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Tool life and wear mechanism of WC–5TiC–0.5VC–8Co cemented carbides inserts when machining HT250 gray cast iron
Author’s Accepted Manuscript Tool life and wear mechanism of WC–5TiC– 0.5VC-8Co cemented carbides inserts when machining HT250 gray cast iron Jian Chen, Wei Liu, Xin Deng, Shanghua Wu www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)30242-5 http://dx.doi.org/10.1016/j.ceramint.2016.03.107 CERI12478
To appear in: Ceramics International Received date: 4 February 2016 Revised date: 14 March 2016 Accepted date: 14 March 2016 Cite this article as: Jian Chen, Wei Liu, Xin Deng and Shanghua Wu, Tool life and wear mechanism of WC–5TiC–0.5VC-8Co cemented carbides inserts when machining HT250 gray cast iron, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.03.107 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tool life and wear mechanism of WC–5TiC–0.5VC-8Co cemented carbides inserts when machining HT250 gray cast iron Jian Chen, Wei Liu*, Xin Deng*, Shanghua Wu
School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou 510006, Guangdong, China E-mail: [email protected] (W. Liu); [email protected] (X. Deng)
Corresponding author, Tel.: +86 20 39322925; fax: +86 20 39322925.
Abstract Cutter development has drawn a lot of attention for cast iron machining in recent years. In this study, a special cemented carbide of WC–5TiC–0.5VC-8Co （WTVC8） was used for a comprehensive HT250 gray cast iron machining test. Compared with the baseline plain WC-8Co（WC8） carbides, WTVC8 shows significantly higher tool life under the same cutting conditions due to significantly higher hardness and red hardness. The worn flank face observation shows that adhesion wear and oxidation are the main wear mechanisms and there is no apparent chipping/breakage and abrasion wear for both WTVC8 and WC8. Based on Taylor’s equation, the accurate tool life models for both WTVC8 and WC8 have been constructed, which shows clearly that cutting speed has the most significant effect on tool life, followed by depth of cut and feed rate. The tool life models can serve as a quantified guidance for cutting performance optimization. Keywords：Cemented carbides; Tool life; Wear; cutting parameters
1. Introduction 1
Cemented carbides are special composites with the skeleton of carbides (e.g. WC, TiC, TaC, or NbC) intertwined with the continuous metal binder (mainly Co or Ni) [13]
. Due to their high wear resistance and considerable toughness, cemented carbides
have been used for metal cutting ever since its very first invention, especially for the machining of cast iron [4-6] Historically, cemented carbides of plain WC-Co have been used as the main cutting tools for cast iron [4]. Because of the high wear resistance of cast iron and the strong metallurgical reaction between cast iron and WC-Co cutting tools, plain WC-Co shows undesirable cutting life and severe crater wearing. In order to improve the cutting performance, more cemented carbide development have been made on adding TiC, VC, TaC, and other types of carbides[7-10]. In recent years, high strength gray cast iron, especially HT250 (gray cast iron with tensile strength 250MPa) has been used widely as cylinder head, cylinder and castings chassis, and other engineering parts
[4,11]
. The machining of HT250 has become a
significant challenge for the development of cemented carbide tools, which has drawn a lot of attention
[11-14]
. Although there have been a couple cemented carbide grades
that have been used for the machining of HT250, there are still a lot of development work to be made to significantly improve the cutting performance
[13-15]
. In this study,
a special cemented carbide, WTVC8, was developed to improve the cutting performance for gray cast iron HT250. TiC is selected due to its can improve hardness and wear resistance of WC-Co cemented carbides
[8,9]
and VC was added to inhibit WC grain
growth . As it is for VC, it’s the most effective grain growth inhibitors due to its high solubility and mobility in the cobalt phase at lower temperatures [7]. 2
A systematic cutting test was made on HT250 cast iron with the cutter of WTVC8. At the same time, the plain cemented carbide WC8 was used as the reference cutter during the test to provide the baseline data for comparison. Both the tool life and the wearing performance of cutters were investigated as the function of the cutting parameters such as cutting speed, cutting depth, and feed rate.
2. Experimental procedures 2.1. Preparation and Mechanical Testing of WTVC8 and WC8 The raw material powders used in the present study include WC powder (purity 99.9 wt.%, average oxygen content of 0.05 wt.%, mean particle size of 0.8 μm), Co (purity 99.9 wt.%, mean particle size of 0.8 μm), TiC (purity 99.9 wt.%, mean particle size of 0.8 μm) and VC (purity 99.9 wt.%, mean particle size of 1 μm). Both WTVC8 and WC8 cemented carbides were prepared with the normal powder metallurgical procedures of powder mixing, pressing, de-waxing, and sintering. Fig. 1 shows the de-waxing and sintering curves for this study. The microstructures of the cemented carbides were observed with SEM. Vickers hardness test was made on HV5-30Z Vickers hardness tester with a load of 98kN . Fracture toughness was measured with palmqvist test with indenting load of 98 kN, and density was measured with Archimedes method. 2.2. Cutting Test The chemical composition and mechanical properties of the HT250 cast iron workpiece are listed in Table 1. The workpiece has the dimension of 100 mm diameter and 150 mm length. As shown in Fig.2, all the cemented carbide cutters were ground to the final dimensions for cutting test. 3
The dry cutting tests were conducted on a CNC lathe with 15 KW of power in the spindle motor. The carbide inserts were clamped in a left-hand tool holder (CSSNL2525M1207) with 45º lead angle and 8°30′ rake angle. A total 7 different combinations of cutting parameters were used for the whole cutting test. The cutting parameters include feed rate (f), depth of cut (ap), and cutting speed (Vc). As shown in Table 2, these 7 cutting parameter combinations can be easily divided into three groups per different feed rate, cutting speed, and depth of cut. For each combination, the cutting test was conducted three times using the three cutting corners of the same cutter. During cutting test, the flank wear was measured with tool maker’s microscope every 2 minutes. Flank wear value (VB) of 300 μm was used as the tool life criterion for each test according to ISO standard 3685. After cutting test, worn surfaces of the cemented carbides inserts were ultrasonically cleaned with acetone, and evaluated with SEM.
