If a piece of steel be subjected to a bright
yellow or white heat its nature is entirely changed, and the workman
says it is burnt. Though this is not actually a fact, it does well
enough to express that condition of the metal. Steel cannot be burnt
unless some portion of it has been oxidized. The carbon would of course
be attacked first, its affinity for oxygen being greatest; but we find
nothing wanting in a piece of burnt steel. It can, by careful heating,
hammering and hardening, be returned to its former excellence. Then what
change has taken place? I should say that two modifications have been
made, one physical, the other chemical. The change chemically is that
of a chemical compound to a mixture of carbon and iron, so that in a
chemical sense it resembles cast iron. The change physically is that of
crystallization, being due partly to chemical change and partly to the
effect of heat. I have procured a specimen of steel showing beautifully
the effect of overheating. The specimen is labeled No. 1, and is a piece
of Park Brothers' steel (one of the best brands made in America). It has
been heated at one end to proper heat for hardening, and at the other is
what is technically called "burnt." It has been broken at intervals
of about 11/2 inches, showing the transition from amorphous or proper
hardening to highly crystalline or "burnt." Malleable or wrought iron
is or should be pure iron.
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