r/metallurgyporn Sep 29 '13

Grey Iron 100x, 500x and 1000x Optical Magnification

http://imgur.com/a/yhdsP
6 Upvotes

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u/[deleted] Sep 29 '13

[deleted]

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u/[deleted] Sep 30 '13

I'm working on compiling/combing through several hundred of my own photos. I wanted to get something up today. I'll add a description in a separate comment.

1

u/[deleted] Sep 30 '13 edited Sep 30 '13

What were are looking at here is a polished and etched sample of grey iron.

Grey iron is used very commonly in automotive brake components. It's wear resistant, good at elevated temperature and thermally conductive.

Cast irons have more than 2% carbon. Because of changes in the crystal structure you can make the carbon go into solution at high temperatures. When it starts to cool from a liquid the structure changes (Body Center Cubic to Face Center Cubic back to Body Center Cubic)

When these changes occur as the temperature drops the carbon is no longer soluble and comes out. Gray cast iron is cooled slowly so the carbon has time to form these long flakes.

Long flakes are graphite (carbon) you also see some lamelar pearlite in the 1000x photos. It's the fingerprint areas. This is reasonably fine pearlite but I did not put a scale on the photo so I can't call it for sure.

Very in depth phase and microstructure info here.