3. Results and discussion 3.1. Microstructure and properties of carbide cutters The microstructures of both WTVC8 and WC8 are given in Fig. 3. With the addition of VC and TiC, the average grain sizes of WC and the amount of the abnormal growth grains decrease remarkably. Mechanical properties and porosity of both WTVC8 and WC8 are listed in Table 3. According to the ISO norm 4505 (Hardmetals Metallographic determination of porosity and uncombined carbon), pores up to 10 μm are designated as "A" and those 4
larger than 10 μm but smaller than 25 μm are called "B" pores[16]. Table 3 shows that there is no pore larger than 10μm for both carbide grades and the A pore concentration is very low (A02), indicating both carbide grades have good density.The WTVC8 cemented carbides show significantly higher hardness but lower toughness. 3.2. Cutting performance Tool life is one of the most important parameters for cutting performance evaluation. During cutting test, tool life can be defined in terms of either crater wear or flank wear if no catastrophic tool failure happens
[17]
. Flank wear, which has a
major negative influence on the dimension accuracy of the workpiece, is more often used to characterize the tool life. As shown in Fig. 4 and Fig. 5, the tool life is quantified as the total cutting time when the flank wear land width VB value reaches 300 μm according to ISO standard 3685.[18]. Fig. 4 and Fig. 5 show the flank wear VB values of WTVC8 and WC8 respectively , as a function of cutting time at different feed rate, cutting speed and depth of cuting, . The tool life per ISO standard 3685 is summarized in Table 4. It shows clearly that the tool life of both WTVC8 and WC8 decreases with each cutting parameter. Fig. 4 and Fig. 5 is the direct tool life comparison between WTVC8 and WC8. For each cutting test, WC8 shows less tool life than WTVC8. The main reason is the higher hardness and red hardness of WTVC8 compared with WC8. 3.3. Tool wear mechanism Fig. 6 shows the SEM images of worn flank face for both WTVC8 and WC8 cemented carbides inserts at the end of cutting test. For both WTVC8 and WC8, no 5
micro-chipping/breakage has been observed on flank face and there is no apparent abrasion wear. As for the WTVC8 insert, abrasive wear and some salient points could be seen on the flank face when the cutting test finished with a cutting speed of 50 m/min (see Fig. 6(a)). It can be concluded that though the hardness of the cemented carbide was largely improved due to refinement of the grains, the oxide and carbide particles of high hardness generated during the cutting of HT250 gray cast iron will result in the aggravation of tool wear. With the cutting speed increased to 80 m/min, serious adhesive wear can be observed, abrasive wear almostly changed to adhesive wear totally. Fig. 7(c) showed the EDS spectrum of the worn flank faces of WTVC8 inserts when cutting test finished at the cutting speed of 100 m/min. There existed intensive W and Fe peaks, compared to the WC8 insert. The EDS spectrum showed fewer Fe and Cr content in the WC8 insert due to the breakage of the cutting edge at an early time. Generally, it is well accepted that several wear mechanisms such as adhesion, abrasion, diffusion, oxidation can operate simultaneously during machining. As for the WC8 inserts, serious adhesive wear can be seen and even spalling when the cutting test was finished with cutting speed of 50 m/min.With the cutting speed increased to 80 m/min, there can be seen thick adhesive of workpieces materials, indicating amazing adhesive wear. With the cutting speed increased to 100 m/min, the cutting force increase accordingly, and the cutting inserts breakage occurred due to the relatively lower properties of WC8 inserts. During cutting of HT250 gray cast iron, Abrasive wear occurs when escaped hard particles from workpieces and tools move across the contact area. With the increase of cutting speed, adhesion of the tool and workpiece increases at higher temperatures, and tool flank adhered to the workpiece surface are periodically sheared off. Fig. 7(d) showed the EDS spectrum of 6
the worn flank faces of WC8 inserts when cutting test finished at the cutting speed of 100 m/min. The presence of Fe and Mn peaks from EDS analysis confirmed the adhesive wear owing to tool wear. Meanwhile, oxidative wear may exist, for O, W and Ti element can be simultaneously detected by the EDS analysis. EDS analysis of the chips is also conducted to investigate the interaction between inserts and workpiece, as shown in Fig. 8. Fe, Mn, Si and C peaks appeared in the spectrum of chips from both cutting tests, but there were no W, Co, Ti and Cr contents relating to cemented carbide inserts. Therefore, there was no diffusion of the cemented carbides content into the chips. It can be concluded from Figs. 7 and 8 that adhesion existed but not any diffusion wear between the cemented carbide inserts and HT250 gray cast iron workpiece materials during cutting.
