Skip to main content



THE IRON-CARBON PHASE DIAGRAM


Of all binary alloy systems the one that is possibly the most important for a metallurgist is the iron and carbon system. We know that both steel and cast iron play a important role in structural applications and they both are iron-carbon system.

Pure iron upon heating experiences changes in its crystal structure until 1538°C and melts there. At room temperature it is present at a stable form called ferrite(α), which has a BCC crystal structure. The ferrite transforms to austenite(ט) at 912°C which has a FCC crystal structure. At 1394°C it again undergoes a phase transformation to                             δ-ferrite which has a BCC crystal structure. Pure iron finally melts at 1538°C. All these changes can be seen in the left vertical axis of the Fe-C phase diagram.

In the composition axis at 6.7% formation of a intermediate compound known as Fe3C(Cementite) or iron carbide is observed. In real world all steels and cast iron have carbon content less than 6.7%. Thus in the Fe-C system consideration only upto 6.7% C is taken. Here, 6.7% C is considered as 100% Fe3C. Between 0.02-0.76% C  it is considered as Hypo eutectoid steel  and between 0.76-2.14% C it is considered as Hypereutectoid steel.

Carbon is a interstitial compound here and forms solid solution with α,δ and ט. In BCC α ferrite only small concentration of carbon is soluble about 0.02% at 727°C. The limited solubility is explained by the size and shape of BCC interstitial position. The maximum solubility of carbon in ט is about 2.14% at 1147°C. The δ-ferrite is same as α except for the fact that it occurs at higher temperature.

Eutectic reaction for Fe-C system(1147°C):

L <=>  ט+ Fe3C  

Eutectoid reaction for Fe-C system(727°C):

<=> ט    α+ Fe3C

Peritectic reaction for Fe-C system(1493°C):

δ + L  <=> ט


Comments

Post a Comment

Popular posts from this blog

  Leaching and its kinetics What is Leaching?  ➢ Also known as liquid-solid extraction.  ➢ Two phases are in intimate contact and the solute or solutes can diffuse from the solid to the liquid phase.  ➢ The process of extracting a substance from a solid material that has come into contact with liquid.  ➢ The liquid that enters will separate the components originally in the solid.  Examples of Leaching: 1) Extracting tea from tea leaves. 2) Production of vegetable oil using hexane. 3) Copper salt leaching from copper ores. 4) Production of nickel from ore.  Kinetics of Leaching:   The above figure shows the concentration profile when a mineral surface dissolves in a leaching medium. Subscripts  S and B refer to interface and bulk respectively. The situation is simple for complete dissolution.   However when a mineral decomposes, and dissolves only partially, then a new solid residue appears. Leaching would continue if this new p...
Post a Comment
Read more
                                                                    THE ELLINGHAM DIAGRAM The Ellingham diagram is the simplest method of representing the relationship between the free energy( Δ G) and temperature of various oxides and sulphides. In metallurgy, the Ellingham diagram is used to   find out a suitable reducing agent. In the Ellingham diagram the highly stable oxides are found at the bottom and the less stable oxides are found at the top. An element occupying a lower position in the diagram can reduce the oxides of another element present above it. For example, Mg lies below Si thus Mg can be used as a reducing agent for oxides of Si. Why Ellingham diagram has all straight lines? Reason for upward slope of line...
Post a Comment
Read more