deep cryogenic treatment of tool steels-a review.pdf

HEAT TREATMENT\ OF METALSV 1996.2 p.40-42

Deep Cryogenic Treatment of Tool Steels: a ReviewD. N. COLLINS National Heat Treatment Centre, Ireland

As opposed to conventional sub-zero treatment to transform retained austenite, deep cryogenic treatment at liquid-nitrogen temperatures has been claimed to enhance the wear resistance o f tool steels by additional hitherto ill-defined phenomena. In summarising the current state o f knowledge, this review clarifies the underlying mechanisms.

INTRODUCTIONFrom time to time over the last few decades, interest has been shown in the effect of low temperatures during the heat treat­ment cycle on the performance of steels, particularly tool steels. Published articles range from the merely promotional, publicising various proprietary processes, to more detailed metallurgical investigations.To the casual reader, there may appear to be some confusion between the conflicting claims of some of the literature. The purpose of this review is to present a bibliography, to sum­marise the current state of knowledge in this area, and point to the underlying mechanisms involved. In a future article, results of work done on several grades of tool steel will be reported.In order to avoid confusion, a distinction will be drawn between "cold treatment", at temperatures down to about -80°C or thereabouts, and "deep cryogenic treatment", at about liquid-nitrogen temperatures (-196°C) since, as will be discussed later, these two temperature ranges result in different effects.

BASIC MICROSTRUCTURAL CONSIDERATIONSThe usual purpose of heat treating a tool steel is to achieve a microstructure consisting of a suitable distribution of carbides of desired type in a matrix of tempered martensite. In most high-carbon and alloy tool steels, significant amounts of austenite are retained after initial hardening, because the Mf may be substantially below room temperature. After comple­tion of the heat treatment cycle, the performance of the com­ponent will depend on the combined effect of each of the microstructural constituents. In order to make any sense of the effects of deep cryogenic treatment on properties, it is necessary to view the process in the context of phenomena relating to each of these main constituents.

Retained AusteniteAlthough the factors affecting retained austenite are numerous and interrelated, the phenomena are reasonably well under­stood:+ Alloying elements, especially carbon, in solution, strengthen

the austenite. More energy is thus required to effect the shearing mechanism to produce martensite. A greater degree of undercooling is thus required, lowering the Ms.

• Strong carbide-formers may tie up carbon as undissolved carbides, having little effect on Ms.

• Increasing austenitising temperature increases austenite grain size, and also alloy and carbon dissolution, further reducing Ms.

• The transformation of austenite to martensite during cool­ing is not time-dependent, but related to the degree of cool­ing below the Ms.

• Slow or interrupted cooling may allow stress relief and possibly diffusion to occur, reducing the driving force for

martensite formation, again reducing Ms.• Some alloying elements promote austenite stabilisation,

whilst others inhibit it.• In plain-carbon and low-alloy steels, retained austenite

transforms to bainite, or is stabilised, at relatively low tempering temperatures.

• In high-alloy steels, austenite remains untransformed at tempering temperatures up to about 450°C or higher, at which temperature it becomes "conditioned" by carbide precipitation, transforming to martensite (of lower carbon and alloy content) on cooling back to room temperature.

MartensiteA full treatment of the metallurgy of martensite is outside the scope of this article. In relation to deep cryogenic treatment, only a few points need to be noted:• Martensite is supersaturated with carbon which, during

tempering, precipitates out as carbides, the nature of which depend on alloy content and tempering temperature.

• The instability of martensite is associated with the strain energy relating to its dislocation/twin structure, and with interfacial energy associated with lath boundaries and martensite/retained austenite boundaries (when the austenite is present as thin inter-lath films).

• Carbon atoms segregate to dislocation sites and interfaces, and tend to cluster at such sites.

• At very low temperatures, the activation energy for carbon diffusion (and alloy diffusion) is too high to permit formation of carbide precipitates as in the final stages of tempering.

• Tempered-martensite embrittlement may result from cementite films precipitating from inter-lath austenite during tempering.

CarbidesAs with martensite, only a few points need to be noted here, with specific reference to deep cryogenic treatment:• The type of carbide formed during tempering depends

mainly on alloy content and tempering temperature.• Some carbides in the final microstructure will be those that

remained undissolved during the austenitising treatment.• The size and distribution of carbides precipitated out from

the martensite (or retained austenite) during tempering will be dependent on nucleation and growth phenomena, influenced in turn by a number of factors, including prior thermal history.

• Nucleation and growth are time-dependent.

Secondary HardeningThe phenomenon of secondary hardening in some high-alloy tool steels is caused by a combination of two main mecha­nisms:• Transformation of "conditioned" retained austenite to

martensite on cooling.• Precipitation of a fine distribution of alloy carbides

(especially the very hard, abrasion-resistant M2C and MC).Other strengthening/hardening mechanisms include solution hardening by the alloying elements, and strengthening due to the prevention of grain coarsening by some of the alloy carbides.

