5cryogenic-processes..

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15 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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CRYOGENIC PROCESSES AND ITS APPLICATIONS



ABSTRACT

This paper deals with the affects of cryogenic processes on metals and its applications in
industries. As it is known the most
important problems faced by the industries are the wear
and tear of the machine parts. This wear of the machine parts not only increases the cost of
production but also the time wasted for the replacement process. In this paper I would like to
explore the

enhancement in strength and durability that would be gained by cryogenically
treating those machine parts.

Cryogenics is the ultra low temperature processing of materials to enhance their desired
metallurgical and structural properties. Cryogenic treatmen
t process is the treating of a wide
variety of materials, such as ferrous and non
-
ferrous metals, metallic alloys, carbides,
plastics, and ceramics. These ultra
-
cold temperatures, below
-
310°F, will greatly increase the
strength and wear life of all types
of vehicle components, castings and cutting tools. In
addition, other benefits include reduced maintenance, repairs and replacement of tools and
components, reduced vibrations, rapid and more uniform heat dissipation, and improved
conductivity.

Here in th
is context I shall purpose to explain about the purpose of cryogenic treatment and
what happens in the metal structure along with its advantag
es and some of its applications





INTRODUCTION


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Cryogenics have been derived from the Greek word “KRUOS” (frost)

and “GENICS”
meaning to produce very low temperatures.

At the end of eighteenth century liquefaction of
some gasses was achieved and that opened the door to deep cryogenic temperatures. Some
experiments with steel started at the beginning of 20th century.

The results were quite
discouraging because, in most of cases, the material shattered or broke due to the thermal
shock when the steel was directly put into liquefied gas.

After the Second World War these tests were abandoned until the seventies when aero
space
industry took up this technology again and the cryogenic treatment started to be developed as
a new industrial process.

Today cryogenic treatment would be regarded as one of the most important processes in the
field of industries, and it is the ultra

modern type of processing to make the metals more
resistant to wear and more durable. The process can be used to improve the properties of a
wide range of materials
. Steel (cold working, hot working, HSS, inox…), aluminium, copper,
carbide, ceramics and e
ven some polymers can be improved with the treatment. The results
which are obtained depend basically on the treated material and on the application. The most
remarkable ones are
wear resistance

increase and fatigue life improvement. The use of this
treatm
ent is extremely
environmentally friendly
, as absolutely no waste is produced during
the process.

Some companies have taken steps to move the industry forward. One step in that direction is
involvement in the Cryogenic Society of America, Inc. (CSA) (Oak P
ark, IL). Compared to
other cryogenic technologies, however, cryogenic processing is considered low tech. Another
step intended to move the industry forward was the recent formation of an ASM International
(formerly American Society for Metals) (Materials
Park, OH) committee on cryogenics to
address the need for more information, standardization, and traini
n
g.



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THEORY OF CRYOGENIC PROCESS

The theory was based on how heat
-
treating metal works and supposed that continuing the
descent would allow for further

strength increases. Using liquid nitrogen, CryoTech
formulated the first early version of the
cryogenicprocessor
. Unfortunately for the newly
-
born
industry, the resu
lts were unstable, as components sometimes experienced
thermalshock

when
they were cooled too fast. Some components in early tests even shattered because of the ultra
-
low tempera
tures. In the late twentieth century, the field improved significantly with the rise
of applied research, which coupled microprocessor based industrial controls to the
cryogenicprocessor

in order to create more stable results.

The property of the cryogenic process is generally gained due to the conversion of austenite
to martensite
. Proper heat treating can transform 85% of the retained austenite to martensite
and the cryogenic

treatment only transforms an addition of 8 to 15%. But a more uniform,
refined microstructure with greater density is formed as a result of cryogenic process
ing.

There are usually two types of cryogenic processes namely deep cryogenic treatment and
shallow cryogenic treatment. Deep cryogenic treatment usually takes place around
-
320
0
F (
-
196
0
C), near the temperature of liquid nitrogen. Shallow cryogenic treatme
nt takes place
around
-
120
0
F near the temperature of dry ice.

