In iron-carbon system, carbon is present either in the form of free carbon or as iron carbide.
Time-Temperature-Transformation (TTT) Diagram
The time-temperature transformation curves correspond to the start and finish of transformations. The horizontal lines that run between the two curves, marks the beginning and end of isothermal transformations.
Constan-Cooling-Temperature (CCT) Diagram
The constant-cooling temperature diagrams, describe the transformations taking place at various constant cooling rates. The CCT Diagrams provide not only the information on the transformation start and finish conditions, but also on the resulting hardness and microstructures for each particular cooling curve.
Heat treatment is the controlled heating and cooling of metals to modify their physical and mechanical properties without changing the shape of the product. Heat treatment is associated with increasing the strength of materials. Heat treating is adopted by many group of industrial and metalworking processes used to alter the physical, properties of a material.
Types of heat treatment
i) Softening – Softening is done to reduce strength or hardness, remove residual stresses, improve toughness, restore ductility, refine grain size or change the electromagnetic properties of steel.
ii) Hardening – Hardening of steels is done to improve the strength and wear properties.
Common Heat Treatments of Steels
A wide variety of heat treatments is generally used for improving the physical as well as mechanical properties of steels for effective applications.
Full annealing is the process of slowly raising the temperature about 50*C above the sustenitic temperature line A3 or line Acm in the case of hypo-eutectoid steels. It is held at this temperature for sufficient time for all the material to transform into austenite. Then slowly cooled at the rate of about 20*C/h.
Normalizing is the process of raising the temperature to over 60*C. It is held at this temperature to fully convert the structure into austenite, and then removed from the furnace and cooled at room temperature.
Tempering is a process done subsequent to quench hardening. Quench-hardened parts are often too brittle. This brittleness is caused by a predominance of martensite. This brittleness is removed by tempering. Tempering results in a desired combination of hardness, ductility, toughness, strength, and structural stability.
The mechanism of tempering depends on the steel and the tempering temperature.
Tempering is done immediately after quench hardening. When the steel cools to about 40*C after quenching, it is ready to be tempered. The part is reheated to a temperature of 150*C to 400*C. In this region, softer and tougher structure Troostite is formed.
Austempering is a quenching technique. The part is not quenched thorough the martensite transformation. Instead the material is quenched above the temperature when martensite forms Ms, around 315*C. Bainite is tough enough so that further tempering is not necessary and the tendency to crack is reduced severely.
Martempering is similar to austempering except that the part is slowly cooled through the martensite transformation. The biggest advantage of martempering over rapid quenching is that there is less distortion and tendency to crack.
Quenching is the act of rapidly cooling the hot steel to harden the steel. A host of medium is used based on the desired properties of the steel.
Quenching by water – Quenching can be done by plunging the hot steel in water. The water adjacent to the hot steel vapourizes, and there is no direct contact of the water with the steel.
By Salt water – It is more rapid quenching medium than plain water.
Quenching by Oil – Oil is used when a slower cooling rate is desired. Since oil has a very high boiling point, the transition from start of martensite formation to the finish is slow and this reduces the likelihood of cracking.
Carburizing is a process of adding carbon to the surface. This is done by exposing the part to a coarbon rich atmosphere at an elevated temperature and allows diffusion to transfer the carbon atoms into steel.
Pack Carburizing – Parts are packed in a high carbon medium, such as carbon powder or cast iron shavings and heated in a furnace for 12 h to 72 h at 900*C.
Gas Carburizing – Gas carburizing is conceptually the same as pack carburizing, except that CO is supplied to a heated furnace and the reduction reaction of deposition of carbon takes place on the surface of the part.
Liquid Carburizing – The steel parts are immersed in a molten carbon rich bath.
Nitriding is a process of diffusing nitrogen into the surface of steel. The parts are heat-treated and tempered before nitriding.
Carbonitriding – It is most suitable for low carbon and low carbon alloy steels. In this process, both carbon and nitrogen are diffused into the surface.
The strength of a material can significantly be improved by the precipitation of a finely dispersed second phase in the matrix. It is common phenomenon in case of some of the aluminium alloys.