Research Article EXPERIMENTAL ANALYSIS OF CRYOGENIC TREATMENT ON COATED TUNGSTEN CARBIDE INSERTS IN TURNING Shivdev Singh ¹*, Dilbag Singh ², Nirmal S Kalsi ² Address for Correspondence ¹Department of Mechanical Engineering, Rayat Bahra College of Engineering and Nanotechnology for Women, Hoshiarpur, Punjab, India. ² Department of Mechanical Engineering, Beant College of Engineering & Technology, Gurdaspur, Punjab, India. ABSTRACT This research work presents the results of an experimental investigation of cryogenically treated, coated and uncoated tungsten carbide cutting tool inserts in turning of AISI 1040 steel. Three different tungsten carbide inserts coated with aluminum chromium nitride (AlCrN), titanium nitride (TiN) and uncoated WC were taken and treated cryogenically. Experiments were performed to evaluate the cutting forces and tool wear at different machining conditions. ANOVA was performed to identify the effect of the input parameters on response variables. Results indicated that treated TiN coated tools have lower tool wear and cutting forces followed by treated AlCrN coated and treated uncoated tools. It has been observed that cutting forces increased with the increase in feed, depth of cut and decreased with increase of cutting speed in all cases. However Flank wear increased with the increase in depth of cut, cutting speed and feed. KEYWORDS: Deep cryogenic treatment (DCT), Turning, Tool wear, Cutting forces. 1. INTRODUCTION Machining is the process of removing material from a workpiece in the form of chips. It involves turning, boring, milling, drilling, shaping, planning, broaching etc. In turning, the chips slide on the rake face of tool and subjected to a high coefficient of friction. Most of mechanical energy used to form chips is converted into heat, which reduce the wear resistance of cutting tool. High cutting temperature and its detrimental effects are generally reduced by proper selection of process parameters, using heat and wear resistant cutting tool materials like carbides, coated tools and cryogenic treated tools. Instead of just seeking out new tool materials, researchers showed interest into other areas such as the development of coatings for tools. Coating technology was introduced in mid-1960. Now-a-days, it has become an integral part of tool technology. Several researchers have established that hard coatings deposited on tool and machine parts by different physical vapor decomposition methods can improve the performance of the parts. These coated materials not only help reducing the wear and increasing the tool life but also improve strength and chemical inertness, reduce friction, and make the parts more stable at high temperatures [1]. Coatings used for tungsten carbide tools include titanium carbide, titanium nitride and aluminum oxide, or combinations of this etc. Although tool life is still less than that of extremely hard cutting tools such as the diamond tools, coatings enable the tool life of the carbide tools to be extended much longer than the uncoated tools. Yet another possible method of improving wear resistance in tools is to subject them to cryogenic treatment. Cryogenic technology on the whole is not a new process and has been used on several types of materials including plastics and composites to improve their performance in their various applications. In cryogenic treatment the inserts are cooled down to cryogenic temperature and maintained at this temperature for a long time and then brought back to room temperature to improve their wear resistance. 2. LITERATURE REVIEW Researchers used different methods to reduce tool wear and cutting forces so as to increase productivity. Earlier, heat treatment was used to improve the wear resistance of cutting tools. Now-a-days hard coating is generally used for this purpose. Some researchers have reported the effect of the coating on the performance of cutting tools. Che Haron et al. [2] studied the wear behavior of coated and uncoated carbide tools in turning cutting steel under dry and wet conditions. They found that in terms of flank wear, the performance of coated tools was found to be better than uncoated. Coated carbide tools showed better results under wet conditions than dry. Avila et al. [3] investigated the performance of TiN coated and uncoated tool in turning AISI 8620 steel. It is clear that TiN coated carbide tool performed better than the uncoated tool with regard to the crater wear resistance. Kalss et al. [4] performed experiment to compare Ti-based PVD hard coating and Ti free AlCrN coating in machining of medium carbon and heat-resistant steels. The results showed that high aluminium content improves oxidation and wear resistance. It increased tool life and provided wide range of cutting speeds as compared to Ti based coating. Yigit et al. [5] studied the effect of cutting speed on nodular cast iron in turning with coated and uncoated cutting tools. Tool performance was evaluated with respect to tool wear, surface finish produced and cutting forces generated during turning. Uncoated WC/Co tool was the worst performing tool with respect to tool wear and surface finish. Coated tool was the most suitable one for turning nodular cast iron especially at high cutting speeds. Titus et al. [6] reviewed that coating with hard substances like TiN, TiC and Al 2 0 3 (aluminium oxide) improves cutting tool capabilities. It increased productivity and reduced power requirements. The wear resistance of the coating itself should be superior to that of the substrate. The coating acts as a heat barrier owing to the lower thermal conductivity compared with that of the substrate. Thus the proportion of frictional heat which dissipates into the substrate is reduced which, in turn, lowers the substrate temperature.