Although these two processes are present but deep cryogenic treatment is the one most
preferred and effective. Deep cryogenics is the ultra low temperature processing of materials
to enhance the
ir desired metallurgical and structural properties. In this case, this is a
temperature about
-
320°F,
-
196°C, or 77°K. These ultra cold temperatures are achieved using
computer controls, a well
-
insulated treatment chamber and liquid nitrogen (LN
2
). The liq
uid
form is the product of air separation, compression and liquefaction. Controlled deep
cryogenic treatment system and process is capable of treating a wide variety of materials,
such as ferrous and non
-
ferrous metals, metallic alloys, carbides, plastics
(including nylon
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and Teflon) and ceramics. The entire process takes between 36 to 74 hours, depending on the
weight and type of material being treated. Strict computer control and proper processing
profiles assure that optimum results will be achieved with

no dimensional changes or chance
of thermal shock. This special process is not a surface treatment; it affects the entire mass of
the tool or component being treated, making it stronger throughout. This means the process
keeps working even after continued

use and/or numerous sharpening. The hardness of the
material treated is unaffected, while its strength is increased.



Figure
-
1 Cryogenic
-
Processing Equipment

The part to be processed is placed in a processor. It is a computer contro
lled process the
system is controlled with proven cooling curves programmed to the computer. Any other
desired cooling curves may be easily programmed into the processor. Computer controlled
processing ensures accurate tempering cycles

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Liquid nitrogen is
used as the cryogen in this process. Liquid nitrogen is converted to a gas
before it enters the chamber, so that at no time liquid nitrogen comes in contact with parts,
assuring that the dangers of cracking from too rapid cooling are eliminated. They are
gradually cooled with nitrogen gas to
-
320 degrees Fahrenheit. That temperature is
maintained for at least eight hours. The length of time varies by material and desired results.
After the cooling cycle is complete, the item is slowly warmed back to room t
emperature.
Then the object is heat
-
treated, with temperatures of 100 to 400 degrees Fahrenheit,
depending on the composition of the item. Finally, the item is gradually returned to room
temperature. The complete process takes a minimum of 24 hours to a ma
ximum of 7 days.

Proper heat treating can transform 85% of the retained austenite to martensite and the
cryogenic

treatment only transforms an addition of 8 to 15%. But a more uniform, refined
microstructure with greater density is formed as a result of cr
yogenic processing.
Additionally carbide precipitation and thermal mechanical stabilization (reduction of residual
stresses in material) occurs during a cryogenic soak. As the material is cooled it passes a
martensitic start temperature (Ms) at which the f
ormation of martensite commences. As the
temperature is lowered further it will eventually reach a martensitic finish temperature (Mf)
at which all the retained autensite has been converted. Cryogenic processing aims to convert
the entire structure to mart
ensite, and it is not a heat treatment process that only affects the
surface of the material
.



EFFECTS ON CRYOGENICALLY PROCESSED MATERIALS



Increases abrasive wear resistance.



Requires only one permanent treatment.



Creates a denser molecular structure. The result is a larger contact surface area
that reduces friction, heat and wear.

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Changes the equipment's entire structure, not just the surface. Subsequent
refinishing operations or re
-
grinds do not affect permanent
improvements.



Eliminates thermal shock through a dry, computer controlled process.



Transforms almost all soft retained austenite to hard martensite.



Forms micro fine carbide fillers to enhance large carbide structures.



Increases durability or wear life.




Decreases residual stresses in tool steels.



Decreases brittleness.



Increases tensile strength, toughness and stability coupled with the release of
internal stresses.




