CRYOGENICTREATMENT OFTOOL STEELSPossible mechanisms a re suggested for the w e a r resistance o r hardness im provem ent obtained b y conventional cold treatment a n d deep cryogenic treatment a t liquid-nitrogen temperature. The austenitizing tem perature plays a m a jo r role.

by David N. Collins*N a tio n a l H eat Treatm ent Centre U niversity C o llege Dublin Ire land

* Member o f ASM International

^ ver the past few decades, interest has been shown in the effect of low-temperature treatment on the performance of steels, particularly tool steels. Published articles have ranged from the merely

^ promotional, publicizing various proprietary processes, to more

detailed metallurgical investigations.The casual reader may be confused

by the conflicting claims made in some of the literature. The purpose of this article is to summarize the current state of knowledge in this area, and to point to the underlying mechanisms involved. To avoid confusion, a distinction will be made between "cold treatment," at temperatures down to about -80C, and "deep cryogenic treatment," at about liquid-nitrogen temperature (~196C), since, as will be discussed, these two temperature ranges produce different effects.

Reported property gainsIn the various articles published

over the years, a wide range of property improvements have been daimed for steels that were low-temperature treated. These include:

Hardness: In many cases, hardness increases of 1 to 3 points HRC have been daimed, although some authors report very little increase in steel hardness.

Toughness: Claims for increases in toughness (usually unnotched Charpy) have been made, but are not widespread.

Wear resistance: One of the most prevalent daims is an increase in wear resistance (with or without a hardness increase). Some daims also have been made for a more uniform wear pattern and an improved surface finish after grinding.

Dimensional stability: This was the original purpose of cryogenic treatment to stabilize part dimensions by eliminating the possibility 6f spontaneous transformation of retained austenite subsequent to the final heat treatment.

Intergranular corrosion resistance. One author has daimed an improvement in intergranular corrosion resistance due to reduced grain- boundary diffusion.

Why the apparent confusion?At first sight, there appears to be

much confusion and conflict between the reported property improvements and the mechanisms postulated as being responsible for these effects. An added complication is that many authors do not provide the exact details of the low-temperature treatments that were used. Some of the apparent anomalies:

Why do some authors imply that the chief improvement is in hardness, due to the transformation of retained austenite, while others claim little change in hardness?

Why, if the main mechanism is transformation of retained austenite (not time-dependent), have slow cooling rates and long treatment times 1 (24 hours or more) been specified?

Many treatments are performed at a temperature around -80C, while others axe performed at -196C.

Microstructure and propertiesThe properties of an alloy, be it a

tool steel or any other, are dependent on its microstructure. If we are to understand how any factor affects properties, we must look at how this factor affects the microstructure of the alloy.

Continued

H23ADVANCED MATERIALS & PROCESSES 1 2 /9 8

Fig. 1 The effect o f a prolonged holding time on the transformation of retained austenite in AISID2 austenitized at 1075C. Ifcryo- genically treated immediately, almost all the retained austenite will transform at about -80C, ivhereas if held for a long time at room temperature, a much lower cryogenic treatment temperature is needed, and even then not all of the austenite transforms, as evidenced by the lower hardness achieved.

perature on the hardness o f a.D2 tool steel. There is a sharp increase in hardness doivn to the Mp as retained austenite transforms. Cooling below this temperature, however, actually results in a slight decrease in hardness. The data are for cryogenic treatment times of 38,120,379, and 1200 minutes.

Effects explained by two quite d iffe ren t

mechanisms.

Throughout history, practice has normally led understanding, but to effectively control and develop technology, a basic understanding is needed. For example, for centuries the effects of quenching and tempering on steel were known, but it was only after a basic metallurgical understanding was gained that these processes could be developed and refined.

This is probably still the case with cryogenic treatment: practice has been leading the way, with understanding lagging behind. Fortunately, information is now available to help fill in this knowledge gap.

Microstractural considerationsThe usual purpose of heat treating

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 the temperature' at which transformation of austenite to martensite is completed during cooling 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. To make any sense of the effects of deep cryogenic treatment on the properties of the steel, it is necessary to view the process in the context of phenomena relating to each of these main microstructural constituents.

Retained austenite: Although the factors affecting retained austenite are numerous and interrelated, the phenomena involved 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 the temperature at which transformation of austenite to martensite starts during cooling.

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

Increasing austenitizing temperature increases austenite grain size, and also alloying element 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. This hold-time effect is illustrated in Fig. 1.

Some alloying elements promote austenite stabilization, while others inhibit it.

In plain-carbon and low-alloy steels, retained austenite transforms to bainite, or is stabilized, at relatively low tempering temperatures.

