harden when cooled rapidly. It is very soft and highly magnetic. This is a Body Centered Cubic iron phase containing little or no carbon.
This is formed when the carbon combines with iron in the form of iron carbide which is extremely hard in nature. Cementite increases generally with increase in carbon percentage. It is found in steel containing more than 0.87% carbon.
This is a mechanical mixture of about 87% ferrite and 13% cementite. It comprises of alternate layers of ferrite and cementite in steel. When seen with the help of a microscope, the surface has appearance like pearl, hence it is called pearlite. Hard steels are mixtures of pearlite and cementite while soft steels are mixtures of ferrite and pearlite.
This is produced by entrapping carbon on decomposition of austenite when cooled rapidly. It is the main constituent of hardened steel. It is magnetic and appears as a needle like fibrous structure. It has carbon content up to 2%. It is extremely hard and brittle. The decomposition of austenite below 320°C starts the formation of martensite.
It is a solid solution of ferrite and iron carbide in gamma iron. It is formed when steel contains carbon up to 1.8% at 1130°C. On cooling below 723°C, it starts transforming into pearlite and ferrite. Austenitic steels cannot be hardened by usual heat treatment methods.
This is another constituent of steel obtained by tempering martensite. It is less hard and brittle than martensite. It is also produced by cooling the metal slowly until transformation begins and then cooling rapidly to prevent its completion. It has a dark appearance on etching. It is weaker than martensite.
This is also produced by the transformation of tempered martensite. It is produced when steel is cooled slowly from the temperature of the solid solution to normal room temperature. It has good strength and is practically pearlite. Its properties are intermediate between those of pearlite and troostite.
Furnaces Used for Heat Treatment
Heat treatment furnace may be defined as a refractory-lined chamber in which the metal part to be heat-treated, is heated to the required temperature. Usually a heat-treatment furnace is a box-like structure i.e., a steel shell with an access door, a refractory lining, temperature controls and indicators.
Furnaces used for heat treatment are classified as:
- Stationary hearth furnace
- Movable hearth furnace
- Salt bath
- Lead bath
- Oil bath
Stationary Hearth Furnaces
These are of two types:
1. Direct Fuel Fired Furnace
It burns the fuel in the space occupied by the charge. It is of low cost and is suitable for all ordinary temperature ranges. This furnace is used for rough heating, such as in forging, and also used for heat treatment, particularly at lower temperature. All the forging jobs can be heat treated in this type of furnace.
2. Indirect Fuel Fired Furnace/Muffle Furnace
Indirect fuel fired furnace has a heating chamber and a muffle that separates the combustion space from the work space. Since the products of combustion do not enter the work chamber, the furnace atmosphere can be controlled. It has an upper temperature limit of about 1100ºC. Reduced scaling and contamination from the fuels are the advantages of this type of furnace. This type of furnaces are used for heat treatment of small size components.
Movable Hearth Furnaces
These are of two types:
1. Car Bottom Furnace
A car bottom furnace has a moveable hearth like a flat car that can be rolled in and out of the furnace for charging and unloading the components. Commonly, the metal charge is placed on heat-resistance alloy or refractory base and spaces are provided for circulation of hot air. Car bottom furnace is used for large and heavy parts (e.g. big casting and forgings).
2. Rotary Hearth Furnace
Rotary furnace is used for continuous production. It has a round shell and horizontal hearth that rotates slowly. Material is charged through a door and taken out from the same or another door. It is used for hardening, tempering and carburizing. Hammers and chisels can be heat treated in this type of furnace. The temperature range of this furnace is 100ºC to 1200ºC.
These are further classified as:
1. Salt Bath
A molten salt bath furnace consist of a pot i.e. a ceramic or metal container filled with molten salt in which small jobs are immersed for either heating or cooling within the temperature range of 150°c- 1315°c. Molten bath contains one or more salts such as Nitrates. Carbonates, Chlorides Cyanides and Caustic soda. In this furnace the heat is developed either by oil, gas or electricity.
2. Lead Bath
Molten lead is used in lead bath. It is used for tempering steel parts. The parts are preheated then immersed in this bath, which is already heated to a tempering temperature. This bath is used for temperature range from 327º C to 1285º C. Heat treatment of Files, Reamers and Drills is carried out in this furnace.
3. Oil Bath
Mineral oils are used for oil baths. Light oil baths are used for temperature up to 230°C. Heavy oil baths are used for temperature range from 340°C– 370°C.