Fe-C Phase Diagram (Phase Diagrams III)

Question 1

What yields pearlite?

Answer 1

Equilibrium (slow) cooling from the
γ region through the eutectoid
composition of 0.76 wt% C yields
pearlite

Question 2

What is pearlite and why is the pearlite structure formed?

Answer 2

Lamellar microstructure is formed
because phase transformation
occurs instantaneously over short
diffusional distances

Question 3

Describe the mechanical properties of pearlite?

Answer 3

Mechanical properties are in-between the soft, ductile ferrite and
the brittle, hard cementite

Question 4

What are the mechanical properties of steel largely dictated by?

Answer 4

1. carbon content, and
2. microstructural transformations upon cooling

Question 5

At what composition is cementite (Fe3C) formed?

Answer 5

At 6.70 wt%C

Question 6

Describe the properties of cementite

Answer 6

Hard, brittle and not used
on its own.

Question 7

Write the reaction where equilibrium cooling yields pearlite

Answer 7

Check slide 8 of Phase Diagrams III slides

Question 8

What type of process is the formation of pearlite?

Answer 8

A diffusion-governed process.

Question 9

Name the four cooling methods that change external heat transport

Answer 9

1. Furnace cooled
2. Air cooled
3. Oil quenched
4. Water quenched.

Question 10

Describe the relative cooling rate of each cooling method

Answer 10

1. Furnace cooled - slow cooling rate
2. Air cooled - moderate cooling rate
3. Oil quenched - fast cooling rate
4. Water quenched - very fast cooling rate

Question 11

What is pearlite a natural composite of?:

Answer 11

 Hard and Brittle Fe3C plates
 Soft and Ductile ferrite plates

Question 12

Why is pearlite an important feature for railway rails?

Answer 12

Pearlite displays good wear resistance for railway rails relative to other
compositions of pro-eutectoid ferrite and cementite

Question 13

What is the relationship between cooling rates, thickness of layers, and inter-laminar spacing of pearlite?

Answer 13

Faster cooling rates = thinner layers = smaller inter-laminar spacing

Question 14

Does fine pearlite or coarse pearlite have higher strength? Why?

Answer 14

Fine pearlite has higher strength since it has a greater phase boundary area than coarse pearlite for a given carbon composition.

Question 15

What is the relationship between phase boundary area, strength of a material and resistance to dislocation movement?

Answer 15

The more phase boundary area = stronger material = more difficulties for dislocations to move through

Question 16

How does coarse and fine pearlite form?

Answer 16

Coarse pearlite forms due to low cooling rates and fine pearlite forms due to high cooling rates.

Question 17

What is the relationship between cooling time, thickness of the layer and time diffusion for pearlite?

Answer 17

The more time given to cool = more time for diffusion = the thicker the layer

Question 18

Compare the yield and ultimate tensile strength of fine and coarse pearlite

Answer 18

Fine Pearlite is stronger (higher yield strength and ultimate tensile strength compared to coarse Pearlite) than coarse Pearlite

Question 19

What types of conditions do phase diagrams apply to?

Answer 19

Slow-cooled (equilibrium)
conditions

Question 20

What is the final structure of microstructural changes in the hyper-eutectoid?

Answer 20

Final structure is comprised of
pro-eutectoid cementite and
pearlite

Question 21

What happens upon cooling in the microstructural changes of hypo-eutectoid of the steel alloy?

Answer 21

Upon cooling, the solid austenite
γ enters a two-phase region

Question 22

Showcase the reaction where the solid austenite
γ enters a two-phase region ( α +γ)

Answer 22

γ ⇒α +γ

Question 23

Where does the pro-eutectoid ferrite form at?

Answer 23

The pro-eutectoid ferrite (α)
forms at γ grain boundaries

Question 24

What is the final microstructure of the hypo-eutectoid steel alloy

Answer 24

Final microstructure consists of
pro-eutectoid ferrite and pearlite

Question 25

What is the range of wt% carbon composition for hypo-eutectoid steel alloy?

Answer 25

(0.022-0.76 wt% C)

Question 26

What happens to the remaining austenite in the hypo-eutectoid steel below the eutectoid isotherm? Write the reaction.

Answer 26

Below the eutectoid isotherm,
the remaining austenite
transforms to pearlite
γ ⇒α + Fe3C

Question 27

What happens upon cooling in the hyper-eutectoid steel alloy? Write the reaction.

Answer 27

Austenite enters a two-phase region of austenite + cementite. γ ⇒ γ + Fe3C

Question 28

What happens below the eutectoid isotherm in the hyper-eutectoid steel alloy?

Answer 28

Below eutectoid isotherm,
remaining austenite transforms
to pearlite

Question 29

What is the range of wt% C composition of the hyper-eutectoid steel alloy?

Answer 29

0.76-2.14 wt% C

Question 30

What is the common composition of railway rails?

Answer 30

Often at the eutectoid composition of 0.76 wt% C (entirely Pearlite)

Question 31

What types of layers does pearlite have?

Answer 31

Pearlite consists of alternating
layers of ferrite and cementite

Question 32

In a hypoeutectoid steel, both eutectoid
and proeutectoid ferrite exist. Explain the
difference between them. What will be the
carbon concentration in each?

Answer 32

For a hypoeutectoid steel, the proeutectoid ferrite is a microconstituent that formed above the eutectoid
temperature.

The eutectoid ferrite is one of the constituents of pearlite that formed at a temperature below the
eutectoid.

The carbon concentration for both ferrites is 0.022 wt% C.

Question 33

What is the relationship between the strength of pearlite and the inter-laminar spacing

Answer 33

The strength of Pearlite increases with decreasing interlaminar spacing, S

Question 34

What mechanism governs higher
strength in fine vs. coarse pearlite?

Answer 34

Phase boundary (interface between the two solid phases) between ferrite and cementite phases resist dislocation movement.

Fine pearlite has a greater phase boundary area than coarse pearlite for a given carbon composition.

Question 35

Rank the strength of cementite, ferrite, and austenite from highest to lowest.

Answer 35

Cementite > Ferrite > Austenite

Question 36

Rank the ductility of austenite, ferrite, and cementite from highest to lowest.

Answer 36

Austenite > Ferrite > Cementite

Question 37

What is the difference in carbon concentration of a “hypoeutectoid steel” and a “hypereutectoid” steel?

Answer 37

“Hypoeutectoid” steel has a carbon concentration less than the eutectoid; on the other hand, “hypereutectoid” steel has a carbon content greater than the eutectoid.