Lot of research work has been done on cryogenic treatment of materials and show that it further improves wear resistance. Molinari et al. [7] reported that deep cryogenic treatment on quenched and tempered high speed tools increases hardness reduces tool consumption and downtime for the equipment set up the leading to cost reduction of about 50%. AISI M2 and AISI H13 steel was confirmed the possibility of increasing the wear resistance and toughness by carrying out the cryogenic treatment after the usual heat treatment. Stewart [8] applied cryogenic treatment to C2 tungsten carbide (WC-6% Co) inserts and compared with untreated carbide inserts to determine if tool wear could be reduced during turning tests with medium density fiberboard (MDF). The cryogenic treatment appeared to have an effect upon the cobalt binder by changing phase or crystal structure so that more cobalt binder was retained during cutting. Yong et al. [9] concluded that cryogenic treated tungsten carbide tools can maintain their superior tool wear resistance if used for just short period of time in turning steel. The cryogenic treatment showed significant effect under mild cutting operations. On contrary to it heavy duty cutting operations, longer durations of heating of cutting tool will not show improvement. Yong et al. [10] analyzed the differences in tool performance between cryogenically treated and untreated tungsten carbide tool inserts during high speed milling of medium carbon steel. It was found that cryogenically treated tools exhibit better tool wear resistance than untreated tools. Firouzdor et al. [11] observed the effect deep cryogenic treatment on M2 HSS drill. They attributed over to the resistance of cryogenically treated drills against diffusion wear mechanism, which was due to the formation of fine and homogeneous carbide particles during cryogenic treatment. Ready et al. [12] investigated the effect of deep cryogenic treatment on chemical vapour deposition (CVD) coated carbide ISO P-30 inserts in machining C45 steel. They concluded that flank wear and cutting forces of deep cryogenic treated coated carbide inserts were less as compared to untreated tools. Deep cryogenic treatment (DCT) of inserts resulted better surface finish on workpiece as compared to untreated tools. Vadivel et al. [13] have studied the microstructure of cryogenically treated (TiCN+Al 2 O 3) coated and untreated inserts in turning nodular cast iron. It is clear that coated and treated carbide inserts exhibits better performance than untreated (UT) carbide tools. The study was based on wear resistance, surface roughness, power consumption and flank wear. Kalsi et al. [14] reviewed different CT approaches and concluded that deep cryogenic treatment has significant effect on the performance of tool steel and other materials. Ramji et al. [15] compared the performance of coated and treated and untreated in turning grey cast iron. They revealed that cryogenically treated tools have performed superior to non-treated in all tests condition, in terms of lesser flank wear, lesser cutting forces and reduced surface roughness. Jiang et al. [16] indicated that cryogenic treatment increased hardness, compressive strength, wear resistance and fatigue resistance of cemented carbide, but there is no effect on bending strength and toughness of cemented carbide. The improvement of mechanical properties is highly dependent on the soaking time. From the literature review, coating acts as a heat barrier and provides lubrication. It improves wear resistance, reduces cutting forces, lowers the friction coefficient and thereby the contact temperature. However under extreme cutting conditions coated tools behave like uncoated ones. The literature reports reveal that cryogenic treatment improves wear resistance, increases hardness, fatigue resistance and reduces residual stresses. Most of the research on cryogenic treatment has been concentrated on steel. Some studies have been done on the cryogenically treated carbide tools and few literatures are available about the effect of cryogenic treatment on coated tools. Since, there are no reports of evaluation on