Cold and Cryogenic Treatment of Steel
Revised by F. Diekman, Controlled Thermal Processing
COLD TREATNG OF STEEL is widelyaccepted within the metallurgical profession asa supplemental treatment that can be used toenhance the transformation of austenite to mar-tensite and to improve stress relief of castingsand machined parts. Common practice identi-fies –84
 
C (–120
 
F) as the optimal tempera-ture for cold treatment. There is evidence,however, that cryogenic treatment of steel (alsoreferred to as deep cryogenic treatment, or DCT), in which material is brought to a temper-ature on the order of –184
 
C (–300
 
F),improves certain properties beyond theimprovement attained at cold treatment tem-peratures. This discussion explains the practicesemployed in the cold treatment of steel and pre-sents some of the results of using cryogenictreatment to enhance steel properties.
Cold Treatment of Steel
Cold treatment of steel consists of exposingthe ferrous material to subzero temperatures toeither impart or enhance specific conditions or properties of the material. Increased strength,greater dimensional or microstructural stability,improved wear resistance, and relief of residualstress are among the benefits of the cold treat-ment of steel. Generally, one hour of cold treat-ment for each 2.54 cm (1 in.) of cross section isadequate to achieve the desired results.All hardened steels are improved by a proper subzero treatment to the extent that there willbe less tendency to develop grinding cracksand therefore they will grind much more easilyafter the elimination of the retained austeniteand the untempered martensite.
Hardening and Retained Austenite 
Whenever hardening is to be done duringheat treating, complete transformation fromaustenite to martensite generally is desired prior to tempering. From a practical standpoint, how-ever, conditions vary widely, and 100% trans-formation rarely, if ever, occurs. Cold treatingmay be useful in many instances for improvingthe percentage of transformation and thus for enhancing properties.During hardening, martensite develops as acontinuous process from start (M
s
) to finish(M
) through the martensite formation range.Except in a few highly alloyed steels, martens-ite starts to form at well above room tem-perature. In many instances, transformationessentially is complete at room temperature.Retained austenite tends to be present in vary-ing amounts, however, and when consideredexcessive for a particular application, must betransformed to martensite and then tempered.
Cold Treating versus Tempering.
 Immedi-ate cold treating without delays at room temper-ature or at other temperatures during quenchingoffers the best opportunity for maximum trans-formation to martensite. In some instances,however, there is a risk that this will causecracking of parts. Therefore, it is important toensure that the grade of steel and the productdesign will tolerate immediate cold treatingrather than immediate tempering. Some steelsmust be transferred to a tempering furnacewhen still warm to the touch to minimize thelikelihood of cracking. Design features such assharp corners and abrupt changes in section cre-ate stress concentrations and promote cracking.In most instances, cold treating is not donebefore tempering. In several types of industrialapplications, tempering is followed by deepfreezing and retempering without delay. For example, such parts as gages, machineways,arbors, mandrils, cylinders, pistons, and balland roller bearings are treated in this manner for dimensional stability. Multiple freeze-drawcycles are used for critical applications.Cold treating also is used to improve wearesistance in such materials as tool steels,high-carbon martensitic stainless steels, andcarburized alloy steels for applications in whichthe presence of retained austenite may result inexcessive wear. Transformation in service maycause cracking and/or dimensional changes thatcan promote failure. In some instances, morethan 50% retained austenite has been observed.In such cases, no delay in tempering after coldtreatment is permitted, or cracking can developreadily.
Process Limitations.
 In some applications inwhich explicit amounts of retained austenite areconsidered beneficial, cold treating might bedetrimental. Moreover, multiple tempering,rather than alternate freeze-temper cycling,generally is more practical for transformingretained austenite in high-speed and high-car-bon/high-chromium steels.
Hardness Testing.
 Lower-than-expectedRockwell C hardness (HRC) readings may indi-cate excessive retained austenite. Significantincreases in these readings as a result of coldtreatment indicate conversion of austenite tomartensite. Superficial hardness readings, suchas HR15N, can show even more significantchanges.
Precipitation-Hardening Steels.
 Specifica-tions for precipitation-hardening steels mayinclude a mandatory deep freeze after solutiontreatment and prior to aging.
Shrink Fits.
 Cooling the inner member of acomplex part to below ambient temperaturecan be a useful way of providing an interfer-ence fit. Care must be taken, however, to avoidthe brittle cracking that may develop when theinner member is made of heat treated steel withhigh amounts of retained austenite, which con-verts to martensite on subzero cooling.
Stress Relief 
Residual stresses often contribute to part fail-ure and frequently are the result of temperaturechanges that produce thermal expansion andphase changes, and consequently, volumechanges.Under normal conditions, temperature gradi-ents produce nonuniform dimensional and vol-ume changes. In castings, for example,compressive stresses develop in lower-volumeareas, which cool first, and tensile stressesdevelop in areas of greater volume, which coollast. Shear stresses develop between the twoareas. Even in large castings and machinedparts of relatively uniform thickness, the sur-face cools first and the core last. In such cases,stresses develop as a result of the phase (vol-ume) change between those layers that trans-form first and the center portion, whichtransforms last.When both volume and phase changes occur in pieces of uneven cross section, normal con-tractions due to cooling are opposed by trans-formation expansion. The resulting residualstresses will remain until a means of relief is
 ASM Handbook 
, Volume 4A,
 Steel Heat Treating Fundamentals and Processes
J. Dossett and G.E. Totten, editorsCopyright
#
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applied. This type of stress develops most fre-quently in steels during quenching. The surfacebecomes martensitic before the interior does.Although the inner austenite can be strained tomatch this surface change, subsequent interior expansions place the surface martensite under tension when the inner austenite transforms.Cracks in high-carbon steels arise from suchstresses.The use of cold treating has proved beneficialin stress relief of castings and machined parts of even or nonuniform cross section. Features of the treatment include:
Transformation of all layers is accomplishedwhen the material reaches –84
 
