Cryogenic
Processing of Wear Control
ABSTRACT:
In today’s world of rapid advancement
in technological process and flexible change in materialistic science. It is
difficult for one to keep in pace with ever changing need of new industrial
application. There is no profound mean to exclaim a process to be fully
advanced and is at it’s felt of it’s advancement in term of technology. Similar
is the case of “cryogenics processing for wear control”.
The quality of most metal products
depends on the condition of their surface and on the surface deterioration due
to use. Surface deterioration is also important in engineering practice .It is
often major factor limiting life and performance of machine components.
The cryogenic processing has great
potential to solve many of the problems faced an industry due to wear and other
detrimental process. Added to it the fact that it is environmentally very sound
makes it the most promising one.
INTRODUCTION:
As we have seen the heat treatment
process as mentioned are only for the surface. But due to the advancement in
science and technology newer and better process have been developed which give
better results.” Cryogenics" is the science that studies phenomenon at
very low temperature. Deep cryogenics is the ultra low temperature processing
of materials to enhance their desired metallurgical and structural properties.
This special process is not a surface treatment; it effects the entire mass of
the tool or component being treated, making it stronger through out. The
hardness of the material is unaffected, while its strength is increased.
Steel tooling and wear parts, which
must exhibit properties as strength, hardness and wear resistance, are
traditionally heat treated to induce a transformation to tempered martensite.
Under carefully controlled laboratory condition, nearly complete transformation
can be attained. However under the conditions of production heat treating,
complete transformation is rarely achieved. With the result that conventionally
heat treated components may contain significant amount of retained austenite,
could be transformed to martensite by cryogenic cooling. It is only recently
however the important time dependent aspects of cryogenic cooling applied to
obtain maximum wear resistance have been understood.
Research has demonstrated that a slow,
gradual cooling, followed by an extended holding period at ultimate temperature
and then a gradual warming results in the maximum increase in the wear
resistance of treated components while eliminating thermal shock.
The thermal treatment of metals must
certainly be regarded as one of the most important developments of the
industrial age. After more than a century, research continues into making
metallic components stronger and more wear-resistant. One of the more modern
processes being used to treat metals (as well as other materials) is cryogenic
tempering. While the science of heat treatment is well known and widely
understood, the principles of cryogenic tempering remain a mystery to most
people in industry. Information regarding this process is full of
contradictions and unanswered questions. Until recently, cryogenic tempering
was viewed as having little value, due to the often-brittle nature of the
finished product. It is only since the development of computer modeled cooling
and reheats curves that the true benefits of cryogenically treated materials
have become available to industry and the general public.
The purpose of this work is not to
break new ground in cryogenic science, nor will it answer all of the questions
surrounding this process. Rather, this is a condensation of much of the
information available concerning the effects cryogenic treatment has on metal
structure, as well as an overview of the actual process involved in treating
parts.
TYPES OF WEAR:
ABRASIVE WEAR:
It occurs when a hard sharp surface
slide on a softer surface creating a series if grooves in it. The material in
the grooves is displaced in the form of the wear particles.
ADHESIVE WEAR:
It occurs when two smooth surface slide
over each other and fragment pulled of one surface adhere to the other. These
fragments come off the surface and are transmitted to the original surface as
loose particle. This occurs because of failure of film failure is caused by
high temperature, pressure, sliding velocity. If the loads are light and the
natural spontaneous oxidation of metal can keep with up rate if it's removal by
wear than the wear rate will be relative low (the oxidation layer acting as a
lubricant) it is called Mild wear. If the loads are high and the protective
oxide is continually disrupted to alloys intimate the metallic contact and
adhesion then wear rate will be high it is called “severe wear”.
Material
that has thin, brittle oxides notably stainless steel, aluminum alloys and
titanium, the protective oxide is easily
disrupted and the consequent massive adhesion and wear is called "
Galling". These usually can be seen in a relation with an non-lubricated
sliding.
CORROSIVE WEAR:
It occurs when environment or
surroundings of a sliding surface interact chemically with it. It forms a film,
but the sliding action wears the film away and corrosive attack continues.