Considering the wear morphology and EDS peaks intensity, it can be concluded that there was major adhesive wear and minor abrasive wear on the rake of WTVC8 inserts, and combination of serious adhesive , abrasive wear and oxidative wear on the rake of WC8 inserts. The dominant wear mechanism on the flank face of both inserts was adhesive wear. For the both inserts, there were abrasion, adhesion, oxidation wear simultaneously during machining. But not any diffusion wear between the cemented carbide inserts and HT250 gray cast iron workpiece materials during cutting. It can be stated that flank wear is produced due to the contact between the workpiece and the edge of the indexable insert.
3.4 Tool life modeling 7
Among the various tool life modeling, Taylor proposed the most influential one [19]
. Taylor’s tool life model initially put cutting speed as the main cutting parameter
and later included both feed rate and depth of cut as well, which can be expressed as [20]
:
(1)
where Vc is the cutting speed, f is the feed rate, ap is the depth of cut, T is the tool life, CT is the tool life constant, which is closely related to the workpiece materials, cutting inserts and other cutting conditions, and m, n, and p are the constants. The testing results in Table 4 provides sufficient data to set up the accurate tool life model for both WTVC8 and WC8 in this study per Eq. (1). With logarithmic transformation of Eq. (1) and the subsequent multiple regression, all the constants in Eq.(1) were easily calculated. The tool life models for WTVC8 and WC8 are presented in Eqs. (2) and (3) respectively
The tool life models agreed well with the experiment results (The R2 value of the above regression results is 0.987292). The tool life models for both WTVC-8 and WC8 show that cutting speed has the most significant effect on tool life, followed by depth of cut and feed rate. The tool life models can serve as a quantified guidance for cutting performance optimization.
8
4. Conclusions The following conclusions can be drawn on turning of HT250 gray cast iron using both WTVC8 and WC8 cemented carbides inserts: (1) WTVC8 cemented carbides inserts showed higher hardness and transverse rupture strength compared to WC8 inserts due to the finer WC grain size and higher TiC content. (2) The turning test on HT250 gray cast iron shows that WTVC8 has significantly higher tool life than WC8 at the same cutting parameters. The main reason is the higher hardness and red hardness of WTVC8. (3) The accurate tool life models for both WTVC8 and WC8 have been constructed based on Taylor’s equation. The models show that cutting speed has the most significant effect on tool life, followed by depth of cut and feed rate. The tool life models can serve as a quantified guidance for cutting performance optimization. (4) For HT250 gray cast iron turning, the worn flank face observation shows that adhension wear and oxidation are the main wear mechanisms and there is no apparent chipping/breakage and abrasion wear for both WTVC8 and WC8.
Acknowledgements The present work was financially supported by the “Project on the Integration of Industry,
Education
and
Research
of
“Guangdong
Province
(Grant
No.
2011A090200080)” and “Research Funding for Introduction of Guangdong Province Leading Talents” (Grant No. 40012001）.
References 9
[1].Upadhyaya G. Materials science of cemented carbides—an overview[J]. Materials & Design. 2001,22(6):483-489. [2].Ren X, Miao H, Peng Z. A review of cemented carbides for rock drilling: An old but still tough challenge in geo-engineering[J]. International Journal of Refractory Metals and Hard Materials. 2013,39:61-77 [3] Su W, Sun Y, Liu J, et al. Effects of Ni on the microstructures and properties of WC–6Co cemented carbides fabricated by WC–6 (Co, Ni) composite powders[J]. Ceramics International, 2015, 41(2): 3169-3177. [4].Thamizhmanii S, Hasan S. Analyses of roughness, forces and wear in turning gray cast iron[J]. Journal of achievement in Materials and Manufacturing Engineering, 2006, 17. [5] Qin J, Long Y, Zeng J, et al. Continuous and varied depth-of-cut turning of gray cast iron by using uncoated and TiN/Al2O3 coated silicon nitride-based ceramic tools[J]. Ceramics International, 2014. 40(8): 12245–12251. [6].Long Y, Zeng J, Yu D, et al. Microstructure of TiAlN and CrAlN coatings and cutting performance of coated silicon nitride inserts in cast iron turning[J]. Ceramics International, 2014, 40(7): 9889-9894. [7].Poetschke J, Richter V, Holke R. Influence and effectivity of VC and VC grain growth inhibitors on sintering of binderless tungsten carbide[J]. International Journal of Refractory Metals and Hard Materials. 2012,31:218-223. [8].van der Merwe R, Sacks N. Effect of TaC and TiC on the friction and dry sliding wear of WC–6wt.% Co cemented carbides against steel counterfaces[J]. International Journal of Refractory Metals and Hard Materials. 