Cryogenic Heat Treatment

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15 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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Cryogenic Heat Treatment

Definition


Cryogenic temperatures are defined by the Cryogenic Society of America as being
temperatures below 120 0 K (
-
244 0 F,
-
153 0 C).



Durability is the most important criterion used to define the quality of a tool steel. Cryogenic
treatment and tempering of metals has been ac
-

knowledge for almost thirty years as an
effective method for increasing durability, or "wear life" and decreasin
g residual stress in tool
steels. Deep cryogenics (below
-
300°F) is creating many new applications in science. High
temperature superconductors, the super
-
conducting super collider, cryo
-
biology, magneto
-
hydrodynamic drive systems for ships, and low temper
ature physics have all developed
recently. The deep cryogenic treatment and tempering process for metals is economical. It is
a one time permanent treatment, affecting the entire part, not just the surface. The treatment
may be applied to new or used tools
, sharp or dull, and reshaping will not destroy the
imparted properties. Benefits achieved from subjecting tools to this treatment include:
increases in tensile strength, toughness, and stability through the release of internal stresses.

Cryogenic Treatme
nt for Improved Properties

A research metallurgist at the National Bureau of Standards in Boulder Colorado, states,
"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 treatment also increases wear resistance." The study concludes:
"...fine carbon carbides and resultant tight lattice structures are precipitated from cryogenic
treatment. These particles ar
e responsible for the exceptional wear characteristics imparted by
the process, due to a denser molecular structure and resulting larger surface area of contact,
reducing friction, heat and wear." There have been skeptics of the cryogenic process for some
time, because it imparts no apparent visible changes to the metal. Since proper heat treating
can transform 85% of the retained austenite to martensite and the deep cryogenic process
only transforms an additional 8 to 15%, the deep cryogenic treatment has
been considered an
inefficient process.

While these percentages are correct, the conclusion drawn from them is inaccurate. In
addition to the trans
-

formation to martensite, the subjected metals also develop a more
uniform, refined microstructure with gre
ater density. Although known to exist, this type of
microstructure was only recently quantified scientifically. Particles known as "binders" are
coupled with the precipitation of the additional micro fine carbide "fillers". The fillers take up
the remainin
g space in the micro
-
voids, resulting in a much denser, coherent structure of the
tool steel. These particles are identified and counted in the above study cited, using a
scanning electron microscope with field particle quanti
-

fiction (an automatic partic
le
counter). It is now believed that these particles are largely responsible for the great gains in
wear resistivity. The permanent irreversible molecular change created is uniform throughout
the tool, unlike coatings, and will last the life of the tool, r
egardless of any subsequent
finishing operations or regrinds.

Deep Cryogenic Treatment Potential

The cryogenic cycle is an extension of standard heat
-
treatment, and creates many outstanding
increases in durability. Some examples are as follows. A major a
ircraft manufacturer testing
deep cryogenic treatment found that with only six different tools treated, the savings in tool
purchases could exceed $5 million. An Arizona State study conducted by Laurel Hunt, used
deep treated C
-
2 debarring tools on INCONEL

alloy 718, achieving a 400% improvement
based on weight, after five cats of .003 in. (.007 cm) on this alloy. This deep cryogenic
treatment of an 8% cobalt end mill has made dramatic improvements in two important ways.

The number of milling cats was incr
eased from three before deep cryogenic processing, to 78
cats after processing (26 times the wear life). Resharpening the end mills after deep cryogenic
treatment required only 1/3 the amount of stock removal to restore the tool geometry.
Rockwell, a major

aircraft manufacturer, using C
-
2 carbide inserts to mill epoxy graphite,
doubles their output after deep cryogenic treatment of the inserts. In a second test, a 400%
improvement was achieved upon milling 4340 stainless steel with cryogenic treated tool.
O
ther applications include: Leading national stock car drivers who previously raced only 4
-
8
races between equipment teardowns, drove in 40+ races before teardown after cryogenically
treating block, crank, cam, pistons and heads.