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Performance evaluation of cryogenically treated tungsten carbide tools in turning
ARTICLE IN PRESS
International Journal of Machine Tools & Manufacture 46 (2006) 2051–2056 www.elsevier.com/locate/ijmactool
Performance evaluation of cryogenically treated tungsten carbide tools in turning A.Y.L. Yong, K.H.W. Seah, M. Rahman Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore Received 28 March 2005; received in revised form 19 October 2005; accepted 3 January 2006 Available online 20 February 2006
Abstract This paper describes a study on the effects of cryogenic treatment of tungsten carbide. Cryogenic treatment has been acknowledged by some as a means of extending the tool life of many cutting tool materials, but little is known about the mechanism behind it. Thus far, detailed studies pertaining to cryogenic treatment have been conducted only on tool steels. However, tungsten carbide cutting tools are now in common use. The main aim of this study is to analyze the differences in tool performance between cryogenically treated and untreated tool inserts during orthogonal turning of steel. This will aid in the quest for optimal cutting conditions for the turning of steel using these inserts, and will also enhance the understanding of the mechanism behind the cryogenic treatment of tungsten carbide, and the changes in its properties after cryogenic treatment. In the process of ascertaining these findings, it was shown in this study that under certain conditions, cryogenic treatment can be detrimental to tool life and performance. It was also shown that cryogenically treated tools perform better while performing intermittent cutting operations. r 2006 Elsevier Ltd. All rights reserved. Keywords: Cryogenic treatment; Tungsten carbide; Wear
1. Introduction Over the past few years, there has been an increase in interest in the application of cryogenic treatment to different materials. Research has shown that cryogenic treatment increases product life, and in most cases, provides additional qualities to the product, such as stress relieving. In the area of cutting tools, extensive study has been done on tool steels, which include high-speed steel (HSS) and medium carbon steels. It has been reported that cryogenic treatment can double the service life of HSS tools, and also increase hardness and toughness simultaneously [1]. Cryogenic treatment of cutting tool materials such as tungsten carbide, have yet to be extensively studied. Tungsten carbide, has been proven to be much more efficient than HSS when machining hard materials such as Corresponding author.
E-mail address: [email protected] (M. Rahman). 0890-6955/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2006.01.002
steel itself. If cryogenic treatments can double the service life of HSS, it could probably do the same for tungsten carbide tools. The main objective of this study is to analyze the effects of cryogenic treatment on tungsten carbide tools.
2. Cryogenic treatment Cryogenic treatment refers to subjecting materials to very low temperatures. This process is not limited in application to metals, but can also be applied to a wide range of materials, with differing results. Many commercial industries have extolled the benefits of cryogenic treatment, but few have extensively studied the mechanism of cryogenic treatment. Several different cryogenic processes have been tested by researchers. These involve a combination of deep freezing and tempering cycles. Generally, they can be described as a controlled lowering of temperature from room temperature
ARTICLE IN PRESS 2052
A.Y.L. Yong et al. / International Journal of Machine Tools & Manufacture 46 (2006) 2051–2056
to the boiling point of liquid nitrogen (196 1C), maintenance of the temperature for about twenty four hours, followed by a controlled raising of the temperature back to room temperature. Subsequent tempering processes may follow. Dong et al. [1] did a detailed study on the effects of varying the deep freezing and tempering cycles on high speed steel. In tool steels, this treatment affects the material in two ways. Firstly, it eliminates retained austenite, and hence increases the hardness of the material. Secondly, this treatment initiates nucleation sites for precipitation of large numbers of very fine carbide particles, resulting in an increase in wear resistance [2]. Most of the research on cryogenic treatment in the area of machining tools and cutting tool materials has concentrated mainly on tool steels, especially high-speed steel. The mechanisms responsible for the improvement in properties of tool steel have also been well documented. However, little research has been done on other cutting tool materials. Seah et al. [3] did some study on the effect of cryogenic treatment on tungsten carbide and found that such treatment increases its wear resistance. They attributed this to an increase in the number of Z-phase particles after cryogenic treatment, a theory which he supported with photographs taken using a scanning electron microscope (SEM). The experimental procedures that were used to perform the cutting on the workpiece were ‘‘repeated turning operations’’. Such ‘‘repeated turning operations’’ refers to using the same cutting edge for subsequent cutting operations, instead of switching to a brand new cutting edge for each new cut. By doing so, they managed to show that cryogenically treated tungsten carbide tools had a much greater resistance to chipping compared to the untreated ones. In addition, the cryogenically treated tools also performed better than the untreated tools at higher cutting speeds. So far, few researchers have proposed other mechanisms that explain the effect of cryogenic treatment on tungsten carbide. Bryson [4] attributes the wear resistance, and hence the increase in tool life, of carbide tools to the improvement in the holding strength of the binder after cryogenic treatment. He believes that cryogenic treatment also acts to relieve the stresses introduced during the sintering process under which carbide tools are produced. However, Bryson also warned that under certain conditions, cryogenic treatment would have little or no effect on carbide tools, such as when reprocessed carbides are used. In this present study, Sumitomo cutting tool inserts were cryogenically treated and tempered in Cryo Nebraska, a company in the USA that specializes in cryogenic treatment of materials. The procedure used is the same as that described by Seah et al. [3], generally outlined as follows:
Inserts are placed in a chamber. Temperature is gradually lowered over a period of 6 h from room temperature to about 184 1C.
