Influence of inserts coating and substrate on TooloxR44 machining

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http://www.diva-portal.org

This is the published version of a paper presented at Swedish Production Symposium '07.

Citation for the original published paper:

Daghini, L., Nicolescu, C. (2007)

Influence of inserts coating and substrate on TooloxR44 machining.

In:

N.B. When citing this work, cite the original published paper.

Permanent link to this version:

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INFLUENCE OF INSERTS COATING AND SUBSTRATE ON TOOLOX®44 MACHINING

L. Daghini, C. M. Nicolescu

KTH, School of Industrial Engineering and Management, Department of Production Engineering, Stockholm, Sweden

lorenzo.daghini@iip.kth.se

Abstract: The objective of the research presented in this paper is to characterize the machinability of TOOLOX 44 during cutting with PALBIT inserts with focus on how different combinations of coatings and substrates influence the machining process in aspects such as tool life, cutting forces, temperature and chip forming process.

The foremost result is that TOOLOX is machinable and when the right tool is chosen high productivity can be achieved. Using the right insert, equipped with chip- breaker, should allow to machine this hardened steel even at higher cutting speeds than the ones used in this investigation.

Keywords: Hard turning, tool life, cutting force, coating, substrate

1. INTRODUCTION

The use of more and more sophisticated materials in today’s industry requires the development of tools that and optimized cutting processes that can guarantee high machining rates. The mechanical properties of a material being cut play an important role for machining economics and therefore the cutting conditions must be capable to meet the specifications of size, shape and surface finish.

SSAB’s TOOLOX 44 is hardened steel that put serious problems on machinability. The high strength of this material allows shaping the desired part directly through machining avoiding the usual heat treatments. The structural properties of this material do not allow choosing operation parameters without constraints. PALBIT, a Portuguese company specialized in carbide production, has provided cutting insert of different substrate and coating that were specially design for external turning of TOOLOX 44.

2. MATERIALS AND TESTING PROCEDURES The cutting inserts¹

PALBIT has delivered seven types of cutting inserts, different combinations of two coatings and four substrates, see table 1.

1 All data are courtesy of Palbit, Minas e metalurgia S.A. , Portugal

Table 1 List of the available inserts.

# Substrate Coating

1 SC21

(P20²) STN

2 SC21 Tinalox

3 SM10

(K10² micrograin) STN

4 SM10 Tinalox

5 SA35

(P40²) STN

6 SA35 Tinalox

7 SM01 (K05² micrograin)

STN

In the continuation of this article the inserts could be referred to this table, for example: SA35+STN will be called insert 5. Table 2 shows the properties of SC21 and SM10 substrates and figure 1 shows a comparison of the two substrate.

All the inserts were delivered with the same geometry, triangular with three available edges, without any chip-breaker.

2 According to ISO513:2004

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Table 2 Properties of SC21 and SM10 substrate.

Property SC21 SM10

%(weight)WC 85.5 90.2

%(weight)TaC/NbC 6.0 2.78

%(weight)Co 6.2 6.8

%(weight)TiC 2.3 - Granulometry of WC

[μm] 3.20 0.8

Theoretical density

[g/cm³] 14.14 14.84

Density [g/cm³] 14.05±0.05 14.74±0.05 Hardness [HV10] 1580±40 1680±50

TRS [N/mm²] 1900 3000

Grain size ASTM Medium micrograin WC-matrix principal

[μm] 2.0 – 4.0 0.5 – 1.5

WC-matrix various

[μm] 4.0 – 5.0 1.5 – 3.0

WC-matrix isolated

[μm] 5.0 – 12.0 3.0 – 9.0

Bind Dispersed Dispersed

Gamma phase Medium Thick

Fig. 1 Difference in grain size between SC21 (left) and SM10 (right) (1500X).

The properties and compositions of the two coatings are shown in table 3, and figure 2 shows how differently the two coatings look like at 1500X enlargement

Table 3 Properties of Tinalox and STN coating.

Property STN Tinalox

Microhardness [HV] 3600 3600 Layer TiN-TiCN-

TiC-TiCN- TiN

TiAlN+Al2O3

Thickness [μm] 2 – 4 6 – 10

Process CVD PVD

Fig. 2 Microstructure of Tinalox (left) and STN (right) (1500X).

The workpiece material

For machining, TOOLOX bars, with diameter ∅150 mm and length 500 mm were used. TOOLOX is a pre-hardened tool steel with a hardness between 410 and 475 HBW (corresponds approximately to 41 – 47 HRC)³. The chemical composition is shown in table 4 and table 5 shows the mechanical properties.

Table 4 Chemical composition of TOOLOX 44 (typical values)⁴.

