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METALLURGICAL CHANGES IN STEELS DUE TO CRYOGENIC PROCESSING ITS APPLICATIONS

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METALLURGICAL CHANGES IN STEELS DUE TO CRYOGENIC PROCESSING & ITS APPLICATIONS

Metallurgical changes in steels due to cryogenic
processing & its applications

Abstract:
Cryogenic processing is a supplementary process to conventional heat treatment process in steels. It is an inexpensive one time permanent treatment affecting the entire section of the component unlike coatings. Though the benefits have been reported widely, there are issues debated upon, in respect of the treatment parameters, extent of benefits experienced in different materials, underlying mechanism and pretreatment conditions. A study on the improvement in wear resistance and the significance of treatment parameters in different materials has been made. It is found that cryogenic treatment imparts nearly 110% improvement in tool life. It is even superior to tin coatings. The underlying mechanism is essentially an isothermal process.
Keywords: Cryo processing; Wear resistance


INTRODUCTION:
The word Cryogenics is derived from the Greek words 'Kryos" (meaning cold) and "Genes" (meaning born). The cryogenic processing is modification of a material or component using cryogenic temperatures. Cryogenic temperatures are defined by the Cryogenic Society of America as being temperatures below 120K (-244F, -153C).
Cryogenic processing makes changes to the crystal structure of materials. The major results of these changes are to enhance the abrasion resistance and fatigue resistance of the materials.

The thermal treatment of metals must certainly be regarded as one of the most important developments of the industrial age. One of the modern processes being used to treat metals (as well as other materials) is cryogenic tempering. 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. Cryo tempering is a permanent, non-destructive, non-damaging process (not a coating) which reduces abrasive wear (edge dulling), relieves internal stress, minimizes the susceptibility to micro cracking due to shock forces, lengthens part life, and increases performance. Cryo treated pieces are also less susceptible to corrosion. The deep cryogenic tempering process is a one-time, permanent treatment affecting the entire part, not just the surface.
In Ferrous metals, cryogenic processing converts retained austenite to martensite and promotes the precipitation of very fine carbides. Fine carbon carbides and resultant tight lattice structures are precipitated from cryogenic treatment. These particles are responsible for the exceptional wear characteristics imparted by the process, due to a denser molecular structure; reducing friction, heat, and wear. Cryogenic Processing is not a coating. It affects the entire volume of the material. It works synergistically with coatings. Furthermore, the cost of cryogenic treatment is said to be less than the cost of coating, which is currently a popular method for improving tool life. Cryogenic Processing has a great effect on High Speed Steel cutting tools. The normal result is that the tools will last considerably longer, typically 2 to 3 times longer. Cryogenic processing establishes a very stable piece of metal that remains distortion free. The process will also stabilize some plastics. The stamping, forming and cutting die industry is one of the first places where cryogenic processing worked its wonders. Cryogenically treated metals form better. Valve spring life can be improved up to seven times over the shot peened life by the use of cryogenics.