Temperature is then held steady for about 18 h. Temperature is gradually raised over a period of 6 h to room temperature. Inserts are tempered.
3. Experimental procedure The tool inserts were tested by performing orthogonal turning on medium carbon steel (ASSAB 760) on an Okuma LH35-N lathe. The cutting tools used were square inserts with chipbreakers (Sumitomo SNGG 230408RUM). This was to avert the long continuous chips that would form if no chipbreaker were used. Such long continuous chips tend to ball-up around the cutting tool after a certain period of cutting, obstructing the free flow of chips away from the cutting edge. This not only affects the wear of the tool insert, but also poses some danger to the machine operator, who has to manually clear the cutting tool of chips that ball-up. The untreated and cryogenically treated cutting tool inserts were tested at various cutting speeds. In addition, the inserts were tested for continuous cuts, whereby the tool performed the entire cutting operation without interruption until it started to chip away, as well as for repeated cuts, whereby the tool cut for predetermined periods of time, before it was stopped for a short interval, then allowed to carry on, repeating this sequence until the tool started to chip. After each cutting operation, the maximum flank wear, VBmax, was immediately measured using a Mitutoyo toolmaker’s microscope. Measurements of maximum flank wear were recorded according to the guidelines provided in ‘‘Turning operations for Single Point Tools’’ (ISO 3685), since the width of flank wear was not regular along the cutting edge, as shown in Fig. 1.
Fig. 1. Flank wear on a tungsten carbide tool insert.
ARTICLE IN PRESS A.Y.L. Yong et al. / International Journal of Machine Tools & Manufacture 46 (2006) 2051–2056
4. Results from continuous cutting without breaks
0.400 Flank Wear (mm)
0.350 0.300 0.250 0.200 0.150 Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm)
0.050 0.000 0
200
400
600 800 Time (s)
1000
1200
1400
Flank Wear (mm)
Fig. 2. Flank wear development for cutting speed ¼ 150 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.1 mm/rev.
0.450 0.400 0.350 0.300 0.250 0.200 0.150 0.100 0.050 0.000
Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm) 0
50
100
150 200 Time (s)
250
300
350
Fig. 3. Flank wear development for cutting speed ¼ 200 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.1 mm/rev.
Flank Wear (mm)
0.350 0.300 0.250 0.200 0.150 0.100
Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm)
0.050 0.000 0
20
40
60 80 Time (s)
100
120
140
Fig. 4. Flank wear development for cutting speed ¼ 250 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.1 mm/rev.
0.350 Flank Wear (mm)
In the first set of experiments, in which metal cutting was continuous without any breaks, the depth of cut and feed were kept constant, while cutting speed was varied between 150 m/min and 300 m/min. The graphs in Figs. 2–5 show the comparison between the untreated and cryogenically treated tools. Generally, the cryogenically treated inserts experience less flank wear than the untreated inserts during the earlier part of the cutting operation, unless the cutting speed is very high (300 m/min) as shown in Fig. 5. As the duration of cut increases, the treated insert gradually loses its wear resistance, and at some point will have about the same wear resistance (measured in terms of flank wear) as the untreated insert. For ease of description, it shall be henceforth referred to as the ‘‘convergence point’’. Once the cutting duration exceeds the convergence point, the treated insert will have a greater flank wear than the untreated tool. For the operation at v ¼ 150 m= min (Fig. 1), the convergence point is about halfway through the maximum tool life. At v ¼ 200 m= min (Fig. 2), this point moves further right, to about three-quarters of the maximum tool life. This point is non-existent for v ¼ 250 m= min (Fig. 3), but it can be seen that the flank wear of the treated tool gradually increases to almost that of the untreated tool. However, at v ¼ 300 m= min (Fig. 4), the
0.100
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0.300 0.250 0.200 0.150 0.100
Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm)
0.050 0.000 0
10
20
30 40 Time (s)
50
60
70
Fig. 5. Flank wear development for cutting speed ¼ 300 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.1 mm/rev.
cryogenically treated tool invariably experiences better wear resistance throughout the entire duration of cutting operation before failure. This phenomenon seems to show that prolonged heating of the cutting tool interface due to the continuous cutting operation has a detrimental effect on the wear resistance of the cryogenically treated tool. This effect is less noticeable at higher cutting speeds, because the tool life becomes very short. (The tool life at v ¼ 150 m= min is 1200 s, whereas that at v ¼ 300 m= min is only 60 s). It can also be deduced that at higher cutting speeds, the time of exposure to high cutting temperatures is rather short, and hence the high temperature does not have sufficient time to produce a detrimental effect on the cryogenic treatment of the cutting tool. It is noteworthy that in Fig. 4, although the cutting speed is higher than Fig. 2, (250 m/min compared to 150 m/ min), the tool wear of the cryogenically treated tool at 250 m/min is always smaller than the tool wear of the untreated tool. This cannot be said for the tool at 150 m/ min, where the tool wear of the cryogenically treated tool is greater than that of the untreated tool towards the end of its tool life. So it is possible to conclude here that cryogenic tools can maintain their superior tool wear resistance if used for just short periods of time. It should be noted also that the time taken for both the treated and untreated tools to chip is approximately the same in all cases mentioned above.
