Untitled Quiz

Iron and its Crystal Structures

• Iron experiences two transformations before melting.
• Iron exists in three crystalline forms: α-Ferrite (BCC), γ-Austenite (FCC), and δ-Ferrite (BCC).
• α-Ferrite is stable at room temperature and is a soft material.
• γ-Austenite is important for heat treatment of steels.
• δ-Ferrite has similar properties to α-Ferrite, but at higher temperatures.

Iron-Carbon Phase Diagram

• The Iron-Carbon phase diagram describes the relationship between temperature, carbon content, and the phases present in alloys of iron and carbon.
• Two important compositions are defined: steels (0.008 wt% C to 2.14 wt% C) and cast irons (>2.14 wt% C).
• This diagram is used to predict the microstructure and properties of steel and cast iron.
• Intermetallic compound, cementite (Fe3C), forms when the solubility limit of carbon in α-Fe is exceeded below 727 ºC.
• Cementite is hard and brittle.

Phases in the Fe-C System

• α-Ferrite (BCC): An interstitial solid solution of carbon in BCC iron with a low carbon concentration and a maximum solubility of 0.022 wt.% at 727 ºC.
• γ-Austenite (FCC): An interstitial solid solution of carbon in FCC iron with a maximum solubility of 2.14 wt.% at 1147 ºC. It is not magnetic.
• δ-Ferrite (BCC): An interstitial solid solution of carbon in BCC iron similar to α-Ferrite but at higher temperatures.

Invariant Reactions

• Three invariant reactions occur on the Fe-C phase diagram:
  • Eutectoid reaction (727ºC): γ(0.76%)  α(0.022%) + Fe3C(6.67%)
  • Eutectic reaction (1147ºC): L(4.3%)  γ(2.14%) + Fe3C(6.67%)
  • Peritectic reaction (1493ºC): δ(0.09%) + L(0.53%)  γ(0.16%)

Pearlitic Transformation

• Pearlite is a two-phase microconstituent formed during the eutectoid reaction.
• It consists of alternating layers (lamellae) of α-Ferrite and cementite.
• Pearlite formation occurs through nucleation and growth processes where ferrite nucleates on the austenite grain boundary, rejecting carbon that diffuses along the interface to form cementite.

Influence of Alloying Elements

• Alloying elements can influence the Fe-C phase diagram.
• γ-stabilizers, such as Mn, Co, Ni, Cu, Zn, and Au, favour the γ phase and displace the eutectoid to lower temperatures and lower carbon content.
• α-stabilizers, such as Si, P, Al, Be, Sn, Sb, As, Ti, Nb, V, Ta, Mo, W, and Cr, favour the α phase and displace the eutectoid to higher temperatures and lower carbon content.

Eutectoid Steels

• Eutectoid steels have a composition of 0.76% wt% carbon.
• They consist of a single microconstituent: pearlite.
• Pearlite is formed during the eutectoid reaction, where austenite transforms into a mixture of ferrite and cementite.
• The microstructure of pearlite is characterized by alternating layers of α-Ferrite and cementite.

Microstructure of Pearlite

• Pearlite has a layered structure with an alternating arrangement of α-Ferrite and cementite.
• Decreasing the spacing between the lamellae increases the hardness of pearlite.
• The lamellae spacing is influenced by the cooling rate during the eutectoid transformation.