He is then able to pick
out its component minerals, ferrite, austenite, martensite, pearlite,
graphite, cementite, and to show how their abundance, shape and
arrangement contribute to the strength or weakness of the specimen. The
last of these constituents, cementite, is a definite chemical compound,
an iron carbide, Fe_{3}C, containing 6.6 per cent. of carbon, so hard as
to scratch glass, very brittle, and imparting these properties to
hardened steel and cast iron.
With this knowledge at his disposal the iron-maker can work with his
eyes open and so regulate his melt as to cause these various
constituents to crystallize out as he wants them to. Besides, he is no
longer confined to the alloys of iron and carbon. He has ransacked the
chemical dictionary to find new elements to add to his alloys, and some
of these rarities have proved to possess great practical value.
Vanadium, for instance, used to be put into a fine print paragraph in
the back of the chemistry book, where the class did not get to it until
the term closed.
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