FACTORS AFFECTING WEARS:
Factors affecting wear are as follows
§ Operating condition
§ Material structure and properties
§ Environmental conditions
OPERATING CONDITION:
§ Loading
§ Velocity
§ Fatigue
LUBRICATION:
The purpose of lubrication is to reduce
friction and to mitigate their effects. There are five types of lubrication:
Hydrodynamic Lubrication:
In this type moving surfaces are
separated by a fluid film resulting from movement of one surface relative to
another. Thus adhesion is prevented.
Hydrostatic Lubrication:
In this type lubrication is supplied
under pressure and is able to sustain higher load without contact taking place
between the surfaces.
Elasto-Hydrodynamic Lubrication:
In this type the pressure between the
surfaces are so high, the lubrication film is so thin that elastic deformation
of surface is likely to occur and is a feature of this kind of lubrication.
Boundary Lubrication:
In this an oil or grease containing a
suitable boundary lubricant separates the surface by what is mean by,” Absorbed
Molecular Films". Appreciable contact between asperities and formation of
junction may occur.
Solid Lubricant:
This provides a solid, low, shear
strength film between surfaces. Lubrication is dictated by circumstances.
Material Structure and Properties:
The wear resistance of any metal
depends on its surface condition and also it's properties.
Environmental Conditions:
The wear or deterioration also depends
on kind of environment the metal is placed in, like the gases present in
surroundings.
Wear Control Techniques:
Wear is impossible to remove but by
using certain methods or techniques it can be kept under control i. e. under
desirable levels. Many materials & methods are available for protection
from wear.
Various techniques for providing
surface protection to wear as follows:
1.
Electroplating
2.
Anodizing
3.
Metal spraying
4.
Hard facing
5.
Selective heat treatment
ELECTROPLATING:
The wear resistance of metal part can
be improved by electroplating a harder metal on its surface. The metals often
plated on the base metal are chromium, nickel & rhodium.
ANODIZING:
In anodizing the work is the anode and
the oxide layers are built on the base metal. Since the newest oxide layers
always forms next to the base metal in order for the process to continue the
previously formed oxide layers must be porous enough to allow the oxygen ions
to pass through them. Anodizing has greatly extended the use of aluminum and
it's alloys.
METAL SPRAYING:
Metal spraying is usually done by
automatically feeding a metal wire at controlled rate of speed through metal
zing tool or gun .Air, oxygen and a combustible gas are supplied to the gun by
means of hoses and form a high temperature high velocity flame around the wire
tip .The wire tip is continuously melted off and liquid metal particles are
directed at the work by the high velocity flame when they strike the surface
this particles flatten out .At the same time they are forced into surface pores
and irregularities to provide some mechanical interlock with previously
deposited material. Cooling is very rapid and thin oxide film forms on the
exposed surface of the deposited.
HARD FACING:
The production of hard wear resistant
surface layer on metals by welding is known as hard facing. This method is
relatively easy to apply requiring only hard facing alloy in the form of
welding rods and an oxy-acetylene flame or an electric arc.
SELECTIVE HEAT TREATMENT:
The methods used for selective heat
treatment are induction hardening and flame hardening.
a) Induction hardening:
The work piece is placed in a rapidly
changing magnetic field. The induced current produces the heat. The time of
heating decides the depth of hardness by quenching the heated sample hardness
is achieved.
b) Flame hardening:
This process does not change the
chemical composition but convert the surface into martensite. Normally the
samples are heated with the help of oxy-acetylene torch and then water or oil
is spread on the surface. Depth of hardness is controlled by flame intensity
and heating time.
Wear resistance can be developed by:
§ The surface smoothness to eliminate the projection
§ Preventing metal-to-metal contact.
§ Increasing the hardness to resist initial indentation.
§ Increasing the toughness to resist the tearing.
