Welding
Non-Ferrous
Metals
Treating
Welding
Cast Iron
Welding
Ferrous
Metals
3
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Flame Hardening
To industry, the most significant
heating application is flame-hardening. The properties
of steel – especially the properties
of medium- and high-carbon steels – are greatly affected by the rate at which
it is cooled from a point above
its critical temperature. All flame-hardening applications involve rapid heating,
followed by quenching, in order
to harden the surface of steel without making the entire mass of metal unnecessarily
hard and excessively brittle.
Let’s assume that we have a medium
sized gear cast or machined from steel containing about 0.40% carbon. If we
place this gear in a forge fire, heat
it until it was bright red, then bury it completely in the fire and allowed it
to cool slowly
as the fire goes out and the ashes cool, we’ll find the steel, after it has
cooled fully, to have about the same hardness
it had before we heated it. However, if we take an identical gear, heat it bright
red, and then plunge it in a tub
of water, we’ll get a different result. The steel will be much harder; it
may have been distorted by the sudden quench;
it may even have small cracks as the result of the quench.
The simple heat-and-quench method of
hardening steel has been known and applied for centuries. The blacksmith
took advantage of this method when
he heated a part bright red in his forge fire and then plunged it in a tub of
water, or sprinkled water over it.
Large quantities of small parts are often heated in a furnace in one batch, and
then quenched in water or oil. Larger
parts are sometimes individually heated and then quenched with an air blast.
In the heat-and-quench methods just
described, there may be some variation in hardness between the surfaces of
the finished parts and the interior
sections. Generally speaking, however, the entire part has been hardened. For
many types of parts, that is not detrimental.
But for some parts that need maximum surface hardness, to make
them resistant to wear, but also maximum
toughness, to resist impact or suddenly-applied load, complete
hardening is undesirable. That’s
where flame-hardening comes in. You use oxy-acetylene flames to heat the part
so rapidly that the surface reaches
a temperature above the critical before the bulk of the part does; then you
withdraw the flames and apply a water
quench to the surface. The surface layer is cooled rapidly, both by the water