2013,41:94-102. [9]Weidow J, Andrén H O. Grain and phase boundary segregation in WC–Co with TiC, ZrC, NbC or TaC additions[J]. International Journal of Refractory Metals and Hard Materials, 2011, 29(1): 38-43. [10]Lin N, Wu C H, He Y H, et al. Effect of Mo and Co additions on the microstructure and properties of WC-TiC-Ni cemented carbides[J]. International Journal of Refractory Metals and Hard Materials, 2012, 30(1): 107-113. [11] Huang X B, Ye Y G, Shen X Q, et al. The Mechanical Properties of Gray Cast Iron and Metallographic Structure Effect on the Chip Shape[C]//Advanced Materials Research. 2011, 339: 200203. [12] Wang W Y, Lu G, Xie J P, et al. Research of Friction and Wear Performance of Large Truck’s Brake Drum[C]//Advanced Materials Research. 2012, 503: 601-605. [13].Pereira A A, Boehs L, Guesser W L. The influence of sulfur on the machinability of gray cast iron FC25[J]. Journal of Materials Processing Technology, 2006, 179(1): 165-171. [14].DeBenedictis K. A short course in cast-iron turning[J]. Manufacturing Engineering(USA), 1997, 119(3): 48-50. [15]Chen M, Zhang X H, Han B, et al. Wear Performance Evaluation of Tungsten Carbide Taps in Blind Hole Tapping Cast Iron[C]//Advanced Materials Research. 2010, 126: 755-759. [16].Bhaumik S K, Upadhyaya G S, Vaidya M L. Full density processing of complex WC-based cemented carbides[J]. Journal of materials processing technology, 1996, 58(1): 45-52. [17]. Dolinšek S, Šuštaršič B, Kopač J. Wear mechanisms of cutting tools in high-speed cutting processes[J]. Wear, 2001, 250(1): 349-356. [18]. Duplak J, Hatala M, Zajac J, et al. The Comprehensive Comparison of the Selected Cutting Materials with Standard ISO 3685 in Machining Process of Steel C60[C]//Applied Mechanics and Materials. 2015, 718: 93-98. [19]. Vagnorius, Z., M. Rausand, and K. Sørby, Determining optimal replacement time for metal cutting tools. European Journal of Operational Research, 2010. 206(2): 407-416. [20].Karandikar, J.M., A.E. Abbas, and T.L. Schmitz, Tool life prediction using Bayesian updating. Part 2: Turning tool life using a Markov Chain Monte Carlo approach. Precision Engineering, 2014. 38(1): 18-27.
10
Figure Captains Fig. 1. De-waxing and sintering curves for this study. Fig. 2. Dimension of the cemented carbide inserts, l=d =12.76( ± 0.025mm), s=4.76(±0.025mm), r=0.8mm. Fig.3. Microstructure of both cemented carbide inserts, (a) WC–8Co, (b) WTVC8. Fig. 4. Flank wear VB values as a function of cutting time of WTVC8 cemented carbides inserts with different (a) feed rate, (b) cutting speed, and (c) depth of cut. Fig. 5. Flank wear VB values as a function of cutting time of WC–8Co cemented carbides inserts Fig. 6. The flank wear morphology of both WTVC8 (a, b, c) and WC–8Co (d, e, f) cemented carbide inserts machined with different cutting speed. Fig. 7. EDS spectrum of the worn flank face for cutting test at the cutting speed of 100 mm/r. (a) WTVC8 insert and (b) WC–8Co insert. Fig. 8. SEM images taken from the chips generated from cutting tests using WTVC8 (a) and WC–8Co inserts (c); EDS patterns of Point 1 in (b) and Point 2 in (d), respectively.
11
Table 1 Chemical composition and mechanical properties of gray cast iron. Chemical composition of gray cast iron (wt.%) C
Si
Mn
S
P
3.16-3.30
1.79-1.93
0.89-1.04
0.094-0.125
0.120-0.170
Mechanical properties Hardness (RH=1) 209 HB
Tensile strength 250 MPa
Table 2 Matrix of Cutting Test. Group
Experiment number
Feed Rate, f (mm/r)
Depth of Cut, ap (mm)
Ⅰ
1 2 3
0.2 0.4 0.6
0.1 0.1 0.1
Cutting Speed, Vc (m/min) 50 50 50
Ⅱ
1 4 5
0.2 0.2 0.2
0.1 0.1 0.1
50 80 100
Ⅲ
1 6 7
0.2 0.2 0.2
0.1 0.2 0.3
50 50 50
Table 3 Porosity rating and mechanical properties of carbide inserts. Inserts
WC8
WTVC8
Porosity Rating
A02B00C00
A02B00C00
Density (g/cm3)
13.16
13.04
Hardness ( HV10)
1780±32
2080±25
12
Fracture toughness( MPa·m1/2)
9.3
8.5
Table 4 The tool life of carbide inserts. F (mm/r)
ap
Vc
Tool life of WTVC8 insert Tool life of WC–3TiN–8Co
(mm)
(m/min)
(min) average
insert (min) Standard deviation
average
Standard deviation
0.2 0.4 0.6
0.1 0.1 0.1
50 50 50
21.5 16.8 15.4
0.08 0.05 0.07
18.2 12.6 11.5
0.05 0.07 0.08
0.2 0.2 0.2
0.1 0.1 0.1
50 80 100
21.5 14.6 11.7
0.06 0.07 0.04
18.2 10.9 8.1
0.04 0.09 0.05
0.2 0.2 0.2
0.1 0.2 0.3
50 50 50
21.5 15.2 13.9
0.08 0.09 0.06
18.2 13.5 9.4
0.07 0.04 0.05
13
14
15
PII: DOI: Reference:
S0272-8842(16)30242-5 http://dx.doi.org/10.1016/j.ceramint.2016.03.107 CERI12478
To appear in: Ceramics International Received date: 4 February 2016 Revised date: 14 March 2016 Accepted date: 14 March 2016 Cite this article as: Jian Chen, Wei Liu, Xin Deng and Shanghua Wu, Tool life and wear mechanism of WC–5TiC–0.5VC-8Co cemented carbides inserts when machining HT250 gray cast iron, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.03.107 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tool life and wear mechanism of WC–5TiC–0.5VC-8Co cemented carbides inserts when machining HT250 gray cast iron Jian Chen, Wei Liu*, Xin Deng*, Shanghua Wu
School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou 510006, Guangdong, China E-mail: [email protected] (W. Liu); [email protected] (X. Deng)
Corresponding author, Tel.: +86 20 39322925; fax: +86 20 39322925.