Cryogenics:
Cryogenic tempering may be
oversimplified into a process of chilling a part down to relatively near
absolute zero and maintaining that condition until the material has
cold-soaked. The temperature is then allowed to rise until ambient equilibrium
is reached. The part may then be subjected to a normal tempering reheat,
although this step is not always included in the process. The complexity of the
process involves determining and achieving the proper duration for the cooling,
soaking, and warming cycles. It is here that developments in computer modeling
and controls have placed cryogenic tempering on the cutting edge of metal
treatment. Scientists in provinces of the former Soviet
Union typically disagree with western methods of cryogenic
treatment, as tests there have revolved around unceremoniously dumping parts into
a flask of liquid nitrogen, removing them, and allowing the material to cool
uncontrolled in ambient air. Predictably, reports of extended tool life have
not been as favorable as those achieved using more tightly controlled processes
(History).
CRYOGENIC PROCESSING:
The specimen placed in a cryogenic
cooling chamber. The process takes the specimen from room temperature of -196 C
at a very slow rate, then holds the specimen at 196 C for a predetermined
amount of time and controls the return to room temperature. During this time
the specimen is subjected to light tempering to complete the process. The
entire process takes between 36 to 70 hours, depending on the weight and type
of the material.
These ultra cold temperatures are
achieved using computer controls, a well insulated treatment chamber and liquid
nitrogen (LN2). Nitrogen is the gas that constitutes 78.03% of the air we breathe.
The liquid formed is the product of air separation, compression and
liquification. These deep cryogenic system are completely environmentally
friendly an actually help reduce waste.
A wide range of experimental techniques
has been applied to investigate the atomic displacement of carbon and micro
structural changes in martensite during tempering. Including X-ray diffraction,
electron microscopy and diffraction, Mossbaur spectroscopy, atom probe field
ion microscopy, electrical resistivity, dilatometric and calorimetric analysis.
According to the present state of
knowledge the structural evolution of martensite on tempering can be divided
into the following sequence of processes: (a) the 0-th stage, the formation of
carbon atom clusters, modulated structures and ordered structures, (b) the
first stage, where the martensite decomposes into low carbon martensite
containing 0.2 to 0.3 wt% C and transition carbide particle, (c) the second
stage, the decomposition of retained austenite into ferrite and cemetite, (d)
the third stage, conversion of the transition carbide into cementite and
complete loss of the tetragonality of martensite.
Although a lot of words about tempering
behavior have been done, a complete and satisfactory understanding of the
mechanisms of the structural changes involved has not yet been obtained. The
0-th stage, i.e., prior to carbide precipitation, and the first stage were of
interest in the last decades. However, not so much attention has been paid to
studying the effects of cryogenic treatment on the carbide precipitation in
martensite during tempering.
The aims of the present study,
therefore, are to investigate metallurgically the wear resistance and the
microstructure of tempered alloy tool steels after quenching, after cold
treatment at 223 K and after cryogenic treatment at 93 K.
Mechanism of
h-carbide Precipitation:
A model of the bet-orthorhombic system
transformation is proposed. It is known that the lattice deformation of
martensite results from cryogenic treatment. Figures 9(a) and (b) represent the
relationship between (010) h-carbide plane and (110) martensite plane. This
existence of the lattice correspondence between two phases implies that (010)
h-carbide plane is derived from (110) martensite plane, and [100], [010] and
[001] h-carbide directions are
Derived from [110]. [110] And [001]
martensite directions respectively. In the h-carbide structure, carbon atoms
are in the octahedral interstices and iron or substitutional atoms take a hep
arrangement. The distance between neighbor iron atoms or substitutions atoms in
h-carbide and martensite is AB (h)>AB (x'), BC (h)<BC (x').
The lattice deformation is supposed to
convert the parent bct lattice into an orthorhombic h-carbide lattice through
the readjustment of iron or substitutional atoms due to contraction along [110
and [110] martensite direction and expansion along [001] martensite direction.
Correspondingly, a slight shift of carbon atoms is required on (110) martensite
planes in order to achieve carbon stacking of h-carbide. This can be present as
follows in bct lattice, carbon atoms at (1/2, 1/2.0) and (1/2, 1/2.1) positions
in (110) martensite planes shift a/12[150], which may be explained
a/6[110]+a/4[110], and ones at (0,1,1/2) and (1,0,1/2) positions shift
a/12[150], i.e., a/6[110]=a/4[110], where a means the lattice parameter of
martensite. Furthermore, before contraction and expansion, a a/6[110] shuffling
of iron or substitutional atoms on alternate (110) martensite plane is
necessary to meet the needs of stocking described by h-carbide structure.
