Performance Study of Cryogenically Treated HSS Drills in Drillilg Gray Cast Iron Using Orthogonal Array Technique

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1 Research Journal of Applied Sciences, Engineering and Technology 2(5): , 2010 ISSN: Maxwell Scientific Organization, 2010 Submitted Date: May 27, 2010 Accepted Date: June 15, 2010 Published Date: August 01, 2010 Performance Study of Cryogenically Treated HSS Drills in Drillilg Gray Cast Iron Using Orthogonal Array Technique 1 B.R. Ramji, 2 H.N. Narasimha Murthy, 2 M. Krishna and 2 M.J. Raghu 1 Department of Manufacturing Engineering, BMS College of Engineering, Bangalore, India 2 Department of Mechanical Engineering, RV College of Engineering, Bangalore, India Abstract: The objective of this research was to study the performance of cryogenically treated HSS drills for drilling gray cast iron. Drilling experiments were conducted with cutting speeds: 560, 710, 900, 1120 rpm, feeds: 0.05, 0.08, 0.12, 0.19 mm/rev and a constant drill diameter: 8 mm. The cryogenic treatment cycle consisted of cooling the test samples from room temperature to cryogenic temperature of ºC in 3 h, soaking at cryogenic temperature for 24 h and warming to room temperature in about 5 h. The thrust force and torque were measured using drill tool dynamometer. The surface roughness (Ra, Rz, Rq and Rt) of the drilled specimens were measured using talysurf. The experimental lay-out was designed using Taguchi s Orthogonal Array technique. Signal-to-Noise Ratio analysis was performed to identify the effect of the parameters on the response variables. The treated drills were found superior to the non-treated in all the test conditions in terms of lesser thrust force, torque and also superior surface roughness of the specimens. The tool wear was studied using SEM. Key words: Cryogenic, drilling, gray CI, surface roughness, Taguchi INTRODUCTION The challenge of modern machining industries is mainly focused on achieving high quality in terms of work-piece dimensional accuracy, surface finish and less tool wear. The harder the material, the more difficult it is to machine. Cast iron has been used in large quantities for years because of desirable properties as good castability, good machinability and low cost. A wide variety of gray cast iron is used in industries for various applications. Machinability of Cast Iron is affected by the amount of carbon. The tool lives may increase significantly after being submitted to subzero temperatures. Cryogenic treatment involves the soaking of cutting tools at very low temperatures, -184ºC or below. The results can be surprisingly good depending upon the application. Reports of % increases in tool lives after they are treated at -196ºC are found (Paulin, 1993). The tool steels submitted to conventional heat treatment presented only a small amount of retained austenite, but those submitted to cryogenic treatment showed better wear performance during machining. This is due to a new mechanism besides the transformation of the retained austenite into martensite. The new mechanism is time and temperature dependent during the long hours of dwelling in cryogenic treatment. The carbide particles smaller in sizes are found after the treatment, which would contribute to the wear resistance of the tools (Gruman Aircraft Engineering, 1965; Barron, 1974; Ahmed, 2004). Chatterjiee (1992) reported the cryogenically treated high speed drills yielded higher tool life compared than that of with untreated ones, for different cutting speeds and for different work materials. Firouzdor et al. (2008) reported the influence of deep cryogenic treatment on wear resistance and tool life of M2HSS drills in high-speed dry drilling of carbon steels. The researchers observed 77 and 126% improvement in cryogenic treated and cryogenic and temper treated drill lives, respectively. The improvement in wear resistance of the drills was mainly attributed to the resistance of cryogenically treated drills against diffusion wear mechanism, due to the formation of fine and homogeneous carbide particles during cryogenic treatment. Additionally, transformation of retained austenite to martensite played an effective role, i.e. improved hardness values. Carter (1956), Shaw (1986) and Adler et al. (2006) reported results of tool performance in case of cryogenically treated tools used for drilling, milling, turning, etc. Based on the literature review it was concluded that experimental studies on the effect of cryogenic treatment on drilling gray cast iron has not been reported yet. Finite Element Analysis for the performance of cryogenically treated drills has been very scarcely reported. Thus, this research was aimed at studying the performance of Corresponding Author: B.R. Ramji, Department of Manufacturing Engineering, BMS College of Engineering, Bangalore, India Tel:

2 cryogenically treated HSS drills in terms of thrust force, torque and the surface roughness in drilling of gray cast iron experimentally based on Taguchi s Orthogonal array technique and FEA. MATERIALS AND METHODS Experimental-drilling studies: The drilling exercises on gray cast iron and the study of the influence of cryogenic treatment of the inserts on the thrust force, torque and the surface roughness of the work piece were undertaken under the experimental conditions presented in Table 1. The experiments were conducted at RV College of Engineering Bangalore, India during September The cryogenic treatment involved cooling the inserts from room temperature to cryogenic temperature (-178.9ºC) in 3 h, soaking at this temperature for 24 h and warming to room temperature in 5 h. Drilling exercises were carried out for each experimental condition to cut 15 mm depth blind hole in the work-piece and for each experimental condition five holes were drilled. Design of experiments using taguchi s orthogonal array technique: The factor level combinations selected for the design of experiments is shown in Table 2. An L'16 OA layout was selected to satisfy the minimum number of experiments condition. The experimental results are shown in Table 3 and 4. Signal-to-Noise ratio analysis of the responses: The signal-to-noise ratio analysis based on the responses obtained in the drilling exercises is shown in Table 5, 6 and 7 for thrust force. To determine the effect each variable has on the output, the signal-to-noise ratio, or the SN number, has been computed for each experiment conducted. Table 1: Experimental conditions used for drilling exercises Machine tool Sensitive drilling machine Workpiece Gray Cast Iron (C %, Mn-0.329%, P , S-0.120, Si %) Size 50 X 40 mm Cutting tool HSS drills Working tool geometry Diameter: 8 mm, Flute length: 75 mm, Overall length: 117 mm Lip angle: 118, Helix angle: 20 Drilling parameters Cutting velocity, A: 560, 710, 900, 1120 rpm Feed rate, B: 0.05, 0.08,0.12, 0.19 mm/rev Depth of drilled blind hole: 15 mm Tool condition, C: Cryogenically treated (CT), non-treated (NT) Measuring instruments Talysurf (SJ-201) for surface roughness (Ra, Rz, Rq, Rt) Drill tool dynamometer for measuring thrust force in Kgf and Torque in Kg-m, Scanning Electron Microscope: JOEL Place of study R V College of Engineering, Bangalore, India Time of study September 2009 Table 2: Parameters, levels and orthogonal array Factor code Parameter No. of levels A Cutting velocity 4 B Feed rate 4 C Tool condition 2 No. of levels considered for study 4 Orthogonal Array chosen corresponding to L'16 four parameters and four levels The calculation of the SN for the first experiment in the array above is shown below for the case of a specific target value of the performance characteristic. (1) Stl = = (2) Stl = Stl-Sml = = 5.2 (3) Table 3: Experimental lay-out and results for thrust force as response Thrust force (Kgf) S.No. Cutting speed (rpm) Feed mm/rev Tool condition NT/CT T NT C NT CT C NT CT NT NT CT NT CT CT NT CT NT

3 Table 4: Experimental results for torque and surface roughness as responses Torque (Kg-m) Surface roughness Ra ( m) S.No. T 1 T Table 5: SN number for each experiment conducted for thrust force S.No. Speed A Feed B Tool condition C S m1 S t1 S e1 V e1 SN NT CT NT CT CT NT CT NT NT CT NT CT CT NT CT NT Table 6: Computed average SN values SN A SN B SN C SN A SN B SN C SN A SN B SN C SN A SN B SN C = 1.97 = 3.81 = Table 7: Range and rank for thrust force, torque and surface roughness Speed Feed Tool condition Thrust force Range, Rank Torque Range, Rank Surface roughness Range, Rank Similarly SN 2, SN 3, SN 4 etc. have been computed. (4) (5) After computing the SN ratio for each experiment, the average SN value is calculated for each factor and level as shown in Table 6. Where: Similarly SN B1, SN C1 were computed and (6) = SN Amax SN Amin = = 1.97 (7) Table 7 shows the range R ( = high SN - low SN) of the SN computed for each parameter for each of the response factors. Larger the R value for a parameter, the larger the effect the variable has on the process. This is because the same change in signal causes a larger effect on the output variable being measured. It can be seen that the tool condition has the largest effect on the thrust force, torque and surface roughness. Speed has the smallest effect on the thrust force and feed has the smallest effect on the torque and the surface roughness. 489