Abstract Cutter development has drawn a lot of attention for cast iron machining in recent years. In this study, a special cemented carbide of WC–5TiC–0.5VC-8Co （WTVC8） was used for a comprehensive HT250 gray cast iron machining test. Compared with the baseline plain WC-8Co（WC8） carbides, WTVC8 shows significantly higher tool life under the same cutting conditions due to significantly higher hardness and red hardness. The worn flank face observation shows that adhesion wear and oxidation are the main wear mechanisms and there is no apparent chipping/breakage and abrasion wear for both WTVC8 and WC8. Based on Taylor’s equation, the accurate tool life models for both WTVC8 and WC8 have been constructed, which shows clearly that cutting speed has the most significant effect on tool life, followed by depth of cut and feed rate. The tool life models can serve as a quantified guidance for cutting performance optimization. Keywords：Cemented carbides; Tool life; Wear; cutting parameters
1. Introduction 1
Cemented carbides are special composites with the skeleton of carbides (e.g. WC, TiC, TaC, or NbC) intertwined with the continuous metal binder (mainly Co or Ni) [13]
. Due to their high wear resistance and considerable toughness, cemented carbides
have been used for metal cutting ever since its very first invention, especially for the machining of cast iron [4-6] Historically, cemented carbides of plain WC-Co have been used as the main cutting tools for cast iron [4]. Because of the high wear resistance of cast iron and the strong metallurgical reaction between cast iron and WC-Co cutting tools, plain WC-Co shows undesirable cutting life and severe crater wearing. In order to improve the cutting performance, more cemented carbide development have been made on adding TiC, VC, TaC, and other types of carbides[7-10]. In recent years, high strength gray cast iron, especially HT250 (gray cast iron with tensile strength 250MPa) has been used widely as cylinder head, cylinder and castings chassis, and other engineering parts
[4,11]
. The machining of HT250 has become a
significant challenge for the development of cemented carbide tools, which has drawn a lot of attention
[11-14]
. Although there have been a couple cemented carbide grades
that have been used for the machining of HT250, there are still a lot of development work to be made to significantly improve the cutting performance
[13-15]
. In this study,
a special cemented carbide, WTVC8, was developed to improve the cutting performance for gray cast iron HT250. TiC is selected due to its can improve hardness and wear resistance of WC-Co cemented carbides
[8,9]
and VC was added to inhibit WC grain
growth . As it is for VC, it’s the most effective grain growth inhibitors due to its high solubility and mobility in the cobalt phase at lower temperatures [7]. 2
A systematic cutting test was made on HT250 cast iron with the cutter of WTVC8. At the same time, the plain cemented carbide WC8 was used as the reference cutter during the test to provide the baseline data for comparison. Both the tool life and the wearing performance of cutters were investigated as the function of the cutting parameters such as cutting speed, cutting depth, and feed rate.
2. Experimental procedures 2.1. Preparation and Mechanical Testing of WTVC8 and WC8 The raw material powders used in the present study include WC powder (purity 99.9 wt.%, average oxygen content of 0.05 wt.%, mean particle size of 0.8 μm), Co (purity 99.9 wt.%, mean particle size of 0.8 μm), TiC (purity 99.9 wt.%, mean particle size of 0.8 μm) and VC (purity 99.9 wt.%, mean particle size of 1 μm). Both WTVC8 and WC8 cemented carbides were prepared with the normal powder metallurgical procedures of powder mixing, pressing, de-waxing, and sintering. Fig. 1 shows the de-waxing and sintering curves for this study. The microstructures of the cemented carbides were observed with SEM. Vickers hardness test was made on HV5-30Z Vickers hardness tester with a load of 98kN . Fracture toughness was measured with palmqvist test with indenting load of 98 kN, and density was measured with Archimedes method. 2.2. Cutting Test The chemical composition and mechanical properties of the HT250 cast iron workpiece are listed in Table 1. The workpiece has the dimension of 100 mm diameter and 150 mm length. As shown in Fig.2, all the cemented carbide cutters were ground to the final dimensions for cutting test. 3
The dry cutting tests were conducted on a CNC lathe with 15 KW of power in the spindle motor. The carbide inserts were clamped in a left-hand tool holder (CSSNL2525M1207) with 45º lead angle and 8°30′ rake angle. A total 7 different combinations of cutting parameters were used for the whole cutting test. The cutting parameters include feed rate (f), depth of cut (ap), and cutting speed (Vc). As shown in Table 2, these 7 cutting parameter combinations can be easily divided into three groups per different feed rate, cutting speed, and depth of cut. For each combination, the cutting test was conducted three times using the three cutting corners of the same cutter. During cutting test, the flank wear was measured with tool maker’s microscope every 2 minutes. Flank wear value (VB) of 300 μm was used as the tool life criterion for each test according to ISO standard 3685. After cutting test, worn surfaces of the cemented carbides inserts were ultrasonically cleaned with acetone, and evaluated with SEM.