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PROPERTY IMPROVEMENTS CLAIMED FOR CRYOGENIC TREATMENTIn the various articles published over the years, a wide range of property improvements have been said to be achieved. These include:• Hardness. In many cases hardness increases of 1-3

Rockwell points have been claimed, although some authors report very little increase in hardness.

• Toughness. Claims for increases in toughness (usually unnotched Charpy) are not widespread.

• Wear resistance/more uniform wear pattern/better ground surface finish. One of the most prevalent claims is an increase in wear resistance (with or without a hard­ness increase). Some claims have also been made of improved uniformity of wear pattern, and also improved surface finish after grinding.

• Dimensional stability. This was the original purpose of cryogenic treatment, to stabilise dimensions by eliminating the possibility of spontaneous transformation of retained austenite subsequent to the final heat treatment.

• Intergranular corrosion resistance. One author claimed an improvement due to reduced grain-boundary diffusion.

Apparent ConfusionAt first sight, there appears to be much confusion and conflict between both the property improvements claimed, and also the mechanisms postulated to cause the effects. An added complication is that many of the authors do not specify the exact details of the treatments undertaken. Some of the apparent anomalies are:• Why do some authors imply that the chief improvement

is in hardness, due to the transformation of retained austenite, whilst others claim little change in hardness?

• Why, if the main mechanism is transformation of retained austenite, have slow cooling rates and long treatment times (24 hours or more) been specified?

• Many treatments are performed in the region of -80°C, whilst some are at-196°C.

AN EXPLANATION OF THE PHENOMENA INVOLVED IN DEEP CRYOGENIC TREATMENTIn order to resolve the apparent anomalies, a research project was undertaken at University College Dublin. Preliminary find­ings have been reported elsewhere, and a fuller report will appear in a subsequent article. The results, based on both cold- work (D2) and high-speed (ASP 23) tool steels indicate that there are two quite different phenomena or mechanisms involved. These two phenomena have distinctly different effects, and it is the confusion between them that has caused the apparently conflicting results in some of the literature.

Mechanism 1: Transformation of Retained AusteniteThis mechanism is well known, and is the result of cooling near or below the effective Mf. The vast majority of the austenite in the structure transforms to martensite with a resulting increase in hardness. The elimination of the retained austenite also stabilises the dimensions of the piece. This effect is largely complete for most steels at temperatures of between -80°C and -110°C, provided that the austenite has not been stabilised, by long holding times at ambient temperatures or above, prior to cryogenic treatment. The effect of this cryogenic treatment is:• an increase in hardness (the larger the amount of austenite

in the structure, the greater the hardness increase);• a reduction in toughness;• only a very modest, if any, improvement in wear resistance;• dimensional stability.

Mechanism 2: Low-temperature Conditioning of Martensite.Continued cooling of martensite, well below its formation

temperature (e.g. liquid-nitrogen temperatures for martensite formed at or above room temperature), and holding at the low temperature for sufficient time, promotes the formation of large numbers of very fine carbide particles on tempering. The result is an improvement in wear resistance and toughness, but little or no increase in hardness. (There is in fact an initial reduc­tion in hardness, but this is largely recovered after a sufficiently long holding time). The appearance of this fine carbide distribu­tion has been reported by a number of authors.The exact mechanism of this "low-temperature conditioning" of martensite is not yet fully understood. It is possible that the continued cooling increases the strain energy and instability of the martensite, and possibly also affects its dislocation structure as the lattice contracts. Given sufficient time at the low temperature, carbon atoms may migrate and cluster (albeit slowly at these temperatures; hence the long holding times necessary, e.g. 24 - 72 hours). On subsequent heating up to or above room temperature, these sites act as nuclei for the formation of the fine carbide particles observed in deep- cryogenically-treated steels.This mechanism has the most beneficial effect on work­pieces with least retained austenite (most martensite) in their structure, indicating that the effect is on martensite, not on the retained austenite. It also indicates that -196°C is not a low enough temperature to condition martensite formed at very low temperatures, as in the first stage of cold treatment. The effect is both temperature and time (holding time at the deep cryogenic temperature) dependent; the lower the temperature and the longer the holding time, the finer is the carbide distribu­tion and the greater the increase in wear resistance. The overall effect of this deep cryogenic treatment mechanism is:• a much greater number of fine carbide particles in the

microstructure;• a different partition of alloying elements between matrix

and carbides, compared with conventionally-treated steels;• an improvement in wear resistance;• an increase in toughness: it is possible that tempered-

martensite embrittlement is eradicated by one or both of these mechanisms, by either eliminating the inter-lath retained austenite, or causing nucleation of fine carbides rather than cementite films;

• little or no increase in hardness (if the most beneficial austenitising process is used, which is different from the conventional optimum);

• no secondary hardening occurs if tempered in the normal secondary-hardening temperature range.

