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Questions and Answers

What is the result of decreasing the pearlitic lamella spacing in steel?

It enhances the toughness of the steel.

It increases the hardness of the steel. (correct)

It increases the ductility of the steel.

It decreases the hardness of the steel.

Which microconstituents are formed in eutectoid steel with a composition of 0.76% C?

Spheroidite

Bainite

Pearlite (correct)

Martensite

In the Fe-C phase diagram, what phases are present in pearlite?

Ferrite and Cementite (correct)

Austenite and Ferrite

Spheroidite and Martensite

Austenite and Delta-Ferrite

What is the carbon content of cementite in the Fe-C phase diagram?

6.67% C Signup and view all the answers

Which property of δ-Ferrite is NOT typically associated with its structure?

Brittleness at high temperatures Signup and view all the answers

What is the carbon content range that defines steels based on composition?

0.008 wt% C to 2.14 wt% C Signup and view all the answers

Which alloying elements are known as γ-stabilizers in the Fe-C system?

Mn, Co, Ni, Cu, Zn, Au Signup and view all the answers

What is the primary characteristic of cementite in the Fe-C phase diagram?

It is represented by the formula Fe3C. Signup and view all the answers

How do α-stabilizers affect the eutectoid reaction temperature in the Fe-C system?

They raise the eutectoid temperature at lower %C. Signup and view all the answers

What are the important phases found on the Fe-C phase diagram?

α-ferrite, γ-austenite, and cementite Signup and view all the answers

What effect do γ-stabilizers have on the A3 and A1 lines in the Fe-C phase diagram?

They lower A3 and A1 lines, shifting eutectoid to lower T. Signup and view all the answers

What happens to the eutectoid point with the introduction of α-stabilizers?

It shifts to higher temperatures at lower carbon content. Signup and view all the answers

Which phase exhibits a body-centered cubic (BCC) structure in the Fe-C phase diagram?

α-ferrite Signup and view all the answers

What occurs during the eutectoid reaction at 727ºC?

γ transforms to α and Fe3C. Signup and view all the answers

Which phase exists in the two-phase regions below 727ºC?

α-Ferrite and Fe3C Signup and view all the answers

At what temperature does δ-Ferrite occur in the Fe-C system?

At 1390ºC Signup and view all the answers

What is a characteristic of cementite (Fe3C)?

Metastable compound Signup and view all the answers

In the pearlitic transformation, what does ferrite nucleate on?

Austenite grain boundary Signup and view all the answers

Which of the following describes the characteristics of δ-Ferrite?

Same properties as α-Ferrite at higher temperatures Signup and view all the answers

What is the maximum carbon solubility in γ-Austenite at 1147ºC?

2.14 wt.% Signup and view all the answers

What is the density of α-Ferrite?

7.88 g/cm³ Signup and view all the answers

Which reaction occurs at 1147ºC in the Fe-C system?

Eutectic reaction Signup and view all the answers

What phase forms when the solubility limit of carbon in α-Fe is exceeded?

Cementite Signup and view all the answers

What is a key feature of the eutectoid transformation?

Produces layers of α-Fe and carbon. Signup and view all the answers

Which phase is magnetic in the Fe-C system?

α-Ferrite Signup and view all the answers

Which of the following phases has the highest carbon solubility?

γ-Austenite Signup and view all the answers

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.