ساختار شناسی فولاد

[Peyman 16,150]

The different microscopic constituents of iron and steels are as follows:

 

 

1. Ferrite:

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[TD=align: left]Ferrite Structure[/TD]

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The pure iron grains or crystals are probably called ferrite. Pure iron means carbon-free iron. It is soft and ductile. Cooling rapidly cannot from a bright red heat at a rapid rate. Low carbon steel and sought iron consists 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:

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[TD=align: left]Cementite Structure[/TD]

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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 is 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 carbide of iron.

 

 

3. Pearlite:

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[TD=align: left]Pearlite Structure[/TD]

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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 color of the mother of pearl when the etching process has removed a part of surrounding softer ferrite. Soft steels contain ferrite and pearlite. The hardness increases with the proportion of cementite.

During the process of cooling a slow rate from a red heat cementie forms a mechanical mixture with ferrite, and appears under high magnificatioin as alternate layers of cementite and ferrite. This constituents is called pearlite. Slow cooling produced coarse pearlite and quicker cooling finer pearlite. The carbon content of pearlite is in the plain carbon steel is 0.85%. pearlite possess a tensile strength in the vicinity and ductile, whilst ferrite is weak soft and ductile.

 

 

4. Martensite:

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[TD=align: left]Martensite Structure[/TD]

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It is 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 hardened site, which corresponds in composition to that of pearlite or martensite with carbon. Martensite is not as tough as austenite.

Marten site, 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 constituent’s austenite to sorbite. Marten site is the hardest and also the most brittle, with little ductility.

 

 

5. Austenite:

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[TD=align: left]Austenite Structure[/TD]

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Austenite is the solid solution of carbon or iron carbide (Fe₃C) in gamma iron. When carbon steel I heated, particularly no change in the constituents 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.