J.Y. Huang * , Y.T. Zhu, X.Z. Liao, I.J. Beyerlein, M.A. Bourke, T.E. Mitchell

Materials Science and Technology Division, Los Alamos National Laboratory, MS G 755, Los Alamos, NM 87545, USA

Abstract

Cryogenic treatment has been claimed to improve wear resistance of certain steels and has been implemented in cutting tools, autos, barrels etc. Although it has been confirmed that cryogenic treatment can improve the service life of tools, the underling mechanism remains unclear. In this paper, we studied the microstructure changes of M2 tool steel before and after cryogenic treatment. We found that cryogenic treatment can facilitate the formation of carbon clustering and increase the carbide density in the subsequent heat treatment, thus improving the wear resistance of steels. #2003 Elsevier Science B.V. All rights reserved.

Keywords: Cryogenic treatment; Steel; Microstructure; Carbide; Wear resistance

1. Introduction

Over the past few decades, interest has been shown in the effect of low-temperature treatment on the performance of steels. Low-temperature treatment is generally classified as either ‘‘cold treatment’’ at temperatures down to about /80 8C (dry ice temperature), or ‘‘deep cryogenic treatment’’ at liquid nitrogen temperature (/ 196 8C). For simplicity, we shall refer to the latter as cryogenic treatment in this paper. In recent years, many small businesses have been set up to cryogenically treat finished steel products, such as drills, cutters, etc., claiming significant improvements on their wear resistance. However, scientific research on cryogenic treatment has been spotty, and only a few academic papers have been published.

One of the most prevalent claims in low-temperature treatment is an increase in wear resistance of certain steels [1/4] . There is no clear-cut understanding of the mechanisms by which cryogenic treatment improves performance of these steels. However, most researchers believe that cryogenic treatment promotes the complete transformation of retained austenite into martensite at cryogenic temperatures, which is attributed to improved wear resistance [1,2] . Others claim that cryogenic treatment facilitates the formation of fine h-carbides in the martensite, thus improving the wear resistance [3,4] . The motivation of this research is to assess the cryogenic effect and to understand the cryogenic mechanism.

* Corresponding author. Tel.: /1-505-665-0835; fax: /1-505-667-

E-mail address: jyhuang@lanl.gov (J.Y. Huang).

2. Experiments

A commercial M2 tool steel rod with a diameter of 6.35 mm was used in the experiment. The composition of sample is (weight%) 0.85 /1 C, 6 W, 5 Mo, 4 Cr, 2 V. A heat treatment was carried out by first preheating at3 0.17 8Cs¹ to 815 8Cina vacuum furnace at 4 /10Pa; then continuously heating to an austenitizing temperature of 1100 8C in a nitrogen atmosphere at 20 Pa, holding for 1 h, followed by quenching to an ambient temperature in a cool nitrogen gas. The cryogenic treatment was performed by soaking the samples in liquid nitrogen for 1 week. For comparison, both the cryogenic treated and non-cryogenic treated samples were tempered at 200 8C in nitrogen atmosphere for 24 h. Thin foils for transmission electron microscope (TEM) samples were prepared by first cutting the M2 steel rod into thin slices with a diamond saw, then mechanical polishing to about 100 mm, and finally polishing in an electrolytic jet-polisher. The electrolyte consists of 62 cc perchloric acid (70%), 700 cc ethanol, 100 cc butyl cellosolve (or butanol), and 137 cc distilled water. The jet polishing was performed at a temperature 0921-5093/03/$ -see front matter #2003 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1-5 0 9 3 ( 0 2 ) 0 0 1 6 5-X between /20 and /10 8C, at a voltage of between 20 and 25 V, and at a current of about 10 mA. TEM observations were carried out in a Philips CM 30 electron microscope operated at 300 kV.

J.Y. Huang et al. / Materials Science and Engineering A339 (2003) 241 /244

3. Results and discussion

As shown in Figs.1 and 2, both the cryogenically treated (Fig.1 ) and non-cryogenically treated samples (Fig.2 a and b) precipitated out spherical Fe4M2(M / W, Mo, Cr, V)C carbides with a face-centered-cubic˚ structure and a lattice parameter a /11.5 A. The microstructure and lattice parameter of the carbide were determined by selected area electron diffraction patterns such as those shown in Fig.3 a /c. And the composition of the carbide was obtained by energy dispersive X-ray spectrum such as that shown in Fig. 4 . As shown in Fig. 5, the particle size ranges from 0.3 to 2 mm, and the particle size distributions in the cryogenic treated and non-cryogenic treated samples are very similar. However, the population, volume fraction and distribution of carbides in the two samples are different. The carbides are distributed more homogeneously in the cryogenic treated sample than in the non-cryogenically treated one. As shown in Fig.1 , carbides with different sizes are homogeneously distributed in the cryogenically treated sample. However, the distribution of carbides in the non-cryogenically treated samples is inhomogeneous. They are localized in certain regions, and their size varies from region to region. Fig.2 a and b are representative regions from the same non-cryogenic treated and tempered sample. It is seen that the carbides in Fig.2 a are relatively small, generally smaller than 1.

