[TheForge] Re: Iron-Iron Carbide Phase Diagram Example

[Kenneth Mayer]

>Date: Mon, 2 Oct 2006 07:12:45 -0500 (CDT)
>From: dann at wctatel.net
>
>As I scanned Bruce's discovery on Wikipedia,  the diagram mostly relates
>to solutions of  pure iron and carbon, and the gradients between cast iron
>and carbon steel. That got me back to an old question.
>
>I am simply guessing, but IS cast iron a virtually pure solution of iron
>and carbon,  and is THAT the reason that the scrap is valued higher than
>vegetable soup scrap steel???

Yes.  Cast iron can be recycled into cast iron or steel.  Steel can only be recycled into steel.  Cast iron contains up to ~4% carbon, which precipitates out of solution as graphite flakes, giving gray iron its characteristic color.  If innoculated with magnesium (iirc), the carbon forms nodules instead; a/k/a nodular iron or ductile iron.  If innoculated with phosphorus, it can be cast into thin sections.

>It is easy for a chef to make a wonderful batch of new soup using all new
>ingredients, but like the old 100 year old soup, where everything from
>yesterday's soup gets mixed back into the pot with everything for today's
>soup, which means that the recipe must always be changed to taste.
>
>Once we have vegetable soup steel, it is only easily as a base mixture for
>more vegetable soup steel.
>
>Anyway, besides the primary cost factors, one of the reasons I like
>blacksmithing with salvaged  old farm machinery from pre- world war II, is
>that it seems to be mostly carbon steel, vs the vegetable soup steels of
>today. Some of the new hot-rolled mild steel seems to have variable
>characteristics, purchase to purchase.   Could just be my imagination.

No, its not your imagination.  When scrap cars are melted into structural steel, the alloys used in the gears, shafts, bearings and other high-stress components are diluted in the melt, but are still present.  This changes the microstructure and metalurgy to make the steel stronger.  Deprnding what went into the melt, some of the recycled steel has yield strength approaching 50KSI.  This is why you should use low-hydrogen electrodes when welding it, as it is much more susceptible to hydrogen embrittlement.  The A-36 specification requires 36KSI minumum yield strength.  Stronger steel meets this requirement, and can be classified as A-36 if it meets the other parts of the specification.  The recycled steel does not meet the specifications for 50ksi steel, so it can't be sold as such.

btw I recently came across an account of an unmelted ball bearing inside a W- beam.  It was discovered because a hole had to be drilled at that location.  Sorry, I didn't bookmark the URL.

Ken
:-)