Alternatively, iron or substitutional atoms, the double circles indicate iron
or substitutional atoms, which belong to two unit cells, and the solid circles
indicate carbon atoms. An arrow shows the shuffling direction of atoms. It is
suggested by TEM observation and crystallographic analysis of carbide nucleates
heterogeneously along the carbon-rich bands, which develop during the spinodal
decomposition of martensite.
It is well known that precipitation of
fine h-carbide enhances strength and toughness of martensite matrix, and
further increase wear resistance.
Alterations in Metal Structures:
An explanation of the effect of deep
cooling on metallic structure requires a connection be drawn to the more
standard elevated temperature treatment processes. When a metal (high carbon
steel, for example) is heated, the increase in energy expands the molecules. A
ferrite (iron) molecule, like everything else, is mostly empty space between
tiny atoms. As the iron atoms separate, atoms of carbon present in the steel
"fall" into the larger empty spaces, creating a condition known as
interstitial solid solution. This hot, carbon enriched iron is known as
austenite. Hardened steel is simply austenite, which has been rapid quenched to
trap the carbon atoms in solution. This hardening process is the first step in
any thermal treatment of steel. Because the grain structure of austenite is
normally unstable at ambient temperatures, simply quenching results in steel
which is brittle and of little value to industry. If the steel is partially
quenched and held at a certain elevated temperature (dependent on actual carbon
content) austenite will re-form into a much more stable structure known as
martensite. After the steel has reached thermal equilibrium at the martensite
start temperature (Ms), it is allowed to cool slowly to promote martensitic
transformation.
It is at this point that cryogenic
tempering becomes important, especially in the hypereutectoid steels (above
0.83% carbon content). Martensite structures do not form at a constant
temperature; rather the austenite is converted to martensite as the steel cools
to ambient temperature. The temperature range for martensite formation is
determined by the particular carbon content of the material. Figure 1 is a
graph of martensite formation temperatures as they relate to carbon content.
The ranges are not exact; they are merely intended to show the concept of
martensite growth with cooling. As the graph shows, the martensite growth
completion line (Mf) drops below 0C at approximately 0.7% carbon content. As
most steel producing plants are considerably warmer than this, it is easy to
see that the higher carbon steels cannot undergo a complete austenite
conversion without artificial refrigeration.
The Mf line is extrapolated below 0C,
but it serves to point out the major reduction in temperature necessary to
completely transform austenite in the higher carbon steels. Obviously, the
cryogenic process is key to continuing the transformation of retained austenite
into martensite in hypereutectoid steels. A study conducted by the Polytechnic
Institute of Jassy, Romania
Electron micrographs of cryogenically
treated steel reveal another phenomenon, which is less easily understood.
During the martensite transformation process (hot or cold), a certain amount of
free carbon atoms will precipitate out of the interstitial solid solution.
These atoms are grouped together by pressures exerted during martensite crystal
growth. These tiny pockets of carbon are known collectively as carbides. Under
the microscope, carbides appear as tiny lumps of coal wedged into the
martensite grain boundaries. These carbides upset the uniform structure of the
martensite crystals, and are a significant factor in the brittleness of
hardened and tempered steel. Cryogenic treatment appears to have the effect of
significantly reducing the size of these carbides. While roughly the same
amount of free carbon is present, the cryogenic process seems to slow the
development of these "lumps of coal", distributing the carbon atoms
more evenly and allowing a tighter overall grain structure with less voids. One
hypothesis is that the very low temperatures inhibit the formation of covalent
atomic bonds in the free carbon, preventing the larger carbide structures from
forming.
