Ferrous Metals

Question 1

Describe the qualities of the microstructural phase of Austenite.

Answer 1

• Stable at higher temperature (above 912℃).

- Face-centre cubic structure (easy to deform).

Question 2

Describe the qualities of the microstructural phase of Ferrite.

Answer 2

• Stable phase for extra low-carbon steel at room temp.
• Body-centred cubic structure.
• Can contain a maximum 0.02% Carbon.

Question 3

Describe the qualities of the microstructural phase of Graphite.

Answer 3

• The thermodynamic stable phase for carbon.

- Seen in cast iron where carbon content is high.

Question 4

Describe the qualities of the microstructural phase of Cementite.

Answer 4

• Intermetallic compound of iron and carbon (Fe3C).

- In carbon steels and cast irons that are slowly cooled, a portion of the carbon is in the form of cementite.

Question 5

Describe the qualities of the microstructural phase of Pearlite.

Answer 5

• Two-phased lamellar (layered) structure, composed of alternating layers of ferrite and cementite (occurs in carbon steels and ast irons).
• Seen in slow cooled carbon steels.

Question 6

Describe the qualities of the microstructural phase of Bainite.

Answer 6

• Plate- like microstructure that forms in steels at around 250℃ (depending on steel grade).
• Fine, non-lamellar structure.
• Composed of Ferrite and Cementite.

Question 7

Describe the qualities of the microstructural phase of Martensite.

Answer 7

• Formed in carbon steels by rapid cooling (quenching) of austentite.
• The atoms cool at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe3C).

Question 8

Describe the qualities of the microstructural phase of Tempered Martensite.

Answer 8

• Transformed from Martensite by a reheat after quenching.
• Fine Structure.
• Ferrite and Cementite.

Question 9

What are the 3 main ways to make steel harder?

Answer 9

• Change the chemistry: Alloying (Solid Solution).
• Change the microstructure: Grain size, precipitation, work hardening.
• Change the crystal structure: Martensite

Question 10

What are the 3 main ways to heat-treat steel?

Answer 10

Annealing: Softens the steel by heating and allowing to cool slowly in the furnace. Results in stress-free, large-grained structure.
Normalising: Faster cooling than annealing. Stress relief without grain growth.
Hardening and Tempering: Hardens the finished component to its required mechanical properties.

Question 11

What is work hardening, and how does it work?

Answer 11

Some materials get stronger (and harder) if they are plastically deformed.
- Dislocations entangle and multiply, thus motion becomes more difficult.

Question 12

What are the 3 main stages of Annealing?

Answer 12

Recovery: Dislocations untangle and annihilate, relieving stress.
Recrystallisation: New stress-free grains nucleate at boundaries and dislocation entanglements.
Growth: Grains grow and consume deformed grains.

Question 13

What is the purpous of solution treatment of metal?

Answer 13

• To dissolve any precipitates present in the material.

- To transform the material at the solution annealing temperature into a single phase structure.

Question 14

Compare the cooling time of Martensite and Pearlite.

Answer 14

Pearlite = Slow Cooling
Martensite = Fast Cooling

Question 15

List 3 properties of Martensite.

Answer 15

• Extremely Hard (Hardenss increases with carbon content)
• A material made of 100% Martensite is brittle and unusable.
• Important for producing quenched and tempered steels.

Question 16

How does quenching and tempering affect the microstructure of steel?

Answer 16

• Forms a strong, tough microstructure of Fe3C precipitates with Ferrite.
  (same constituents as in pearlite)

Question 17

What is the hardness and Ductility of Pearlite?

Answer 17

Hardness = Low
Ductility = High

Question 18

What is the hardness and Ductility of Martensite?

Answer 18

Hardness = Very High
Ductility = Very Low

Question 19

What is the hardness and Ductility of Tempered Martensite?

Answer 19

Hardness = Medium
Ductility = Medium

Question 20

What are the effects of Hardening and Tempering

Answer 20

• Crystallographic transformation to body-centred tetragonal Martensite.
• Tempering at low temperature maintains strength while recovering some ductility.

Question 21

What does slowly-cooled Austentite produce?

Answer 21

Pearlite

Question 22

What does moderately-fast cooled Austentite produce?

Answer 22

Bainite

Question 23

What does rapidly-cooled Austentite produce?

Answer 23

Martensite

Can be tempered again to form tempered martensite

Question 24

Name three types of surface hardening which only heat the surface.

Answer 24

• Flame Hardening
• Induction Hardening
• Laser Hardening

Question 25

Name three types of surface hardening which modify the chemistry of the surface.

Answer 25

• Carburising
• Nitriding
• Nitrocarburising

Question 26

Explain some characteristics of galvanized steels.

Answer 26

• Applies a layer of zinc to the surface by hot dipping.
• Corrosion of a zinc coating will proceed at a slow rate because the ratio of the anode-to-cathode surface area is quite large.

Question 27

How does grain size affect metal properties?

Answer 27

The smaller the grain size, the higher the strength and toughness.