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Page 1: Cryogenic Hardning

Cryogenic Hardening

For more visit- http://www.akfunworld.co.nr 1

Page 2: Cryogenic Hardning

Cryogenic Hardening

For more visit- http://www.akfunworld.co.nr 2

Chapter-1

INTRODUCTION

Heat treating is what gives metals its hardness as well as its toughness, wear

resistance and ductility. Even performed properly, heat treating cannot remove all of the

retained austenite (large, unstable particles of carbon carbide) from steel. Proper heat treating

is a key part in increasing a part's toughness, durability, wear resistance, strength and

hardness.

The beneficial changes that occur as a result of the heat treat process do not actually

take place during the heating, but, rather from the cooling or "quenching" from the high

temperature. The benefits of the quench do not stop at room temperature, as many alloys will

continue to show significant improvements if the quench temperature is reduced further to

subzero levels approaching absolute zero. While it is impossible to actually achieve 0K or

-273°C, (a molecular zero movement state that eliminates all stress).

Hence to reduce the amount of retained Austenite in the metals they need to be cooled

below room temperature. This is where cryogenic hardening comes into picture, cryogenic

temperatures i.e. below −150°C, −238°F or 123K are very efficient and cost effective in

increasing dimensional stability, increasing wear resistance and performance of most metals

and alloys by reducing austenite content of metals and alloys.

Some of the benefits of cryogenic hardening include longer part life, less failure due

to cracking, improved thermal properties, better electrical properties including less electrical

resistance, reduced coefficient of friction, less creep and walk, improved flatness, and easier

machining.

1.1 DEFINITION

Cryogenic Hardening

A cryogenic hardening is the process of treating work pieces to cryogenic

temperatures (below −150°C, −238°F or 123K) to remove residual stresses and improve wear

resistance on steels by transforming all the austenite into martensite.

Page 3: Cryogenic Hardning

Cryogenic Hardening

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1.2 HISTORY

In the past toolmakers would bury components in snow banks for weeks or even

months to improve wear resistance. Castings were always left outside in the cold for months

or years to age and stabilize. Swiss watchmakers noticed that extreme cold changed the

properties of their metal clock parts for the better. They would store them in cold caves and

let them freeze during the winter. Because of the secret use of cold treating metals and the

resulting increase in watch quality lifted the Swiss watch making to mystic levels.

1930s German records tell of aircraft engine manufacturers testing cryogenics on their

products with some success. During World War II American bomber manufacturers used this

method of cold tempering to stress relieve aluminum superstructures. This allowed the

airplanes to be made from thinner materials of lesser weight. This allowed airplanes to carry

heavier ammunition and bomb loads. Increasing the bomb load dramatically increased the

effectiveness of the airplanes.

Today cryogenic tempering is used to some degree in many industries. Its positive

effects are not just limited to metals. They include nylons and other plastics, lighting, high

voltage/amperage electrical systems, soldered connections, computer memory, circuit boards

and components, well drilling, machining processes, casting and forging, ceramics, farming,

transportation fleets, construction, excavation, etc.

Figure 1.1 Ice storage cave in Bliesdahlheim Figure 1.2 Ice harvesting

Page 4: Cryogenic Hardning

Cryogenic Hardening

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Chapter-2

PROCESSES IN CRYOGENIC HARDENING

1. Lowering the temperature of the object (RAMP DOWN).

2. Holding the temperature low (SOAK).

3. Bringing the temperature back up to room temperature (RAMP UP).

4. Elevating the temperature to above ambient (TEMPER RAMP UP).

5. Holding the elevated temperature for a specific time (TEMPER HOLD).

Figure 2.1 Stages in Cryogenic hardening

2.1 RAMP DOWN: Lowering the temperature of the object. A typical cryogenic cycle will bring the temperature of the part down to -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. Assume if an object is dropped in a vat of liquid nitrogen. The outside

of the object wants to become the same temperature as the liquid nitrogen, which is near -

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.

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2.2 SOAK: Holding the temperature low.

A typical soak segment will hold the temperature at -320°F 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

2.3 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.

2.4 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 300°F on up to 1100°F, depending on

the metal and the hardness of the metal.

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Cryogenic Hardening

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2.5 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.

Page 7: Cryogenic Hardning

Cryogenic Hardening

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Chapter-3

MICROSTRUCTURAL CHANGES IN MATERIAL

Cryogenic processing makes changes to the structure of materials being treated, depending on

the composition of the material it performs three things:

1. Conversion of retained austenite into martensite.

2. Fine Carbide Precipitation.

3. Stress relieves

3.1 Conversion of Retained Austenite to Martensite

In many steels, the transformation of austenite to martensite is complete when the object

reaches room temperature. 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.

