What is Ferrite, Cementite, Pearlite , Martensite, Austenite – Constituents of Iron and Steel

The different microscopic constituents of iron and steel which commonly occur are:

1. Ferrite

2. Cementite

3. Pearlite

4. Martensite

5. Austenite

6. Troostite

7. Sorbite.

The other constituents comprise the three allotropic forms of nearly pure iron, graphite and slag.

Read More: Introduction to Iron-Carbon Equilibrium Diagram | Structures in Fe-C Diagram

1. Ferrite:

Iron which contains little or no carbon is called ferrite. It is very soft and ductile and is known as alpha iron by the metallurgists. Ferrite is present to some extent in a great range of steels, particularly those low in carbon content, and it is also present, in soft cast iron. Ferrite does not harden when cooled rapidly. It forms smaller crystals when cooled from a bright red heat at a rapid rate.

2. Cementite:

This is a definite carbide of iron (Fe₃C) which is extremely hard, being harder than ordinary hardened steel or glass. Cementite increases generally with the proportion of carbon present, and the hardness and also the brittleness of cast iron is believed to be due to this substance.

It contains 6.6 percent carbon and occurs either in the form of a network or in globular or massive form, depending on the analysis of the steel and the heat treatment to which it is subjected. It is magnetic below 25°C. Its presence in iron or steel decreases the tensile strength but increases the hardness and cutting qualities.

3. Pearlite:

Pearlite is the name given to a mixture of about 87.5 percent ferrite and 12.5 percent cementite. It consists of alternate layers of ferrite and cementite in steel. Under high magnification the ferrite and cementite can be seen to be arranged in alternate laminations or plates.

When seen in the microscope the surface has appearance like mother of pearl, hence the name pearlite. The thickness of alternate plates and the distance between them is governed by the rate of cooling, slow cooling produces a coarser structure than rapid cooling. Pearlite is eutectoid of steel.

It has been found that the proportion of pearlite increases from nothing in the case of pure carbonless iron upto 100%, or saturation, for steel containing 0.90% of carbon thus a 0.3 percent carbon steel will consist of about 33 percent pearlite and rest ferrite. It is the characteristic of soft steels that they contain ferrite and pearlite, and the hardness increases with the proportion of pearlite. Hard steels are mixtures of pearlite and cementite.

4. Martensite:

It is hard brittle mass of fibrous or needle like structure and is the chief constituent of hardened steel. The vickers pyramid numeral is anything upto 900 for an original carbon content of 0.9 percent. It has been found that martensite is produced by the rapid quenching of high carbon steel from a slightly higher temperature than the maximum temperature of critical interval. It is not as tough as austenite. It differs from austenite in being magnetic.

5. Austenite:

It is a solid solution of iron-carbon which is stable only within a particular range of composition and temperature, and is non-magnetic. On cooling below 700°C it is completely transformed into ferrite which is magnetic and cementite to form the eutectoid pearlite, together with free ferrite or free cementite, depending on whether the carbon content is less or greater than 0.87 percent respectively.

It is formed when carbon steel with more than 1.1 percent carbon is quenched rapidity from about 1000°C. The amount of austenite increases with the proportion of carbon, 0 upto 1.1 percent carbon, upto 70 percent for 1.6 to 1.8 percent carbon. Austenitic steels cannot be hardened by usual heat treatment methods and are non-magnetic.

6. Troostite:

It is a structure in steel (consisting of very finely divided iron carbide in what is known at “alpha-iron”) produced either by tempering a martensitic steel at between 250 and 450°C or by quenching steel at a speed insufficient to suppress the thermal change point fully. The structure produced by the latter method should be more accurately termed very fine pearlite.

7. Sorbite:

It is a structure which consists of evenly distributed carbide of iron particles in a mass of ferrite, formed when a fully hardened steel is tempered at between 550 and 650°C. A sorbitic structure is characterised by strength and a high degree of toughness.

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