post-cryogenic treated AlCrN, TiN coated and uncoated tools. Hence, it was decided to investigate the performance of deep cryogenic treatment on AlCrN, TiN coated and uncoated carbide tool by evaluating the effect of various parameters on cutting forces and tool wear. 3. EXPERIMENTATION The planning of experiments is very much essential to minimize the experiments. The classical experimental design methods are too complex and time consuming. A large number of experiments have to be performed when the number of process parameters increases. Taguchi s method uses a special design of orthogonal arrays to study the entire parameter space with minimum experiments. In present work, machining parameters such as speed, feed, depth of cut and coating type were considered for analyzing the effect of these parameters on cutting forces and tool wear. According to Taguchi method, a robust design and L 27 orthogonal array was employed for experimentation. Three set of rhombic shaped ISO specification CCMT09T0304 single point cutting tools were used in our experiments. These tools were procured from Mitishbushi Pvt. Ltd. (Gurgaon) India. TiN and AlCrN PVD coatings were deposited on two sets of cutting inserts and one set of tool was uncoated. Both of these coatings are monolayered, have a cubic structure and thickness of around 3µ m. All these inserts were treated cryogenically. Deep cryogenic treatment can be described as a controlled lowering of temperature from room temperature to the boiling point of liquid nitrogen (-196 0 C) and maintained it for about 24h, followed by a controlled raising of temperature to the ambient temperature. Subsequently, the inserts were subjected to tempering cycles to relieve the stresses induced by cryogenic treatment. AlCrN coated, TiN coated and uncoated inserts were denoted by C1, C2 and UC respectively. A high power rigid lathe of HMT, LB-17 model was used for experimental investigation. AISI 1040 steel was used as a workpiece material. AISI 1040 steel was heat treated up to hardness of 33 HRC. The diameter and length of workpiece material were 63 mm and 540 mm respectively. The composition of workpiece material is shown in Table 1.
Table1. Chemical composition of AISI 1040 steel (wt %) The material (AISI 1040) has many industrial applications such as shafts, gears, bolts etc. For each experiment, a fresh cutting edge was used. A dynamometer was mounted on the tool post for measurement of cutting forces. Figure.1. Experimental set up The calibration of the dynamometer was done before starting the experiment. The experimental set up is shown in Figure 1. The flank wear was measured using metallurgical microscope (Yucon Japan). The experimental design is shown in Table 2, which is using L 27 Orthogonal array. Table 2. Experimental combinations of the machining parameters using L 27 orthogonal array. 4. RESULTS AND DISCUSSION Taguchi approach was applied to find out the factors affecting the cutting forces and tool wear. The effect of input parameters i.e. cutting speed, feed, depth of cut and coating type and some of their interactions were evaluated using ANOVA. The purpose of the ANOVA is to investigate the process parameters which significantly affect the performance characteristics. MINITAB 14.1 statistical software was used to plot main effect for the means of cutting force and tool wear. 4. 1 Cutting Forces Cutting forces are very important factors in machining process, it determine the power consumption and surface roughness. The results obtained from the experiments were statistically analyzed. Table 3 gives the ANOVA and F test values with percentage contribution, i.e. effectiveness of individual turning parameters on cutting forces. It also shows that all calculated values on the basis of experimentally obtained results for feed and two factor interaction are insignificant. The rank of importance for various factors in the terms of their relative significance is given in the Table 4. The Depth of cut has the highest rank, signifying highest contribution to cutting forces and feed has the lowest rank and was observed to have insignificant affect on cutting forces. With the