The Ideal Role of Cryogenic
Tempering:
As previously discussed, the
transformation of austenite into useful martensite is dependent on two factors,
carbon content and temperature. The steel must first be heated to a temperature
sufficient to allow carbon atoms to enter into solution. This temperature must
then be maintained until enough time has passed for a complete austenitic
reaction to take place. This is known as soak time, and is dependent on the
mass of the part being treated. Next, the part must be partially quenched down
to the martensite start temperature for the particular carbon content of the
steel. The part is then held at this temperature until thermal equilibrium is
reached (the entire thickness of the part is the same temperature). Under ideal
circumstances, the part would immediately be placed into the cryogenic chamber
and cooled to the relevant Mf temperature for carbon content and maintained at
that temperature until thermal equilibrium was once again reached at Mf. It is
at this point that complete martensite transformation will have occurred and
the part may be allowed to return to ambient temperature at a rate, which will
minimize internal stresses.
Cryogenic tempering of room temperature
steels is effective at transforming retained austenite, however, the resulting
crystalline structure is not as uniform as martensite, which has all formed at
the same time, as in the above example. Integral cryogenic tempering is not
cost effective for volume production parts, as the logistics of having a large
cryo chamber in close proximity to a steel furnace would be interesting, to say
the least. Interrupted tempering or tempering of room temperature steels are
much more practical ways of obtaining a majority of the benefits of cryogenic
treatment.
CASE STUDY 1:
IMPROVEMENT IN CASE OF RIFEL BARRELS FIREARMS:
Cryogenic treatment of firearms is a
proven reliable process, which relives stress in firearm barrels. Inherent
internal stresses cause movement in the barrels as it heats from the repeated
firing. Our ultra low temperatures and ultra slow temperatures change allows
the molecules in the barrel to compress and then expand in uniform homogeneous
manner and then realign in more coherent fashion, thus reducing internal stress
and increasing barrel accuracy and life.
Riffles treated by CRYOGENIC PROCESS
produced the enclosed range test targets. The best groups from these riffles
while using match ammunition prior to CRYOGENIC TREATMENT. Then after many
groups were fired while testing numerous brands. No other singular work or
effort to achieve greater accuracy has yielded more positive results then the
simple act of CRYOGENIC TREATMENT. Its fact!
The observed changes were:
1) Increased accuracy
2) Increased barrel life
3) Eliminate pattern shift
CASE STUDY 2:
IMPROVEMENT IN CASE OF GUN BARREL FIREARMS:
Stress in a steel barrel is uneven; the
Stresses in steels are created by mechanical methods such as machining, boring
and forming. Residual stresses are also created in castings or forgings as a
result of differential cooling. Thermal stresses are created in steels after
heat-treating through the quench hardening process. An ice cube when dropped
into a cup of hot coffee illustrates this effect. The heating creates expansive
stress on the exterior of the ice cube while the core is still frozen. The
result is stress shear or cracking due to the differing rates of thermal
"growth" caused by the coefficient of expansion. Dropping a rifle
barrel into liquid nitrogen would have the same effect most barrels have
residual stresses and distortions throughout their entire length. While
shooting, heating and cooling occurs which will cause the barrel to warp
unevenly and cause random shot placement. A barrel that has been treated by
Cryogenics International will dissipate heat quickly and more evenly and allow
the shooter to achieve the tightest groups possible. The primary reason for
dramatic accuracy improvement is the elimination of residual stresses, which
are inherent in the barrel from the manufacturing process. The barrel will
become more uniform, free from stresses and tighter microstructure after the
Cryogenics International deep cryogenic process.
The observed changes were:
1) Tighter Groups
2) Improved Durability
3) Stress Relief
4) Improved Accuracy
5) Reduced Wear
6) Extended Barrel Life
7) Quick Turnaround
8) Easier Cleaning
BENEFITS:
§
Increasing the durability of components
in the vehicles is the main reason for using cryogenic processing. The great
thing about cryogenic processing is that it allows an increase in durability
without an increase in weight or major modifications to component design.
§
Thermal and mechanical stresses are
relieved, reducing the possibility of uneven warping while firing your gun,
improving its accuracy. The metals porosity is reduced, they’re by reducing
friction, excessive heat and wear. The tighter surface structure also makes the
barrel and complete gun much easier to clean. Cryogenic processing affects the
entire mass of the part. It is not a coating. This means that parts can be
machined after treatment without losing the benefit of the process.