ARTICLE IN PRESS A.Y.L. Yong et al. / International Journal of Machine Tools & Manufacture 46 (2006) 2051–2056
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5. Results from repeated cuts with breaks in between
Flank Wear (mm)
0.600 0.500 0.400 0.300 0.200 Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm)
0.100 0.000 0
50
100 Time (s)
150
200
Fig. 6. Flank wear development for cutting speed ¼ 250 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.1 mm/rev.
0.700 0.600 0.500 0.400 0.300 0.200
Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm)
0.100 0.000 0
100
200
300
400
500
Time (s)
Fig. 7. Flank wear development for cutting speed ¼ 200 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.1 mm/rev.
Flank Wear (mm)
Thus far, it can be seen that while the trends in maximum flank wear somewhat agree with those recorded by Seah et al. [3], the time taken for chipping to occur does not agree at all. In the study by Seah et al. [3], the cryogenically treated tool chips much later than that of the untreated tool. This was not observed in the study described in the previous section. As such, another series of experiments were designed to test the efficacy of cryogenic treatment on the cutting tool inserts. Here the cutting operations were of short predetermined durations, and between each duration of cutting, there was a 5-min break to allow the tool to cool. This set of operations will be known as ‘‘repeated cuts with breaks in between’’. The 5-min break used in this set of experiments is meant to simulate events such as tool change, workpiece change or periods during the machining cycle where no cutting is performed. 5 min may seem to be a long break in some situations, but the intention is to deliberately allow the tool inserts to cool down after short periods of cutting, to study how the tool is affected by this break. Figs. 6 and 7 show the comparison between the cryogenically treated and untreated tools, both with constant depth of cut of 1 mm and feed of 0.1 mm/rev, but for cutting speeds of 250 and 200 m/min, respectively. The horizontal axis shows the total machining time, which excludes the duration of the breaks. From Fig. 6, it can be seen that at a cutting speed of 250 m/min and repeated cuts of 10 s duration with 5 min breaks in between, there is almost no difference in performance between the treated and untreated tools. When the cutting speed is reduced to 200 m/min with repeated cuts of 15 s duration with 5 min breaks in between (Fig. 7), the cryogenically treated tool shows better resistance to chipping, lasting notably longer than the untreated tool, roughly 1.05 times longer. Flank wear was also slightly lower for the cryogenically treated tool. At the point of failure of the untreated tool, the flank wear of the cryogenically treated tool was approximately 4% lesser than that of the untreated tool.
Flank Wear (mm)
0.800
1.800 1.600 1.400 1.200 1.000 0.800 0.600 0.400 0.200 0.000
Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm) 0
200
400
600 800 Time (s)
1000
1200
1400
Fig. 8. Flank wear development for cutting speed ¼ 200 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.05 mm/rev.
Compared to the continuous cutting operations of similar cutting conditions described in Section 4, it can be easily seen that the tools that were used for repeated cuts began chipping much later than those that were used for continuous cuts, regardless of whether the tools had been cryogenically treated or not. This shows that allowing the tool insert time to cool down periodically helps to delay the onset of chipping. Furthermore, the graphs also suggest that this cooling down time is more beneficial to the cryogenically treated tool as the onset of chipping is later than the untreated tool. For the next test (Fig. 8), the cutting speed was kept at 200 m/min, but the feed was reduced to 0.05 mm/min, and the repeated cuts were of 60 s duration with 5 min breaks in between. From the graph, it can be seen that chipping of the cryogenically treated tool occurred much later than that of the untreated tool, roughly 1.3 times longer. Flank wear of the treated tool was also lesser than that on the untreated tool. At the point of failure of the untreated tool, the flank wear of the cryogenically treated tool was approximately 10% lesser than that of the untreated tool. It is noteworthy that no convergence points exist in the experiments with the repeated cuts. Since the tool was allowed to cool down for 5 min in between cuts, not only was the maximum temperature experienced by the tool reduced, but it seemed to also allow the cryogenically treated tool to retain its better wear resistance. Although tool inserts are seldom used to an extent where the flank wear reaches 1.5 mm, this experiment shows that at mild
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cutting conditions, with repeated cuts where the tool is allowed to cool, cryogenically treated tools perform far better than the untreated counterparts. Another observation which is not shown in the figures is that there is a substantial difference between the amounts of flank wear experienced after continuous cutting and after a series of repeated cuts with breaks in between. For a cutting speed of 250 m/min and 12 sessions of 10 s long repeated cuts with 5 min intervals in between, the flank wear for the cryogenically treated tools was 0.401 mm for repeated cutting and 0.486 mm for continuous cutting. For the untreated tool, the flank wear was 0.404 mm for repeated cutting and 0.492 mm for continuous cutting (compare Figs. 4 and 6). To cite another example, for a cutting speed of 200 m/min and 12 sessions of 15 s cutting with 5 min intervals in between, the flank wear for the treated tools was 0.332 mm for repeated cutting and 0.398 mm for continuous cutting. For the untreated tool, the flank wear was 0.341 mm for repeated cutting and 0.400 mm for continuous cutting (compare Figs. 3 and 7). Evidently, the 5-min breaks reduced the flank wear by about 20% in each case.
