wear of coated and uncoated carbides in turning tool steel

8/8/2019 Wear of Coated and Uncoated Carbides in Turning Tool Steel

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Wear of coated and uncoated carbides in turning tool steel

C.H. Che Haron*, A. Ginting, J.H. GohFaculty of Engineering, Department of Mechanical and Materials Engineering, National University of Malaysia, 43600 Bangi, Selangor DE, Malaysia

Abstract

The 358-diamond-shaped insert with simple grooves of coated and uncoated carbide tools were used in turning tool steel bars (23 HRC)

with the objective of describing the wear behaviour of these tools based on the ¯ank wear data. Machining tests were performed under wet

and dry cutting conditions at various cutting speeds, while the feed rate and depth of cut were kept constant. A certain strategy was

established in order to obtain smooth initial wear and avoid concentrated impact load that could trigger chipping when machining was

started by making a 5 mm precut entry. It was found that the coated carbide tools were superior to the uncoated carbide tools and their ¯ank

wear grew smoothly. By linking the machining operations and the tool life curves obtained using the ¯ank wear data, the wear behaviour of

coated and uncoated carbide tools was described. # 2001 Elsevier Science B.V. All rights reserved.

Keywords: Carbides; Turning; Tool steel; Flank wear

1. Introduction

Coated and uncoated carbides are widely used in the

metal-working industry and provide the best alternative

for most turning operations. When machining using carbides

under typical cutting conditions, the gradual wear of the¯ank and rake faces is the main process by which a cutting

tool fails. Venkatesh [1] carried out tool wear investigations

on some cutting tool materials. He plotted tool life curves

using the ¯ank wear criterion and obtained that the tool life

of carbides decreased quickly at higher speed.

Some authors af®rm that the ¯ank wear in carbide tools

initially occurs due to abrasion and as the wear process

progresses, the temperature increases causing diffusion to

take place [1±5]. Actually, the fact that abrasive wear may

occur in metal cutting is not surprising since there are many

hard abrasive particles present in metals, especially in steel

[6,7].

The use of coolant to increasetoollifeis anissue withmany

differing views. In contrast, others have found that coolant

promotes tool wear in machining. The inherent brittleness of

carbides renders them susceptible to severe damage by crack-

ing if sudden loads of thermal gradients are applied to their

edge [8]. KoÈnig and Klinger [9] also claimed that better

performance of carbides was obtained under dry cutting.

This paper is a contribution towards understanding the

behaviour of carbides, mainly with the aim of describing the

wear behaviour of coated and uncoated carbide tools based

on the ¯ank wear data. For that purpose, the 358-diamond-

shaped insert with simple grooves of coated and uncoated

carbide tools were used in turning tool steel bars (23 HRC).

Machining tests were performed under wet and dry cutting

conditions at various cutting speeds (75±350 m/min), while

feed rate (0.16 mm/rev) and depth of cut (1 mm) were keptconstant.

2. Experimental details

2.1. Workpiece material

In this study, tool steel with ISO designation 95MnCrW 1

[10] was selected as the workpiece material. The material

was supplied in fully annealed condition, cylindrical in

shape, 100 mm diameter and 1 m length in size. In order

to meet the requirement of ISO 3685 [11] that the length/

diameter ratio of the workpiece material to be used should be

less than 10 during testing, the bar was cut into three pieces

(330 mm length). Each bar was checked for its hardness

across the diameter at each end prior to the tests and the

average value of the hardness measurements was 23 HRC.

The chemical compositions of workpiece material are

0.95%C, 0.3%Si, 1.2%Mn, 0.5%Cr, 0.5%W, 0.1%V.

2.2. Cutting tools and tool geometry

Coated and uncoated carbide inserts were used for the

turning tests. The inserts were manufactured by Kennametal

Journal of Materials Processing Technology 116 (2001) 49±54

* Corresponding author.