C (–120
 
F).
The increase in volume of the outer martens-ite is counteracted somewhat by the initialcontraction due to chilling.
Rewarm time is controlled more easily thancooling time, allowing equipment flexibility.
The expansion of the inner core due to trans-formation is balanced somewhat by theexpansion of the outer shell.
The chilled parts are handled more easily.
The surface is unaffected by lowtemperature.
Parts that contain various alloying elementsand that are of different sizes and weightscan be chilled simultaneously.
Advantages of Cold Treating 
Unlike heat treating, which requires that tem-perature be controlled precisely to avoid rever-sal, successful transformation through coldtreating depends only on the attainment of theminimum low temperature and is not affectedby lower temperatures. As long as the materialis chilled to –84
 
C (–120
 
F), transformationwill occur; additional chilling will not causereversal.
Time at Temperature.
 After thorough chill-ing, additional exposure has no adverse effect.In heat treating, holding time and temperatureare critical. In cold treatment, materials of dif-ferent compositions and of different configurationsmay be chilled at the same time, even thougheach may have a different high-temperature trans-formation point. Moreover, the warm-up rate of a chilled material is not critical as long asuniformity is maintained and large temperaturegradient variations are avoided.The cooling rate of a heated piece, however,has a definite influence on the end product. For-mation of martensite during solution heat treat-ing assumes immediate quenching to ensurethat austenitic decomposition will not result inthe formation of bainite and cementite. In largepieces comprising both thick and thin sections,not all areas will cool at the same rate. As aresult, surface areas and thin sections may behighly martensitic, and the slower-cooling coremay contain as much as 30 to 50% retainedaustenite. In addition to incomplete transforma-tion, subsequent natural aging induces stressand also results in additional growth aftemachining. Aside from transformation, no other metallurgical change takes place as a result of chilling. The surface of the material needs noadditional treatment. The use of heat frequentlycauses scale and other surface deformations thatmust be removed.
Equipment for Cold Treating 
A simple home-type deep freezer can be usedfor transformation of austenite to martensite.Temperature will be approximately –18
 
