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Cryogenic
Tempering Research:
Researchers
at the National Bureau of Standards, speaking about
cryogenic tempering,
stated, "When carbon precipitates form, the
internal stress in the martensite is reduced, which
minimizes the susceptibility to micro cracking.
The
wide distribution of very hard, fine carbides,
from deep cryogenic
tempering, also increases wear resistance." The
study concludes, "... fine carbides and
resultant tight lattice structures are precipitated
from cryogenic treatment.
These
particles are responsible for the exceptional
wear characteristics
imparted to materials by the process, due to
a denser structure and resulting larger surface
area of contact, reducing friction, heat and
wear."
Deep
cryogenic tempering is not a coating or a surface
treatment, but
a one-time, permanent, irreversible process that
penetrates completely through the entire material
structure.
Deep
cryogenic temperatures are required to effect
a complete molecular change
in most alloy steels, converting the retained
austenite into martensite ( a more refined grain
structure, which is more uniform than austenite
).
Cryogenic tempering transforms the microstructure
into a more uniform structure that is more durable,
stronger, longer lasting, and more dimensionally
stable.
Cryogenics: Deep cryogenic tempering can significantly
reduce retained internal stresses on most alloys.
Stress
imparted unequally can in most alloys cause a
decrease in strength
and durability.
Stress boundary areas are more susceptible to micro
cracking, which can lead to premature fatigue and
even eventual failure of the stressed part. |
Residual
stresses exist in all types of parts from engines
to tooling. The stress is introduced into the parts
at the time of forging, casting, heat treating and
final machining.
These stresses create
a complex, invisible ( to the naked eye ) random
pattern in the alloy. As parts expand from the
heat generated during operation, the retained
stresses cause uneven expansion, increased dimensional
instability, and increased wear as well as decreased
performance.
Stress relief takes
place when the entire mass of the part is at
an equal temperature ( surface and core ), and
then slowly cycled ( less than one degree per
minute ) through a wide temperature range. By
cycling parts to ultra low temperatures for a
prolonged period a very dense molecular state
is created. Absolute zero ( -459.67 Fahrenheit
) is known as the zero motion molecular state
of mass. It is this slow rate of temperature
change that allows what is known as thermal compression
and thermal expansion to occur, which is what
actually effects the release of stress. The result
is a dimensionally stabilized part, which will
resist distortion and warpage increasing
performance and durability.
Precipitation of eta Carbide: In a study performed at
the Jassy Institute in Romania, researchers used a scanning electron
microscope with a microscopic particle counter to evaluate additional
changes in the structure of cryogenically treated steel.
The study concluded
that the number of countable small carbides increased
throughout heat treatable steel, from 33,000
particles per square millimeter to over 80,000
particles per square millimeter as a result of
the cryogenic treatment. The increase in the
carbides adds greatly to the wear resistance
of the part. The carbides make a refined flat "super
hard" surface on the steel which is smoother
and decreases friction and heat as well. |
