Why is this iron-age tool considered steel and not cast iron?


“The 6cm-long punch has a carbon content (2%) rarely seen in iron-based objects from the region at the time.”

““We think it is the earliest ultra-high carbon steel from Europe,” Godfrey, of the University of Bradford, UK, told BBC News Online.”

I thought that if the carbon content was above 1.7% it was cast-iron, not steel. Is there some other factor besides carbon contect that differentiates between steel and cast-iron?

It is probably clear that I know very little about metallurgy.

It is possible to get cast irons with concentrations of C at 1.7%C, but 2% is a more usual cut-off. The difference is whether one gets carbon (graphite) rich phase in the material - if so then it is cast iron. This can vary according to the other elements present (e.g. silica)

I suspect in the present case it may be a lazy reporter?

Here is a good place to start exploring cast iron http://www.key-to-steel.com/Articles/Art63.htm

Ah, so it’s the form of the carbon that can make the difference? If it’s not “carbon (graphite) rich phase” then it becomes steel?

I’ll check out that link, thanks!

I’m not a metallurgist but I think that’s the key. In cast iron the carbon is in the form of elemental carbon. In steel the carbon is in something like iron carbide.

The carbon aspect of iron vs. steel is a bit confusing and somewhat counter intuitive. Cast iron has more carbon that steel. Steel is made by removing excess carbon from iron not vice versa. What we call cast iron has more carbon than steel, greater than 2%. Steel came along when various furnace processes were able to burn away the excess carbon content to make an alloy that has a finer grain structer and is actually stronger. High carbon steels up near 2% can be heat treated to be extremely hard with lower carbon steels, mild steels, being more suitable for welding, forging and stamping. When all the carbon is taken out the metal is called wrought iron.

Thanks folks.