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with repeated cuts of 30 s duration with 5 min breaks experienced it at a flank wear of 0.47 mm at 150 s. Performing the repeated cuts for a shorter duration before breaks prevented the tool from heating up as much as while performing repeated cuts for longer durations. The former case results in the tool chipping later, an observation which further supports the theory propounded so far. The results in Figs. 2–5, which show graphs of flank wear against time for continuous cuts, demonstrate that cryogenic treatment apparently does not seem to increase the resistance to chipping of cryogenically treated tools. This seems to contradict the results obtained by Seah et al. [3]. However, the experimental results for repeated cuts (Figs. 6–9) show trends similar to the results of Seah et al. [3], i.e. The onset of chipping is delayed as a result of cryogenic treatment. What their research and the results in this section have proven is that cryogenically treated tools, if allowed time to cool during machining operations, will retain their superiority in terms of resistance to chipping over the untreated tools. 7. Conclusions
6. Effect of cutting duration before a break Experiments were conducted to determine if the cutting duration before a break affected the performance of the cryogenically treated tools. Keeping cutting conditions fixed at cutting speed of 250 m/min, depth of cut of 1 mm and feed of 0.1 mm/rev, a comparison was made between cutting durations of 10 s and 30 s, respectively, before a break was allowed. The graphs in Fig. 9 show the comparison between the cryogenically treated and untreated tools, for the two different cutting durations before a break. It would be expected that under such cutting conditions, there would not be a significant difference in the amount of tool wear between the cryogenically treated and untreated tools. Nevertheless, for the run with repeated cuts of 10 s duration with 5-min breaks in between, the cryogenically treated tool experienced chipping at a flank wear of 0.53 mm at 180 s, whereas the run
Flank Wear (mm)
0.600 0.500 0.400 0.300 30s intervals (Treated) 10s intervals (Treated) 30s intervals (Untreated) 10s intervals (Untreated)
0.200 0.100 0.000 0
50
100
150
Time (s)
Fig. 9. Effect of cutting duration before a break.
200
Cryogenic treatment no doubt improves the resistance to chipping of tools and to a less significant extent, improves flank wear resistance. However, under certain conditions, such as prolonged exposure to high temperatures during long continuous cutting operations, cryogenically treated tools can lose their superior properties. Cryogenically treated tools perform better than untreated tools when performing continuous cuts for short periods of time, or when performing repeated cuts with breaks in between, both in terms of decreased tool wear, and increased resistance to chipping. Cryogenically treated tools subjected to prolonged periods of high temperature at the cutting edge lose their wear resistance, suggesting that high temperatures alters the property of the treated tool. Hence the state of cryogenically treated materials is not a permanent state, but a metastable one. Cryogenically treated tools performing repeated cuts with breaks in between are able to cool down between cuts, thereby recovering some property that allows them to retain their superior wear resistance. This property also gives it superior resistance to chipping, compared to untreated tools. In light of the fact that cryogenically treated tools perform best when the tool temperature is kept low, their effectiveness can be extended if coolants or suitable methods of cooling are used to keep the tool temperatures low. Hence, the validity of claims that cryogenic treatment can improve the lifespan of cutting tools would depend a lot on the cutting conditions. Tools under mild cutting conditions stand to gain from cryogenic treatment, but heavy duty cutting operations with long periods of heating of the cutting tool will not benefit from it.
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References [1] Y. Dong, X. Lin, H. Xiao, Deep cryogenic treatment of high-speed steel and its mechanism, Heat Treatment of Metals 3 (1998) 55–59. [2] D.N. Collins, J. Dormer, Deep cryogenic treatment of a D2 cold-work tool steel, Heat Treatment of Metals 3 (1997) 71–74.
[3] K.H.W. Seah, M. Rahman, K.H. Yong, Performance evaluation of cryogenically treated tungsten carbide cutting tool inserts, Proceedings of the Institution of Mechanical Engineers Part B–Journal of Engineering Manufacture 217 (1) (2003) 29–43. [4] W.E. Bryson, Cryogenics, Hanser Gardner Publications, Cincinnati, OH, 1999, pp. 81–107.
