- Email: [email protected]
Effect of tool nose radius and tool wear on residual stress distribution in hard turning of bearing steel
Journal of Materials Processing Technology 150 (2004) 234–241
Effect of tool nose radius and tool wear on residual stress distribution in hard turning of bearing steel Meng Liu a,∗ , Jun-ichiro Takagi a , Akira Tsukuda b a
Department of Mechanical Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-Ku, Yokohama 240-8501, Japan b Kagawa Prefectural Industrial Technology Center, 587-1 Goto-Cho, Takamatsu 761-8031, Japan Received 20 September 2002; accepted 12 February 2004
Abstract This study presents a experimental investigation to clarify the effects of tool nose radius and tool wear on residual stress distribution in hard turning of bearing steel JIS SUJ2. Three types of CBN tools with different nose radius (0.4, 0.8 and 1.2 mm) were used in this study. The residual stresses beneath the machined surface were measured using X-ray diffraction technique and electro-polishing technique. The results obtained in this study show that the tool nose radius affects the residual stress distribution significantly. Especially the effect on the residual stresses at the machined surface at early stage of cutting process is remarkable. For the tool wear, as the tool wear increases, the residual stress at the machined surface shifts to tensile stress range and the residual compressive stress beneath the machined surface increases greatly. © 2004 Elsevier B.V. All rights reserved. Keywords: Hard turning; Residual stress; CBN tool; Nose radius; Bearing steel
1. Introduction Hard turning attracts great interests since it potentially provides an alternative to conventional grinding process for machining high hardness, high precision components in small production. Several advantages of hard turning are described in terms of flexibility, low-cost, environmentally friendly production in comparison with the grinding process [1–8]. In order to substitute grinding process, the profile of residual stress induced in hard turning process becomes an important factor for assuring the quality of the machined components. As well known, in the case of contact components, e.g. roller bearings, the profile of residual stress including the magnitude and the depth beneath the machined surface, etc. can greatly affect the fatigue life of a roller bearing. The residual compressive stresses at the rolling contact surface are more favorable than tensile residual stresses for fatigue life [9,10]. Therefore, the formation of the residual stress and the effect of the cutting process parameters on the residual stress must be understood for applying the hard turning technique. Many studies about the residual stress induced in hard turning process have been conducted [11–23]. These investigations show that the residual stress depends on the ther∗ Corresponding author. Fax: +81-45-3316593. E-mail address: [email protected] (M. Liu).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.02.038
moplastic deformation of the workpiece. Numerous studies about the residual stress were conducted to determine the relation between the cutting parameters and the residual stress [5,6,14–18]. The effect of workpiece hardness was also discussed in several reports because the workpiece hardness can be modified to change thermoplastic deformation characteristics [12–15]. The effects of the tool wear in hard turning were also studied [10]. In order to substitute grinding process and minimize tool wear, cutting parameters in hard turning are generally adapted for finishing operations. Small depth of cut and low feed rates are chosen to improve finished surface and reduce the mechanical and thermal impacts on the tools to acceptable limits. Thus, in hard turning process, the undeformed chip thickness is an order of magnitude as the radius of the cutting edge or the size of the edge chamfer. Consequently, the chips are formed along the nose of the tool in the region of the edge chamfer or the cutting-edge radius. Therefore, the cutting-edge geometry contributes to thermoplastic deformation during hard turning process. About the geometry of the tool, the effect of cutting-edge geometry or edge preparation on the residual stress was discussed mainly [9,20–22]. The experimental investigations on the effect of the cutting-edge preparation (cutting-edge geometry) on surface residual stress show that the cutting-edge geometry has a significant effect on the surface residual stress and microstructure in hard turning. The large edge hone (ra-
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Table 1 Experimental conditions
Fig. 1. Geometry of workpiece.