C(0
 
F). In some instances, hardness tests canbe used to determine if this type of cold treatingwill be helpful. Dry ice placed on top of thework in a closed, insulated container also iscommonly used for cold treating. The dry icesurface temperature is –78
 
C (–109
 
F), butthe chamber temperature normally is approxi-mately –60
 
C (–75
 
F).Mechanical refrigeration units with circulat-ing air at approximately –87
 
C (–125
 
F) arecommercially available. A typical unit has thesedimensions and operational features:
Chamber volume, up to 2.7 m
3
(95 ft
3
)
Temperature range, 5 to –95
 
C (40 to –140
 
F)
Load capacity, 11.3 to 163 kg/h (25 to360 lb/h)
Thermal capacity, up to 8870 kJ/h(8400 Btu/h)Although liquid nitrogen at –195
 
C (–320
 
F)may be employed, it is used less frequentlythan any of the previous methods because of its cost.
Cryogenic Treatment of Steels
Cryogenic treatment, also referred to as cryo-genic processing, deep cryogenic processing,deep cryogenic treatment (DCT), cryogenictempering, and deep cryogenic tempering, is adistinct process that uses extreme cold to mod-ify the performance of materials. (The use of the word
 tempering
 is a misnomer, because thisis not a tempering process.)The process is differentiated from cold treat-ment by the use of lower temperatures, thepresence of distinct time/temperature profiles,and its application to materials other than steel(Ref 1). Cryogenic treatment has been in exis-tence only since the late 1930s, making it a rel-atively new and emerging process. The latedevelopment of the process is mainly due tothe fact that cryogenic temperatures have beenavailable in useful commercial quantities onlysince the early 1900s.Cryogenic treatment can provide wear-resis-tance increases several times those created bycold treatment with hardened steels (Ref 2).The process is not confined to hardened steels,but also shows results with most metals,cemented carbides, and some plastics (Ref 3– 5). Use of the process on metals other than steelproduces similar affects as with steel. Results of the process include relief of residual stresses(Ref 6); reduced retained austenite (in hardenedsteel); the precipitation of fine carbides in fer-rous metals (Ref 7, 8); and increased wear resistance, fatigue life, hardness, dimensionalstability, thermal and electrical conductivity,and corrosion resistance (Ref 9).What are cryogenic temperatures? The scien-tific community generally defines cryogenictemperatures as temperatures below
 
150
 
C(
238
 
F, or 123 K) (Ref 10). This, admittedly,is an artificial upper limit. Temperatures usedpresently in cryogenic treatment are generally –185
 
C (–300
 
F, or 89 K). These are tempera-tures easily reached with liquid nitrogen. Somework is being done with liquid helium at tem-peratures down to –268
 
C (–450
 
F, or approx-imately 6 K).Cryogenic treatment was made easier toachieve and more successful by the develop-ment of microprocessor-based temperature con-trols in the 1960s and 1970s and by thepioneering research by Randall Barron of Louisiana Tech University. Research into theprocess has been accelerating. The CryogenicSociety of America maintains a database of peer reviewed research papers (Ref 11).
Cryogenic Treatment Cycles 
One distinct difference from cold treatmentis that cryogenic processing requires a slowdrop in temperature in order to reap all benefitsof the process. The ramp down in temperatureusually is on the order of 0.25 to 0.5
 
C/min(32.5 to 32.9
 
F/min). The object of this slowramp down is to avoid high-temperature gradi-ents in the material that can create harmfulstresses, and to allow time for the crystal latticestructure to accommodate the changes that areoccurring.Typical cryogenic treatment consists of aslow cool-down from ambient temperature toapproximately –193
 
C (–315
 
F), where it isheld for an appropriate time. Hold periodsrange from 4 to 48 h depending on the material.At the end of the hold period, the material isbrought back to ambient temperature at a rateof approximately 2.5
 