International Journal of Machine Tools & Manufacture 46 (2006) 2051–2056 www.elsevier.com/locate/ijmactool
Performance evaluation of cryogenically treated tungsten carbide tools in turning A.Y.L. Yong, K.H.W. Seah, M. Rahman Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore Received 28 March 2005; received in revised form 19 October 2005; accepted 3 January 2006 Available online 20 February 2006
Abstract This paper describes a study on the effects of cryogenic treatment of tungsten carbide. Cryogenic treatment has been acknowledged by some as a means of extending the tool life of many cutting tool materials, but little is known about the mechanism behind it. Thus far, detailed studies pertaining to cryogenic treatment have been conducted only on tool steels. However, tungsten carbide cutting tools are now in common use. The main aim of this study is to analyze the differences in tool performance between cryogenically treated and untreated tool inserts during orthogonal turning of steel. This will aid in the quest for optimal cutting conditions for the turning of steel using these inserts, and will also enhance the understanding of the mechanism behind the cryogenic treatment of tungsten carbide, and the changes in its properties after cryogenic treatment. In the process of ascertaining these findings, it was shown in this study that under certain conditions, cryogenic treatment can be detrimental to tool life and performance. It was also shown that cryogenically treated tools perform better while performing intermittent cutting operations. r 2006 Elsevier Ltd. All rights reserved. Keywords: Cryogenic treatment; Tungsten carbide; Wear
1. Introduction Over the past few years, there has been an increase in interest in the application of cryogenic treatment to different materials. Research has shown that cryogenic treatment increases product life, and in most cases, provides additional qualities to the product, such as stress relieving. In the area of cutting tools, extensive study has been done on tool steels, which include high-speed steel (HSS) and medium carbon steels. It has been reported that cryogenic treatment can double the service life of HSS tools, and also increase hardness and toughness simultaneously [1]. Cryogenic treatment of cutting tool materials such as tungsten carbide, have yet to be extensively studied. Tungsten carbide, has been proven to be much more efficient than HSS when machining hard materials such as Corresponding author.
E-mail address: [email protected] (M. Rahman). 0890-6955/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2006.01.002
steel itself. If cryogenic treatments can double the service life of HSS, it could probably do the same for tungsten carbide tools. The main objective of this study is to analyze the effects of cryogenic treatment on tungsten carbide tools.
2. Cryogenic treatment Cryogenic treatment refers to subjecting materials to very low temperatures. This process is not limited in application to metals, but can also be applied to a wide range of materials, with differing results. Many commercial industries have extolled the benefits of cryogenic treatment, but few have extensively studied the mechanism of cryogenic treatment. Several different cryogenic processes have been tested by researchers. These involve a combination of deep freezing and tempering cycles. Generally, they can be described as a controlled lowering of temperature from room temperature
ARTICLE IN PRESS 2052
A.Y.L. Yong et al. / International Journal of Machine Tools & Manufacture 46 (2006) 2051–2056
to the boiling point of liquid nitrogen (196 1C), maintenance of the temperature for about twenty four hours, followed by a controlled raising of the temperature back to room temperature. Subsequent tempering processes may follow. Dong et al. [1] did a detailed study on the effects of varying the deep freezing and tempering cycles on high speed steel. In tool steels, this treatment affects the material in two ways. Firstly, it eliminates retained austenite, and hence increases the hardness of the material. Secondly, this treatment initiates nucleation sites for precipitation of large numbers of very fine carbide particles, resulting in an increase in wear resistance [2]. Most of the research on cryogenic treatment in the area of machining tools and cutting tool materials has concentrated mainly on tool steels, especially high-speed steel. The mechanisms responsible for the improvement in properties of tool steel have also been well documented. However, little research has been done on other cutting tool materials. Seah et al. [3] did some study on the effect of cryogenic treatment on tungsten carbide and found that such treatment increases its wear resistance. They attributed this to an increase in the number of Z-phase particles after cryogenic treatment, a theory which he supported with photographs taken using a scanning electron microscope (SEM). The experimental procedures that were used to perform the cutting on the workpiece were ‘‘repeated turning operations’’. Such ‘‘repeated turning operations’’ refers to using the same cutting edge for subsequent cutting operations, instead of switching to a brand new cutting edge for each new cut. By doing so, they managed to show that cryogenically treated tungsten carbide tools had a much greater resistance to chipping compared to the untreated ones. In addition, the cryogenically treated tools also performed better than the untreated tools at higher cutting speeds. So far, few researchers have proposed other mechanisms that explain the effect of cryogenic treatment on tungsten carbide. Bryson [4] attributes the wear resistance, and hence the increase in tool life, of carbide tools to the improvement in the holding strength of the binder after cryogenic treatment. He believes that cryogenic treatment also acts to relieve the stresses introduced during the sintering process under which carbide tools are produced. However, Bryson also warned that under certain conditions, cryogenic treatment would have little or no effect on carbide tools, such as when reprocessed carbides are used. In this present study, Sumitomo cutting tool inserts were cryogenically treated and tempered in Cryo Nebraska, a company in the USA that specializes in cryogenic treatment of materials. The procedure used is the same as that described by Seah et al. [3], generally outlined as follows:
Inserts are placed in a chamber. Temperature is gradually lowered over a period of 6 h from room temperature to about 184 1C.