Element Quantity

C 0.31%

Si 0.60%

Mn 0.90%

P, max 100 ppm

S, max 40 ppm

Cr 1.35%

Ni 0.70%

Mo 0.80%

V 0.145%

CEV (IIW) 0.96

CET 0.57 Table 5 Mechanical properties of TOOLOX 44

(typical values).⁴

Temp Rm Rp0,2 Elongation A5

Impact toughness

°C [MPa] [MPa] [%] [J]

20 1450 1300 13 30

200 1380 1200 10

TOOLOX 44 has a compressive strength (Rc0,2) of about 1040 MPa after the material has been kept at 400°C for 175 minutes and impact toughness of 85 J when the has been heated up at 400° C⁵.

Testing method

The tests were carried out in turning with the cutting edge angle (κ) of 90°. In first phase the tool life was investigated for all inserts. In the second phase, cutting force and temperature were recorded along with tool life for selected inserts. Finally, the quick- stop test has been carried out in order to study the chip formation process. The tool life investigation was carried out on a SMT Swedturn 6 turning centre

3 Data from

http://www.toolox.com/Pages/English/Main%20Fra me_eng.htm

4 Data from

http://www.toolox.com/PDF/English/ENG_Anvdndn ing_T44.pdf

5 Courtesy of Dr. Per Hansson, Senior Research Metallurgist at SSAB Oxelösund AB, Sweden.

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(Fig. 3). The tool-holder was a MIRCONA CTGPR- 0025*16 (Fig. 4) specially designed for use with hydrostatic tool holders type Hydrofix (SPIREX- TOOLS). The force measurement and quick stop tests were performed on a TJECKO-SVEA SUI 63 lathe.

Tool life. The tool life test has been carried out in two stages, at first the inserts underwent a comparison at one set of cutting parameters (vc = 80 m/min, f = 0.3 mm/rev and ap = 1.5 mm), then the three that gave longest tool life were compared at different sets of parameters (see table 6). The inserts were considered to be worn out when maximum flank wear (VB) reached 0.3 mm.

Table 6 Cutting parameters used in the tool life investigation.

f ↓ vc →

[m/min] 40 60 80

0.3 mm/rev X X

0.225 mm/rev X

0.15 mm/rev X X

Tool wear has been measured using a Nikon SMZ800 stereo microscope and Primage software for image processing.

Cutting force. Cutting force was measured with a Kistler dynamometer plate type 9441 connected to three charge amplifiers type 5011, one for each channel. Feed (x-direction), tangential (z-direction) and radial (y-direction) components have been measured. The software used for the data acquisition was LMS Test.Lab. Further data, such as calculating the average over the tool engagement time, has been performed by help of Matlab. The cutting parameters at which the forces were recorded are feed 0.3 mm/rev depth of cut 2 mm and cutting speed 40, 60, 80 and 100 m/min. The averaged magnitudes of the three components have then been used to calculate the magnitude of the resultant force using equation 1:

2 2 2

z y

x F F

F

F = + + (1)

Fig. 3 SMT Swedturn 6 machining centre

Fig. 4 The Hydrofix tool holder

Temperature. Temperature was measured manually with a thermocouple positioned in the proximities of the cutting zone.

Chip forming process. Chip formation process has been investigated by measuring the chip thickness and by using quick-stop procedure (Fig. 5) for freezing the process. Samples of partially formed chips so obtained have been used to produce micrographs of the chip at the incipient formation.

Chip thickness has been measured manually for all combinations of cutting parameters and inserts with a micrometer. Quick-stop tests were carried out at two combinations of parameters vc=60 to 80 m/min, f=

0.15 mm/rev and ap = 2 mm for some of the inserts.

Fig. 5 The quick stop device

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3. RESULTS Tool life

The first test gave as a result that insert 2 and 4 (SC21+Tinalox and SM10+Tinalox) were the ones with longer tool life, see Fig. 6.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

0 10 20 30 40

Cutting time (min)

VB (mm)

SC21+TiNAlOx SNG SA35+TiNAlOx SNG SM10+TiNAlOx SNG

SM01+STN SC21+STN SM10+STN

SA35+STN

Fig. 6 Result of the first stage of the tool life investigation, all inserts were tested at vc =

80 m/min and f = 0.3 mm/rev.

The inserts number 5 and 6 (both SA35 substrate inserts) failed almost instantaneously due to plastic deformation, see Fig. 7.

Fig. 7 Example of plastic deformation of the tool.

Therefore insert 3, which has the same substrate of insert 4 (SM10), was chosen for the second stage of this tool life investigation.

When machining at a cutting speed of 40 m/min and and a feed of 0.3 mm/rev the inserts 2 and 3 reached a flank wear of 0.30 mm when insert 4 was still under 0.10 mm (see Fig. 8).

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

0 5 10 15 20 25 30

VB (mm)

SC21+TiNAlOx SNG SM10+TiNAlOx SNG SM10+STN

Fig. 8 Tool life when machining at vc = 40 m/min and f = 0.3mm/rev.