Tool [24]
CBN tool (Mitsubishi Materials Co., Japan) Grade: MB825 Composition: CBN, TiC and Al2 O3 Shape: NP-TNMA160404G, NP-TNMA160408G, NP-TNMA160412G (nose radius: 0.4, 0.8, 1.2 mm) Chamfer: 0.25 mm × 25◦ Cutting-edge radius: 0.01 mm Tool-holder: PTGNR2020K16
Workpiece
Material: JIS-SUJ2 (bearing steel) Composition: C 0.95–1.10%, Si 0.15–0.35%, Mn ≤0.50%, P ≤0.025%, S ≤0.025%, Cr 1.30–1.60% Hardness: HRC60 Diameter: 100 mm, length of cut: 50 mm
Cutting conditions
Cutting speed: 120 m/min Feed speed: 0.1 mm/rev Depth of cut: 0.1, 0.2 mm Coolant condition: dry
dius) tools produce more compressive residual stress than small edge hone [20–22]. And a double chamfer tool was designed and applied in hard turning. Using the double chamfer tool, the compressive residual stress was larger and deeper than that induced by a sharp tool [9]. Although many studies about the residual stress have been conducted, but about the geometry of tools, the abovementioned cutting-edge geometry was discussed only. There exist merely reports about the effect of the nose radius of tools on the residual stress. On the other hand, the nose radius of tools is very important in determining the roughness of machined surface. A smooth surface comparable with a ground surface was realized using a tool with a large nose radius [23]. Then, the tool nose radius plays an important role in deciding the shape of cutting cross-section in combination with the depth of cut and feet rate. Thus, the nose radius relates to the chip morphology very strongly. It can be considered that the tool nose radius plays an important role not only in determining the surface roughness but also in thermoplastic deformation of workpiece in hard turning process. Therefore, in this study, an experimental investigation was conducted to clarify the effect of the nose radius of CBN tool on the residual stress distribution in hard turning of bearing steel. And the effect of tool wear on the residual stress distribution in the case of different tool nose radius was also discussed.
tools were observed using scanning electron microscope (SEM) and CCD microscope. The flank wear of CBN tools was evaluated using the photographs of CCD microscope. The measurements of residual stress were conducted using X-ray diffraction technique. The measurement conditions used in this study are listed in Table 2. The measurements of residual stress were performed by using “sin φ” method and were conducted in the direction of the circumferential direction (the cutting direction). To determine the residual stress beneath the machined surface, an electro-polishing technique was utilized, and the maximum electro-polished depth from the machined surface is over 200 m.
2. Experimental procedures
3. Effect of tool nose radius on residual stress
Turning tests were conducted in continuous dry conditions. The specification of the workpiece is illustrated in Fig. 1. The material of the workpiece is JIS SUJ2 bearing steel with the hardness of HRC60. The detail conditions about CBN tools, workpiece and cutting conditions are listed in Table 1. Cutting forces were measured during turning operations using a strain-gauge type dynamometer. The worn CBN
3.1. Cutting forces
Table 2 Conditions of X-ray diffraction Characteristic X-ray Diffraction plane Diffraction angle Tube voltage Tube current Divergent angle Step angle Fixed time Irradiated area Stress constant
Cr K␣ (2 1 1) 156.08◦ 30 kV 10 mA 1.0◦ 0.5◦ 0.4 s per step 10 mm × 20 mm −297.23 MPa/◦
Figs. 2 and 3 show the results of cutting forces measured during cutting process in the condition of the depth of cut of 0.1 and 0.2 mm. The results of cutting forces shown in the figures were average values at the early stage of turning process, which the maximum cutting length was about 47 m. In all tests, the thrust forces (Fp ) show larger value than the
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Fig. 2. Results of cutting forces (depth of cut 0.1 mm).
Fig. 5. Schematic model of cutting cross-section.
Fig. 3. Results of cutting forces (depth of cut 0.2 mm).
cutting forces (Fc ). The thrust forces increase greatly as the nose radius of CBN tool increases. And the feed forces (Ff ) decrease a little as the nose radius of CBN tool increases. There exists a tendency that cutting forces increase with the increase of the nose radius, but this tendency is not so remarkable in comparison with the increases of thrust forces. The results of the ratio of thrust force to cutting force (Fp /Fc ) and the ratio of thrust force to feed force (Fp /Ff ) are shown in Fig. 4. As shown in the figure, the ratio of Fp /Fc and the ratio of Fp /Ff increase greatly as the nose radius of CBN tool increases. The reason for these results can be considered due to a pure geometric effect of the tool nose radius on the cutting cross-section. In machining hardened steel, because of small depth of cut, the chips were formed by the region of the edge chamfer or the radius of cutting edge. A large negative rake angle increases the cutting forces only a little, whereas increases the thrust forces remarkably.