Temperature is then held steady for about 18 h. Temperature is gradually raised over a period of 6 h to room temperature. Inserts are tempered.
3. Experimental procedure The tool inserts were tested by performing orthogonal turning on medium carbon steel (ASSAB 760) on an Okuma LH35-N lathe. The cutting tools used were square inserts with chipbreakers (Sumitomo SNGG 230408RUM). This was to avert the long continuous chips that would form if no chipbreaker were used. Such long continuous chips tend to ball-up around the cutting tool after a certain period of cutting, obstructing the free flow of chips away from the cutting edge. This not only affects the wear of the tool insert, but also poses some danger to the machine operator, who has to manually clear the cutting tool of chips that ball-up. The untreated and cryogenically treated cutting tool inserts were tested at various cutting speeds. In addition, the inserts were tested for continuous cuts, whereby the tool performed the entire cutting operation without interruption until it started to chip away, as well as for repeated cuts, whereby the tool cut for predetermined periods of time, before it was stopped for a short interval, then allowed to carry on, repeating this sequence until the tool started to chip. After each cutting operation, the maximum flank wear, VBmax, was immediately measured using a Mitutoyo toolmaker’s microscope. Measurements of maximum flank wear were recorded according to the guidelines provided in ‘‘Turning operations for Single Point Tools’’ (ISO 3685), since the width of flank wear was not regular along the cutting edge, as shown in Fig. 1.
Fig. 1. Flank wear on a tungsten carbide tool insert.
ARTICLE IN PRESS A.Y.L. Yong et al. / International Journal of Machine Tools & Manufacture 46 (2006) 2051–2056
4. Results from continuous cutting without breaks
0.400 Flank Wear (mm)
0.350 0.300 0.250 0.200 0.150 Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm)
0.050 0.000 0
200
400
600 800 Time (s)
1000
1200
1400
Flank Wear (mm)
Fig. 2. Flank wear development for cutting speed ¼ 150 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.1 mm/rev.
0.450 0.400 0.350 0.300 0.250 0.200 0.150 0.100 0.050 0.000
Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm) 0
50
100
150 200 Time (s)
250
300
350
Fig. 3. Flank wear development for cutting speed ¼ 200 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.1 mm/rev.
Flank Wear (mm)
0.350 0.300 0.250 0.200 0.150 0.100
Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm)
0.050 0.000 0
20
40
60 80 Time (s)
100
120
140
Fig. 4. Flank wear development for cutting speed ¼ 250 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.1 mm/rev.
0.350 Flank Wear (mm)
In the first set of experiments, in which metal cutting was continuous without any breaks, the depth of cut and feed were kept constant, while cutting speed was varied between 150 m/min and 300 m/min. The graphs in Figs. 2–5 show the comparison between the untreated and cryogenically treated tools. Generally, the cryogenically treated inserts experience less flank wear than the untreated inserts during the earlier part of the cutting operation, unless the cutting speed is very high (300 m/min) as shown in Fig. 5. As the duration of cut increases, the treated insert gradually loses its wear resistance, and at some point will have about the same wear resistance (measured in terms of flank wear) as the untreated insert. For ease of description, it shall be henceforth referred to as the ‘‘convergence point’’. Once the cutting duration exceeds the convergence point, the treated insert will have a greater flank wear than the untreated tool. For the operation at v ¼ 150 m= min (Fig. 1), the convergence point is about halfway through the maximum tool life. At v ¼ 200 m= min (Fig. 2), this point moves further right, to about three-quarters of the maximum tool life. This point is non-existent for v ¼ 250 m= min (Fig. 3), but it can be seen that the flank wear of the treated tool gradually increases to almost that of the untreated tool. However, at v ¼ 300 m= min (Fig. 4), the
0.100
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0.300 0.250 0.200 0.150 0.100
Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm)
0.050 0.000 0
10
20
30 40 Time (s)
50
60
70
Fig. 5. Flank wear development for cutting speed ¼ 300 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.1 mm/rev.
cryogenically treated tool invariably experiences better wear resistance throughout the entire duration of cutting operation before failure. This phenomenon seems to show that prolonged heating of the cutting tool interface due to the continuous cutting operation has a detrimental effect on the wear resistance of the cryogenically treated tool. This effect is less noticeable at higher cutting speeds, because the tool life becomes very short. (The tool life at v ¼ 150 m= min is 1200 s, whereas that at v ¼ 300 m= min is only 60 s). It can also be deduced that at higher cutting speeds, the time of exposure to high cutting temperatures is rather short, and hence the high temperature does not have sufficient time to produce a detrimental effect on the cryogenic treatment of the cutting tool. It is noteworthy that in Fig. 4, although the cutting speed is higher than Fig. 2, (250 m/min compared to 150 m/ min), the tool wear of the cryogenically treated tool at 250 m/min is always smaller than the tool wear of the untreated tool. This cannot be said for the tool at 150 m/ min, where the tool wear of the cryogenically treated tool is greater than that of the untreated tool towards the end of its tool life. So it is possible to conclude here that cryogenic tools can maintain their superior tool wear resistance if used for just short periods of time. It should be noted also that the time taken for both the treated and untreated tools to chip is approximately the same in all cases mentioned above.
