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QUICK FACT GUIDES FOR MACHINISTS
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GUIDES |
FLAME HARDENING This method of hardening is
especially applicable to the selective hardening of large steel forgings or
castings which must be finish-machined prior to heat-treatment, or which because
of size or shape cannot be heat-treated by using a furnace or bath. An
oxy-acetylene torch is used to heat quickly the surface to be hardened; this
surface is then quenched to secure a hardened layer which may vary in depth from
a mere skin to 1/4 inch and with hardness ranging from 400 to 700 Brinell. A
multi-flame torch-head may be equipped with quenching holes or a spray nozzle
back of the flame. This is not a carburizing or a case-hardening process as the
torch is only a heating medium. Most authorities recommend tempering or drawing
of the hardened surface at temperatures between 200 and 350 degrees F. This
treatment may be done in a standard furnace, an oil bath, or with a gas flame.
It should follow the hardening process as close as possible. Medium-carbon and
many low-alloy steels are suitable for hardening. Plain carbon steels ranging
from 0.35 to 0.60 per cent carbon will give hardness of from 400 to 700 Brinell.
Steels in the 0.40 to 0.45 per cent carbon ranges are preferred, a they have
excellent core properties and produce hardness of from 400 to 500 Brinell
without checking or cracking. Higher carbon steels will give greater hardness,
but extreme care must be taken to prevent cracking. This requires careful
control of the quenching operation. |
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Copyright © 2002 QUICK FACT GUIDES FOR MACHINISTS
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