Thus, the ratio of Fp /Fc shows a value that is larger than 1.0 [25]. Under the condition adopted in this study, i.e. the depth of cut is less than 0.2 mm, the nose radius of CBN tool is larger than 0.4 mm, the chips was formed mainly in the region of the tool nose. Thus, the cutting-edge angle varies. A reference cutting-edge angle κref (defined as shown in Fig. 5) is used to evaluate and understand the cutting process. The thrust force increases remarkably as the nose radius of CBN tool increase due to the decrease of the cutting-edge angle κref . As the depth of cut decreases or the nose radius increases, the cutting-edge angle κref decreases. Consequently, the thrust forces increase and the feed forces decrease. In the condition used in this study, the reference cutting-edge angle changes from 10.58◦ to 26.41◦ . These values are great smaller than conventional cutting process. As the cuttingedge angle κref increases, the arc length of machined region (lc in Fig. 5) increases and the width of machined region (amax in Fig. 5) decreases. Namely, the cross-section region machined in cutting process will be thinner and wider. Therefore, in the cutting using a CBN tool with a larger nose radius, the ratio of Fp /Ff increases greatly. On the other hand, area of machined region is almost constant for the different nose radius. Comparison with the thrust forces, the increase of the cutting forces due to the increase of the nose radius is only a little. Thus, the ratio of Fp /Fc increases greatly as the nose radius increases. 3.2. Residual stress
Fig. 4. Ratio of cutting forces vs. corner radius.
Fig. 6 shows the results of residual stress measured in the depth of cut of 0.1 and 0.2 mm. These results were measured
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Fig. 6. Results of the residual stress distribution.
at the cutting length of 47 m. From the figures, the residual stresses show different depth profiles for the different nose radius. The residual stress at the machined surface and the maximum residual compressive stress induced beneath the machined surface are different greatly. The results of the nose radius of 0.4 mm, there exists a larger residual compressive stress at the machined surface. With the increase of the depth beneath the machined surface, these residual compressive stresses increase and reach the maximum compressive stress. Then the compressive residual stresses decrease with the increase of the depth and back to the status without the residual stress. Comparison of the different the depth of cut, the residual compressive stress induced at the machined surface in the depth of cut of 0.1 mm is larger than that in the depth of cut 0.2 mm. There exists no remarkable differences in the profile of the residual stress. About the results of the nose radius of 0.8 mm, the residual stress at the machined surface was also a residual compressive stress induced in the both depth of cut of 0.1 and 0.2 mm. But the magnitude of these residual compressive stresses is small in comparison with the nose radius of 0.4 mm. And
the influence of the depth of cut on the residual stress is not remarkable as same as the nose radius of 0.4 mm. The shape of the residual stress profile beneath the machined surface is similar to that of the nose radius of 0.4 mm. About the results of the nose radius of 1.2 mm, the residual stresses at the machined surface were residual tensile stress in the both depth of cut of 0.1 and 0.2 mm. But the magnitude of these residual tensile stresses is small near the state without the residual stress. These residual tensile stresses shifted to the residual compressive stress and reached the maximum compressive stress with the increase of the depth beneath the machined surface. The shape of the residual stress profile beneath the machined surface is similar to that of the nose radius of 0.4 mm. From above-mentioned results, it can be considered that the nose radius of CBN tool affects the residual stress greatly, especially the residual stress at the machined surface. There exists a tendency that the residual stresses induced at the machined surface shift from compressive stress to tensile stress as the nose radius increases in the cutting condition of this study.
