Wednesday 4 April 2007


It may be kind of poor that it took me 2 and a half years in chemistry to find out what the difference between cast iron and steel is. Iron just never excited me. But if you leave aside the blast furnace [1], it can be pretty cool. And everything becomes more fun if you have a phase diagram.

This is the inorganic technology lab again, metallography exercise. We did our best to produce some nice pictures without wasting too much SiC and diamond [2]. This post is mainly to show those pictures. I am not going to cover the whole chemistry. But remember we are not van der Voort [3].

Here's (globular grey) cast iron

and here's a steel (at a higher magnification, etched with FeCl3).

The difference is mainly that cast iron has more carbon in it (more than 2.1 %). When it cools down, graphit forms (unless you cool it down really fast and you get white cast iron). In steel there is ferrite (iron with a little bit of C dissolved) and metastable cementite Fe3C. Those are probably the two phases above but it's hard to really see something.

We etched the cast iron with nital solution (nitric acid in alcohol). You can see that there are three areas: carbon spheres, carbon depraved ferrite around them and a third one which is a mixture of ferrite and and cementite called ledeburite or perlite.

Perlite is formed at the eutectoid where solid austenite decays. Ledeburite is formed when the eutectic liquid freezes. In both cases ferrite and cementite form and stay in close contact. With 500 fold magnification you can see the two phases.
This is a copper alloy (4% tin, 4% zink, 4% lead). We etched it with FeCl3.

You can see typical twin crystallites in there.The grey spots are lead that is not miscible with copper in the solid phase.

[1] I don't think it is possible for me ever to remember every part of a blast furnace.

[2] Actually "diamond" sounds better than it is. One carat of it which goes for something like 15,000 Euros if used for jewelry, costs less than a Euro if you produce it industrially.

[3] Van der Voort is the man when it comes to metallography. He has never taken a foto with a scratch.

1 comment:

Wavefunction said...

Stunning...makes one realise that there is beauty around us not just in the form of all those organic molecules, but also metals.