At half the feed (f = 0.15 mm/rev) the three inserts survived for longer than 40 minutes, no major difference could be noticed, see Fig. 9.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

0 20 40 60 80

VB (mm)

SM10+TiNAlOx SNG SM10+STN SC21+TiNAlOx SNG

Fig. 9 Tool life when machining at vc = 40 m/min and f = 0.15 mm/rev.

When the cutting speed was doubled with the feed kept constant (vc = 80 m/min, f = c0.15 mm/rev) the inserts did not reach the flank wear criterion after 15 minutes of machining. A difference could be noticed though, insert 4 did not show any increase in flank wear for about 25 minutes, while the other two revealed a clear dependency to wear (see Fig. 10).

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

0 5 10 15 20 25 30

VB (mm)

Fig. 10 Tool life when machining at vc = 80 m/min and f = 0.15mm/rev.

The effect of increasing the feed (vc = 80 m/min, f = 0.30mm/rev) was evident, insert 3 and 4 showed the same tool life (about 30 minutes) while insert 2 worn out after barely 15 minutes (see Fig. 11)

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0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

0 10 20 30 40

VB (mm)

Fig. 11 Tool life when machining at vc = 80 m/min and f = 0.3 mm/rev.

At a cutting speed of 60 m/min and a feed of 0.225 mm/rev the three inserts behaved similarly, none of them reached the tool life end criterion, although insert 4 still showed lower flank wear than the other inserts (see Fig. 12).

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

0 10 20 30 40

VB (mm)

Fig. 12 Tool life when machining at vc = 60 m/min and f = 0.225 mm/rev.

Cutting force

A previous research (Chen and Tsao, 1997) demonstrated that different tool coatings do not give substantial difference in cutting force, the results here achieved confirms those. In figure 10 is summarized the result of the force measurement at the different cutting parameters for the different inserts. A substantial difference is noticeable at higher cutting speed (80 and 100 m/min), where the SC21 substrate combined with Tinalox coating gave much lower readings (see Fig. 13), even lower than the force measured at lower speed.

Fig. 13 Cutting force at four different cutting speed settings.

This result can be explained by the falling characteristic of the cutting force (Trent, 1984) taking into account the lower range of cutting speed used for machining of TOOLOX. Also it can be mentioned that the force transmitted from the cutting interface to the point of measurement is influenced by the structural path formed by the cutting insert, the tool holder, the dynamometer etc. In figure 14 is shown how differently insert 2 (SC21+Tinalox) and insert 4 (SM10+Tinalox) behaved under cutting conditions after the same machining time.

Fig. 14 Comparison between insert 2 (left) and insert 4 (right) after approximately the same machining time. The “wrinkles” visible under the tip of insert 2 are the sign of the deformation.

Temperature

The results shown in table 7 give an idea on how the coating influence the temperature. By comparing the results for the first two inserts in this table, it appears that the Tinalox coating develops higher temperature than the STN.

Table 7 Temperatures measured near the cutting zone.

Insert type↓ vc →

[m/min] 100 80 60 40

SA35+STN Breakage 266° 220° 218°

SA35+Tinalox Breakage 278° 252° 359°

SC21+STN Breakage 234° 312° 288°

SM01+STN 270° 265° 262° 256°

Chip forming process

The results of the chip thickness measurement is represented in figure 15 as function of cutting speed.

The feed is 0.3 mm/rev and the values represent the average of different measures, the deviation is shown as well. A certain trend can be observed especially for the SM10+Tinalox insert (3 in the graph), it appears that the chip thickness diminishes quite drastically from 40 to 60 m/min for then stabilize.

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1a

1b 1c 1d

2b

2c

2d 3a2a

3b 3c 3d

0.3 0.35 0.4 0.45 0.5

30 40 50 60 70 80 90 100 110

cutting speed [m/min]

chip thickness [mm]

1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d

Fig. 15 Chip thickness as function of cutting speed.

The feed is 0.3 mm/rev. 1, 2 and 3 are respectively SC21+Tinalox, SM10+STN and SM10+Tinalox; a,b,c and d are indicating the cutting speed.

When machining at half the feed (f = 0.15 mm/rev) the same trend could be observed. Figure 16 shows how the chip thickness diminishes with the higher cutting speed.

1a

1b 2a

2b 3a

3b

0.15 0.17 0.19 0.21 0.23 0.25 0.27 0.29 0.31 0.33

30 40 50 60 70 80 90

cutting speed [m /m in]

chip thickness [mm]

1a 1b 2a 2b 3a 3b

Fig. 16 Chip thickness as function of cutting speed.

The feed is 0.15 mm/rev. 1, 2 and 3 are respectively SC21+Tinalox, SM10+STN and SM10+Tinalox.

