Constituents of Steel

Constituents of Steel Points : Constituents of Steel, Ferrite, Cementite, Pearlite, Martensite, Austenite, Troostite, Sorbite The different microscopic constituents of iron and steels are as follows:
1. Ferrite
2. Cementite
3. Pearlite
4. Martensite
5. Austenite
6. Troostite
7. Sorbite

All these constituents have been described here in brief. Other constituents comprise the allotropic forms of nearly pure iron, graphite and slag. 1. Ferrite The pure iron grains or crystals are probably called ferrite. Pure iron means carbon-free iron (Fe) It is soft and ductile. Cooling rapidly cannot harden it. It forms smaller crystals when cooled from a bright red heat at a rapid rate Low carbon steel and sought iron consist chiefly of ferrite Low carbon steel examined under the microscope will be observed to contain regions of ferrite and regions of pearlite. Alpha iron is pure ferrite. The soft and ductile crystals of ferrite are just opposite of iron carbide (cementite Fe₃C) which are extremely hard and brittle. 2. Cementite It Is carbide of iron (Fe₃C). it is extremely hard. It increases generally with the proportion of carbon content. The hardness and brittleness of cast iron is believed to be due to the presence of the cementite. It s magnetic below 2.50°C. Its presence in iron and steel increases hardness but decreases tensile strength. Its composition by weight is 14 parts of iron and one part are carbon or 93.45% iron and 6.55% carbon its tensile strength is below 3.50 kgf/cm². It occurs in steel which has cooled slowly from a high temperature as a constituent of pearlite. The carbon is almost completely combined with a definite amount of iron to form a carbide of iron. 3. Pearlite It is a mixture of about 87.5% ferrite and 12.5% cementite. It occurs particularly in medium and low carbon steels in the form of fine lamellae which are usually curved and inter-stratified with those of ferrite. The name pearlite is derived from the fact that it shows oblique lighting, under the microscope the rainbow colour of the mother-of- pearl when the etching process has removed a part of surrounding softer ferrite. Soft steels contain ferrite and pearlite. The hardens increases with the proportion of cementite. During the process of cooling a slow rate from a red heat cementite forms a mechanical mixture with ferrite, and appears under high magnification as alternate layers of cemeiltite and ferrite. This constituent is called pearlite. Slow cooling produced coarse pearlite and quicker cooling finer pearlite. The carbon content of pearlite is in plain carbon steel is 0.85%. Pearlite possesses, a tensile strength in the vicinity of 8800 kgf7cm2. pearlite is relatively strong, hard and ductile, whilst ferrite is weak soft and ductile. 4. Martensite It s the chief constituent of hardened steel. It is of fibrous or needle like structure. It is very hard and consists of iron with carbon in varying proportion up to about 2%. When it contains iron and 0.9% carbon it is termed as Harden site, which corresponds in composition to that of pearlite or marten site saturated with carbon. Martensite is not as tough as austenite. Martensite, generally regarded as a solid solution of carbon or carbide in alpha ferrite, is the chief and characteristic constituent of hardened steel when cooled rapidly from temperature above the critical range of the transition constituents-austenite to Sorbite. Martensite is the hardest and also the most brittle, with little ductility. 5. Austenite Austenite is the solid solution of carbon or iron carbide (Fe₃ C) in gamma iron. When carbon steel is heated, particularly no change in the constituent occur during the heating until a temperature corresponding to the lower critical Ac₁ is reached; this is at about 724°C to 727°C. Here there is a complete change in the nature and structure of the pearlite, and it is known under the general term solid solution usually called “Austenite”.
As the temperature is raised from the lower critical Ac₁ to the upper critical Ac₂, which ends at about 852°C to 854°C, the least remaining excess of ferrite or cementite depending on whether it is a low or high carbon steel, will be absorbed by the austenite, so that above the upper critical range. Ac₂ the steel is composed entirely of solid solution-the austenite. Austenite is hard and non-magnetic. 6. Troostite It is another constituent of steel obtained by quenching during transformation period. Cooling the steel rapidly and then Tempering it may also form it. It may be regarded as a stage in the transformation of austenite. It occurs as a slightly granular, somewhat amorphous, laminated structure. Whether it is primary troostite, formed directly from austenite of secondary Troostite resulting from tempera ring or breaking down of Martiensite, Troostite is intermediate in hardness between Martiensite and Sorbite. 7. Sorbite When a fully hardened steel is finally drawn at about 677₀ C, Sorbite is formed. As the reheating or drawing temperature is increased still further beyond the temperature range, another stage in the decomposition of steel begins the change of troostite into Sorbite. Like the change from Martiensite to Troostite the formation of Sorhite does not take place spontaneously throughout the steel, but only increases gradually and progressively. Although scorbutic steel is slightly less ductile than pearlite steel, its tensile strength and yield point are so high that a higher combination of these three properties can be had in Sorbite than in pearlite steel. Consequently, Steels so treated as to contain Sorbite are often known as toughened steel.