3. Results and discussion 3.1. Microstructure and properties of carbide cutters The microstructures of both WTVC8 and WC8 are given in Fig. 3. With the addition of VC and TiC, the average grain sizes of WC and the amount of the abnormal growth grains decrease remarkably. Mechanical properties and porosity of both WTVC8 and WC8 are listed in Table 3. According to the ISO norm 4505 (Hardmetals Metallographic determination of porosity and uncombined carbon), pores up to 10 μm are designated as "A" and those 4
larger than 10 μm but smaller than 25 μm are called "B" pores[16]. Table 3 shows that there is no pore larger than 10μm for both carbide grades and the A pore concentration is very low (A02), indicating both carbide grades have good density.The WTVC8 cemented carbides show significantly higher hardness but lower toughness. 3.2. Cutting performance Tool life is one of the most important parameters for cutting performance evaluation. During cutting test, tool life can be defined in terms of either crater wear or flank wear if no catastrophic tool failure happens
[17]
. Flank wear, which has a
major negative influence on the dimension accuracy of the workpiece, is more often used to characterize the tool life. As shown in Fig. 4 and Fig. 5, the tool life is quantified as the total cutting time when the flank wear land width VB value reaches 300 μm according to ISO standard 3685.[18]. Fig. 4 and Fig. 5 show the flank wear VB values of WTVC8 and WC8 respectively , as a function of cutting time at different feed rate, cutting speed and depth of cuting, . The tool life per ISO standard 3685 is summarized in Table 4. It shows clearly that the tool life of both WTVC8 and WC8 decreases with each cutting parameter. Fig. 4 and Fig. 5 is the direct tool life comparison between WTVC8 and WC8. For each cutting test, WC8 shows less tool life than WTVC8. The main reason is the higher hardness and red hardness of WTVC8 compared with WC8. 3.3. Tool wear mechanism Fig. 6 shows the SEM images of worn flank face for both WTVC8 and WC8 cemented carbides inserts at the end of cutting test. For both WTVC8 and WC8, no 5
micro-chipping/breakage has been observed on flank face and there is no apparent abrasion wear. As for the WTVC8 insert, abrasive wear and some salient points could be seen on the flank face when the cutting test finished with a cutting speed of 50 m/min (see Fig. 6(a)). It can be concluded that though the hardness of the cemented carbide was largely improved due to refinement of the grains, the oxide and carbide particles of high hardness generated during the cutting of HT250 gray cast iron will result in the aggravation of tool wear. With the cutting speed increased to 80 m/min, serious adhesive wear can be observed, abrasive wear almostly changed to adhesive wear totally. Fig. 7(c) showed the EDS spectrum of the worn flank faces of WTVC8 inserts when cutting test finished at the cutting speed of 100 m/min. There existed intensive W and Fe peaks, compared to the WC8 insert. The EDS spectrum showed fewer Fe and Cr content in the WC8 insert due to the breakage of the cutting edge at an early time. Generally, it is well accepted that several wear mechanisms such as adhesion, abrasion, diffusion, oxidation can operate simultaneously during machining. As for the WC8 inserts, serious adhesive wear can be seen and even spalling when the cutting test was finished with cutting speed of 50 m/min.With the cutting speed increased to 80 m/min, there can be seen thick adhesive of workpieces materials, indicating amazing adhesive wear. With the cutting speed increased to 100 m/min, the cutting force increase accordingly, and the cutting inserts breakage occurred due to the relatively lower properties of WC8 inserts. During cutting of HT250 gray cast iron, Abrasive wear occurs when escaped hard particles from workpieces and tools move across the contact area. With the increase of cutting speed, adhesion of the tool and workpiece increases at higher temperatures, and tool flank adhered to the workpiece surface are periodically sheared off. Fig. 7(d) showed the EDS spectrum of 6
the worn flank faces of WC8 inserts when cutting test finished at the cutting speed of 100 m/min. The presence of Fe and Mn peaks from EDS analysis confirmed the adhesive wear owing to tool wear. Meanwhile, oxidative wear may exist, for O, W and Ti element can be simultaneously detected by the EDS analysis. EDS analysis of the chips is also conducted to investigate the interaction between inserts and workpiece, as shown in Fig. 8. Fe, Mn, Si and C peaks appeared in the spectrum of chips from both cutting tests, but there were no W, Co, Ti and Cr contents relating to cemented carbide inserts. Therefore, there was no diffusion of the cemented carbides content into the chips. It can be concluded from Figs. 7 and 8 that adhesion existed but not any diffusion wear between the cemented carbide inserts and HT250 gray cast iron workpiece materials during cutting.
