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

  • Thread starter Tiberious
  • Start date
  • #1
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Homework Statement


[/B]
(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.
 

Answers and Replies

  • #2
Baluncore
Science Advisor
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Use a phase diagram such as; https://en.wikipedia.org/wiki/Steel#Material_properties
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
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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) .
 

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