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Soaking changes the steel grain structure of the steel into Austenite,
which allows the carbon to dissolve into the solid iron crystals.
After about 15 minutes the blanks are rapidly air-cooled in the
tunnel to 150 degrees to harden the steel (air quenching). The
cooling rate largely determines the structure of the steel. If
the steel is not cooled quickly enough, the carbon and iron will
begin to separate, forming an undesirable stucture (pearlite or
bainite instead of Martinsite) which will not be hard enough.
The hardening process takes the steel blade blanks from the HRC
20 to HRC 60. However, the heating, crystalization, and molecular
changes cause the steel to shrink and to develop internal stresses.
Within 45 minutes of reaching 150°F, the cooled blades must be
put into special ovens for tempering, discussed on the next page.
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