C/min (36.5
 
F/min).The temperature-time plot for this cryogenictreatment cycle is shown in Fig. 1. By conduct-ing the cool-down cycle in gaseous nitrogen,temperature can be controlled accurately,and thermal shock to the material is avoided.Single-cycle tempering usually is performedafter cryogenic treatment to improve impactresistance, although double or triple temperingcycles sometimes are used.It is worthy of note that most time-tempera-ture profiles have been empirically developed.Some research is being done to optimize theprofiles for individual steels. For instance, someresearch indicates the holding time for AISI
Cold and Cryogenic Treatment of Steel / 383
 
T42 steel should not be longer than 8 h(Ref 12). In contrast, research indicates thatthe hold time should be 36 h for AISI D2 (Ref 13). This indicates there is much research tobe done to optimize the process for all materi-als. Research into the optimal ramp downtimes, hold times, and ramp up times wouldmaximize the effect of the process and mini-mize the time needed to accomplish its results.There are several theories behind the effectsof cryogenic treatment. One theory involvesthe more nearly complete transformation of retained austenite into martensite. Thistheory has been verified by x-ray diffractionmeasurements. Another theory is based onthe strengthening of the material broughtabout by precipitation of submicroscopic car-bides as a result of the cryogenic treatment(Ref 7, 8). Allied with this is the reductionin internal stresses in the martensite that hap-pens when the submicroscopic carbide precip-itation occurs. A reduction in microcrackingtendencies resulting from reduced internalstresses also is suggested as a reason foimproved properties. Studies also show reduc-tion in residual stresses. Another theory postu-lates that the extreme cold reduces thefree energy of the crystal structure and createsa more orderly structure. Another area to con-sider is the basic effect of cold on the crystalstructure of metals. Point defects in the crystalstructure are temperature dependent. Lower-ing the temperature of the crystal structurewill cause the number of point defects in thecrystal structure to change according to:
 N 
d
 ¼
 N 
 exp
 
 E
d
=
kT 
ð Þ
where
 N 
d
 is the number of defects present,
 N 
 isthe total number of atomic sites,
 E
d
 is the acti-vation energy needed to form the defect,
 
 isthe Boltzmann constant, and
 
 is the absolutetemperature. Reducing the temperature at asuitably slow rate drives the point defectsout of the structure to the grain boundaries. Inother words, the solubility of vacancies andother point defects in the matrix drops. Thiscould account for some of the effects seenin DCT.In the past, the absence of a clear-cut under-standing of the mechanism by which cryogenictreatment improves performance had hamperedits widespread acceptance by metallurgists.Some confusion has arisen from the fact thatthere are a number of different effects onmetals, many of which cannot be seen in simplemicrostructural examination of the materialwith a light microscope. The lack of easilydetected microstructural changes led many todiscount the process. Another reason was thegenerally accepted belief that nothing happensto solid objects as the temperature drops.Extreme cold has been available on earth onlyfor about 100 years. Understanding of materialsscience developed with the observation thatheat changes properties. Much of the earlyresearch was centered around determiningwhether or not cryogenic treatment actuallyprovided the advantages claimed. Because theearly research and actual industry usage haveproven the validity of the process, researchnow is turning to determine why the resultsare seen and how to maximize those results.
Uses of Deep Cryogenic Treatment 
Deep cryogenic treatment is used in manyways to reduce wear. It is in common use tocontrol distortion of metal objects, modify thevibrational characteristics of metals, increasefatigue life, reduce abrasive wear, and reduceelectrical resistance. It is safe to say the appli-cations for this process are extremely broad.The process is in commercial use for high-speed steel (HSS) and carbide cutting tools,knives, blanking tools, forming tools, and more.Research as far back as 1973 indicates that deepcryogenic processing results in over three timeslife increase in end mills, 82 times life improve-ment in punches, over two times the life inthread dies, six times the life in copper resistanceelectrodes, six times the life in progressive dies,and over four to five times the life in broaches(Ref 2). Research estimates a 50% reductionin tooling costs with H13 and M2 steels thathave been deep cryogenically treated (Ref 14).Other studies have shown that DCT increasesabrasion resistance of cast iron. Cast iron brakerotors consistently show a three to five timeslife increase when tested to SAE2707 brakedynamometers (Ref 15). This has been validatedagainst real-world experience in passenger cars,racing cars, trucks, and mining vehicles.Deep cryogenic treatment also has beenproven to create a phase change in cementedcarbide (Ref 5). A study by the National Aero-nautics and Space Administration (NASA)proved the release of residual stresses in weldedaluminum (Ref 6), and other studies proveincreases in fatigue life in steel springs (Re16) and in load capacity of gears (Ref 17).Deep cryogenic treatment is used in the auto-motive racing industry to increase life in virtuallyevery engine component. Drive line componentssuch as transmission and differential gears, sus-pension springs, torsion bars, axles, suspensionmembers, and, of course, brakes are treated.Deep cryogenic treatment also is in commer-cial use by musical instrument makers. YamahaWind Instruments has done extensive testing of DCT and offers the process on its wind instru-ments (Ref 18). There is much activity in thehigh-performance stereo industry in treatingvacuum tubes, wire, power cords, vacuumtubes, transformers, connectors, and more.
Equipment for Cryogenic Treatment 
All cryogenic treatment equipment is com-prised of a thermally insulated container andsome means of extracting the latent heat of the payload to reach the desired low tempera-ture. In most cases the insulation is a solidmaterial that contains small closed cells of trapped still air. The thermal conductivity of such insulation essentially is that of still, non-convecting air, assuming that the solid material
Fig. 1
 Plot of temperature vs. time for the cryogenic treatment cycle. Tempering may or may not be necessary,depending on the material treated. Some materials require multiple tempering cycles. Some companies arenow treating materials down to –268
 
