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a very low hardness on the workpiece surface. Most
Flame hardening is a rapid, economical method for
parts can be reheated after cooling to correct this
selectively hardening specific areas on the surface of a
part. The process is applied only to flame-hardenable
materials, principally carbon and alloy steels, certain
Too much heat can cause low surface hardness by
stainless steels, and cast irons. Flame hardening is
burning carbon out of the surface (decarburization).
used to harden parts to depths of 1/64 -1/4 in.
The workpiece will have a high carbon steel core and a
soft, low-carbon iron skin. Decarburized workplaces
How Flame Hardening Works
usually cannot be reheated without cracking.
Flame hardening is any process that uses an oxy-fuel
gas flame to heat carbon or alloy steel, tool steel, cast
iron, or hardenable stainless steels above a certain
Cooling speed during quenching depends on the type
"critical" temperature, followed by a rapid quenching.
and temperature of quenching medium used, and how
This causes the heated material to harden to some
fast it is agitated. Self-quenching, when a part has a
depth below its surface.
sufficiently large, cool mass to draw heat away from
the surface causing it to quench itself, is the slowest
Carbon Content and Hardness
method and produces the lowest surface hardness.
Flame hardening seldom requires tempering because
Forced air is a mild quenchant that rapidly cools a
the hardening effect is shallow. Tempering also tends
workpiece with minimal risk of surface cracking,
to reduce maximum hardness to some degree.
especially in higher-carbon steels. Oil and soluble-oil
However, insufficient heating (not up to the critical
mixtures give relatively high hardness without too
temperature, or not long enough above it), will produce


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