Considering the wear morphology and EDS peaks intensity, it can be concluded that there was major adhesive wear and minor abrasive wear on the rake of WTVC8 inserts, and combination of serious adhesive , abrasive wear and oxidative wear on the rake of WC8 inserts. The dominant wear mechanism on the flank face of both inserts was adhesive wear. For the both inserts, there were abrasion, adhesion, oxidation wear simultaneously during machining. But not any diffusion wear between the cemented carbide inserts and HT250 gray cast iron workpiece materials during cutting. It can be stated that flank wear is produced due to the contact between the workpiece and the edge of the indexable insert.
3.4 Tool life modeling 7
Among the various tool life modeling, Taylor proposed the most influential one [19]
. Taylor’s tool life model initially put cutting speed as the main cutting parameter
and later included both feed rate and depth of cut as well, which can be expressed as [20]
:
(1)
where Vc is the cutting speed, f is the feed rate, ap is the depth of cut, T is the tool life, CT is the tool life constant, which is closely related to the workpiece materials, cutting inserts and other cutting conditions, and m, n, and p are the constants. The testing results in Table 4 provides sufficient data to set up the accurate tool life model for both WTVC8 and WC8 in this study per Eq. (1). With logarithmic transformation of Eq. (1) and the subsequent multiple regression, all the constants in Eq.(1) were easily calculated. The tool life models for WTVC8 and WC8 are presented in Eqs. (2) and (3) respectively
The tool life models agreed well with the experiment results (The R2 value of the above regression results is 0.987292). The tool life models for both WTVC-8 and WC8 show that cutting speed has the most significant effect on tool life, followed by depth of cut and feed rate. The tool life models can serve as a quantified guidance for cutting performance optimization.
8
4. Conclusions The following conclusions can be drawn on turning of HT250 gray cast iron using both WTVC8 and WC8 cemented carbides inserts: (1) WTVC8 cemented carbides inserts showed higher hardness and transverse rupture strength compared to WC8 inserts due to the finer WC grain size and higher TiC content. (2) The turning test on HT250 gray cast iron shows that WTVC8 has significantly higher tool life than WC8 at the same cutting parameters. The main reason is the higher hardness and red hardness of WTVC8. (3) The accurate tool life models for both WTVC8 and WC8 have been constructed based on Taylor’s equation. The models show that cutting speed has the most significant effect on tool life, followed by depth of cut and feed rate. The tool life models can serve as a quantified guidance for cutting performance optimization. (4) For HT250 gray cast iron turning, the worn flank face observation shows that adhension wear and oxidation are the main wear mechanisms and there is no apparent chipping/breakage and abrasion wear for both WTVC8 and WC8.
Acknowledgements The present work was financially supported by the “Project on the Integration of Industry,
Education
and
Research
of
“Guangdong
Province
(Grant
No.
2011A090200080)” and “Research Funding for Introduction of Guangdong Province Leading Talents” (Grant No. 40012001）.
References 9
[1].Upadhyaya G. Materials science of cemented carbides—an overview[J]. Materials & Design. 2001,22(6):483-489. [2].Ren X, Miao H, Peng Z. A review of cemented carbides for rock drilling: An old but still tough challenge in geo-engineering[J]. International Journal of Refractory Metals and Hard Materials. 2013,39:61-77 [3] Su W, Sun Y, Liu J, et al. Effects of Ni on the microstructures and properties of WC–6Co cemented carbides fabricated by WC–6 (Co, Ni) composite powders[J]. Ceramics International, 2015, 41(2): 3169-3177. [4].Thamizhmanii S, Hasan S. Analyses of roughness, forces and wear in turning gray cast iron[J]. Journal of achievement in Materials and Manufacturing Engineering, 2006, 17. [5] Qin J, Long Y, Zeng J, et al. Continuous and varied depth-of-cut turning of gray cast iron by using uncoated and TiN/Al2O3 coated silicon nitride-based ceramic tools[J]. Ceramics International, 2014. 40(8): 12245–12251. [6].Long Y, Zeng J, Yu D, et al. Microstructure of TiAlN and CrAlN coatings and cutting performance of coated silicon nitride inserts in cast iron turning[J]. Ceramics International, 2014, 40(7): 9889-9894. [7].Poetschke J, Richter V, Holke R. Influence and effectivity of VC and VC grain growth inhibitors on sintering of binderless tungsten carbide[J]. International Journal of Refractory Metals and Hard Materials. 