0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 8 4 1 - X



with an ISO designation of VBMT 160408 (358-diamond-

shaped insert with simple grooves), grade KC 9025 for the

coated carbides and grade K 313 for the uncoated carbides.

The substrate material of KC 9025 is the same as that for K

313. KC 9025 is coated with a TiCN underlayer, an inter-

mediate layer of Al2O3 and a TiN outerlayer. The inserts

were rigidly mounted on a tool holder with an ISO designa-

tion of SVJBR 2525 M16. The assembled tool geometry is

given in Table 1.

2.3. Cutting conditions

Turning experiments were performed under wet and dry

cutting at various cutting speeds, while feed rate (0.16 mm/

rev) and depth of cut (1 mm) were kept constant. Oil-based

coolant, RATAK SAN 211-P with a density (158C) of

0.868 g/ml, viscosity (408C) of 3.8 cst and ¯ashpoint of

2168C, was used in wet cutting.

Based on ISO 3685 [11], four different cutting speeds

were used during testing, the coated carbide tools were

tested at the following cutting speeds: 200, 250, 300 and

350 m/min, while the uncoated carbide tools were tested atthe following cutting speeds: 75, 100, 125, and 150 m/min.

Cutting speeds corresponding to 350 m/min for the coated

carbide tools and 150 m/min for uncoated carbide tools were

approximately the upper limit of the application range, since

any further increment resulted in extremely short tool life or

even premature tool breakage soon after the tests were

started.

Chubb and Billingham [2] claimed that they performed

high speed machining tests when turning annealed EN24

steel with coated carbide tools at cutting speed of 244 m/

min. The feed rate and depth of cut used were 0.185 mm/rev

and 2 mm, respectively. In that sense, cutting speeds of 250±

350 m/min used in this study for coated carbide tools can be

considered as high speed machining. However, in a recent

review of high speed machining, other authors had classi®ed

the criteria of high speed machining based on the material to

be cut [12]; thus machining steel at cutting speeds of 250±

350 m/min fall in the transitional range, between the con-

ventional and high speed range.

2.4. Experimental techniques

The assembled tool and workpiece were mounted on a

Cincinnati Milacron's Avenger 200T CNC turning centre.

The CNC turning centre was operated at the speci®ed cutting

conditions described previously. Tool wear was observed

and measured using Mitutoyo's Absolute Digimatic digital

vernier microscope, with a magni®cation ranging from 5 to

10 times.

ISO 3685 [11] was used as a guide in establishing the wear

criterion. Some preliminary cuts were also conducted toestablish this tool wear criterion. The observations and

measurements obtained from these preliminary trials

showed that the ¯ank face of the coated (KC 9025) and

uncoated (K 313) carbide tools was regularly worn in zone C

and therefore, VBmax of 0.6 mm was taken as the wear limit

to determine tool life.

Flank wear was observed and measured at various cutting

intervals throughout the experiments. In order to obtain

smooth initial wear and avoid concentrated impact load that

could trigger chipping when machining started, a 5 mm

precut entry was made for every new pass of cutting. A

separate insert was used to machine this precut entry for each

test using the same cutting condition as that of the insert to betested. This strategy was established since the total failure or

breakage occurred in some cases after a certain amount of

chipping had occurred, and in particular, it occurred rather

soon after the beginning of the tests [13]. An experiment was

terminated upon the detection of signi®cant chattering result-

ing from roughening of the machined surface and before the

insert is totally worn (detected visually after each pass of

machining) since it could increase the ¯ank wear rapidly.

3. Results and discussion

3.1. Tool wear

The ¯ank wear values of the coated and uncoated carbide

tools for the different cuttingspeedswere presented in Figs. 1

and 2, respectively. From the ®gures, it can be seen that the

¯ank wear curves were generally in three stages: at the initial

stage, followed by the gradual stage, and ®nally the abrupt

stage of wear. This behaviour was also discussed in detail

and reported by other researchers [2,5,14,15].