C (–450
 
F, or 6 K)
384 / Steel Heat Treatment Processes
 
that encloses the air pockets is of thin cross sec-tion and low conductivity. \Examples are poly-urethane foam, aerogel, and expanded glassfoam. Fifteen centimeters (6 in.) of any of thesewill conduct approximately (15 Btu/h.ft
2
)across a temperature differential of 204
 
C(400
 
F), which exists between the interior of a refrigerator at –195
 
C (–320
 
F) and ambienttemperature of 26
 
C (80
 
F).These solid insulating materials are relativelyinexpensive and, in the case of foamed-in-placepolyurethane, can readily fill irregularly shapedcavities. They all suffer from one importantdrawback: temperature cycling establishes atemperature gradient across the insulating slabthat results in differential contraction in thematerial. Repeated temperature cycles ulti-mately result in fatigue cracking of the insula-tion. Energy expenditure to sustain thetemperature difference goes up, and tempera-ture uniformity within the refrigerator maydeteriorate.The use of vacuum insulation in cryoproces-sor design avoids these problems. A vacuuminsulated container consists of two concentricshells, usually cylindrical, separated by a smalldistance relative to their diameters, which are joined around the perimeter of one end of theshells. The space between the shells containsreflective insulation and is evacuated to a pres-sure of approximately 533 Pa (10
 –6
torr). Thisessentially eliminates heat flow by conductionand convection because most of the conductingor convecting gas has been removed. Heat gainvia infrared radiation is minimized by multiplereflective layers placed in the vacuum space.Heat flow across a vacuum-insulated space,given a temperature difference across the wallsof 204
 