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The residual stresses in a ground surface have been reported that it is primarily generated due to the thermal expansion and contraction, the phase transformations and the plastic deformation [26–29]. The residual stresses are mainly induced by the mechanical, thermal and chemical impacts during the cutting process as the same as the grinding process. These impacts are acting at the same time with overlapping effects. This leads to the residual stress distribution which cannot be correlated to one impact only. The existence of the tensile or compressive residual stress will depend to a great extent on the depth of the permanent plastic deformation zone which penetrates into the workpiece. This zone depends on the stress generated by the mechanical and thermal impact which exist during cutting process [1,2]. In hard turning, the cutting energy is mainly consumed for shearing to form chips and friction between tool–chips, tool–workpiece. This enables the produced heat, the cutting heat flows not only into the workpiece but also into the chips. The heat absorbed in the workpiece is generally small ratio of total cutting consumption energy. During cutting process, the thermal stress on the surface layer is capable of producing only tensile residual. This is because during cutting process, the material near the surface is heated and elongated more than the bulk material. Thus the surface layer expands and experiences a compressive stress resulting from the bulk material beneath. If this compressive stress exceeds the yield stress of the material, then the tensile stress will remain in the surface after cooling. The deformation of the surface layer due to the applied mechanical impact may produce both tensile and compressive residual stresses [1,4,6]. From above-mentioned results, the experimental results obtained in this study can be explained by the different effect of the mechanical and thermal impacts due to the difference of the nose radius of CBN tool. a larger nose radius
causes not only a higher thermal impact but also a higher mechanical impact. In hard turning, the machined surface is formed by the tool nose. It has been proved that the friction between tool and workpiece induces the increase of forces and the cutting temperature increase due to the tool nose. With a larger nose radius tool, this results in producing higher friction and thus the cutting temperature increases [23]. On the other hand, the cutting process using a tool with a larger nose radius, the cross-section region machined will be thinner and wider, and a smaller chip thickness will be produced. Thus, the tool with larger nose angle produces a larger thrust force (as shown in Figs. 2 and 3) and a higher degree of the plastic deformation which results in decreasing of surface tensile stress and increasing of the compressive stresses beneath the machined surface.
4. Effect of the tool wear on residual stress 4.1. Cutting forces and tool wear In order to evaluate the effect of the tool wear on the residual stress for various nose radius, cutting tests were carried out until cutting length reached 550 m in the condition which the depth of cut was 0.2 mm. The residual stress distributions for various nose radius were measured after each cutting test. Fig. 7 shows the photographs of the worn CBN tools after cutting tests. The flank wear VBmax is less than 0.2 mm at the cutting length of 550 m for three types of CBN tools. Fig. 8 shows the variation of the cutting forces with the cutting length for various nose radius. From Fig. 8, there exists a tendency that the cutting force and the thrust force, especially the thrust force, increase gradually with the increase of the tool wear, and this tendency is remarkable in the cutting process using a large nose radius especially.
Fig. 7. Photographs of worn CBN tools (depth of cut 0.2 mm, cutting length 550 m).
M. Liu et al. / Journal of Materials Processing Technology 150 (2004) 234–241
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Fig. 8. Variation of cutting forces (depth of cut 0.2 mm).
4.2. Residual stress Fig. 9 shows the comparison of the effect of tool wear on the residual stress for various nose radius. For various nose radius, as the tool wear increases, the residual stresses in-
duced at the machined surface show tensile stresses and the maximum residual compressive stress was induced deeply with a lager magnitude. On the other hand, as the tool wear increases, there exist no remarkable differences in the residual stress distribution among various nose radius. Namely,
Fig. 9. Results of residual stress distribution as a function of tool wear.
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the effect of the nose radius on the residual stress decreases greatly with the increase of the tool wear. In prior studies, about the effect of the tool wear on residual stress, it has been concluded that the tool wear leads to higher thermal process energy related to the increase of the friction. The residual tensile stresses at the machined surface are caused by the thermal impacts due to the friction and plastic deformation. And, the mechanical impacts lead to induce the compressive residual stress which admittedly influence deeper material regions. Therefore, the increase of the tool wear leads to the shift to tensile stress range at the machined surface, and the compressive residual stresses are induced beneath the machined surface [30,31]. The experimental results obtained in this study can been explained by the reason as stated above. In all experimental conditions, the tool wear leads to the increase of the friction between tool and workpiece. The results of increase of the thrust forces and the cutting forces (as shown in Fig. 8) indicate the increase of the friction. The friction between tool and workpiece causes an increase of the thermal process energy. On the other hand, the tool wear causes a larger thrust force, consequently, a larger mechanical impact is produced to the workpiece. Therefore, the residual stress distribution was produced due to the influence of the thermal impact and the mechanical impact.
5. Conclusions In this study, the effect of the tool nose radius and the tool wear on the residual stress induced in hard turning process was discussed experimentally. The main experimental results are as follows: (1) The increase of the tool nose radius leads to an increase of the thrust force greatly. The ratio of the thrust force to cutting force and the ratio of the thrust force to feed force increase with the increase of the tool nose radius. (2) The tool nose radius affects the residual stress at the machined surface significantly at early cutting stage. There exists a tendency that the residual stress at the machined surface shift to tensile range with the increase of the tool nose radius. (3) With the increase of the tool wear, the residual tensile stresses at the machined surface increases, and the residual compressive stress beneath the machined surface increases remarkably. The effect of the nose radius on the residual stress distribution decreases greatly with the increase of the tool wear.