EFECTS ON VARIOUS MATERIALS

AISI#

Description

At
-
110cF

(
-
79cC)

At
-
310c F

(
-
190cC)

430

Ferritic stainless

116%

119%

303

Austenitic stainless

105%

110%

8620

Nickel
-
chromium
-
moly ally steel

112%

104%

C
-
1020

Carbon steel

97%

98%

AQS

Graphitic cast iron

96%

97%

T
-
2

Tungsten high
-
speed steel

72%

92%

Table 1

Materials that did
not show significant improvement


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AISI#

Description

At
-
110F

(
-
79°C)

At
-
310 F

(
-
190°C)

D
-
2

High carbon/chromium die steel

316%

817%

S
-
7

Silicon tool steel

241%

503%

52100

Standard steel

195%

420%

0
-
1

Oil hardening cold work die steel

221%

418%

A
-
10

Graphite tool steel

230%

264%

M
-
1

Molybdenum high
-
speed steel

145%

225%

H
-
13

Chromium/moly hot die steel

164%

209%

M
-
2

Tungsten/moly high
-
speed steel

117%

203%

T
-
1

Tungsten high
-
speed tool steel

141%

176%

CPM
-
10V

Alloy steel

94%

131%

P
-
20

Mold steel

123%

130%

440

Martensitic stainless

128%

121%

Table 2 Materials that showed significant improvement

When carbon precipitates form, the internal stress in the martensite

is reduced; this
minimizes the susceptibility to micro cracking. The wide distribution of very hard, fine
carbides from deep cryogenic treatment also increases wear resistance. In addition to
the transformation to martensite, the subjected metals also dev
elop a more uniform,
refined microstructure with greater density. Particles known as binders are coupled
with the precipitation of the additional micro fine carbide fillers. The fillers take up the
remaining space in the micro voids, resulting in a much de
nser, coherent structure of
the tool steel. These particles are largely responsible for the gains in wear resistivity
.

CHANGES IN THE METAL STRUCTURE

Martensite
, named after the German
metallurgist

Adolf Martens

(1850

1914), is any crystal
structure that is formed by
displacive

transformation, as opposed to much slower
diffusive

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t
ransformations. The martensite is formed by rapid cooling (
quenching
) of
austenite

which
traps carbon atoms that do not ha
ve time to diffuse out of the crystal structure.

Martensite has a different crystalline structure (tetragonal) than the face
-
centered
-
cubic
austenite

from which it is formed, but identi
cal chemical or alloy composition. The transition
between these two structures requires very little
thermal activation

energy because it occurs
displacively or martensi
ticly by the subtle but rapid rearrangement of atomic positions, and
has been known to occur even at
cryogenic

temperatures. Martensite has a lower density than
austenite, so that the ma
rtensitic transformation results

in a relative change of volume.

this
can be seen vividly in the Japanese
katana
, which is straight before quenching. Differential
quenching causes martensite t
o form predominantly in the edge of the blade rather than the
back; as the edge expands, the blade takes on a gently curved shape.

Martensite is not shown in the equilibrium
ph
ase diagram

of the iron
-
carbon system because it
is a metastable phase, the kinetic product of rapid cooling of steel containing sufficient
carbon. Since chemical processes (the attainment of equilibrium)
accelerate

at higher
temperature, martensite is easily destroyed by the application of heat. This process is called
tempering
. In some alloys, the effe
ct is reduced by adding elements such as
tungsten

that
interfere with
cementite

nucleation, but, more often than not,
the phenomenon is exploited
instead. Since quenching can be difficult to control, many steels are quenched to produce an
overabundance of martensite, then tempered to gradually reduce its concentration until the
right structure for the intended application

is achieved. Too much martensite leaves steel
brittle
, too little leaves it
soft
.


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Materials structure after treatment Materials structure before treatme
nt


ADVANTAGES

It has been proven to increase
the strength and durability of the material being treated, relieve
stress, create a more uniform material, and micro
-
smooth surface. The molecular structure is
"filled in" increasing the strength of the material by up to 400%! Also, the material will not
b
e damaged and it will retain its shape. The sharpness will not only last longer, but you will
be able to sharpen more times with less removal of material. Each sharpening will
demonstrate the benefits of the treatment.


Since cryogenics is a on
e
-
time treatment affecting the part to the core, we can re
-
sharpen the tool as many times as necessary without having to re
-
cryogenically treat the part.
Therefore, you may wish to treat new tools or tools that have not been worn excessively to
obtain the
maximum amount of life for the least amount of cost. There is no need of re
-
cryogenically treating a part unless it has been reheated to its fluid state. Not only the cutting
tools last longer the same performance is obtained.