Cryogenic processing tinkers with materials at the molecular level at cryogenic stillness, resulting in:
Homogenizes the Crystal Structure
Grain Structure refinement
Improved structural compactness
Prevents concentrated Heat Built-up
Increases Resistance to Deformation
Reduces Deformation significantly
Retained austenite is converted to a fine martensite matrix
Mechanical Properties like micro-hardness, Tensile Strength etc. are the same across any cross-section
Significant improvement in dimensional stability
Relieves residual Stresses
Several fold improvement in hot hardness
Significant improvement in material toughness
Binder Materials like Cobalt Nickel and in some cases additives of tantalum,
Tungsten or Titanium are advantageously affected
Big decrease in the amount of catastrophic shattering
Produces stronger, denser parts for better performance and longer service life
There is no official definition of the process, the process parameters vary widely from one company to the next. With the use of cryogenically treated M7 high-speed steel drill bits for drilling holes in titanium alloys, the estimated annual savings was $350,000 for $1,000,000.
Increase productive life of engineering components by 25-100%
¢ Decrease perishable tooling consumption by 25% and add to profits
¢ Increase service life of tools by 50-200%
2. Cryogenics at a glance:
Cryogenics is the study of how to get to low temperatures and of how materials behave when they get there. Besides the familiar temperature scales of Fahrenheit and Celsius (Centigrade), cryogenicists use other temperature scales, the Kelvin and Rankine temperature scales. One interesting feature of materials at low temperatures is that the air condenses into a liquid. The two main gases in air are oxygen and nitrogen. Liquid oxygen, "lox" for short, is used in rocket propulsion. Liquid nitrogen is used as a coolant. Helium, which is much rarer than oxygen or nitrogen, is also used as a coolant.
In 1942, researchers at the Massachusetts Institute of Technology found that a certain favorable combination of properties could be achieved only by including a cold treatment in the processing cycle of a tool steel. Several years later, moderate to large improvements in tool steel performance were reported when cold treatments were used. A study conducted at Louisiana Technical University, indicated that holding at “310™F (-190™C) for longer times (20 hours, compared with 8, 10, 12, and 16 hours) produced greater improvement in wear resistance. That result probably accounts for the use of holding times of 1 or 2 days at the cryogenic temperature.
It has been observed that the process provides the materials a stronger, denser and more-coherent structure thus increasing the abrasive resistance and thermal and electrical conductivity. For steels, the explanation of the phenomena in Laymanâ„¢s terms is as follows: Super cooling the steel refines the carbides in the steel by expanding the carbide structure to fill any voids in the metal. Then as the higher temperatures return, everything relaxes into where it wants to be thus providing stability to the steel. Every step in the treatment is carefully controlled else the temperature extremes will shock the steel into delaminating.

Cryogenic processing will not in itself harden metal like quenching and tempering. It is not a substitute for heat-treating. It is an addition to heat-treating. Most alloys will not show much of a change in hardness due to cryogenic processing. The abrasion resistance of the metal and the fatigue resistance will be increased substantially. Cold processes have been used for years to stabilize fixtures and tooling. The process will relieve stresses and that will help to machine parts to the proper size and shape. Cryogenic processing establishes a very stable piece of metal that remains distortion free. The process will also stabilize some plastics.
3. Typical Cryogenic Cycle:
RAMP DOWN: Lowering the temperature of the object

A typical cryogenic cycle will bring the temperature of the part down to -300F 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 -323F. 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.
SOAK: Holding the temperature low
A typical soak segment will hold the temperature at 123K 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. 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
RAMP UP: Bringing the temperature back up to room temperature
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.
TEMPER RAMP UP: Elevating the temperature to above ambient
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 300F on up to 1100F, depending on the metal and the hardness of the metal
TEMPER HOLD: Holding the elevated temperature for a specific time
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.
4. Metallurgy of cryogenic processing:
In many steels, the transformation of austenite to martensite is complete when the part reaches room temperature. (I.e. other steels, however, including many tool steels, some of the softer austenite phase is retained). Subsequent cooling to a lower temperature can cause additional transformation of the soft austenite to hard martensite. However, it is possible also to transform all (or nearly all) of the retained austenite in the steel by appropriate elevated-temperature tempering treatments that carry the added benefit of reducing the brittleness of the martensite. Transformation of retained austenite at low temperatures in tool steels generally is believed to be dependent only on temperature, not on time. Thus, merely reaching a suitably low temperature for an instant would produce the same effect as holding for several days.
Cryogenic treatments can produce not only transformation of retained austenite to martensite, but also can produce metallurgical changes within the martensite. The martensitic structure resists the plastic deformation mush better than the austenitic structure, because the carbon atoms in the martensitic lattice lock together the iron atoms more effectively than in the more open-centered cubic austenite lattice. Tempering the martensite makes it tougher and better able to resist impact than un-tempered martensite. Secondly, cryogenic treatment of high alloy steels, such as tool steel, results in the formation of very small carbide particles dispersed in the martensite structure between the larger carbide particles present in the steel. This strengthening mechanism is analogous to the fact that the concrete made of cement and large rocks is not as strong as concrete made of cement, large rocks and very small rocks, (Coarse sand). The small & hard carbide particles within the martensitic matrix help support the matrix and resist penetration by foreign particles in abrasion wear.
The reported large improvements in tool life usually are attributed to this dispersion of carbides in conjunction with retained austenite transformation. . This cryogenic processing step causes irreversible changes in the microstructure of the materials, which significantly improve the performance of the materials. The treatment calls for a precise temperature control during the processing, usually up to one-tenth of one degree, necessitating elaborate controls and sophisticated instrumentation.
Further explanation to the Concrete effect is as follows:
Cryogenic treatment of alloy steels causes transformation of retained austenite to martensite. Freshly formed martensite changes its lattice parameters and the c/a ratio approaches that of the original martensite. Etta (h) carbide precipitates in the matrix of freshly formed martensite during the tempering process. This h carbide formation favors a more stable, harder, wear-resistant and tougher material. This strengthens the material without appreciably changing the hardness (macro hardness).
The other major reason for the improvement is stress relief. The densification process leads to an elimination of vacancies in the lattice structure by forcing the material to come to equilibrium at “196™C and lowering the entropy in the material. This lower entropy leads to the establishment of long range order in the material which leads to the minimization of galvanic couples in the material thus improving the corrosion resistance of materials including Stainless Steels. Besides, there is some amount of grain size refinement and grain boundary realignment occurring in the material. These two aspects lead to a tremendous improvement in the electrical and thermal conductivity of the material thus transporting the heat generated during the operation of the tool away from the source and increasing its life.

