Flame
Hardening: 
A high intensity oxy-acetylene flame is applied to the
selective region. The temperature is raised high enough
to be in the region of Austenite transformation. The
"right" temperature is determined by the operator
based on experience by watching the color of the steel.
The overall heat transfer is limited by the torch and
thus the interior never reaches the high temperature.
The heated region is quenched to achieve the desired
hardness. Tempering can be done to eliminate brittleness.
The depth of hardening can be increased by increasing
the heating time. As much as 6.3 mm (0.25 in) of depth
can be achieved. In addition, large parts, which will
not normally fit in a furnace, can be heat-treated.
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Induction
Hardening:
In Induction hardening, the steel part is placed inside
a electrical coil which has alternating current through
it. This energizes the steel part and heats it up. Depending
on the frequency and amperage, the rate of heating as
well as the depth of heating can be controlled. Hence,
this is well suited for surface heat treatment. The
heated region is quenched to achieve the desired hardness.
Tempering can be done to eliminate brittleness.
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