TOTAL HEAT TREATMENT CYCLENeither cold treatment nor deep cryogenic treatment should be considered as an "add-on" to a conventional heat treatment cycle, if optimum properties are to be expected. In particular, careful selection of austenitising treatment is all-important:• If the objective is to maximise hardness, then a high

austenitising temperature should be selected, to maximise carbon and alloy solution, accepting the increased per­centage of retained austenite, which is then transformed to martensite by cold treatment. This can be followed by a single low-temperature temper. Using this type of treatment, it is quite feasible to achieve 65HRC from a D2 steel, for example (but at the loss of some toughness, it should be noted).

• If an increase in wear resistance is the desired objective, then this can be achieved by using a lower-than-normal austenitising temperature (to minimise the amount of retained austenite) and then deep-cryogenically treating. This results in a significant improvement in wear resistance, (for example at a hardness of 58HRC in a D2 steel), and toughness similar to or better than conventionally-treated samples.

Heat Treatment o f Metals 1996.2 41

There is thus a choice between hardness and wear resistance:cryogenic/deep cryogenic treatment can give signifi­cant improvements in either of these properties, but not both at the same time.

BIBLIOGRAPHYBayer H.E. Can I benefit from the use of low temperature treatment? Steel Processing. Oct. 1953,502-508.Nordquist W.N. Low-temperature treatment of metals. Tooling and Production. July 1953, 72-100.Morris V. Below zero chilling toughens metals, increases tool life. Machine and Tool Blue Book. Jan. 1955, 124-134.Andrews K.W. Empirical formulae for the calculation of some trans­formation temperatures. Journal o f the Iron & Steel Institute, 1965, Vol.203, No. 7, 721-727.Cryogenic quenching cuts warpage. Iron Age. May18, 1967,88-89. Moore C. Development of the BOC Ellenite process (cold treatment of metals with liquid nitrogen). Heat Treatment '73. The Metals Society, 1975, Book no. 163, 157-161.Barron R.F. Effect of cryogenic treatment on lathe tool wear. Progress in Refrigeration Science and Technology. 1973, Vol. 1,529-534.Barron R.F. Yes, cryogenic treatment can save you money! Here's why. Tappi Corrugated Containers Conference, Denver, Colorado, 1973,35-40.Bowes R.G. The theory and practice of sub-zero treatment of metals. HEAT TREATMENT OF METALS. 1974.1, Vol.1,29-32.Barron R.F. Cryogenic treatment produces cost savings for slitter knives. Tappi J. May 1974, Vol. 57, No. 5, 137-139.Taylor J. Cold plunge gives tools an extra lease of life. Metalworking Production. May 1978, 73-77.Barron R.F. and Mulhem C. Cryogenic treatment of AISI-T8 and C1045 steels. Advances in Cryogenic Engineering Materials. 1980, Vol. 26, 171-179.Miller P. Cryogenics: deep cold solves. Tooling and Production. 1980, Vol 45, No. 11,82-86.Popandopulo A.N. and Zhukova L.T. Transformations in high­speed steels during cold treatment. Metal Science and Heat Treatment. 1980, Vol.22, 708-710.Barron R.F. Cryogenic treatment of metals to improve wear resistance. Cryogenics. Aug. 1982, Vol.22, No.5,409-413.Keen A.R. Cryogenic treatment to improve wear resistance of steel by the "Cryotough" process. Metals Australasia. Aug. 1982, Vol. 14, No.7, 12-13,21.Alexandru I., Picos C. and Ailincai G. Contributions on the study of the increase of durability of the high-alloyed tool steels by thermal treatments at cryogenic temperatures. Proceedings o f the 2nd International Congress on Heat Treatment o f Materials (Florence, Sept.20/24, 1982), 573-579.Frey R. Cryogenic treatment improves properties of drills and P/M parts. Industrial Heating. Sept 1983, Vol. 50, No.9,21-23.Leonard L. Enhancing metals properties with supercold; fact or fancy. Materials Engineering. 1985, Vol. 102, No. 2,29-32.Sweeney T.P. Deep cryogenics: the great cold debate. Heat Treating. Feb. 1986, Vol. 18, No.2,28-32.Pillai R.M., Pai P.C. and Satyanarayana K. Deep cryogenic treatment of metals. Tool and Alloy Steels. June 1986,205-208.New dry refrigerent treatments improve characteristics and wear resistance of metal parts. Industrial Heating. Mar. 1986, Vol. 53, No.3, 36-38.Alexandru I., Coman G. and Bulancea V. The change of the substructure elements and the redistribution of the alloying elements by means of cryotreatments in alloy tool steels. Proceedings o f the 5th International Congress on Heat Treatment o f Materials (Budapest, Oct. 20/24, 1986), Vol.2,901-908.Gilmore V.E. Frozen tools. Popular Science. June 1987,64-67, 106-109. Reasbeck R.B. Improved tool life by the Cryotough treatment. Metallurgia. Apr. 1989, Vol. 56, No.4, 178-179.Alexandru I., Ailincai G. and Baciu C. Influence of cryogenic treatments on life of alloyed high-speed steels. Memoires et Etudes Sci. Rev. Metall. 1990, Vol 87, No.6, 383-389.Carlson E.A. Cold treating and cryogenic treatment of steel. ASM Handbook. 1991, Vol. 4,203-206.Paulin P. Mechanism and applicability of heat treating at cryogenic temperatures. Industrial Heating. Aug. 1992, Vol. 59, No.8,24-27.Albert M. Cutting tools in the deep freeze. Modern Machine Shop. Jan. 1992,54-61.Moore K. and Collins D.N. Cryogenic treatment of three heat-treated tool steels. Key Engineering Materials. 1993, Vol. 86-87, 47-54.