The initial wear pattern on the ¯ank face and the nose of

the coated carbide tools were similar to the uncoated carbide

tools. The width of the wear increased rapidly, accompanied

by the formation of severe abrasive marks, with further

cutting. Upon attaining a certain wear value, the ¯ank wear

was relatively constant and this was followed by abrupt wear

until the wear criterion was reached. Clear examples of these

stages can be observed from the curves of ¯ank wear in

Fig. 1(b), at a cutting speed of 350 m/min and in Fig. 2(a), at

a cutting speed of 100 m/min. It was found that the ¯ank face

of the coated and uncoated carbide tools were regularly worn

in zone C. In the case of coated carbide tools, VBmax

occurred at the edge of nose region, while for the uncoated

carbide tools it was in the range 0.4±0.7 mm from the cutting

edge. The typical wear of coated and uncoated carbide tools

Table 1

Cutting tool geometry

Back rake angle (8) 0

End relief angle (8) 5

End cutting-edge angle (8) 52

Side cutting-edge angle (8) 3

Side rake angle (8) 0

Side relief angle (8) 5Nose radius (mm) 0.8

50 C.H. Che Haron et al. / Journal of Materials Processing Technology 116 (2001) 49±54



were shown in Figs. 3 and 4, respectively. Other wear

patterns, e.g. notch wear, was found on the coated and

uncoated carbide tools. At the ®nal stage of cutting; atcutting speeds of 250±350 m/min, chipping occurred at

the nose edge of the coated carbide tools as shown in Fig. 3.

Although machining was not performed using identical

cutting speeds, it can be concluded that the ̄ ank wear rate of

uncoated carbide tools was greater than the coated carbide

tools as shown in Table 2. Dearnley [4] also reported similar

results with regards to the behaviour of coated and uncoated

carbide tools when machining steel.

From Fig. 1(a) and (b), at a cutting speed of 200 m/min,

the wear progression was relatively slow and the cutting time

recorded to reach the wear limit was signi®cantly longer

than for the other speeds. Although the cutting time wasmuch less than the coated carbide tools, the behaviour of

¯ank wear of uncoated carbide tools at a cutting speed of

75 m/min was similar to using the coated carbide tools at

200 m/min (Fig. 2(a) and (b)). The curves plotted using the

¯ank wear data at cutting speeds of 250 m/min (for coated

carbide tools) and 100 m/min (for uncoated carbide tools)

had similar trend with the curves plotted using data for

Fig. 1. Flank wear curves for coated carbide tools: (a) wet cutting; (b) dry

cutting.

Fig. 2. Flank wear curves for uncoated carbide tools: (a) wet cutting; (b)

dry cutting.

Fig. 3. Typical wear of coated carbide tool (cutting speed 300 m/min

under wet cutting condition).

Fig. 4. Typical wear of uncoated carbide tool (cutting speed 125 m/min

under wet cutting condition).

C.H. Che Haron et al. / Journal of Materials Processing Technology 116 (2001) 49±54 51



cutting speeds of 200 and 75 m/min described previously.

This showed that the wear progression at these cutting

speeds was similar even when the cutting times at these

cutting speeds were less than when using the lower cutting

speeds (75 and 200 m/min).Wear progression of coated carbide tools at cutting speeds

of 300 and 350 m/min (Fig. 1(a) and (b)) was similar to the

uncoated carbide tools at cutting speeds of 125 and 150 m/

min (Fig. 2(a) and (b)). At these cutting speeds, the curves

were almost linear, ¯ank wear was very rapid and cutting

time was much shorter than the others. In particular, the

cutting time values of the uncoated carbide tools at these

cutting speeds were extremely short (less than 1 min). Based

on ISO 3685 [11], the cutting speeds should actually be

chosen such that the cutting time at the highest speed of

testing was not less than 5 min. However, cutting speeds of

75±150 m/min were selected in order to obtain the upperlimit of the application range of uncoated carbide tools.