Because austenite and martensite have different size crystal structures, there will be stresses built in to the crystal structure where the two co-exist. Cryogenic processing eliminates these stresses by converting most of the retained austenite to martensite. This also creates a possible problem. If there is a lot of retained austenite in a part, the part will grow due to the transformation. This is because the austenitic crystals are about 4% smaller than the martensitic crystals due to their different crystal structure.

The process also promotes the precipitation of small carbide particles in tool steels and steels with proper alloying metals. A study in Rumania found the process increased the countable small carbides from 33,000 per mm to 80,000 per mm. The fine carbides act as hard areas with a low coefficient of friction in the metal that greatly adds to the wear resistance of the metals. Cryogenic processing will not in itself harden metal like quenching and tempering. It is not a substitute for heat-treating. It is an addition to heat-treating. Most alloys will not show much of a change in hardness due to cryogenic processing. The abrasion resistance of the metal and the fatigue resistance will be increased substantially.

A Japanese study (Role of Eta-carbide Precipitations in the Wear Resistance Improvements of Fe-12Cr-MO-V-1.4C Tool Steel by Cryogenic Treatment; Meng, Tagashira, et al, 1993) concludes the precipitation of fine carbides has more influence on the wear resistance increase than does the removal of the retained austenite. Note that the hardness of a piece of metal becomes more even during the process. When multiple hardness readings are taken before and after the process, the standard deviation of those readings will drop a significant amount.
Unlike coated tools, a cryogenically treated tool can be sharpened, dressed, or modified. The change brought about by cryogenic processing is permanent. The process works synergistically with most coatings. This is because coatings generally work by decreasing the coefficient of friction and by preventing metals from galling. Coatings start to fail when the metal underneath them fails. It is not unusual to find wear particles with coating on one side and base metal on the other. The coating did not fail; the base metal under it failed. Cryogenic processing keeps the metal under the coating from failing while the coating protects the metal.
Life increased 2x or greater (up to 10x) 50
Life increased in some cases but was Not unaffected in others 18
Life increased in some cases but decreased In others 3
No Effect 24
Negative results 5
A comparison study conducted among 204 manufacturing plants that used cryogenic treatments (shock cooling) on steel tools, found the following:

Results in Cryogenic Treatment Percent of Plants

It is seen that about 70 percent of the plants observed tool life improvements
5. APPLICATIONS:
a) Gun barrels:
One of the truths about rifles and guns is their erratic shooting after heating up. The Cryo-Accurizing process remedies this. Cryogenic treatment increases the wear life of the barrel and makes cleaning easier and faster. All firearms develop mechanical and residual stresses during manufacturing, even with the most careful processes. These stresses cause twisting and arcing as the barrel heats up from repeated firing. Cryo-Accurizing permanently relieves the internal stresses with no risk of damage to the barrel or the action of a fine gun.
Cryo-Accurizing:
Cryo-Accurizing relieves stress in firearm barrels through deep cryogenic tempering. Stresses causes a barrel to bend or warp as it heats from repeated firing -- warping causes walking, stringing or wandering in the shot group. Deep cryogenic tempering process relieves internal stress in the firearm so the barrel will no longer bend or warp. In addition, your firearm will be easier to clean and give you increased performance, increased accuracy and extended barrel life.
The Process:
Cryogenic accurizing is a one-time, computer-controlled process where metal is cooled slowly to deep cryogenic temperatures (-300 F), and slowly returned to room temperature. The metal is triple-tempered as the final step in the process. This dry process permanently refines the grain structure of a firearm barrel at the atomic level, producing a homogeneously stabilized barrel. The denser, smoother surface reduces friction, heat and wear. The result is better shot groups in handguns and rifles and more consistent coverage and placement of shotgun patterns. Your barrel will last longer, be stronger, shoot better and be easier to clean.