4 RESULTS AND DISCUSSION Thrust force and torque are indicators of tool performance. Greater the thrust force and torque, greater is the tool wear and hence lower drill tool life. The drill tool wear is caused by abrasive, diffusive and adhesive wear mechanisms. The heat generated during drilling also affects drill tool life. At higher speeds and feeds, the thrust force and torque of the drill tool increases for a given depth of hole drilled. Surface finish is an important index of machinability because life and performance of the machined components are influenced by their surface finish, residual stresses and surface defects (Paul et al., 2001). The signal-to-noise ratio analysis indicated that, the cryogenic treatment has the largest effect on the thrust force, torque and surface roughness. Speed has the smallest effect on the thrust force and feed has the smallest effect on the torque and surface roughness (Ra). Scanning electron microscopy: The scanning electron micrographs of the HSS drills are shown in Fig 1. Figure 1a and c show non treated and treated drills respectively. In this after drilling condition Fig. 1(b), the non-treated drills showed plastic deformation, which was due to the lack of matrix strength indicating the absence of carbide particles i.e., martensitic matrix. The treated drills showed (Fig. 1d) micro damages at the chisel edge indicating the presence of carbide particles i.e., martensitic matrix. The cryogenic treatment of HSS drills toughened the tool material and increased its strength, which was also indicated by the increase in hardness. The treated drills showed decreased thrust force and torque and better surface roughness than that obtained by non-treated drills. CONCLUSION Drilling exercises were performed using cryogenically treated and non-treated HSS drills to study the tool performance. Based on the study the following conclusions were drawn: The extents of influence of cutting velocity, feed and the condition of the inserts were examined by conducting drilling exercised based on Orthogonal Array Experimentation technique. Cryogenic treatment resulted in lesser thrust force, torque and yielded superior surface finish compared to that of non-treated drills. The responses obtained after drilling were analyzed using signal-to-noise ratio analysis.the tool condition had the largest effect on the Fig. 1: SEM of HSS drills (a) Non-treated before drilling, (b) Non-treated after drilling showing excessive wear on cutting edge, (c) treated before drilling and (d) treated drill after drilling showing less wear of the cutting edge 490

5 thrust force, torque and surface roughness. Speed had the smallest effect on the thrust force and feed had the smallest effect on the torque and the surface roughness. Based on the results it was concluded that cryogenic treatment can yield significant improvement in both productivity and product quality and hence overall machining economy offsetting the cost of cryogenic cooling. REFERENCES Adler, D.P., W. W-S. Hii, D.J. Michalek and J.W. Sutherland, Examining the role of cutting fluids in machining and efforts to address associated environmental/health concerns. Mach. Sci. Technol., 10(1): Ahmed, M., Cryogenic drilling of Kevlar composite laminated. M.Sc. Thesis, Mechanical Engineering, King Fahd University of Petroleum and Minerals. Barron, F.R., Yes-Cryogenic Treatments Can Save You Money! Here's Why. Tappi J., 57(5): Carter W.A., Metal Machining, Part VI, Overseas Edition, Cutting Fluids, Machinery Lloyd. Chatterjee, S., Performance characteristics of cryogenically treated high-speed drills.int. J. Prod. Res., 32(4): Firouzdor, V., E. Nejati and F. Khomamizadeh, Effect of deep cryogenic treatment on wear resistance and tool life of M2 HSS drill. J. Mater. Process. Technol., 206(1-3): Gruman Aircraft Engineering, Cryogenic coolants speed titanium machining. Machinery, pp: Paul, S., N.R. Dhar and A.B. Chattopadhyay, Beneficial effects of cryogenic cooling over dry and wet machining on tool wear and surface finish in turning AISI 1060 steel. J. Mater. Process. Tech., 116(1): Paulin, P., Frozen gears. Gear Technol., pp: Shaw, M.C., Metal Cutting Principles. Oxford University Press, Oxford. 491