AUTHOR'S ADDRESSDavid Collins is with the National Heat Treatment Centre, University College Dublin, Department of Mechanical Engineering, Dublin 4, Ireland. 0

HEAT TREATMENT OF METALS

reviewSURFACE MODIFICATION TECHNOLOGIES VIIIT.S. Sudarshan and M. Jeandin (Eds.), Book 617, The Institute o f Materials, 1 Carlton House Terrace, London SW1Y 5DB. 1995. pp976. ISBN 0-901716-69-3. £100 ($200 to non-EU purchasers).

This volume contains 103 papers presented at the Eighth International Conference on Surface Modification Technologies held in Nice during September 1994. Previous confcrcnccs have similarly had proceedings published by the Institute of Materials. In this instance, the contents are divided into the following main subject areas:• Wear, • Advanced Coatings,• Advanced Investigation • Advanced Processes,

Techniques, • Modelling,• Lasers, • Thermal Spraying,• Ion Beam and Electron • Biomedical Applications,

Beam Techniques, • Corrosion and Miscellaneous.The range of subject matter covered by these ten headings is extremely wide, with much of the research work reported directed at assessing in detail the improvement in properties that can be obtained from the application of individual surface modification techniques to specific materials, both metallic and non-metallic. Other papers concentrate on the problems faced by particular components that suffer wear and/or corrosion, for example orthopaedic implants, and aim to establish surface treatments for improving life and reducing costs.Conventional heat treatments, as well-established and researched processes, barely rate a mention in this volume. This is quite under­standable, however, since many of the materials being studied are not suited to or do not respond to surface heat treatment techniques, or can only be treated by niche processes, such as laser treatments, to improve their surface characteristics. There are occasional papers of direct heat treatment interest, (covering such diverse topics as the surface hardening of copper alloy by laser boronising, electron beam surface melting to increase hardness and wear resistance of aluminium bronze, and the kinetics of gas nitriding of steel) but these are a distinct minority.The strength of surface engineering, as represented by this volume, lies in the diversity of processes available and under development, the ultimate aim being to obtain the best match between substrate, surface condition and the working environment.

C.G. Williams. S3

HEAT TREATMENT OF METALS

notes for contributorsHEAT TREATMENT OF METALS welcomes informative articles on all aspects of industrial practice and innovationin heat treatment.• MANUSCRIPTS, typewritten on one side of international A4 paper with double-line spacing, should be submitted to the Editor. A summary of up to 100 words should accompany each contribution. When prepared on a wordprocessor, manuscripts should also be supplied on disk (Microsoft Word 5).• ILLUSTRATIONS. Line drawings should consist of bold black lines and photographs should be of high quality with good contrast. Polaroid photographs and printed reproduc­tions can seldom be reproduced satisfactorily.• REFERENCES should be indicated in the text by a super- script(8) and in the bibliography as:8. Hick A.J. What's new in surface heat treatment? The

Metallurgist and Materials Technologist. Dec. 1979, Vol.11, No.12,685-691.

Authors are requested to use full journal titles or to abbreviate them according to the conventions of the "World List of Scientific Periodicals".• SI UNITS should be employed except where current prac­tice allows the retention of those in common use, e.g. °C not K. Previously-used units may follow the SI units, in brackets, if required.• PROOFS will not be sent to authors before publication.• 25 REPRINTS will be sent free of charge to each author. Additional reprints are obtainable: scale of charges available on application.

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