DAY 11 – INTERMETALLIC COMPOUNDS AND THE IRON-CARBON PHASE DIAGRAM

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Note the MG2Pb line.

Intermetallic compounds show up as vertical lines. Their bonding predicts that they will be hard britlle, “rock-like” phases

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Ferrite () Low temperature form of Fe – it is BCC. Can only hold up to 0.022% C by weight. This carbon is interstitial. It is very soft ductile material.

Austenite (). High temperature form of Fe – not in equilibrium below 727C. It is FCC. Can hold up to 2.14% C by weight. This is also very soft ductile material.

Cementite (Fe3C). A compound of Fe and C which always has 6.70% C by weight. The crystal structure is complicated. It is very hard brittle material.

VOCABULARY

Ferrite () Low temperature form of Fe – it is BCC. Can only hold up to 0.022% C by weight. This carbon is interstitial. It is very soft ductile material.

Austenite (). High temperature form of Fe – not in equilibrium below 727C. It is FCC. Can hold up to 2.14% C by weight. This is also very soft ductile material.

Cementite (Fe3C). A compound of Fe and C which always has 6.70% C by weight. The crystal structure is complicated. It is very hard brittle material.

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USING THE FE-C DIAGRAM

We can use the diagram to predict microstructure, and thereby mechanical properties in equilibrium (i.e. slow) cooled steels.

This depends on a reaction known as the Euctectoid Reaction. Here is that reaction CFeC 3%77.0

As we cool below 727C

The resulting microconstituent is called Pearlite, P.

PEARLITE

Here are some micrograph images.

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Hypoeutectoid – Below Eutectoid Composition, i.e. less than 0.77% carbon.

Hypereutectoid – Above Eutectoid Composition, i.e. greater than .77% carbon

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FINDING THE RELATIVE AMOUNT OF PEARLITE Steel has 0.30% C by wt. Use the Fe-C diagram to

predict microstructure if we cool it slowly.

0.30 %CWP at low temp = W at 727

%100022.076.0

022.030.0

PW

%38 WWP

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WHAT IS STEEL?

A vast family of materials Usually composed of ferrite + cementite.

There are some exceptions. The way that the cementite is distributed in

the ferrite is very important. For example,1. Very coarse roundish cementite particles

widely separated soft ductile steel.2. Very fine roundish cementite particles closely

spaced hard strong steel. (Has some ductility)

3. And so on – there are many ways to distribute cementite in ferrite.