Fe-C Phase Diagram (Phase Diagrams III) Flashcards
What yields pearlite?
Equilibrium (slow) cooling from the
γ region through the eutectoid
composition of 0.76 wt% C yields
pearlite
What is pearlite and why is the pearlite structure formed?
Lamellar microstructure is formed
because phase transformation
occurs instantaneously over short
diffusional distances
Describe the mechanical properties of pearlite?
Mechanical properties are in-between the soft, ductile ferrite and
the brittle, hard cementite
What are the mechanical properties of steel largely dictated by?
- carbon content, and
- microstructural transformations upon cooling
At what composition is cementite (Fe3C) formed?
At 6.70 wt%C
Describe the properties of cementite
Hard, brittle and not used
on its own.
Write the reaction where equilibrium cooling yields pearlite
Check slide 8 of Phase Diagrams III slides
What type of process is the formation of pearlite?
A diffusion-governed process.
Name the four cooling methods that change external heat transport
- Furnace cooled
- Air cooled
- Oil quenched
- Water quenched.
Describe the relative cooling rate of each cooling method
- Furnace cooled - slow cooling rate
- Air cooled - moderate cooling rate
- Oil quenched - fast cooling rate
- Water quenched - very fast cooling rate
What is pearlite a natural composite of?:
Hard and Brittle Fe3C plates
Soft and Ductile ferrite plates
Why is pearlite an important feature for railway rails?
Pearlite displays good wear resistance for railway rails relative to other
compositions of pro-eutectoid ferrite and cementite
What is the relationship between cooling rates, thickness of layers, and inter-laminar spacing of pearlite?
Faster cooling rates = thinner layers = smaller inter-laminar spacing
Does fine pearlite or coarse pearlite have higher strength? Why?
Fine pearlite has higher strength since it has a greater phase boundary area than coarse pearlite for a given carbon composition.
What is the relationship between phase boundary area, strength of a material and resistance to dislocation movement?
The more phase boundary area = stronger material = more difficulties for dislocations to move through
How does coarse and fine pearlite form?
Coarse pearlite forms due to low cooling rates and fine pearlite forms due to high cooling rates.
What is the relationship between cooling time, thickness of the layer and time diffusion for pearlite?
The more time given to cool = more time for diffusion = the thicker the layer
Compare the yield and ultimate tensile strength of fine and coarse pearlite
Fine Pearlite is stronger (higher yield strength and ultimate tensile strength compared to coarse Pearlite) than coarse Pearlite
What types of conditions do phase diagrams apply to?
Slow-cooled (equilibrium)
conditions
What is the final structure of microstructural changes in the hyper-eutectoid?
Final structure is comprised of
pro-eutectoid cementite and
pearlite
What happens upon cooling in the microstructural changes of hypo-eutectoid of the steel alloy?
Upon cooling, the solid austenite
γ enters a two-phase region
Showcase the reaction where the solid austenite
γ enters a two-phase region ( α +γ)
γ ⇒α +γ
Where does the pro-eutectoid ferrite form at?
The pro-eutectoid ferrite (α)
forms at γ grain boundaries
What is the final microstructure of the hypo-eutectoid steel alloy
Final microstructure consists of
pro-eutectoid ferrite and pearlite
What is the range of wt% carbon composition for hypo-eutectoid steel alloy?
(0.022-0.76 wt% C)
What happens to the remaining austenite in the hypo-eutectoid steel below the eutectoid isotherm? Write the reaction.
Below the eutectoid isotherm,
the remaining austenite
transforms to pearlite
γ ⇒α + Fe3C
What happens upon cooling in the hyper-eutectoid steel alloy? Write the reaction.
Austenite enters a two-phase region of austenite + cementite. γ ⇒ γ + Fe3C
What happens below the eutectoid isotherm in the hyper-eutectoid steel alloy?
Below eutectoid isotherm,
remaining austenite transforms
to pearlite
What is the range of wt% C composition of the hyper-eutectoid steel alloy?
0.76-2.14 wt% C
What is the common composition of railway rails?
Often at the eutectoid composition of 0.76 wt% C (entirely Pearlite)
What types of layers does pearlite have?
Pearlite consists of alternating
layers of ferrite and cementite
In a hypoeutectoid steel, both eutectoid
and proeutectoid ferrite exist. Explain the
difference between them. What will be the
carbon concentration in each?
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.
What is the relationship between the strength of pearlite and the inter-laminar spacing
The strength of Pearlite increases with decreasing interlaminar spacing, S
What mechanism governs higher
strength in fine vs. coarse pearlite?
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.
Rank the strength of cementite, ferrite, and austenite from highest to lowest.
Cementite > Ferrite > Austenite
Rank the ductility of austenite, ferrite, and cementite from highest to lowest.
Austenite > Ferrite > Cementite
What is the difference in carbon concentration of a “hypoeutectoid steel” and a “hypereutectoid” steel?
“Hypoeutectoid” steel has a carbon concentration less than the eutectoid; on the other hand, “hypereutectoid” steel has a carbon content greater than the eutectoid.