References [1] H.K. Tonshoff, C. Arendt, R. Ben Amor, Cutting of hardened steel, Ann. CIRP 49 (2) (2000) 547–566. [2] W. Konig, R. Komanduri, H.K. Tonshoff, G. Ackershott, Machining of hardened materials, Ann. CIRP 33 (2) (1984) 417–427.
[3] W. Konig, A. Berktold, K.-F. Koch, Turning versus grinding—a comparison of surface integrity aspects and attainable accuracies, Ann. CIRP 42 (1993) 39–43. [4] N. Narutaki, Y. Yamane, Tool wear and cutting temperature of CBN tool in machining of hardened steels, Ann. CIRP 28 (1) (1979) 23– 28. [5] T. Hodgson, P.H. Trendler, Turning hardened tool steels with cubic boron nitride inserts, Ann. CIRP 30 (1) (1981) 63–66. [6] W. Konig, M. Klinger, R. Link, Machining hard materials with geometrically defined cutting edges—field on applications and limitations, Ann. CIRP 39 (1) (1990) 61–64. [7] C.R. Liu, S. Mittal, Single-step superfinishing using hard turning resulting in superior surface integrity, J. Manuf. Syst. 14 (2) (1995) 124–133. [8] H.A. Kishawy, M.A. Elbestawi, Effects of process parameters on material side flow during hard turning, Int. J. Mach. Tools Manuf. 39 (1999) 1017–1030. [9] Y. Matsumoto, F. Hashimoto, G. Lahoti, Surface integrity generated by precision hard turning, Ann. CIRP 48 (1) (1999) 59–62. [10] A.M. Abrao, D.K. Aspinwall, The surface integrity of turned and ground hardened bearing steel, Wear 196 (1996) 279–284. [11] T.I. EI-Wardany, H.A. Kishawy, M.A. Elbestawi, Surface integrity of die material in high speed hard machining. Part 2. Microhardness variations and residual stresses, ASME J. Manuf. Sci. Eng. 122 (2000) 632–641. [12] Y. Matsumoto, M.M. Barash, C.R. Liu, Effects of hardness on the surface integrity of AISI4340 steel, ASME J. Eng. Ind. 108 (1986) 169–175. [13] D.W. Wu, Y. Matsumoto, The effect of hardness on residual stress in orthogonal machining of AISI 4340 steel, ASME J. Eng. Ind. 112 (1990) 245–252. [14] J.D. Thiele, S.N. Melkote, R.A. Peascoe, T.R. Watkins, Effect of cutting edge geometry and workpiece hardness on surface residual stress in finish hard turning of AISI52100 steel, Manuf. Sci. Technol. ASME 122 (2000) 642–649. [15] T.I. El-Wardany, H.A. Kishawy, M.A. Elbestawi, Surface integrity of die material in high speed hard machining. Part 1. Micrographical analysis, ASME J. Manuf. Sci. Eng. 122 (2000) 620–631. [16] S.R. Agha, C.R. Liu, Experimental study on the performance of superfinish hard turned in rolling contact, Wear 244 (2000) 52– 59. [17] C.R. Liu, S. Mittal, Optimal pre-stressing the surface of a component by superfinish hard turning for maximum fatigue life in rolling contact, Wear 219 (1998) 129–410. [18] P. Munoz-Escalona, Z. Cassier, Influence of the critical cutting speed on the surface finish of turned steel, Wear 218 (1998) 103–109. [19] K. Jacobus, R.E. Devor, S.G. Kapoor, Machining-introduced residual stress: experimental and modeling, ASME J. Manuf. Sci. Eng. 122 (2000) 20–31. [20] J.D. Thiele, S.N. Melkote, Effect of cutting edge geometry and workpiece hardness on surface generation in the finish hard turning of AISI 52100 steel, J. Mater. Process. Technol. 94 (1999) 216–226. [21] J.D. Thiele, S.N. Melkote, R.A. Peascoe, T.R. Watkins, Effect of cutting edge geometry and workpiece hardness on surface residual stress in finish hard turning of AISI52100 steel, Manuf. Sci. Technol. ASME 122 (2000) 642–649. [22] E. Brinksmeier, J.T. Cammett, W. Konig, P. Leskovar, K. Tonshoff, Residual stress-measurement and causes in machining processes, Ann. CIRP 31 (1982) 491–510. [23] C.R. Liu, S. Mittal, Single-step superfinish hard machining: feasibility and feasible cutting conditions, Robot. Comput.-Integr. Manuf. 12 (1996) 15–27. [24] Mitsubishi Material Co. Ltd., Japan. http://www.mitsubishicarbide. com.mmus/en/index.htm. [25] K. Nakayama, M. Arai, T. Kanda, Machining characteristics of hard materials, Ann. CIRP 37 (1) (1988) 89–92.