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

Martensite: A full treatment of the metallurgy of martensite is outside the scope of this article. However, 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 alloying element content and tempering temperature.

The instability of martensite is associated with the strain energy relating to its dislocation/twin structure, and with interfadal 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 alloying element 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.

Carbides: As with martensite, only a few points need to be noted with respect to deep cryogenic treatment:

The type of carbide formed during tempering depends mainly on alloying element content and tempering temperature.

Some carbides in the final microstructure will be those that remained undissolved during the

H 24 ADVANCED MATERIALS & PROCESSES 1 2 /9 8

austenitizing treatment of the steel. The size and distribution of car

bides 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 hardening: The phenomenon of secondary hardening in some high-alloy tool steels is caused by a combination of two main micro- structural mechanisms:

Transformation of "conditioned" retained austenite to martensite on cooling.

Precipitation of a fine distribution of alloy carbides (especially the very hard and abrasion-resistant M2C and MC carbides, where "M" is a metallic element).

Other strengthening/hardening mechanisms that can occur in high- alloy tool steels include solution hardening by the alloying elements, and strengthening due to the prevention of grain coarsening by some of the alloy carbides.

Explanation of the phenomenaTo resolve the apparent anomalies,

a research project was undertaken at University College Dublin. Some findings have been reported elsewhere (see selected references). Results based on studies of both cold-work and high-speed tool steels indicate that there are two quite different phenomena or mechanisms involved: transformation of retained austenite and low-temperature conditioning of martensite. These phenomena have distinctly different effects, and it is probably the confusion between them that has caused the apparently conflicting results in some of the technical literature.

The cold-work steel was AISID2 (UNS T30402). The high-speed steel was ASP 23, a powder metallurgy steel made by Erasteel Kloster AB of Sweden using the ASEA-Stora process (ASP). It is equivalent to AISI M3, Class 2 (UNS T11323).

Transformation of austeniteThe first mechanism is the transfor

mation of retained austenite. This mechanism is well known, and is the result of cooling near or below the ef

fective 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 stabilizes the dimensions of the workpiece. This effect is largely complete for most steels at temperatures between -80 and -130C (Fig. 2), provided that the austenite has not been stabilized prior, to cryogenic treatment by long holding times at ambient temperatures or above.

The effects of this treatment are: An increase in hardness. (The

larger the amount of austenite in the microstructure, the greater the hardness increase.)

A reduction in toughness. Only a very modest, if any, im

provement in wear resistance. Dimensional stability. The absence of a secondary hard

ening peak on the tempering curve following cryogenic treatment (Fig. 3).

Conditioning of martensiteThe second mechanism is the low-

temperature conditioning of martensite. Continued cooling of martensite to well below its formation temperature such as, to Kquid-nitrogen tem-

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ondary hardening. The tempering curves for cryogenically treated (~196C) and untreated (20C) D2 steel illustrate the disappearance of the secondary hardening peak in the cryogenically treated samples, and the convergence o f the hardnesses o f treated and untreated samples at high tempering temperatures.

Fig. 4 Development o f fine carbides after cryogenic treatment. This graph illustrates both the time-dependence (hours at -196C) of the cryogenic treatment, and also the effect of tempering temperature. The data indirectly imply that the carbides actually form during tempering, but are nucleated at the cryogenic treatment temperature.

Fig. 5 Effect of fine carbide count on wear rate. Deep cryogenic treatment increases the number o f fine carbides, which play a major role in improving wear resistance.

H 26

perature 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 car-

resistance (Fig. 5) 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 and alloying element atoms may migrate and cluster (albeit slowly at these temperatures, which may explain the requirement for long, 24 to 72 hour holding times). On subsequent heating, these sites act as nuclei for the formation of the fine carbide particles observed in deep cryogenically treated steels. It has also been reported that the composition (alloying element content) of the carbides subsequently formed on tempering may also be influenced by this treatment.

This mechanism has the most beneficial effect on workpieces having the least retained austenite (most martensite) in their microstructure, indicating that the effect is on the martensite, not on the retained austenite. It also indicates that -196C 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- dependent (holding time at the deep cryogenic temperature) 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 effects of this deep cryogenic treatment mechanism are:

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 of the steel.

Fig. 6 Effect o f austenitizing temperature and cryogenic treatment on hardness. Peak hardness ofnoncryogenically treated D2 (20C) is reached when the austenitizing temperature is'about 1030C. Above this temperature, increasing amounts of retained austenite reduce hardness. When cryogenic treatment at -196C is used to transform this retained austenite, higher hardnesses can be obtained at higher austenitizing temperatures.

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 austenitizing process is used, which is different from the conventional optimum process).

