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Cryogenics
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Cryogenic treatment
A cryogenic treatment is the process of treating workpieces to cryogenic temperatures (i.e. below
−190 °C (−310 °F)) to remove residual stresses and to improve its wear resistance characteristics.
Especially on the specimens or tools made up of steels. Although its utilites are extended to other
metals, non-metals and polymers.
The process has a wide range of applications from industrial tooling to improvement of musical signal
transmission. Some of the crucial benefits of cryogenic treatment include longer part life, less failure
due to cracking, improved thermal properties, better electrical properties, including less electrical
resistance, better magnetic characteristics, reduced coefficient of friction, less creep and walk,
improved flatness, and easier machining.
It was also observed that post-cryogenic treatment, fine machining (such as grinding) is easier. Hence
the cost of finishing operation comes down due to cryogenic treatment.
HISTORY of Cryogenics
Cryogenics or deep freezing have been around for quite some time. There is documented research from
as far back as the 1930’s where German companies used it on components of jumbo aircraft engines.
Cryogenic processing had its US origins in the 1940s.
Cryogenics is a relatively new process and to eliminate retained austenite, the temperature has to be
lowered, but one that using correct procedures can bring substantial economic benefits. This Cold
treatments or sub zero treatments are done to make sure there is no retained austenite during quenching
Historical breakthroughs in field of Cryogenic sciences
Year Breakthrough
1877 Cailletet and Pictet liquefied oxygen. This was really the beginning of “cryogenics” as an area
separate from “refrigeration.”
1884 Wroblewski (Kracow University, Poland) first liquefied hydrogen as a mist.
1892 Sir James Dewar (England) developed the vacuum-insulated vessel for storage of cryogenic
fluids
1895 Heike Kamerlingh Onnes (Holland) established the Leiden Cryogenic Lab, and Karl von Linde
(Germany) obtained a basic patent for air liquefaction
1898 James Dewar produced liquid hydrogen in bulk at the Royal Institute of London
1902 Georges Claude developed the first air-liquefaction system using an expansion engine
1908 H.K. Onnes first liquefied helium–the last of the so-called “permanent gases” to be liquefied
1911 H.K. Onnes discovered superconductivity
1916 First commercial American-made air liquefaction plant completed
Year Breakthrough
1877 Cailletet and Pictet liquefied oxygen. This was really the beginning of “cryogenics” as an area
separate from “refrigeration.”
1884 Wroblewski (Kracow University, Poland) first liquefied hydrogen as a mist.
1892 Sir James Dewar (England) developed the vacuum-insulated vessel for storage of cryogenic
fluids
1895 Heike Kamerlingh Onnes (Holland) established the Leiden Cryogenic Lab, and Karl von Linde
(Germany) obtained a basic patent for air liquefaction
1898 James Dewar produced liquid hydrogen in bulk at the Royal Institute of London
1902 Georges Claude developed the first air-liquefaction system using an expansion engine
1908 H.K. Onnes first liquefied helium–the last of the so-called “permanent gases” to be liquefied
1911 H.K. Onnes discovered superconductivity
1916 First commercial American-made air liquefaction plant completed
1966 He3/He4 dilution refrigerator developed
1969 3250-hp dc superconducting motor constructed for ship drive application
1986 Georg Bednorz and Alex Muller discover high-transition-temperature ceramic superconductor
with a Tc of about 30K
1987 Paul Chu (Univ. of Houston) and Maw-Kuen Wu (Univ. of Alabama at Huntsville) develop the
1-2-3 yttrium based high-Tc superconductor with a Tc of about 90K
Cryogenic processing
The field of cryogenics advanced during World War II when scientists found that metals frozen to low
temperatures showed more resistance to wear. Based on this theory of cryogenic hardening, the commercial cryogenic processing industry was founded in 1966 by Ed Busch. With a background in the
heat treating industry, Busch founded a company in Detroit called CryoTech in 1966 which merged
with 300 Below in 1999 to become the world's largest and oldest commercial cryogenic processing
company. Busch originally experimented with the possibility of increasing the life of metal tools to
anywhere between 200%-400% of the original life expectancy using cryogenic tempering instead of
heat treating. This evolved in the late 1990s into the treatment of other parts (that did more than just
increase the life of a product) such as amplifier valves (improved sound quality), baseball bats (greater
sweet spot), golf clubs (greater sweet spot), racing engines (greater performance under stress), firearms
(less warping after continuous shooting), knives, razor blades, brake rotors and even pantyhose. The
theory was based on how heat-treating metal works (the temperatures are lowered to room temperature
from a high degree causing certain strength increases in the molecular structure to occur) and supposed
that continuing the descent would allow for further strength increases. Using liquid nitrogen, CryoTech
formulated the first early version of the cryogenic processor.