M. Liu et al. / Journal of Materials Processing Technology 150 (2004) 234–241 [26] E. Brinksmeier, J.T. Cammett, W. Konig, P. Leskovar, J. Peters, H.K. Tonshoff, Residual stresses—measurement and causes in the machining process, Ann. CIRP 31 (2) (1982) 491–510. [27] S.O.A. El-Helieby, G.W. Rowe, A quantitative comparison between residual stresses and fatigue properties of surface-ground bearing steel (EN31), Wear 58 (1980) 155–172. [28] W.B. Rowe, M.N. Morgan, S.C.E. Black, B. Mills, A simplified approach to control of thermal damage in grinding, Ann. CIRP 45 (1) (1996) 299–302.
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[29] X. Chen, W.B. Rowe, D.F. McCormack, Analysis of the transitional temperature for tensile residual stress in grinding, J. Mater. Process. Technol. 107 (2000) 216–221. [30] Y. Matsumoto, M.M. Barash, C.R. Liu, Cutting mechanisms during machining of hardened steels, Mater. Sci. Technol. 3 (1987) 299– 305. [31] M.M. Barash, C.R. Liu, The mechanical state of the sublayer of a surface generated by chipremoval process, J. Eng. Ind. 98 (4) (1976) 1192–1208.
Effect of tool nose radius and tool wear on residual stress distribution in hard turning of bearing steel Meng Liu a,∗ , Jun-ichiro Takagi a , Akira Tsukuda b a
Department of Mechanical Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-Ku, Yokohama 240-8501, Japan b Kagawa Prefectural Industrial Technology Center, 587-1 Goto-Cho, Takamatsu 761-8031, Japan Received 20 September 2002; accepted 12 February 2004
Abstract This study presents a experimental investigation to clarify the effects of tool nose radius and tool wear on residual stress distribution in hard turning of bearing steel JIS SUJ2. Three types of CBN tools with different nose radius (0.4, 0.8 and 1.2 mm) were used in this study. The residual stresses beneath the machined surface were measured using X-ray diffraction technique and electro-polishing technique. The results obtained in this study show that the tool nose radius affects the residual stress distribution significantly. Especially the effect on the residual stresses at the machined surface at early stage of cutting process is remarkable. For the tool wear, as the tool wear increases, the residual stress at the machined surface shifts to tensile stress range and the residual compressive stress beneath the machined surface increases greatly. © 2004 Elsevier B.V. All rights reserved. Keywords: Hard turning; Residual stress; CBN tool; Nose radius; Bearing steel
1. Introduction Hard turning attracts great interests since it potentially provides an alternative to conventional grinding process for machining high hardness, high precision components in small production. Several advantages of hard turning are described in terms of flexibility, low-cost, environmentally friendly production in comparison with the grinding process [1–8]. In order to substitute grinding process, the profile of residual stress induced in hard turning process becomes an important factor for assuring the quality of the machined components. As well known, in the case of contact components, e.g. roller bearings, the profile of residual stress including the magnitude and the depth beneath the machined surface, etc. can greatly affect the fatigue life of a roller bearing. The residual compressive stresses at the rolling contact surface are more favorable than tensile residual stresses for fatigue life [9,10]. Therefore, the formation of the residual stress and the effect of the cutting process parameters on the residual stress must be understood for applying the hard turning technique. Many studies about the residual stress induced in hard turning process have been conducted [11–23]. These investigations show that the residual stress depends on the ther∗ Corresponding author. Fax: +81-45-3316593. E-mail address: [email protected] (M. Liu).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.02.038
moplastic deformation of the workpiece. Numerous studies about the residual stress were conducted to determine the relation between the cutting parameters and the residual stress [5,6,14–18]. The effect of workpiece hardness was also discussed in several reports because the workpiece hardness can be modified to change thermoplastic deformation characteristics [12–15]. The effects of the tool wear in hard turning were also studied [10]. In order to substitute grinding process and minimize tool wear, cutting parameters in hard turning are generally adapted for finishing operations. Small depth of cut and low feed rates are chosen to improve finished surface and reduce the mechanical and thermal impacts on the tools to acceptable limits. Thus, in hard turning process, the undeformed chip thickness is an order of magnitude as the radius of the cutting edge or the size of the edge chamfer. Consequently, the chips are formed along the nose of the tool in the region of the edge chamfer or the cutting-edge radius. Therefore, the cutting-edge geometry contributes to thermoplastic deformation during hard turning process. About the geometry of the tool, the effect of cutting-edge geometry or edge preparation on the residual stress was discussed mainly [9,20–22]. The experimental investigations on the effect of the cutting-edge preparation (cutting-edge geometry) on surface residual stress show that the cutting-edge geometry has a significant effect on the surface residual stress and microstructure in hard turning. The large edge hone (ra-
M. Liu et al. / Journal of Materials Processing Technology 150 (2004) 234–241
235
Table 1 Experimental conditions
Fig. 1. Geometry of workpiece.