Figure 3.1 Matensite formation temperatures

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Cryogenic hardenings 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 much 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.

3.2 Fine Carbide Precipitation

Cryogenic hardening 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. The small & hard carbide particles within the martensitic matrix

help support the matrix and resist penetration by foreign particles in abrasion wear.

The 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

Cryogenic hardening of alloy steels causes transformation of retained austenite to

martensite. Eta carbide precipitates in the matrix of freshly formed martensite during the

tempering process. This Eta carbide formation favors more stable, harder, wear-resistant and

tougher material. This strengthens the material without appreciably changing the hardness.

Figure 3.2 Micrographs of M2 tool steel containing carbides (black dots)

a. after non-cryogenic treatment b. after tempering

CCCC

c. after cryogenic treatment

and tempering

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3.3 Stress relieves.

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 –320°F 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.

Figure 3.3 Structures of Austenite and Martensite

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.

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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.

A Japanese study “Role of Eta-carbide Precipitations in the Wear Resistance

Improvements of 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.

A comparison study conducted among 204 manufacturing plants that used cryogenic

treatments (shock cooling) on steel tools, found the following:

Results in Cryogenic Hardening Percent of Plants

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

Table 3.1 Result of Japanese study

It is seen that about 70 percent of the plants observed tool life improvements.

Figure 3.4 Microstructure of HSS tool

before the cryogenic treatment.

Figure 3.5 Microstructure of cryogenic

treated HSS tool

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Cryogenic Hardening

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Chapter 4

OBSERVATIONS OF CRYOGENIC HARDENING

4.1 Wear Resistance

Figure 4.1 Wear Resistance comparisons of samples soaked at 189°K and 83°K

Figure 4.1 shows a comparison of wear resistance rates of the sample tool steels when

soaked at 189K and 83K. The samples were observed to have relatively the same amount of

retained austenite before and after the soak periods hence another mechanism was at work for

the greatly increased wear resistance of the sample treated at deep cryogenic temperatures

(183K). A study in 1994 showed that the reduction in retained austenite was only part of the

reason for the increased wear resistance. This research confirmed the precipitation of fine

(eta) carbide particles reason for the increased wear resistance.

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4.2 Variation of the wear rate with sliding speeds

The Variation of the wear rate with sliding speeds is shown in Fig. 4.2 for specimens

austenitized at 1293K, quenched and ultra-subzero treated at 93K. Finally tempering was

carried out at 453K for 1.8Ks. The wear rate of specimens after cryogenic hardening is

smaller than that of as-quenched specimens (without any subzero hardening) for whole

sliding speeds. Furthermore, it decreases dramatically at high sliding speed. The cryogenic

hardening results show 110 to 600% improvements. The wear rate shows a minimum at the

sliding speed of 1.14 and 1.63m/s for specimens without and with cryogenic hardening,

respectively.

Figure 4.2 Wear rate vs. sliding speed for specimens both with and without cryogenic

treatment austenitized at 1293K and tempering at 453 K for 1.8Ks.

4.3 Variation of the wear rate with sliding distance

In Fig. 4.3, the variation of the wear rate with sliding distance is shown for specimens

quenched, subzero treated at 223K and ultra-subzero treated at 93K. The wear rate as of

quenched specimens is larger than that of specimens after cold hardening and cryogenic

hardening at sliding distance of 200m. At sliding distance of 400 and 600m, the

specimens after cold treatment have almost the same wear rate as quenched specimens.

However, the specimens after cryogenic hardening have a smaller wear rate than as

quenched specimens and specimens after cold treatment for any sliding distance.

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Figure 4.3 Wear rate vs. sliding distance for specimen quenched, subzero treated at 223 K

and Ultra subzero treated at 93 K, austentization at 293 K and tempering at 453 K

4.4 X-Ray Diffraction Analysis

The volume fraction of retained austenite is plotted against the subzero treatment

temperature in Fig. 4.4 for specimens austenitized at 1293K and 1373K. The volume

fraction of retained austenite is 12% for as quenched specimens after austenization at a

1293K and approximately 6% for specimens after cold and cryogenic treatment.

However, it decreases with treating temperature going down for specimens’ austenitized

as 1373K. Cold treatments reduce the volume fraction of retained austenite drastically.

Nevertheless, cryogenic hardening reduces it slightly relative to cold treatment.