Thus, it can be concluded that the coated carbide tools was

superior to the uncoated carbide tools and its ¯ank wear

grew smoothly compared to the uncoated carbide tools. In

addition, it can be stated that the uncoated carbide tools

performed best at cutting speeds below 75 m/min in order to

obtain a cutting time of not less than 5 min, while the coated

carbide tools can be used with a cutting time in excess of

5 min at cutting speeds of up to 350 m/min.

3.2. Tool life

Tool life curves obtained from the ¯ank wear data at

various cutting speeds under wet and dry cutting conditions

were shown in Figs. 5 and 6. From these curves, it was

observed that the tool life of coated and uncoated carbide

tools decreases quickly at higher speeds. Although the tool

life of coated carbide tools was much longer than the

uncoated carbide tools in magnitude, their curves were

similar in trend. By this reason, it can be concluded that

the behaviour of tool life against cutting speed for coated and

uncoated carbide tools was similar in nature. Venkatesh [1]

carried out tool wear investigations on some cutting tool

materials. He plotted tool life curves using the ¯ank wear

criterion and obtained that the tool life of carbide tools

decreased quickly at higher speed.

The performance of machining related to the tool life of

coated and uncoated carbide tools under wet and dry cuttingconditions at various cutting speeds were presented in Figs. 7

and 8, respectively.

From Fig. 7, it can be seen that the performance of coated

carbide tools under wet cutting was signi®cantly better than

under dry cutting for the all the selected cutting speeds. In

case of uncoated carbide tools (Fig. 8), the oil-based coolant

was found effective in increasing the tool life compared to

machining without coolant at cutting speeds of 75 and

150 m/min, the approximate increase was 33 and 14%,

Table 2

Wear rate data after cutting tool steel (23 HRC) at VBmax 0.6 mm using a

feed rate of 0.16 mm/rev and depth of cut of 1 mm

Tool type Cutting speed

(m/min)

Wear rate (mm/min)

Wet cutting Dry cutting

Coated carbides (TiCN,

Al2O3, TiN)

200 0.011 0.012

250 0.019 0.025

300 0.033 0.040

350 0.040 0.080

Uncoated carbides 75 0.127 0.169

100 0.269 0.250

125 0.706 0.632

150 1.224 1.395Fig. 5. Tool life curves for coated carbide tools.

Fig. 6. Tool life curves for uncoated carbide tools.

Fig. 7. Tool life of coated carbide tools. Wet and dry cutting in

comparison.




respectively. In contrast, dry cutting was found better than

wet cutting at cutting speeds of 100 and 125 m/min where

the approximate increase was 8 and 12%, respectively.

The result for the coated carbide tools, where wet

cutting was better than dry cutting, is in agreement withthe claim that the use of coolant could increase the tool life.

Wet cutting is better for coated carbide tools probably

because of the effect of coatings. Schintlmeister et al.

[16] had summarised the effect of coatings in the following

statements: (1) reduction in friction, in generation heat, and

in cutting forces; (2) reduction in the diffusion between

the chip and the surface of the tool, especially at higher

speeds (the coating acts as a diffusion barrier); (3) preven-

tion of galling, especially at lower cutting speeds. In this

sense, the use of coolant in wet cutting was actively sup-

porting the effect of coatings, particularly in reducing fric-

tion and heat; and therefore, longer tool life could beattained. Further, as the machining process progresses, the

