Objective 02: Iron-Carbon Equilibrium Diagram

Question 1

What are the properties of pure iron?

Answer 1

Relatively soft, ductile element with low strength

Question 2

What is the major aim of the Science of Steelmaking?

Answer 2

To attempt to understand and control the phase changes of iron, in the presence of carbon and other alloying metals, in order to make high-strength steels for industry

Question 3

Carbon Steel

Answer 3

Steel alloy containing 2% or less carbon

Question 4

Cast Iron

Answer 4

Steel alloy containing 2% - 6% carbon

Question 5

What are the 3 grades of Carbon Steel?

Answer 5

1. Low (0.30% C max)
2. Medium (0.30 - 0.60% C max)
3. High (0.60 - 1.00% C max)

Question 6

What are the factors that affect the physical properties of Carbon Steel?

Answer 6

1. The form that Carbon assumes in the iron matrix, i.e. single atoms, graphite flakes, spheres, or molecular combinations, e.g. cementite
2. The carbon atom’s final resting place, i.e. inside the unit cell structures or in the exterior intra-granular spaces
3. The Carbon concentration

Question 7

How does the amount of carbon affect steel?

Answer 7

1. As carbon content increases, hardness and brittleness increase
2. As carbon content decreases, softness, ductility and weldability increase

Question 8

What factors determine the iron-carbon equilibrium in steel?

Answer 8

1. Initial concentration of carbon in the ferrite mixture
2. Temperature
3. Cooling rate
4. Presence of other alloying metals

Question 9

What does the area below the Solidus Line represent in an Iron-Carbon Equilibrium Diagram?

Answer 9

Solid state phase

Question 10

What does the area above the Liquidus Line represent in an Iron-Carbon Equilibrium Diagram?

Answer 10

Molten state phase

Question 11

What does the area between the Solidus and Liquidus lines represent in an Iron-Carbon Equilibrium Diagram?

Answer 11

Pasty state phasese where the mixtures exist in a solid-liquid state at the corresponding temperature

Question 12

Austenite

Answer 12

Structural name of iron in a unit cell of FCC form that can contain dissolved carbon, up to 2%, i.e. gamma iron

Question 13

What phase must all quenching heat treatment procedures begin with?

Answer 13

Austenite

Question 14

Cementite

Answer 14

Iron-Carbon in the form of molecular iron carbide (Fe3C)

Question 15

Eutectic Reaction Point

Answer 15

The point at which liquid alloy changes directly into solid austenite and cementite without going through a pasty state phase

Question 16

When is the only point that a Eutectic Reaction takes place?

Answer 16

An alloy composition of 4.0% carbon and a temperature of 1130ºC

Question 17

Ferrite

Answer 17

Structural name for iron in the BCC form that can contain a max of 0.025% of carbon at 723ºC

Question 18

Lower Critical Change Line

Answer 18

Temperature at which an iron alloy of any carbon composition returns to BCC, i.e. 723ºC

Question 19

Pearlite

Answer 19

Dark-grained, layered structure of ferrite and cementite that forms in iron-carbon alloys below the lower critical change line.

Question 20

Peritectic Reaction Point

Answer 20

The point where liquid delta iron in BCC form changes directly into solid austenite without going through a pasty state phase, at 1492ºC

Question 21

Hypoeutectoid Steels

Answer 21

Low and Medium Carbon Steels (below 0.8% carbon) that contains ferrite and pearlite

Question 22

At room temperature, what are always the components of Hypoeutecoid Steels and the properties they bestow?

Answer 22

1. Ferrite - toughness and ductility

2. Pearlite - hardness

Question 23

Eutecoid Steel

Answer 23

Steel having 0.8% carbon that is 100% pearlite

Question 24

What is an application of Eutecoid Steel?

Answer 24

Railway rails

Question 25

Hypereutectoid Steel

Answer 25

Steel having above 0.8% carbon and at 723ºC consists of a mixture of cementite and pearlite

Question 26

What are the properties of Hypereutectoid Steel?

Answer 26

1. Hardness

2. High Tensile Strength

Question 27

What are some applications of Hypereutectoid Steel?

Answer 27

Tools such as axes and chisels