How are the following properties of Iron affected when alloyed with carbon?

In summary: The ferrite in (3) has smaller grains and is more cemented than the ferrite in (4). The austenite in (4) has more carbon due to its higher carbon content, this results in a higher hardness and a lower ductility . In summary, the addition of carbon to iron affects the following properties: -Hardness -Strength -Ductility
  • #1

Homework Statement

(a) How are the following properties of Iron affected when carbon is alloyed with it?Illustrate your answer with reference to the microstructures of steel. o Hardness

o Strength

o Ductility

The Attempt at a Solution

I've looked at this question a few times and wonder how it should be answered. Would it be more prudent to go for diagrams of grain structure ? Or, should I instead go for the differences in the internal atom layout?

It seems a broad question, any assistance is appreciated.
Physics news on
  • #2
Use a phase diagram such as;
to identify the micro-structure by following the thermal path taken by the cooling steel.
The properties are dependent on the crystals present, their size and orientation.
You can ignore the atomic positions.
Study the different properties of the constituents such as ferrite, graphite or iron.
Then you can show how the properties of those determine the characteristics of the steel.
  • #3
Unable to attach the illustrations. But, would you be able to review the below and let me know what you think so far ? I need to summarise still. The austenite has a higher carbon content due to it's configuration and thus I assume it is more brittle (less ductile) but harder.

(1) has pure austenite.
(2) has ferrite forming.
(3) Has larger grains of ferrite.
(4) has pearlite.

Illustrated above (Reference, III) is the eutectoid reaction as denoted by the iron-carbon phase diagram, the material is cooled slowly from over 〖800〗^o C where the Iron is in a phase of equilibrium, austenite. Austenite has a face-centred cubic (fcc) lattice forming as denoted in illustration (1) a single solid phase.

Whilst the temperature of the material decreased from over 〖800〗^o C to 〖723〗^o C (following the solidus line until the eutectoid temperature) the austenite will slowly decompose. This decomposition allows for the formation of ferrite on the austenite's grain boundary as denoted in illustration (2). The ferrite has a body centred cubic (bcc) lattice, as the interstitial spacing reduces the carbon will be rejected by the newly forming ferrite.

Eutectoid point, at 〖723〗^o C the austenite has a carbon content of 0.8wt% (increased from its initial 0.3wt%). The material decomposed almost entirely to pearlite a dual phase of ferrite and cementite, this change can be observed by the differences in illustration (3) and (4) .

1. How does the addition of carbon affect the strength of iron?

The addition of carbon to iron increases its strength and hardness. This is because carbon atoms are smaller than iron atoms and can fit into the spaces between them, preventing them from sliding and making the alloy more resistant to deformation.

2. What happens to the ductility of iron when it is alloyed with carbon?

Alloying iron with carbon decreases its ductility, or ability to be stretched or bent without breaking. This is because the carbon atoms can create barriers that prevent the metal from being easily deformed, making it more brittle.

3. Does alloying iron with carbon affect its corrosion resistance?

Yes, the addition of carbon can decrease the corrosion resistance of iron. This is because carbon atoms can create gaps and imperfections in the iron's structure, making it easier for corrosive substances to penetrate and cause damage.

4. How does the melting point of iron change when it is alloyed with carbon?

The melting point of iron increases when it is alloyed with carbon. This is because the carbon atoms disrupt the orderly arrangement of iron atoms, making it harder for them to move and reducing the metal's tendency to melt at high temperatures.

5. What effect does carbon have on the electrical conductivity of iron?

The addition of carbon decreases the electrical conductivity of iron. This is because carbon atoms can create barriers and impurities in the metal's structure, making it harder for electrons to flow and reducing its ability to conduct electricity.

Suggested for: How are the following properties of Iron affected when alloyed with carbon?