layers of coatings (TiN, Al2O3, TiCN) cracked because of

abrasion (¯ank wear) and thermal stresses [17]. Once the

coatings were removed from a speci®c region, steel adhered

to the substrate and then diffusion wear began to play a more

important role than abrasion [2]. From the observation made

while measuring the ¯ank wear, it was found that a small

amount of chip adhered at the region where the coating was

removed and chipping at the nose region of the coated

carbide tools occurred when VBmax was approximately

0.6 mm (Fig. 3). Chipping occurred at the end of tool life

probably due to thermal shock. It occurred when coolant was

applied to the cutting edge where the coatings were com-

pletely removed from that particular region. This reason was

also supported by the results of uncoated carbide tools,

where dry cutting, at cutting speeds of 100 and 125 m/

min, was found to be better than wet cutting. KoÈnig and

Klinger [9] also claimed that better performance of carbide

tools was found under dry cutting condition. In addition,

Wright et al. [8] have reported that the inherent brittleness

of carbides renders them susceptible to severe damage

by cracking if sudden loads of thermal gradients were

applied to their edge. In the case of 150 m/min, although

wet cutting was found to be better than dry cutting, the

different was not signi®cant, which was 14% or in actual

cutting time about 3.6 s improvement. Wet cutting was

found to be better at 75 m/min since the temperature was

lower than at higher cutting speeds; thus, thermal shock

had not occurred yet. In other words, at 75 m/min, the wear

process has not changed from mechanical wear to thermal

wear [18]. Mechanical wear, or abrasion, is typically domi-nant during initial cutting and at lower cutting speeds. At

higher cutting speeds, the temperature of the tool increases

and the thermal wear process can be dominant. The chips

collected throughout the machining tests showed a variety

of colours, which changed with cutting speed and tool wear.

The colours were usually caused by oxidation and provide

a limited indication of the relative temperatures involved

during machining.

4. Conclusions

The following conclusions could be made to describe thewear behaviour of 358-diamond-shaped insert with simple

grooves of coated and uncoated carbide tools used in turning

tool steel bars (23 HRC):

1. Wear progression of coated and uncoated carbide tools

are generally in three stages: at the initial stage,

followed by the gradual stage, and finally the abrupt

stage of wear. However, the wear rate of uncoated

carbide tools is much higher than coated carbide tools.

2. Flank face of coated and uncoated carbide tools are

regularly worn in zone C. In the case of coated carbide

tools, wear occurs at the edge of nose region, while foruncoated carbide tools it is in the range 0.4±0.7 mm

from the cutting edge (still below the nose radius).

3. Cutting speeds above 75 m/min are considered the upper

limit of the application range for uncoated carbide tools

since the tool life at cutting speeds of 75 m/min and

above is less than 5 min.

4. Coated carbide tools can be used at cutting speeds of up

to 350 m/min with tool life of more than 5 min

(approximately 7 min under dry cutting and 15 min

under wet cutting). Cutting speeds of 250±350 m/min,

when machining steel, are in the transitional range,

between the conventional and high speed range.

5. Coated carbide tools is superior to uncoated carbide

tools and its flank wear grows smoothly than uncoated

carbide tools.

6. Wet cutting is better than dry cutting for coated carbide

tools. The use of oil-based coolant can increase the tool

life of coated carbide tools since the coolant is actively

supporting the effect of coatings, particularly in

reducing friction and heat. Chipping occurs at the end

of the tool life of coated carbide tools probably because

of thermal shock occurring when the coolant is being

applied to the cutting edge where the coatings (TiN,

Al2O3, TiCN) are completely removed from that

Fig. 8. Tool life of uncoated carbide tools. Wet and dry cutting in

comparison.

C.H. Che Haron et al. / Journal of Materials Processing Technology 116 (2001) 49±54 53



particular region. In other words, the wear process has

changed from mechanical wear (abrasion) to thermal

wear.

7. Better performance of uncoated carbide tools is attain-

able with wet cutting and below cutting speeds of 75 m/

min. This is because at these speeds the wear process

has not changed from mechanical wear (abrasion) tothermal wear. In the case of cutting speeds of more than

75 m/min, better performance of uncoated carbide tools

is attainable under dry cutting.

8. The chips collected throughout machining tests show a

variety of colours, which changed with cutting speed

and tool wear. The colours are caused by oxidation and

provide a limited indication of the relative temperatures

involved during machining.

Acknowledgements

This research has been carried out with ®nancial supportfrom National University of Malaysia through FK 0010/99,

which is gratefully acknowledged.

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wear of coated and uncoated carbides in turning tool steel

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