Unfortunately for the newly born industry, the results were unstable, as components sometimes
experienced thermal shock when they were cooled too quickly. Some components in early tests even
shattered because of the ultra-low temperatures. In the late twentieth century (1992), the field improved
significantly with the rise of applied research, which coupled microprocessor based industrial controls
to the cryogenic processor in order to create more stable results. The first cryogenic processor to use
computer-aided thermal cycling was invented by an aeronautical engineer named Pete Paulin, who
concurrently founded 300 Below in Decatur, IL.
GENERAL OVERVIEW OF CRYOGENIC TREATMENT:
In cryogenic treatment, the material is to be deep freeze temperatures of as low as -185 °C. The
austenite is unstable at this temperature and the whole structure become martensite. This is the region
to use cryogenic treatment. Processing is not a substitute for heat treating, if the product is properly
treated or if the product is over heated during remanufacturing or if it is over stressed during use.
Cryogenic processing will not in itself harden the metal like quenching and tempering, it is an
additional treatment to heat treating. In the present study, the cryogenic technique is applied by
exposing the steel to deep freezing environment (-186 °C) for 24 hours and slowly raised to room
temperature. The results are correlated with standard results.
Cryogenic treatment is a one-time, irreversible treatment that permanently changes the entire
molecular structure of the material being treated, not just the surface, creating a denser molecular
structure, resulting in a smoother contact surface area that reduces friction, heat, and wear. The
engineerig benefits of this process includes: Reduction of abrasive and adhesive wear, improved
machining properties resulting from the permanent change of the structure of the metal, reduction of
the frequency and cost of tool remanufacturing and reduction of likelihood of catastrophic tool failure
due to stress fracture.
Cryogenic processing makes changes to the structure of the materials being treated and dependent on
the composition of the material, It performs three crucial things: 1. Retained austenite turned to
martensite. 2. Carbide structures are refined. 3. Stress is relieved.
The dramatic improvement in wear resistance in deep cryogenically treated tools steels, with no loss in
toughness is most likely explained by the formation of molecular eta carbides and the formation of fine
cementite particles in the final tempered structure. It would appear that the conversion of additional
martensite, although often present, is probably a secondary mechanism. This understanding also
supports the increase wear resistance in materials that don’t readily form martensite.
CRYOGENIC TREATMENT CYCLE:
1.RAMP-DOWN
A typical cryogenic cycle will bring the temperature of the part down to -185 °C (-300 °F) over a
period of six to ten hours. This avoids thermally shocking the part. There is ample reason for the slow
ramp down. Think in terms of dropping a cannon ball into a vat of liquid nitrogen. The outside of the
cannon ball wants to become the same temperature as the liquid nitrogen, which is near -198 °C (-323
°F) . The inside wants to remain at room temperature. This sets up a temperature gradient that is very
steep in the first moments of the parts exposure to the liquid nitrogen. The area that is cold wants to
contract to the size it would be if it were as cold as the liquid nitrogen. The inside wants to stay the
same size it was when it was room temperature. This can set up enormous stresses in the surface of the
part, which can lead to cracking at the surface. Some metals can take the sudden temperature change,
but most tooling steels and steels used for critical parts cannot.
2. SOAK
A typical soak segment will hold the temperature at -185 °C for some period of time, typically eight to
forty hours. During the soak segment of the process the temperature is maintained at the low
temperature. Although things are changing within the crystal structure of the metal at this temperature,
these changes are relatively slow and need time to occur. One of the changes is the precipitation of fine
carbides.
It is believed that, this time in the process also provides time for the crystal structure to react to the low
temperature and for energy to leave the crystal structure. In theory a perfect crystal lattice structure is in
a lowest energy state. If atoms are too near other atoms or too far from other atoms, or if there are
vacancies in the structure or dislocations, the total energy in the structure is higher. By keeping the part
at a low temperature for a long period of time, we believe we are getting some of the energy out of the
lattice and making a more perfect and therefore stronger crystal structure,
3. RAMP UP
A typical ramp up segment brings the temperature back up to room temperature. This can typically take
eight to twenty hours. The ramp up cycle is very important to the process. Ramping up too fast can
cause problems with the part being treated. Think in terms of dropping an ice cube into a glass of warm
water. The ice cube will crack. The same can happen to your parts.