Actual Ruger M77 group at 100 yards


After cryo processing before cryo processing

b) Grinding:
Grinding is a useful and valuable process. But it can induce problems into the part being made that will be very costly. Grinding can induce residual stresses into a part that will be high enough to cause cracking. This residual stress can reduce die life considerably.
Cryogenics can assist in grinding through the following:

1. Cryogenically treated grinding wheels cut more cleanly. We believe that we are affecting the crystal structure of the abrasive, making it more resistant to breaking down. This in turn allows a better cut, less wheel dressing, a better finish, and less tensile residual induced into the work piece.
2. Cryogenic processing greatly reduces or eliminates retained austenite in the part to be ground. Retained austenite in a part will increase the propensity of the part to suffer grinding damage.
3. If pieces to be ground are cryogenically treated before heat treating them, there will be less distortion as a result of heat treat and consequently there will be less need to grind large amounts off the piece in order to bring the part back into specification. In the production of stamping dies with large plates, this can be important.
4. Pieces treated after heat treat will also warp less during grinding. This reduces the cost of grinding the tool to make it flat and increases the amount of the tool left after grinding. It also allows more of the tool to be used, as tool life is not ground away in order to make the plate flat.
5. Cryogenic processing of the plates will reduce the warping that happens when large profiles are wire EDM'd from of the plate. We have seen heat treated plates crack or warp severely during the edm process. This creates delays in tool delivery. It also requires the plate to be ground flat after edm. Not using cryogenic processing causes tool life and delivery schedules suffer due to unnecessary rework
c).Engine parts
Knowledge of the effect of cryogenic processing on engines and power plants comes mainly from automotive racing applications. Racing applications are one of the first applications that the process was put to. There are quite a few non-racing possibilities also. The following are noted:
1. There is up to a four percent increase in the torque across the rpm range.
2. There is an increase in peak pressure in the combustion chamber.
3. Engines turn more freely.
4. Crankshafts do not break as often.
5. Crankshaft journals do not wear as readily.
6. Pistons can be run at higher levels of detonation.
7. Piston skirts do not gall as much.
8. Piston rings provide better sealing.
9. Piston ring wear is reduced.
10. Cylinder wall wear is reduced.
11. Connecting rod failure is reduced.
12. Wrist pins wear less.
13. Valves stems wear less.
14. Valve guides wear less.
15. Valve springs lose less spring constant.
16. Valve spring fatigue life is greatly improved.
17. Cylinder heads can be run at higher levels of detonation.
18. Camshaft wear is diminished.
19. Cam shafts breakage is reduced.
20. Timing gears wear less.
21. Timing chains wear less and stretch less.
22. Rocker arms breakage is reduced.
23. Push rods do not flex as much.
24. Head bolts do not "relax", and maintain their torque.
25. Bearing caps maintain their alignment; distortion is reduced. Cap bolts do not stretch as readily or lose as much torque.