increase in depth of cut, material removal rate increases resulting in increase in cutting forces. The average values of the means for cutting forces at different levels are plotted in Fig. 2, keeping the objective smaller is better which shows the variation of cutting force with the input parameters. Cutting forces increased frequently with increase in depth of cut and feed. Cutting forces decreased as cutting speed increases. This is due to the fact that the cutting temperature is higher at higher cutting speeds, resulting in softening effect of workpiece at higher cutting speed. In case of coated and cryogenic treated, the lowest cutting force was achieved with coating TiN as compared to AlCrN coating and uncoated tools respectively. The highest cutting force was obtained in case of uncoated tools. Coated and cryogenic treated tools exhibited less force due to thin hard surface coating which reduces friction and wear. [12], [15]. It also provides stability at high temperature. Figure 2. Main Effect Plots for Cutting Forces Mean
Table 3. Analysis of Variance for Means Table 4. Response Table for Means of Cutting Forces (Smaller is better) 4.2 Tool Wear Flank wear occurs on the relief face of cutting tool and is generally attributed to rubbing of tool along the machined surface and high temperatures causing abrasive and adhesive wear, thus affecting tool materials properties as well as workpiece surface. Abrasion, diffusion and adhesion are the main wear mechanism in flank wear. The flank wear is usually characterized by the abrasive grooves and ridges on the flank face. The average values of the means for tool wear at different levels are plotted in Fig. 3. The smaller is better criteria was followed for tool wear. The depth of cut has steeper slope in main effect plot, so it has significant effect on tool wear. Tools wear increases frequently with increase in cutting speed and feed. Increase in cutting speed causes higher cutting temperature. This caused concentration of high cutting temperatures very close to the cutting edge. So there is more flank wear. The results were analyzed using ANOVA for identifying the significant factors affecting the performance measures. ANOVA Table 5 shows that depth of cut, cutting speed and coating type are the factors that significantly affecting the tool wear. The rank of importance for various factors in the terms of their relative significance is given in the Table 6. The depth of cut has the highest rank, signifying highest contribution to tool wear and feed has the lowest rank and was observed to have insignificant affect on tool wear. In case of coating, the lowest tool wear was achieved with TiN coating as compared to AlCrN coating and uncoated tools respectively. Reported by Coelho [17] that thermal conductivity of chromium is 94W/m K and titanium is 22W/m K respectively. A more conductive layer would allow higher heat flux to be conducted through it, resulting lower temperature at the chip tool interface. That would contribute to keep the AlCrN layer below its oxidation temperature and would accelerate softening effect on the substrate, further increasing tool wear. The highest tool wear was obtained in case of uncoated tools. It is noted that cryogenic treatment exhibit positive effects for both uncoated and coated carbide inserts. AlCrN coated and treated tools have lower cutting forces and tool wear as compare to uncoated treated inserts. Because AlCrN coating has higher oxidation resistance. So it has maximum service temperature up to 1100 0 C, which is much higher than the uncoated inserts. AlCrN monolayer structure gives it higher hot hardness and results in excellent abrasion resistance as compare to uncoated inserts. Figure 3. Main Effect Plot (Tool Wear) for mean