No secondary hardening occurs if the workpiece is tempered in the normal secondary-hardening temperature range.

Key role of austenitizingNeither cold treatment nor deep

cryogenic treatment should be considered as an "add-on" to a conventional heat treatment cyde, if optimum properties are to be expected. In most of the metallurgical investigations to date, the greatest benefit has been derived when cryogenic treatment has been inserted between hardening (quenching) and tempering. Cryogenic processing of fully heat treated workpieces, already multiple tempered at high temperature, is not as effective as treating immediately after hardening. This is hardly surprising, since at this stage there is little or no retained austenite, and the martensite will already be fully tempered, with a fairly stable distribution of alloy carbides in place. ContinuedAn increase in toughness. It is

ADVANCED AAATERIALS & PROCESSES 1 2 /9 8

Fig. 7 Effect o f austenitizing temperature and cryogenic treatment on wear rate. In contrast to the effect on hardness (Fig. 6), improved wear resistance (lower wear rate) is obtained when cryogenically treating samples of D2 that were austenitized at lower temperatures. (More martensite in the as-quenched microstructure leads to more fine carbides in the final microstructure after cryogenically treating and tempering:)

Careful selection of the austenitizing treatment is all-important:

If the objective is to maximize hardness, select a high austenitizing temperature to maximize carbon and alloying elements in 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-tem- perature temper. Using this type of treatment, it is quite feasible to achieve 65 HRC from a D2 steel (Fig. 6), for example (but at the loss of some toughness, it should be noted).

If the objective is to increase wear resistance, select a lower-than-normal austenitizing temperature to minimize the amount of retained austenite, and then deep cryogenically treat (Fig. 7). This results in a significant improvement in wear resistance at the same hardness level (for example, at a hardness of 58 HRC in a D2 steel), and toughness similar to or better than conventionally treated samples.

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

For more information: Mr. Collins is director, National Heat Treatment Centre, Dept, of Mechanical Engineering, University College Dublin, Engineering Building,

Belfield, Dublin 4, Republic of Ireland; tel: +353-1-7061745 or 7061760; fax: +353-1- 7061736; e-mail: david.collins@ucd.ie or heat.treatment@ucd.ie.

Selected referencesThese papers deal with the metallur

gical effects of deep cryogenic treatment of tool steels. "The Theory and Practice of Sub-Zero Treatment of Metals," by R.G. Bowes: Heat Treatment Of Metals, Vol. 1, No. 1,1974, p. 29-32. "Transformations in High-Speed Steels During Cold Treatment," by A.N. Popan- dopulo and L.T. Zhukova: Metal Science and Heat Treatment, Vol. 22, 1980, p. 708-710. "Cryogenic Treatment of Metals to Improve Wear Resistance," by R.F. Barron: Cryogenics, Vol. 22, No. 5, August 1982, p. 409-113. "Contributions on the Study of the Increase of Durability of the High-Alloyed Tool Steels by Thermal Treatments at Cryogenic Temperatures," by I. Alexandra, C. Picos, and G. Ailincai: Proceedings o f the 2nd International Congress on Heat Treatment of Materials (Florence, Italy, 20- 2^4 Sept. 1982), Associazione Italiana di Metallurgia (AIM), Milan, Italy, 1983, p. 573-579. "The Change of the Substructure Elements and the Redistribution of the Al-

loying Elements by Means of Cryotreat- ments in Alloy Tool Steels," by I. Alexandru, G. Coman, and V. Bulancea: Proceedings o f the 5th International Congress on Heat Treatment o f Materials (Budapest, Hungary, 20-24 Oct. 1986), Vol. 2, Scientific Society of Mechanical Engineers (GTE), Budapest, Hungary, 1987, p. 901-908. "Influence of Cryogenic Treatments on Life of Alloyed High-Speed Steels," by I. Alexandru, G. Ailincai, and C. Badu: Mmoires et Etudes Scientifiques de la Revue de Mtallurgie, Vol. 87, No. 6,1990, p. 383-389. "Cryogenic Treatment of Three Heat- Treated Tool Steels," by K.E. Moore and D.N. Collins: Key Engineering Materials, Vol. 86-87,1993, p. 47-54. "Deep Cryogenic Treatment of a D2 Cold-Work Tool Steel," by D.N. Collins and I. Dormer: Heat Treatment o f Metals, Vol. 24, No. 3,1997, p. 71-74. "Deep Cryogenic Treatment of High- Speed Steel and its Mechanism," by Dong Yun, Lin Xiaoping, and Xiao Hongshen: Heat Treatment o f Metals, Vol. 25, No. 3, 1998, p 55-59.

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