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5. Results from repeated cuts with breaks in between
Flank Wear (mm)
0.600 0.500 0.400 0.300 0.200 Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm)
0.100 0.000 0
50
100 Time (s)
150
200
Fig. 6. Flank wear development for cutting speed ¼ 250 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.1 mm/rev.
0.700 0.600 0.500 0.400 0.300 0.200
Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm)
0.100 0.000 0
100
200
300
400
500
Time (s)
Fig. 7. Flank wear development for cutting speed ¼ 200 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.1 mm/rev.
Flank Wear (mm)
Thus far, it can be seen that while the trends in maximum flank wear somewhat agree with those recorded by Seah et al. [3], the time taken for chipping to occur does not agree at all. In the study by Seah et al. [3], the cryogenically treated tool chips much later than that of the untreated tool. This was not observed in the study described in the previous section. As such, another series of experiments were designed to test the efficacy of cryogenic treatment on the cutting tool inserts. Here the cutting operations were of short predetermined durations, and between each duration of cutting, there was a 5-min break to allow the tool to cool. This set of operations will be known as ‘‘repeated cuts with breaks in between’’. The 5-min break used in this set of experiments is meant to simulate events such as tool change, workpiece change or periods during the machining cycle where no cutting is performed. 5 min may seem to be a long break in some situations, but the intention is to deliberately allow the tool inserts to cool down after short periods of cutting, to study how the tool is affected by this break. Figs. 6 and 7 show the comparison between the cryogenically treated and untreated tools, both with constant depth of cut of 1 mm and feed of 0.1 mm/rev, but for cutting speeds of 250 and 200 m/min, respectively. The horizontal axis shows the total machining time, which excludes the duration of the breaks. From Fig. 6, it can be seen that at a cutting speed of 250 m/min and repeated cuts of 10 s duration with 5 min breaks in between, there is almost no difference in performance between the treated and untreated tools. When the cutting speed is reduced to 200 m/min with repeated cuts of 15 s duration with 5 min breaks in between (Fig. 7), the cryogenically treated tool shows better resistance to chipping, lasting notably longer than the untreated tool, roughly 1.05 times longer. Flank wear was also slightly lower for the cryogenically treated tool. At the point of failure of the untreated tool, the flank wear of the cryogenically treated tool was approximately 4% lesser than that of the untreated tool.
Flank Wear (mm)
0.800
1.800 1.600 1.400 1.200 1.000 0.800 0.600 0.400 0.200 0.000
Untreated Insert Flank Wear (mm) Treated Insert Flank Wear (mm) 0
200
400
600 800 Time (s)
1000
1200
1400
Fig. 8. Flank wear development for cutting speed ¼ 200 m/min, depth of cut ¼ 1 mm, feed rate ¼ 0.05 mm/rev.
Compared to the continuous cutting operations of similar cutting conditions described in Section 4, it can be easily seen that the tools that were used for repeated cuts began chipping much later than those that were used for continuous cuts, regardless of whether the tools had been cryogenically treated or not. This shows that allowing the tool insert time to cool down periodically helps to delay the onset of chipping. Furthermore, the graphs also suggest that this cooling down time is more beneficial to the cryogenically treated tool as the onset of chipping is later than the untreated tool. For the next test (Fig. 8), the cutting speed was kept at 200 m/min, but the feed was reduced to 0.05 mm/min, and the repeated cuts were of 60 s duration with 5 min breaks in between. From the graph, it can be seen that chipping of the cryogenically treated tool occurred much later than that of the untreated tool, roughly 1.3 times longer. Flank wear of the treated tool was also lesser than that on the untreated tool. At the point of failure of the untreated tool, the flank wear of the cryogenically treated tool was approximately 10% lesser than that of the untreated tool. It is noteworthy that no convergence points exist in the experiments with the repeated cuts. Since the tool was allowed to cool down for 5 min in between cuts, not only was the maximum temperature experienced by the tool reduced, but it seemed to also allow the cryogenically treated tool to retain its better wear resistance. Although tool inserts are seldom used to an extent where the flank wear reaches 1.5 mm, this experiment shows that at mild
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cutting conditions, with repeated cuts where the tool is allowed to cool, cryogenically treated tools perform far better than the untreated counterparts. Another observation which is not shown in the figures is that there is a substantial difference between the amounts of flank wear experienced after continuous cutting and after a series of repeated cuts with breaks in between. For a cutting speed of 250 m/min and 12 sessions of 10 s long repeated cuts with 5 min intervals in between, the flank wear for the cryogenically treated tools was 0.401 mm for repeated cutting and 0.486 mm for continuous cutting. For the untreated tool, the flank wear was 0.404 mm for repeated cutting and 0.492 mm for continuous cutting (compare Figs. 4 and 6). To cite another example, for a cutting speed of 200 m/min and 12 sessions of 15 s cutting with 5 min intervals in between, the flank wear for the treated tools was 0.332 mm for repeated cutting and 0.398 mm for continuous cutting. For the untreated tool, the flank wear was 0.341 mm for repeated cutting and 0.400 mm for continuous cutting (compare Figs. 3 and 7). Evidently, the 5-min breaks reduced the flank wear by about 20% in each case.