C (400
 
F), is (0.008 Btu/h.ft
2
), a factor of 1900 better than solid insulation of 15 cm(6 in.) thickness (Ref 19). The principle modeof heat transmission into the interior of a vac-uum-insulated container is metallic conductionthrough the perimeter that joins the inner andouter shells.In addition to providing a barrier to heatflow relative to solid insulation, the vacuum-insulated vessel is immune to thermal cyclingfatigue. Additionally, the vacuum-insulatedvessel can sustain elevated operating tempera-tures far in excess of that permissible with theuse of polyurethane. This permits the post-refrigeration tempering of components in onedevice, eliminating the need for a separate tem-pering oven.Heat extraction from the payload is effectedby the phase change of a low-boiling-pointfluid. If mechanical refrigeration is used, ahigh-pressure fluid is allowed to expand andbecome a gas within an evaporator coil insidethe insulated space. The evaporator coil is aheat exchanger that absorbs heat from the pay-load via convection, natural or forced, withinthe chamber. This ensures the relatively slowcooling of the payload and avoids thermalshock resulting from too rapid cooling. Rapidcooling can cause shrinkage of the outside of the cooled component while the relativelywarm interior does not shrink. Tensile stressinduced this way can lead to cracking or the ini-tiation of residual stress, especially at sharpedges. Reaching cryogenic temperatures bymechanical refrigeration for industrial size pay-loads requires multistage refrigeration. Theseare very expensive machines to build andmaintain.Fortunately, liquid nitrogen is abundant,readily available, and relatively inexpensive. Ithas a boiling point of –196
 
C (–321
 
F) anda heat of vaporization of approximately150 Btu/liter. It is produced in huge industrialgas production facilities and delivered to thefacility where the expansion and phase changeoccurs, free of the capital and maintenanceexpense demanded by in-house mechanicalrefrigerators.Two other approaches have been tried buthave difficulties: a hybrid of mechanicalrefrigeration and LN
2
 (liquid nitrogen) cooling,and a controlled immersion of components intoLN
2
.The hybrid approach uses mechanical refrig-eration to do an initial cooling of the payload tosome sub-atmospheric temperature that is wellabove the desired cryogenic range. At that pointa spray of LN
2
 droplets is showered onto thepayload to bring the temperature down to thedesired point. Unless the mechanical refrigera-tion has sufficient Btu removal rate, the payloadwill be substantially warmer than indicated bythe thermocouple that monitors chamber tem-perature. This causes the LN
2
 spray to comeon prematurely, with the resultant rapid coolingof parts and the increased possibility of cracking.The controlled immersion of componentsinto LN
2
 has been tried in two versions: thepayload is lowered slowly into a pool of LN
2
,or a chamber is slowly flooded with LN
2
 sothe liquid level rises to and eventually coversthe payload.Both versions suffer from a serious weaknessarising from the effects of fundamental physics.First, the temperature gradient above a pool of LN
2
 is very steep. Second, the rate of heattransport between warm solid and a cold gasat –195
 
C (–320
 
F) is much slower than therate between the same warm solid and a liquidat –195
 
C. Therefore, in either of the aboveversions, a slow decrease in the distance separ-ating the part and the liquid does not ensure aslow rate of cooling of the part. The risk of thermal shock is increased by the steep temper-ature gradient above the liquid and the suddenincrease in the heat transfer rate when liquidcontact is made.Cryogenic treatment is a process that holdsgreat promise to modify and improve productsin many markets, including reducing wear andextending the service life of many components.Continuing research efforts are being underta-ken to understand the underlying science of DCT so process improvements can be madeand the technology advanced.
ACKNOWLEDGMENT
Article revised and updated from E.A. Carl-son, Cold Treating and Cryogenic Treatmentof Steel,
 Heat Treating
, Vol 4,
 ASM Handbook 
,ASM International, 1991, p 203–206.
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 A Study of the Effects of Cryogenic Treatment on Tool Steel Properties
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 Fall Corrugated Containers Conference
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 J. LowTemp. Phys
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 Adv. Mater. Process
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 The MacMillan Encyclopedia Of Chemistry
,New York, 2002, http://cryogenics.nist.gov/AboutCryogenics/about%20cryogen-ics.htm (accessed July 17, 2013)11.
 Cryogenic Treatment Database
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, Nov2010, www.cryogenictreatmentdatabase.org/article/influence_of_cobalt_on_the_cryogenically_treated_w-mo-v_high_speed_steel/ (accessed July 17, 2013)13. D. Das, A.K. Dutta, and K.K. Ray, Influ-ence of Varied Cryotreatment on the Wear Behavior of AISI D2 Steel,
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