4. TEMPER RAMP UP
A typical temper segment ramps the temperature up to a predetermined level over a period of time.
Tempering is important with ferrous metals. The cryogenic temperature will convert almost all retained
austenite in a part to martensite. This martensite will be primary martensite, which will be brittle. It
must be tempered back to reduce this brittleness. This is done by using the same type of tempering
process as is used in a quench and temper cycle in heat treat. We ramp up in temperature to assure the
temperature gradients within the part are kept low. Typically, tempering temperatures are from -185 °C
on up to 594 °C (1100 °F), depending on the metal and the hardness of the meta
5. TEMPER HOLD
The temper hold segment assures the entire part has had the benefit of the tempering temperatures.
A typical temper hold time is about 3 hours. This time depends on the thickness and mass of the part.
There may be more than one temper sequence for a given part or metal. We have found that certain
metals perform better if tempered several times.
CRYOGENIC TEMPERATURES AND THE WAYS TO GENERATE SUCH
TEMPERATURES
The temperatures well below room temperature, i.e. 0 to –269 °C, are called cryogenic temperatures.
Normally these temperatures can be generated using solid carbon dioxide or mechanical refrigeration
or liquefied gas system.
A cryogen is any fluid that operates at cryogenic temperatures (below roughly -125 °C)
• The solid carbon dioxide method is the oldest method and is capable of cooling components
down to -80 °C.
• The mechanical refrigeration method may be capable of cooling to about -100 °C using freon as
a convection fluid.
• The last and very important method in cryogenic technology is the liquefied gas system which
is capable of cooling to around -250 °C. The gases that are used for generating the cryogenic
temperatures are oxygen, nitrogen, neon, hydrogen and helium.
ADVANTAGES
Cryo-treating at the plant level can be used to address problems. Cryogenically treated tools will last
longer – saves money in the long run. Cryo-treatment has three major benefits:
• increases dimensional stability
• removes stress
• improves wear and abrasive resistance
Controlling machine wear is a fundamental function of lubrication. By properly specifying lubricant
viscosity, using appropriate modifying additives, and controlling contamination, wear rates can be
minimized to extend equipment life. However, other influencing factors can still result in less than
optimal component wear performance, including equipment loads, design issues, and environmental
factors. Cryogenic treatment of tooling steels is a proven technology to increase wear resistance and
extend intervals between component replacements for blades, bits, and machining mills.
Recent work has also shed light on the effects of cryogenic treatment on bearings, gears and engine
components to reduce wear and improve performance. Combining optimized lubrication, correct
mechanical configuration, and cryogenic treatment of wearing parts results in the maximum
performance of lubricated components, and can significantly extend component's life.
Reliability of operating systems is influenced by 5 factors: component design, manufacture,
specification, installation, and maintenance. Each of these stages can be influenced by separate
individuals or teams, but ultimately the responsibility for performance of the assembled system falls to
the plant maintenance team.
It has been said that machines don’t die, people kill them. In many cases this is true, and many of
the contributing factors to premature failure can be controlled by the end user. However, if the
equipment has been properly installed and maintained, exerting influence on the other factors may be
difficult or impossible for the end-user. They may be “stuck”, as it were, with a poorly specified,
designed or manufactured machine. The poor design may also be a function of the available technology
for the level of funding. In these cases, short time between failures may become accepted as the norm,
in some cases at great cost.
CONCLUSION
This brief report has presented the basic ideas and principles of the most important aspects of
cryogenics, i.e. cryogenic fluids, the effects of cryogenic processing and treatments on tools and other
engineering components, also the engineering gains of this treatment. It has also provided the reader
with typical idea of the relevant parameters involved in this stream, enabling him to perform some
estimates and apply his engineering judgment. There is of course much more to say on each of these
topics, some of which have significantly developed over the years and still constitute areas of technical
progress.
Many other subjects not addressed here also pertain to cryogenic engineering, such as Heat transfer,
thermal design and refrigeration, Cooling by the application of Magneto-caloric Effect (R13). Behavior
of Non-metals and polymer materials at low temperature (R8,R9), storage, handling and transfer of
fluids(R7), two-phase flow and discharge, vacuum and leak-tightness technology, instrumentation (in
particular Thermometry (R11)), process control, impurity control and safety. In all cases, the interested
reader is referred to the selected bibliography for detailed information and to the proceedings of the
cryogenic engineering conferences for recent developments.