GENERAL
APPLICATION DEFINITIONS
A glossary of common terms used in heat treating
Air Hardening Steel - A steel containing sufficient
carbon to harden fully during cooling in air from a temperature
above its transformation range. Annealing
- Heat treat process consisting of heating to and holding at
a temperature above the transformation temperature followed
by slow cooling in the furnace at a suitable rate primarily
for the purpose to soften metallic materials.
Brazing - Heat treat process that joins solid materials
together by heating them to a suitable temperature using a filler
metal with a liquidus temperature below the solidus temperature
of the base materials.
Carburizing - Diffusion of carbon into solid ferrous
alloys by heating to desired temperature in contact with a suitable
carbonaceous material. A form of case hardening that produces
a carbon gradient extending inward from the surface, enabling
the surface layer to be hardened either by quenching directly
from the carburizing temperature or by cooling to room temperature.
Decarburization - Loss of carbon from the surface
layer of the alloy due to reaction with one or more chemical
substances which contact the surface.
Hardening - Heat treat process which increases hardness
by heating and cooling. The formation of martensite in alloy
when quenched rapidly from a temperature above the transformation
temperature.
Induction Heating - Heating by combining electrical
resistance and hysteresis losses induced by subjecting a metal
to the varying magnetic field surrounding a coil carrying alternating
current.
Normalizing - Heat treat process consisting of heating
to a temperature above the transformation range followed by
rapid cooling in air. Normalizing refines grain size and makes
structure more uniform for better machinability.
Preheating - Heating to an intermediate temperature
before either further thermal or mechanical treatment or before
final austenitizing.
Protective Atmospheres - Gases used to displace the
oxygen in the heating chamber to prevent or minimize scale and
oxidation on the surface of the work. Atmosphere can also be
used to add carbon or remove carbon from the surface of steels.
Protective atmospheres are generally classified as being inert
(Nitrogen, Argon) or reducing (Hydrogen, Dissociated Ammonia.)
Reducing atmospheres are flammable and explosive and require
additional safety equipment for operation with furnaces.
Quenching - Hardening alloys by heating above the
transformation temperature and then cooling at a rate to transform
the austenite to martensite. Typical quench mediums include
air, oil, water, salt and polymers.
Sintering - Heat treat process which bonds powdered
metals under high temperatures but below the melting temperature
of the material.
Stress Relieve - Heat treat process consisting of
heating steel below the transformation temperature to relieve
internal stresses followed by slow cooling.
Tempering - Heat treat process consisting of reheating
quenched steel at a temperature below the transformation range
to decrease hardness and increase toughness.
Transformation Temperature - The temperature
at which a change in the molecular structure of the steel occurs.
The transformation temperature varies depending on the carbon
and alloy content of the steel.
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