2012,31:218-223. [8].van der Merwe R, Sacks N. Effect of TaC and TiC on the friction and dry sliding wear of WC–6wt.% Co cemented carbides against steel counterfaces[J]. International Journal of Refractory Metals and Hard Materials. 2013,41:94-102. [9]Weidow J, Andrén H O. Grain and phase boundary segregation in WC–Co with TiC, ZrC, NbC or TaC additions[J]. International Journal of Refractory Metals and Hard Materials, 2011, 29(1): 38-43. [10]Lin N, Wu C H, He Y H, et al. Effect of Mo and Co additions on the microstructure and properties of WC-TiC-Ni cemented carbides[J]. International Journal of Refractory Metals and Hard Materials, 2012, 30(1): 107-113. [11] Huang X B, Ye Y G, Shen X Q, et al. The Mechanical Properties of Gray Cast Iron and Metallographic Structure Effect on the Chip Shape[C]//Advanced Materials Research. 2011, 339: 200203. [12] Wang W Y, Lu G, Xie J P, et al. Research of Friction and Wear Performance of Large Truck’s Brake Drum[C]//Advanced Materials Research. 2012, 503: 601-605. [13].Pereira A A, Boehs L, Guesser W L. The influence of sulfur on the machinability of gray cast iron FC25[J]. Journal of Materials Processing Technology, 2006, 179(1): 165-171. [14].DeBenedictis K. A short course in cast-iron turning[J]. Manufacturing Engineering(USA), 1997, 119(3): 48-50. [15]Chen M, Zhang X H, Han B, et al. Wear Performance Evaluation of Tungsten Carbide Taps in Blind Hole Tapping Cast Iron[C]//Advanced Materials Research. 2010, 126: 755-759. [16].Bhaumik S K, Upadhyaya G S, Vaidya M L. Full density processing of complex WC-based cemented carbides[J]. Journal of materials processing technology, 1996, 58(1): 45-52. [17]. Dolinšek S, Šuštaršič B, Kopač J. Wear mechanisms of cutting tools in high-speed cutting processes[J]. Wear, 2001, 250(1): 349-356. [18]. Duplak J, Hatala M, Zajac J, et al. The Comprehensive Comparison of the Selected Cutting Materials with Standard ISO 3685 in Machining Process of Steel C60[C]//Applied Mechanics and Materials. 2015, 718: 93-98. [19]. Vagnorius, Z., M. Rausand, and K. Sørby, Determining optimal replacement time for metal cutting tools. European Journal of Operational Research, 2010. 206(2): 407-416. [20].Karandikar, J.M., A.E. Abbas, and T.L. Schmitz, Tool life prediction using Bayesian updating. Part 2: Turning tool life using a Markov Chain Monte Carlo approach. Precision Engineering, 2014. 38(1): 18-27.
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Figure Captains Fig. 1. De-waxing and sintering curves for this study. Fig. 2. Dimension of the cemented carbide inserts, l=d =12.76( ± 0.025mm), s=4.76(±0.025mm), r=0.8mm. Fig.3. Microstructure of both cemented carbide inserts, (a) WC–8Co, (b) WTVC8. Fig. 4. Flank wear VB values as a function of cutting time of WTVC8 cemented carbides inserts with different (a) feed rate, (b) cutting speed, and (c) depth of cut. Fig. 5. Flank wear VB values as a function of cutting time of WC–8Co cemented carbides inserts Fig. 6. The flank wear morphology of both WTVC8 (a, b, c) and WC–8Co (d, e, f) cemented carbide inserts machined with different cutting speed. Fig. 7. EDS spectrum of the worn flank face for cutting test at the cutting speed of 100 mm/r. (a) WTVC8 insert and (b) WC–8Co insert. Fig. 8. SEM images taken from the chips generated from cutting tests using WTVC8 (a) and WC–8Co inserts (c); EDS patterns of Point 1 in (b) and Point 2 in (d), respectively.
11
Table 1 Chemical composition and mechanical properties of gray cast iron. Chemical composition of gray cast iron (wt.%) C
Si
Mn
S
P
3.16-3.30
1.79-1.93
0.89-1.04
0.094-0.125
0.120-0.170
Mechanical properties Hardness (RH=1) 209 HB
Tensile strength 250 MPa
Table 2 Matrix of Cutting Test. Group
Experiment number
Feed Rate, f (mm/r)
Depth of Cut, ap (mm)
Ⅰ
1 2 3
0.2 0.4 0.6
0.1 0.1 0.1
Cutting Speed, Vc (m/min) 50 50 50
Ⅱ
1 4 5
0.2 0.2 0.2
0.1 0.1 0.1
50 80 100
Ⅲ
1 6 7
0.2 0.2 0.2
0.1 0.2 0.3
50 50 50
Table 3 Porosity rating and mechanical properties of carbide inserts. Inserts
WC8
WTVC8
Porosity Rating
A02B00C00
A02B00C00
Density (g/cm3)
13.16
13.04
Hardness ( HV10)
1780±32
2080±25
12
Fracture toughness( MPa·m1/2)
9.3
8.5
Table 4 The tool life of carbide inserts. F (mm/r)
ap
Vc
Tool life of WTVC8 insert Tool life of WC–3TiN–8Co
(mm)
(m/min)
(min) average
insert (min) Standard deviation
average
Standard deviation
0.2 0.4 0.6
0.1 0.1 0.1
50 50 50
21.5 16.8 15.4
0.08 0.05 0.07
18.2 12.6 11.5
0.05 0.07 0.08
0.2 0.2 0.2
0.1 0.1 0.1
50 80 100
21.5 14.6 11.7
0.06 0.07 0.04
18.2 10.9 8.1
0.04 0.09 0.05
0.2 0.2 0.2
0.1 0.2 0.3
50 50 50
21.5 15.2 13.9
0.08 0.09 0.06
18.2 13.5 9.4
0.07 0.04 0.05
13
14
15