Fig. 1. TEM micrographs of M2 tool steel after cryogenic treatment and tempering.

mm. However, the carbides in Fig.2 b are generally larger than 1 mm. Using image analysis, we can estimate the volume fraction of the carbides. By analyzing seven representative negatives for each sample, we found that the volume fraction of carbides in the cryogenically treated sample is higher than that in the non-cryogeni-cally treated one, 11% in the former and 5% in the latter. The standard deviation is about 1%.

As mentioned in the introduction, there are two rival hypotheses concerning reasons for the effect of cryogenic treatment on wear resistance. One claims that the only change in steel during the cryogenic treatment is the transformation of retained austenite into martensite, which is the primary reason for the observed improvement in wear resistance. However, it has been proven that a cold treatment at /80 8C is sufficient for transforming most of the retained austenite into martensite [3,5] . We could not detect retained austenite by either TEM or X-ray diffraction in our samples, possibly due to the content of the retained austenite being very small. As shown in Refs. [3,5] the cold treated samples have almost the same volume fraction of retained austenite as the cryogenic treated samples, nevertheless, the wear resistance of the latter was superior to that of the former. These results indicate that factors other than the transformation of the retained austenite into martensite contribute to the improvement of the wear resistance.

Another suggestion is that the precipitation of fine carbides as a result of cryogenic treatment is responsible for the improved wear resistance [3,4] . It is suggested that a reduction in microcracking tendency results from reduced internal stress when the fine carbide precipitation occurs. The present investigation in M2 tool steel favors this hypothesis for two reasons, (1) the distribution of the carbides in the cryogenic treated samples are more homogeneous than that in the non-cryogenic treated samples, and (2) carbide volume fraction in the cryogenically treated samples is almost twice as much as that in the non-cryogenically treated ones. The precipitation of more hard carbides in the cryogenically treated samples can reduce the carbon and alloy contents in the matrix, which improves the toughness of the matrix. We believe the combination of higher carbide content and tougher matrix enhanced the wear resistance.

It is unclear how the cryogenic treatment promotes the precipitation of carbide in martensite. The only clues we have are two engineering observations [3,4,6]: first, martensite needs to be cooled well below the Ms * the temperature at which transformation of austenite to martensite starts during cooling; and second, the longer the holding time, the higher the carbide population and volume fraction [6]. We suggest that crystal defects suchas dislocations and twins are generated by microscopic internal stress during cooling. This stress is caused by the spatial variation in composition and microstructure,

J.Y. Huang et al. / Materials Science and Engineering A339 (2003) 241/244

Fig. 2. TEM micrographs of M2 tool steel after non-cryogenic treatment and tempering. Note that the carbides in (a) are small, while those in (b) are large.

Fig. 3. Representative electron diffraction patterns from carbides. The indexes in Fig.3 a /c were based on an fcc latticewith lattice parameter of˚about 11.5 A.

which leads to different thermal contraction, as well as by the transformation of retained austenite to martensite. The martensite needs to be cooled below a certain temperature to develop internal stress high enough to generate crystal defects. The required long holding time suggests a localized diffusion process, possibly the clustering of carbon and alloying elements to the defects. The martensite becomes more supersaturated with decreasing temperature. This increases the lattice distortion and thermodynamic instability of the martensite, both of which drive carbon and alloying atoms to segregate to nearby defects. These clusters may act as or grow into nuclei for the formation of carbide on subsequent warming up or tempering. unfortunately, the defect density in martensite is generally so high that it is practically difficult to directly observe the segregation or clustering process of the carbon or alloying elements, or whether more dislocations or twins are generated during the cryogenic treatment process. However, a recent in-situ neutron diffraction study indicated that the lattice parameters a and c of the martensite behave differently during the cooling and warming-up processes [7]. The lattice parameter a changes with the temperature almost linearly, following almost the same curve during the cooling and warming-up process, indicating a pure thermal elastic effect. The lattice parameter c , on the other hand, first decreases with the cooling temperature,

Fig. 4. A typical energy dispersive X-ray spectrum from carbide. The elements are indicated.

J.Y. Huang et al. / Materials Science and Engineering A339 (2003) 241 /244

Fig. 5. Size distribution of carbide in M2 steel after cryogenic treatment (left) and non-cryogenic treatment (right) and tempering at 200 8C. The mean particle size is about 0.9 mm (left) and 0.6 mm (right). but it does not follow the same slope while warming up, and increases only very slightly during the warming-up process, indicating it is not only a pure thermal effect. The above result infers that carbon atoms segregation did occur during the cold treatment process. Because the carbon atoms predominantly occupy the octahedral or tetrahedral sites in the martensitic lattice, the segregation of carbon atoms from the octahedral or tetrahedral sites to the defect regions mainly affects the c lattice parameter.

4. Conclusions

In summary, cryogenic treatment cannot only facilitate the carbide formation and increase the carbide population and volume fraction in the martensite matrix, but can also make the carbide distribution more homogeneous. Our results are consistent with previous studies that show increases in carbide density and volume fraction, which may be responsible for the improvement in wear resistance.

Acknowledgements

The authors wish to thank R.M. Dickerson for preparing the electrolyte and J.A. Valdez for cutting the TEM samples. This research is supported by the Laboratory Directed Research and Development (LDRD) office of Los Alamos National Laboratory.

References

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[6] D.N. Collins, J. Dormer, Heat Treatment of Metals 3 (1997) 71.
[7] M.A. Bourke, private communication.