Aluminum piston alloy structure


Cryogenically treated Non- Cryogenically treated
Magnified 3500x
26. The cryogenically processed piston has a more wear resistant surface, higher yield and ultimate strength. This alloy will display structural, thermal and metallurgical stability not found in the untreated condition, as well as significant abrasive wear improvement. The contact and fretting fatigue will be reduced due to the tightening of the surface microstructure. In addition, the corrosion resistance to hot reactive gases and moisture in the combustion chamber will be improved.
COMPACT DISCS:
Compact disks respond to cryogenic treatment. Understanding this is hard to fathom, but it is quite true. The effect is a permanent increase in the quality of sound coming from the disk. The effect has been noted by numerous audio experts and by numerous "average" listeners.
Industrial Applications:
Extended Life and Durability
¢ Machining: lathes, drill bits, cutting and milling tools
¢ Pulp and Paper: saws, chippers, millers and cutters
¢ Oil and Gas: drilling, compression, pumps, pump jack gears, valves and fittings
¢ Mining: drill bits, drilling steel, slasher teeth and face cutters
¢ Food Processing: grinders, knives and extruding dies
¢ Textiles: scissors, needles, shears and cutting tools
¢ Wood Fabricating: saws, drill bits, routing bits and planes
¢ Dental and Surgical Instruments
Testimonials:
Application Tool Material Work Material Improvement
Drilling M42 Titanium Alloy 2 to 1
Forming Die A2 RC 60-62 Electric Iron 2 to 1
Abrasive Wear 52100 Alumina Wheel 165%
Abrasive Wear D2 Alumina Wheel 178%
Abrasive Wear A2 Alumina Wheel 225%
Drilling C2 Carbide Graphite 2 to 1
Face Milling C2 Carbide 4340 4 to 1
Milling M7 Titanium Alloy Significant
De-burring C2 Carbide Inconel 718 4 to 1
Hobs M2 & M7 Hi-Ni-Alloy 3 to 1
Key Cutters M2 & M7 Hi-Ni-Alloy Replace Carbide
Punching M2 & M7 Hi-Ni-Alloy 6 to 1
Punching D2 302 Stainless 3 to 1
Milling 8% Co 347 Stainless 375%
Wood Cutting HSS & Carbides Hardwoods 5 to 1
Disposable Razor Stainless Human Whiskers 15 to 1
Die C2 Carbide 400Stainless 2 to 1
Can Die D2 Aluminum 2.5 to 1
Stamping Die D2 Mid Steel 10 to 1

Why isnâ„¢t cryogenic processing well known
It is empirically developed.
There has been very little research into the theories of why it works
There are people out in industry with beer coolers and liquid nitrogen that claim to do this process, but know nothing about metallurgy or tooling.
The process does not show up as an easily demonstrated change in microstructure.
Cryogenic Processing is relatively new
Conclusion:
Cryogenic Processing is not a substitute for heat-treating. Cryogenic Processing is not a coating. It affects the entire volume of the material. It works synergistically with coatings. These benefits extend to cast iron, aluminum, stainless steels, and other materials. The scope of cryogenics has expanded widely from basic military and space applications to various civil applications. Cryogenic processing is mainly applicable to steels. Cryogenic treatments can produce not only transformation of retained austenite to martensite, but also can produce metallurgical changes within the martensite. this offers many benefits where ductility and wear resistance are desirable in hardened steels While various experts dispute the benefits of time-at-temperature 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 limits the usefulness of this process in the production phase of the materials industry.
Recent advancements:
Currently extensive research is being conducted in an effort to better the available cryocooler technology in fields like materials for the regenerator, cylinder heads, etc., refrigerants used, size of cryocooler, increasing the efficiency. Stirling technology is used to produce miniature cooling systems
Superconductivity occurs in a wide variety of materials, including simple elements like tin and aluminium, various metallic alloys, some heavily-doped semiconductors, and certain ceramic compounds containing planes of copper and oxygen atoms is achieved by this cryogenic processing . dilution refrigerators is One important application of superfluidity and this state is also achieved by crogenic processing
Cryogenics has been successfully tested on flexible circuits to reduce the residual stress between layers of the circuit. This helps keep the circuit from curling and separating
Treated transformers show a lack of hysterisis. The magnetic core saturates less.
Ball and roller bearings respond beautifully to cryogenic processing. Increases of wear life of two to three hundred percent are not uncommon.
Cryogenically treated resistance welding electrodes will last about 3 to six times longer than untreated electrodes.
References:
Advances in Cryogenic engineering ----Plenum (1967)
Thornton, Peter A., and Vito J. Colangelo. Fundamentals of Engineering Materials. Englewood Cliffs: Prentice-Hall. 1985.
Relevant web pages:
http://irtek.arc.nasa.gov/ARCS&T.html
http://www.asm-intl.org/
http://www.metal-wear.com/index.htm
http://diversifiedcryogenics.com
http://www.apexknives.com

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