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Questions and Answers
What is the result of decreasing the pearlitic lamella spacing in steel?
What is the result of decreasing the pearlitic lamella spacing in steel?
Which microconstituents are formed in eutectoid steel with a composition of 0.76% C?
Which microconstituents are formed in eutectoid steel with a composition of 0.76% C?
In the Fe-C phase diagram, what phases are present in pearlite?
In the Fe-C phase diagram, what phases are present in pearlite?
What is the carbon content of cementite in the Fe-C phase diagram?
What is the carbon content of cementite in the Fe-C phase diagram?
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Which property of δ-Ferrite is NOT typically associated with its structure?
Which property of δ-Ferrite is NOT typically associated with its structure?
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What is the carbon content range that defines steels based on composition?
What is the carbon content range that defines steels based on composition?
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Which alloying elements are known as γ-stabilizers in the Fe-C system?
Which alloying elements are known as γ-stabilizers in the Fe-C system?
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What is the primary characteristic of cementite in the Fe-C phase diagram?
What is the primary characteristic of cementite in the Fe-C phase diagram?
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How do α-stabilizers affect the eutectoid reaction temperature in the Fe-C system?
How do α-stabilizers affect the eutectoid reaction temperature in the Fe-C system?
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What are the important phases found on the Fe-C phase diagram?
What are the important phases found on the Fe-C phase diagram?
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What effect do γ-stabilizers have on the A3 and A1 lines in the Fe-C phase diagram?
What effect do γ-stabilizers have on the A3 and A1 lines in the Fe-C phase diagram?
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What happens to the eutectoid point with the introduction of α-stabilizers?
What happens to the eutectoid point with the introduction of α-stabilizers?
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Which phase exhibits a body-centered cubic (BCC) structure in the Fe-C phase diagram?
Which phase exhibits a body-centered cubic (BCC) structure in the Fe-C phase diagram?
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What occurs during the eutectoid reaction at 727ºC?
What occurs during the eutectoid reaction at 727ºC?
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Which phase exists in the two-phase regions below 727ºC?
Which phase exists in the two-phase regions below 727ºC?
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At what temperature does δ-Ferrite occur in the Fe-C system?
At what temperature does δ-Ferrite occur in the Fe-C system?
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What is a characteristic of cementite (Fe3C)?
What is a characteristic of cementite (Fe3C)?
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In the pearlitic transformation, what does ferrite nucleate on?
In the pearlitic transformation, what does ferrite nucleate on?
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Which of the following describes the characteristics of δ-Ferrite?
Which of the following describes the characteristics of δ-Ferrite?
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What is the maximum carbon solubility in γ-Austenite at 1147ºC?
What is the maximum carbon solubility in γ-Austenite at 1147ºC?
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What is the density of α-Ferrite?
What is the density of α-Ferrite?
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Which reaction occurs at 1147ºC in the Fe-C system?
Which reaction occurs at 1147ºC in the Fe-C system?
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What phase forms when the solubility limit of carbon in α-Fe is exceeded?
What phase forms when the solubility limit of carbon in α-Fe is exceeded?
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What is a key feature of the eutectoid transformation?
What is a key feature of the eutectoid transformation?
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Which phase is magnetic in the Fe-C system?
Which phase is magnetic in the Fe-C system?
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Which of the following phases has the highest carbon solubility?
Which of the following phases has the highest carbon solubility?
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Study Notes
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.
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