§
Cryogenics has successfully been used
to relieve the stress in castings and forged metals; even freezing cutting
tools has resulted in increased wear characteristics.
§
Cryogenic will not turn a poorly made
or designed instrument into a good one. But it can help improve resonance
characteristics and make a good instrument just a little better.
ADVANTAGES:
The important observed advantages of
cryogenic treatment are as follows:
§ Increase abrasive wears 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.
§ Changes the item's entire structure, not just the surface.
Subsequent refinishing operations or regrinds do not affect permanent
improvements.
§ Eliminates thermal shock through a dry, computer-controlled
process.
§ Transforms almost all-soft retained austenite to hard
martensite in steel.
§ Forms micro fine carbide fillers to enhance large carbide
structures.
§ Increases durability or wear life.
§ Decreases residual stresses in tool steels, induces
dimensional stability.
§ Decreases brittleness.
§ Increases tensile strength, toughness and stability coupled
with release of internal stresses.
§ Increases fatigue life.
§ Decreases in chipping and flaking.
FIELDS OF APPLICATION:
Cryogenics international's controlled
deep cryogenic process has many proven and possible applications, such as:
SHOOTING/MILITARY:
§
Guns and gun barrels, firearms,
bullets, dies, actions etc
KNIVES:
§
Tim Hancock custom knives, some of the
best in the world.
RACING/MOTOR SPORTS:
§
Engines, axles, gears, radiators,
rotors, brake pads, spark plugs and wires.
TOOLING AGRICULTURE:
§
Plow points, spades, blades,
plowshares, roller chain, chain saw, cycles.
SPORTS:
§
Aluminum baseballs bats, golf balls and
clubs, nylon string, fishing line, fishing hooks etc.
ELECTRICAL COMPONENTS:
§
Copper wire, plug strips, any
conductors etc.
AEROSPACE/AIRCRAFT:
§
Casting and components, engines,
landing gear, supports etc.
MEDICAL/DENTAL:
§
Stainless steel hygienist's tools,
scissors, knives, saws, UHMW Polyethylene for implants etc.
RECYCLING:
§
Shredding screens, knives, shredders,
grinders, pulverizes etc.
MUSICAL INSTRUMENTS:
§
Brass horns, cymbals, strings etc.
MISCELLANEOUS APPLICATIONS OF MATERIAL:
§
Some examples of materials that are
routinely cryogenically treated include:
All types of steel:
§ Titanium
§ Plastics
§ Copper
§ Aluminum
§ Brass
§ Various alloys
§ Glass
§ Complete assemblies' i.e. whole chain saws, engines, motors
etc.
CONCLUSION:
This has been an attempt to put forward
an understanding of cryogenic processing for wear control. This is only an
overview of the whole process. This has been condensed because of limitations.
The cryogenic processing has great potential to solve many of the problems
faced an industry due to wear and other detrimental process. Added to it the fact
that it is environmentally very sound makes it the most promising one.
The study of microstructure changes
induced by cryogenic processing is a fertile area for research, the complete
story is yet not well known. For example the mechanisms responsible for
cryogenically induced improvements are not yet completely understood. The
observed benefits are however well established.
Cryogenic treatment increases wear
resistance dramatically, especially at high sliding speed. The specimens after
cryogenic treatment show a minimum of wear rate. Unlike cold treatment,
cryogenic treatment promotes preferential precipitation of fine carbides. The
formation mechanism of carbide is supposed to be as follows: iron or
substitutional atoms expand and contract, and carbon atoms shift slightly due
to lattice deformation as a result of cryogenic treatment.
The mechanism that cryogenic treatment
contributes to wear resistance is through the precipitation of fine h-carbide,
which enhances strength and toughness of martensite matrix, rather than the
removal of the retained austenite.
REFERENCES:
1)
A textbook of material science and
metallurgy by- O.P.Khanna.
2)
Introduction to physical metallurgy by-
Avner
3)
Cryogenic system by-Rendall Barron
4)
Cryogenic International Friction, Wear
and Lubrication a tribology Hand book
5)
www.google .com
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