DISADVANTAGES

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Insufficient
soak time, cooling or warming too quickly, and skipping the post
-
soak temper
can hamper the effectiveness of cryogenic treatment. Any one of these factors can cause
inconsistent results
--
a problem that has dogged the cryogenic processing industry.
Fortunat
ely, today's cryogenic processors are able to provide more consistent results than
older equipment.


APPLICATIONS

APPLICATION


SOURCE

TOOLS

RESULT

STEEL SPRING

PLASTIC
MANUFACTURING

SPRINGS FOR
OPENING MOULD

FATIGUE LIFE IS
INCREASED FROM
800 TO 2000
CYCLES

WHEEL ASSEMBLY

CAR
MANUFACTURER

BOX KEY FOR
WHEEL NUTS

TOOLS LAST
LONGER

HEAT DISSIPATION

COMPUTER
DISTRIBUTER

COPPER HEAT SINK
IN COMMPUTERS

TAMPERATURE
DECREASE FROM 46
DEGRRE TO 42
DEGREE CELCIUS

SLITTING

STEEL
MANUFACTURER

COLD WORK STEEL
SLI
TTERS

CUT2.5 TIMES MORE
STEEL SHEET

PUNCHING

SHEET METAL
WORKING

HSS M2 PUNCH AND
DIE

LESS GALLING

SHAPING

KITCHEN
HOUSEHOLD
MANUFACTURE

HSS POWDER
METALLURGICAL
PRESS ROLL

THE TOOLS LAST 4
TIMES LONGER

CUTTING OF STEEL

HAND TOOL
MANUFACTURER

COLD WORK
STEEL
TOOLS(1.2379) DIE

TOOLS
PERFORMANCE
INCREASED MORE
THAN 50%

DEEP DRAWING

BATTERY
CARBIDE DEEP
DIE LIFE IS
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MANUFACTURER

DRAWING DIE

INCREASED 2.5
TIMES LONGER

DRILLING

CAPITAL
EQUIPMENT
MANUFACTURER

CARBIDE DRILLS

NUMBER OF HOLES
IS DOUBLED

TURNING

OF STEEL

AUTOMOTIVE
SUPPLIER

CARBIDE INSERTS

INSERTS LAST MORE
THAN TWICE
LONGER

GEAR CUTTING

GEARBOX
MANUFACTURER

HSS PM30 HOB

DOUBLED NUMBER
OF PIECES CUT
BETWEEN
RESHARPENING


CONCLUSION

The process of investigating the cryogenic process and finally
materializing
its effects on
materials
was overall a valuable learning experience
. From this paper it is clear that this
process can create a premium more profitable tool line for a manufacturer. It is also
saving considerable tool expense for the end user
. Among the properties which define
cutting qualities of tool steel, durability is the highest importance. Results in this regard
are decisive in establishing the benefits of cryogenic treatment.

While various experts dispute the benefits of time
-
at
-
temper
ature control; available
research, along with a correlation with standard heat treating processes indicates that this
control is the key to maximizing the potential of cryogenic tempering. As is the case with
many scientific discoveries, the cost factor li
mits the usefulness of this process in the
production phase of the materials industry.


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REFERENCE

1.

Barron, Randall F. “Refrigeration and Cryogenics”, Louisiana Tech University, CRC

Press LLC, 1998.

2.

EfundaHeatTreatments

-
http://www.efunda.com/processes/heat_treat/hardening/direct.cfm

3.

Hogarth, Sharon ed. “Cryogenics: A Technology Seeks

Legitamacy”


www.sme.org/manufacturingengineering


4.

Integrated Cryogenic System
s


http://www.cryogenictempering.net


5.

Koepfer, Chris. “Bringing Cryogenics in from the Cold”


http://www.mmsonline.com/articles/0301rt2.html


6.

Metallurgical Consultants


http
://www.materialsengineer.com/E
-
Titanium.htm



7.

Pederson, Robert. “Microstructure and Phase Transformation” Lulea

University
of Technology, 2002.