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

  BORON FIBER   Introduction Normally boron is a brittle material. Weintraub was the one who obtained the first Boron fiber in 1911 by the reduction of boron halide with hydrogen   on a hot wire (substrate). Boron fibers were developed after 1960s to increase the strength and stiffness of composite by reinforcement. Fabrication Ø Boron fibers are produced by CVD on a substrate by following two methods.                       1.     Thermal reduction of boron halide o   Low temperature. o   Carbon coated glass fiber used as a substrate. o   Weak and less dense boron fiber   is produced by this method because of gas entrapment. 2.     Reduction of Boron halide by hydrogen o   High temperature. o   Tungsten wire (12micron) used as a substrate which has a high density and melting point. o   Uniform quality of Boron fiber is th...
Post a Comment
Read more
  CRYSTAL DEFECTS (IMPERFECTIONS) INTRODUCTION : Most Materials When solidified consist of many crystals and grains. Single crystals can be manufactured by withdrawing a crystal fragment under carefully controlled conditions just above freezing point. In case if single crystal approach a nearly perfect lattice structure it is possible to calculate theoretical strength of a metal by the force required to separate bonds between adjoining atoms. But due to defects in crystals ,   This theoretical value of strength   obtained from single crystal   would be 1000 times smaller than original crystal structure. Hence it is important to study the defects in crystals to know how they affect properties of materials   TYPES OF DEFECTS Defect in a crystal can be defined as a deviation from its ideal structure. Defects in crystals can be broadly classified into 3 types based on their dimensionality ·       Zero dimensional defects o...
Post a Comment
Read more