The ratio between the real chip thickness (hch) and the nominal chip thickness (hD) gives, an indication on the machinability of the material. As Trent (1984) writes, the greater is this ratio the lower is the value of the shear plane angle (φ) and the larger is the area of the shear plane, and therefore the cutting force becomes greater. In the cases here reported the nominal chip thickness is equal to the feed, since the entering angle is 90°, so, the chip thickness ratio (Λh), calculated according to equation 2, using the average of the measured thickness, kept being quite low.

D ch

h h

= h

Λ (2)

The real thickness was never near the double of the nominal one when machining with a feed of 0.3 mm/rev (see table 8). And when machining with a feed of 0.15 mm/rev the real chip thickness barely reached the double of the nominal one (see Fig. 16, where the real chip thickness never exceeded 0.3 mm).

Table 8 Chip thickness ratio for the different inserts at different cutting speed settings. Feed f = 0.3

mm/rev

Figure 17 shows the quick stop specimen when machining with SC21+Tinalox at vc = 60 m/min and f

= 0.15 mm/rev. The real chip thickness is not more than double the nominal one.

Fig. 17 Quick stop, SC21+Tinalox at v = 60 m/min f = 0.15 mm/rev.

The chip obtained during machining was though extremely long and curly due to the absence of chip- breaker, as shown in Fig. 18.

Fig. 18 Example of curly chip obtained during the tool life investigation.

The chip was working as a saw-blade for the insert during machining and sometimes it caused the tool to break by literarily cutting off the tool tip. In figure 19 it is shown this effect on SM10+Tinalox insert.

Vc SC21+Tinalox SM10+STN SM10+Tinalox

40 1.45 1.54 1.58

60 1.35 1.23 1.31

80 1.34 1.41 1.32

100 1.38 1.28 1.34

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Fig. 19 Saw-blade effect on the tool-tip.

The chip thickness (see Fig. 15 and 16) will help PALBIT to implement a chip breaker for this insert (together with the results from contact length measurement).

4. DISCUSSIONS AND CONCLUSIONS The tool life test results show that there can be an enormous difference among different coatings and substrates. The importance of the substrate was apparent when machining at all cutting parameters, the SA35 substrate did not survive for more than few seconds at 80 m/min and 0.3 mm/rev in either the types of coating, due to evident plastic deformation.

The SC21 substrate weakness was not as evident at a first look when investigating on tool life, only after the force measurement, when the result of a lower cutting force at a higher speed made necessary a further inspection of the pictures used for mapping tool life. The difference between the two coatings was more noticeable when machining at low speed and high feed (vc = 40 m/min f = 0.3 mm/rev) where the SM10+Tinalox insert showed a much longer tool life than the SM10+STN. At higher speed the difference was less noticeable.

No major difference could be noticed in terms of cutting forces with the exception of the already mentioned behavior of the SC21+Tinalox insert.

For what it concerns temperature, a difference could be noticed between the two coatings, it seemed that Tinalox developed slightly higher temperature. This result should be taken with the due caution since it was not possible to get a complete measurement. A further investigation on this aspect should be taken in account in order to get a more precise picture of the situation.

The chip-thickness measurement did not show major differences between the different insert types. A certain trend was though common for all the inserts, the chip-thickness tended to diminish when increasing the cutting speed. This measurement gave as well a qualitative indication on the good machinability of TOOLOX 44, the chip thickness ratio is rather low (below 2) and this is generally considered to be a sign for good machinability. The data obtained here is going to be very important for PALBIT in order to implement a tailor-made chip- breaker for machining hardened steel such as TOOLOX 44.

The conclusion of this investigation is that the best combination of substrate and coating for machining TOOLOX 44, among the ones here tested, is SM10+Tinalox. Based on the tool life results, this type of insert could be used even at higher speed than the one used in this tests, granting higher productivity, but before testing that possibility, a chip-breaker should be implemented. The machinability of TOOLOX 44 is out of discussion, the right choice of tooling equipment is though important in order to obtain a satisfactory level of productivity.

ACKNOWLEDGMENT

The project was financed by EUREKA program within the Dampcomat project (project nr. 25953-1).

The authors wish to thank PALBIT and SSAB for providing the necessary material for this research as well as the technical personnel at the department of production engineering for their continuous support and help.

REFERENCES

Chen, W-C. and C-C Tsao (1997). Cutting performances of different coated twist drills.

Journal of Materials Processing Technology, 88, 203-207

ISO 513:2004 Classification and application of hard cutting materials for metal removal with defined cutting edges - Designation of the main groups and groups of application

Primage, Prima image and measure system, version 1.0.13, developed by Lars Svensson

Trent E.M. (1984) Metal cutting, 39-42, Butterworths

& Co, Great Britain