Tool [24]
CBN tool (Mitsubishi Materials Co., Japan) Grade: MB825 Composition: CBN, TiC and Al2 O3 Shape: NP-TNMA160404G, NP-TNMA160408G, NP-TNMA160412G (nose radius: 0.4, 0.8, 1.2 mm) Chamfer: 0.25 mm × 25◦ Cutting-edge radius: 0.01 mm Tool-holder: PTGNR2020K16
Workpiece
Material: JIS-SUJ2 (bearing steel) Composition: C 0.95–1.10%, Si 0.15–0.35%, Mn ≤0.50%, P ≤0.025%, S ≤0.025%, Cr 1.30–1.60% Hardness: HRC60 Diameter: 100 mm, length of cut: 50 mm
Cutting conditions
Cutting speed: 120 m/min Feed speed: 0.1 mm/rev Depth of cut: 0.1, 0.2 mm Coolant condition: dry
dius) tools produce more compressive residual stress than small edge hone [20–22]. And a double chamfer tool was designed and applied in hard turning. Using the double chamfer tool, the compressive residual stress was larger and deeper than that induced by a sharp tool [9]. Although many studies about the residual stress have been conducted, but about the geometry of tools, the abovementioned cutting-edge geometry was discussed only. There exist merely reports about the effect of the nose radius of tools on the residual stress. On the other hand, the nose radius of tools is very important in determining the roughness of machined surface. A smooth surface comparable with a ground surface was realized using a tool with a large nose radius [23]. Then, the tool nose radius plays an important role in deciding the shape of cutting cross-section in combination with the depth of cut and feet rate. Thus, the nose radius relates to the chip morphology very strongly. It can be considered that the tool nose radius plays an important role not only in determining the surface roughness but also in thermoplastic deformation of workpiece in hard turning process. Therefore, in this study, an experimental investigation was conducted to clarify the effect of the nose radius of CBN tool on the residual stress distribution in hard turning of bearing steel. And the effect of tool wear on the residual stress distribution in the case of different tool nose radius was also discussed.
tools were observed using scanning electron microscope (SEM) and CCD microscope. The flank wear of CBN tools was evaluated using the photographs of CCD microscope. The measurements of residual stress were conducted using X-ray diffraction technique. The measurement conditions used in this study are listed in Table 2. The measurements of residual stress were performed by using “sin φ” method and were conducted in the direction of the circumferential direction (the cutting direction). To determine the residual stress beneath the machined surface, an electro-polishing technique was utilized, and the maximum electro-polished depth from the machined surface is over 200 m.
2. Experimental procedures
3. Effect of tool nose radius on residual stress
Turning tests were conducted in continuous dry conditions. The specification of the workpiece is illustrated in Fig. 1. The material of the workpiece is JIS SUJ2 bearing steel with the hardness of HRC60. The detail conditions about CBN tools, workpiece and cutting conditions are listed in Table 1. Cutting forces were measured during turning operations using a strain-gauge type dynamometer. The worn CBN
3.1. Cutting forces
Table 2 Conditions of X-ray diffraction Characteristic X-ray Diffraction plane Diffraction angle Tube voltage Tube current Divergent angle Step angle Fixed time Irradiated area Stress constant
Cr K␣ (2 1 1) 156.08◦ 30 kV 10 mA 1.0◦ 0.5◦ 0.4 s per step 10 mm × 20 mm −297.23 MPa/◦
Figs. 2 and 3 show the results of cutting forces measured during cutting process in the condition of the depth of cut of 0.1 and 0.2 mm. The results of cutting forces shown in the figures were average values at the early stage of turning process, which the maximum cutting length was about 47 m. In all tests, the thrust forces (Fp ) show larger value than the
236
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Fig. 2. Results of cutting forces (depth of cut 0.1 mm).