Figure 4.4 Volume fraction of retained austenite vs. subzero treatment temperatures

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Cryogenic Hardening

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Chapter-5

BENEFITS OF CRYOGENIC PROCESS

• Cryogenically treated materials show a marked increase in wear resistance

without any discernable change in dimensional or volumetric integrity.

• Machining of material is easier and cleaner.

• Redressing or regrinding treated tools removes less stock material resulting in

longer tool life.

• The material shows little or no change in yield or tensile strength.

• The treated material becomes less brittle, without a change in original

hardness.

• The most significant and consistent change is the increased toughness,

stability and wear resistance.

• The Process is also used extensively to relieve residual stresses.

• In abrasive wear situations, both the martensite formation and the fine carbide

formation work together to reduce wear. The additional fine carbide particles

help support the martensite matrix. This makes it more difficult to dig out

lumps of the material.

• Almost any kind of tool steel or dynamic part, for whatever application, will

exhibit some kind of life increase. As less tools or parts are needed, there is

substantial savings in money. Additional savings include less downtime and

short runs, less maintenance and change-over, which allows for lower

production costs.

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Chapter-6

APPLICATIONS

6.1 GUN BARRELS:

One of the truths about rifles and guns is their erratic shooting after heating up. The

Cryo-Accurizing process remedies this. Cryogenic hardening 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 cause 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.

Figure 6.1 Actual Ruger M77 group at 100m

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After cryo processing before cryo processing

6.2 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. Because 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

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.

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6.3 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. Crankshaft journals do not wear as readily.

4. Pistons can be run at higher levels of detonation.

5. Piston ring wear is reduced.

6. Cylinder wall wear is reduced.

7. Connecting rod failure is reduced.

8. Valves stems wear less.

9. Valve spring fatigue life is greatly improved.

10. Cylinder heads can be run at higher levels of detonation.

11. Cam shafts breakage is reduced.

12. Timing gears wear less.

13. Timing chains wear less and stretch less.

14. Push rods do not flex as much.

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Engine horsepower before cryo treatment

Engine horsepower after cryo treatment

Engine torque before cryo treatment

Engine torque after cryo treatment

Figure 6.2 Variation of Engine Horsepower/ Torque with speed before and after

cryogenic treatment

The engine gained a 5% increase in power with a peak increase of 13 horsepower after

the cryogenic treatment from NW Cryogenics. The most noticeable gain was at 7900

RPM with a net gain of 18.1 horsepower.

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Aluminum piston alloy structure

Figure 6.3 Cryogenically treated Non- Cryogenically treated

Magnified 3500x

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.

6.4 COMPACT DISCS:

Compact disks respond to cryogenic treatment. Understanding this is hard to believe,

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.

6.5 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, 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

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CONCLUSION

1. Cryogenic Processing is not a substitute for heat-treating.

2. It affects the entire volume of the material.

3. Cryogenic hardenings 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

4. Cryogenic Hardening increases wear resistance dramatically, especially at high

sliding speed.

5. The specimens after cryogenic hardening show a minimum of wear rate.

6. Unlike cold treatment, cryogenic hardening promotes preferential precipitation of fine

eta carbides.

7. The formation mechanism of eta-carbides is supposed to be as follows: iron or

substitution atoms expand and contract, and carbon atoms shift slightly due to lattice

deformation as a result of cryogenic hardening.

8. The mechanism that cryogenic hardening contributes to wear resistance is through the

precipitation of fine eta-carbide, which enhances strength, and toughness of

martensite matrix, rather than the removal of the retained austenite.

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BIBLIOGRAPHY

JOURNALS

1. The Microstructure In The Cryogenic Treated Bearing Steels Vasile Bulancea “The

“Gh.Asachi” Technical University of Iassy, Romania.

2. “Improving Component Wear Performance Through Cryogenic Treatment” Richard

N. Wurzbach, OMA-1, CLS William DeFelice Maintenance Reliability Group

Brogue, Pennsylvania.

3. “Metallurgical Investigation On Cryogenic Treated HSS Tool” s. sendooran

Department of Mechanical Engineering, Adhiyamaan college of Engineering, Hosur,

Tamil Nadu, India

4. Dynometer test results performed by Loynings Engine Service Inc., Portland, OR

5. Introduction to Cryogenic Engineering G. Perinić, G. Vandoni, Niinikoski, CERN

6. Metallurgical Changes In Steels Due To Cryogenic Processing & Its Applications

BOOKS

1. “Cryogenic Engineering: Fifty Years of Progress” Timmerhaus, Klaus D. and Reed,

Richard P.

2. Materials Science and Engineering Debdulal Das, Apurba Kishore Dutta, Kalyan

Kumar Ray