Table 5. Analysis of Variance (Tool Wear) for Means Table 6. Response Table for Means of Tool Wear (Smaller is better) 5. CONCLUSION This experimental investigation was performed using cryogenically treated coated and uncoated tools in turning AISI 1040 steel. On the basis of various experiments and analysis of the results, the following conclusions have been drawn. 1. Cryogenic treatment of TiN and AlCrN coated tungsten carbide inserts resulted better performance than uncoated when treated cryogenically. 2. TiN coated inserts gave better performance followed by AlCrN coated as compared to uncoated of cryogenic treatment. 3. Lower cutting forces have been observed in case of cryogenically treated TiN coated as compare to AlCrN coated and uncoated treated tools. 4. The cutting forces increased with both depth of cut and feed and decrease with the increase in the speed. 5. The flank wear increased with the increase in speed, feed and depth of cut. ACKNOWLEDGEMENT The authors gratefully acknowledge the grant provided by All India Council for Technical Education, New Delhi, India under Research Promotion Scheme, file no. 8023/BOR/RID/RPS- 143/2008-09 and 8023/BOR/RID/RPS-73/2009-10 to carry out this research. REFERENCES 1. Okumiya M, Griepentrog M, (1999), Mechanical properties and Tribological behavior of TiN-CrAlN and CrN-CrAlN multilayer coatings. Surface Coatings & Technology Volume 112, pp. 123-128. 2. Haron C H C, Ginting A, Goh J H, (2001) Wear of coated and uncoated carbides in turning tool steel. Journal of Materials Processing Technology, Volume 116, pp. 49-54. 3. Avila R F, Abrao A M, Godoy G, Cristina D, (2006), The performance of TiN coated carbide tools when turning AISI 8620 steel. Journal of Materials Processing Technology, Volume 46, pp. 782-800. 4. Kalss W, Reiter A, Derflinger V, Gey C, Endrino J L, (2006), Modern coatings in high performance cutting applications, International Journal of Refractory Metals & Hard Materials, Volume 24, pp. 399-404. 5. Yigit R, Celik E, Findik F, Koksal S, (2008) Effect of cutting speed on the performance of coated and uncoated cutting tools in turning nodular cast iron, Journal of material processing technology, Volume 204, pp. 80-88. 6. Watmon T B, Anthony C I, (2010) Coating Cutting Tools with Hard Substance Lowers Friction Co-efficient and Improves Tool Life - A Review, Proceeding of International Conference of Engineer and computer Scientist, Volume III. 7. Molinari A, Pellizari M, Gialanella S, Straffelini G, Stiasny K H, (2001) Effect of deep cryogenic treatment on the mechanical properties of tool steels. Journal of Materials Processing Technology, Volume 118, pp. 350-355. 8. Stewart H A, (2004) Cryogenic treatment of tungsten carbide reduces tool wear when machining medium density fiberboard. For Prod. J. Volume 54, pp. 53-56. 9. Yong A Y L, Seah K H W, Rahman M, (2006) Performance evaluation of cryogenically treated tungsten carbide tools in turning. International Journal of Machine Tools & Manufacture, Volume 46, pp. 2051 2056. 10. Yong A Y L, Seah K H W, Rahman M, (2007) Performance of cryogenically treated tungsten carbide tools in milling operations, International Journal of Adv Manuf Technol, Volume 32, pp. 638-643. 11. Firouzdor V, Nejati E, Khomamizadeh F, (2008) Effect of deep cryogenic treatment on wear resistance and tool life of M2 HSS drill. Journal of Materials Processing Technology, Volume 206, pp. 467 472. 12. Reddy T V S, Sornakumar T, Reddy M V, Venkatram R, (2008) Machining performance of low temperature treated P-30 tungsten carbide cutting tool inserts. Cryogenics, Volume 48 (2008), pp. 458-461. 13. Vadivel K, Rudramoorthy R, (2009) Performance analysis of cryogenically treated coated carbide inserts. Int J Adv Manuf Technol, Volume 42, pp. 222 232. 14. Kalsi N S, Sehgal R, Sharma V S, (2010) Cryogenic Treatment of Tool Materials: A Review. Materials and Manufacturing Processes, Volume 25, pp. 1077-1100. 15. Ramji B R M, Narasimha H N, Krishna M,(2010) Analysis of forces, roughness, wear and temperature in turning cast iron using cryotreated carbide inserts. International Journal of Engineering Science and Technology, Volume. 2, Issue 7, pp. 2521-2529, 16. Jiang Y, Chen D, (2011) Effect of cryogenic treatment on WC Co cemented carbides. Material Science Engineering, Volume 528, pp. 1735-1739. 17. Coelho R T, Eu-G N, Elbestawi M A, (2007) Tool wear when turning hardened AISI 4340 with coated PCBN tools using finishing cutting conditions. International Journal of Machine Tools & Manufacture Volume 47, pp. 263-272.