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with repeated cuts of 30 s duration with 5 min breaks experienced it at a flank wear of 0.47 mm at 150 s. Performing the repeated cuts for a shorter duration before breaks prevented the tool from heating up as much as while performing repeated cuts for longer durations. The former case results in the tool chipping later, an observation which further supports the theory propounded so far. The results in Figs. 2–5, which show graphs of flank wear against time for continuous cuts, demonstrate that cryogenic treatment apparently does not seem to increase the resistance to chipping of cryogenically treated tools. This seems to contradict the results obtained by Seah et al. [3]. However, the experimental results for repeated cuts (Figs. 6–9) show trends similar to the results of Seah et al. [3], i.e. The onset of chipping is delayed as a result of cryogenic treatment. What their research and the results in this section have proven is that cryogenically treated tools, if allowed time to cool during machining operations, will retain their superiority in terms of resistance to chipping over the untreated tools. 7. Conclusions
6. Effect of cutting duration before a break Experiments were conducted to determine if the cutting duration before a break affected the performance of the cryogenically treated tools. Keeping cutting conditions fixed at cutting speed of 250 m/min, depth of cut of 1 mm and feed of 0.1 mm/rev, a comparison was made between cutting durations of 10 s and 30 s, respectively, before a break was allowed. The graphs in Fig. 9 show the comparison between the cryogenically treated and untreated tools, for the two different cutting durations before a break. It would be expected that under such cutting conditions, there would not be a significant difference in the amount of tool wear between the cryogenically treated and untreated tools. Nevertheless, for the run with repeated cuts of 10 s duration with 5-min breaks in between, the cryogenically treated tool experienced chipping at a flank wear of 0.53 mm at 180 s, whereas the run
Flank Wear (mm)
0.600 0.500 0.400 0.300 30s intervals (Treated) 10s intervals (Treated) 30s intervals (Untreated) 10s intervals (Untreated)
0.200 0.100 0.000 0
50
100
150
Time (s)
Fig. 9. Effect of cutting duration before a break.
200
Cryogenic treatment no doubt improves the resistance to chipping of tools and to a less significant extent, improves flank wear resistance. However, under certain conditions, such as prolonged exposure to high temperatures during long continuous cutting operations, cryogenically treated tools can lose their superior properties. Cryogenically treated tools perform better than untreated tools when performing continuous cuts for short periods of time, or when performing repeated cuts with breaks in between, both in terms of decreased tool wear, and increased resistance to chipping. Cryogenically treated tools subjected to prolonged periods of high temperature at the cutting edge lose their wear resistance, suggesting that high temperatures alters the property of the treated tool. Hence the state of cryogenically treated materials is not a permanent state, but a metastable one. Cryogenically treated tools performing repeated cuts with breaks in between are able to cool down between cuts, thereby recovering some property that allows them to retain their superior wear resistance. This property also gives it superior resistance to chipping, compared to untreated tools. In light of the fact that cryogenically treated tools perform best when the tool temperature is kept low, their effectiveness can be extended if coolants or suitable methods of cooling are used to keep the tool temperatures low. Hence, the validity of claims that cryogenic treatment can improve the lifespan of cutting tools would depend a lot on the cutting conditions. Tools under mild cutting conditions stand to gain from cryogenic treatment, but heavy duty cutting operations with long periods of heating of the cutting tool will not benefit from it.
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References [1] Y. Dong, X. Lin, H. Xiao, Deep cryogenic treatment of high-speed steel and its mechanism, Heat Treatment of Metals 3 (1998) 55–59. [2] D.N. Collins, J. Dormer, Deep cryogenic treatment of a D2 cold-work tool steel, Heat Treatment of Metals 3 (1997) 71–74.
[3] K.H.W. Seah, M. Rahman, K.H. Yong, Performance evaluation of cryogenically treated tungsten carbide cutting tool inserts, Proceedings of the Institution of Mechanical Engineers Part B–Journal of Engineering Manufacture 217 (1) (2003) 29–43. [4] W.E. Bryson, Cryogenics, Hanser Gardner Publications, Cincinnati, OH, 1999, pp. 81–107.