Fig. 5. Schematic model of cutting cross-section.
Fig. 3. Results of cutting forces (depth of cut 0.2 mm).
cutting forces (Fc ). The thrust forces increase greatly as the nose radius of CBN tool increases. And the feed forces (Ff ) decrease a little as the nose radius of CBN tool increases. There exists a tendency that cutting forces increase with the increase of the nose radius, but this tendency is not so remarkable in comparison with the increases of thrust forces. The results of the ratio of thrust force to cutting force (Fp /Fc ) and the ratio of thrust force to feed force (Fp /Ff ) are shown in Fig. 4. As shown in the figure, the ratio of Fp /Fc and the ratio of Fp /Ff increase greatly as the nose radius of CBN tool increases. The reason for these results can be considered due to a pure geometric effect of the tool nose radius on the cutting cross-section. In machining hardened steel, because of small depth of cut, the chips were formed by the region of the edge chamfer or the radius of cutting edge. A large negative rake angle increases the cutting forces only a little, whereas increases the thrust forces remarkably.
Thus, the ratio of Fp /Fc shows a value that is larger than 1.0 [25]. Under the condition adopted in this study, i.e. the depth of cut is less than 0.2 mm, the nose radius of CBN tool is larger than 0.4 mm, the chips was formed mainly in the region of the tool nose. Thus, the cutting-edge angle varies. A reference cutting-edge angle κref (defined as shown in Fig. 5) is used to evaluate and understand the cutting process. The thrust force increases remarkably as the nose radius of CBN tool increase due to the decrease of the cutting-edge angle κref . As the depth of cut decreases or the nose radius increases, the cutting-edge angle κref decreases. Consequently, the thrust forces increase and the feed forces decrease. In the condition used in this study, the reference cutting-edge angle changes from 10.58◦ to 26.41◦ . These values are great smaller than conventional cutting process. As the cuttingedge angle κref increases, the arc length of machined region (lc in Fig. 5) increases and the width of machined region (amax in Fig. 5) decreases. Namely, the cross-section region machined in cutting process will be thinner and wider. Therefore, in the cutting using a CBN tool with a larger nose radius, the ratio of Fp /Ff increases greatly. On the other hand, area of machined region is almost constant for the different nose radius. Comparison with the thrust forces, the increase of the cutting forces due to the increase of the nose radius is only a little. Thus, the ratio of Fp /Fc increases greatly as the nose radius increases. 3.2. Residual stress
Fig. 4. Ratio of cutting forces vs. corner radius.
Fig. 6 shows the results of residual stress measured in the depth of cut of 0.1 and 0.2 mm. These results were measured
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Fig. 6. Results of the residual stress distribution.
at the cutting length of 47 m. From the figures, the residual stresses show different depth profiles for the different nose radius. The residual stress at the machined surface and the maximum residual compressive stress induced beneath the machined surface are different greatly. The results of the nose radius of 0.4 mm, there exists a larger residual compressive stress at the machined surface. With the increase of the depth beneath the machined surface, these residual compressive stresses increase and reach the maximum compressive stress. Then the compressive residual stresses decrease with the increase of the depth and back to the status without the residual stress. Comparison of the different the depth of cut, the residual compressive stress induced at the machined surface in the depth of cut of 0.1 mm is larger than that in the depth of cut 0.2 mm. There exists no remarkable differences in the profile of the residual stress. About the results of the nose radius of 0.8 mm, the residual stress at the machined surface was also a residual compressive stress induced in the both depth of cut of 0.1 and 0.2 mm. But the magnitude of these residual compressive stresses is small in comparison with the nose radius of 0.4 mm. And
the influence of the depth of cut on the residual stress is not remarkable as same as the nose radius of 0.4 mm. The shape of the residual stress profile beneath the machined surface is similar to that of the nose radius of 0.4 mm. About the results of the nose radius of 1.2 mm, the residual stresses at the machined surface were residual tensile stress in the both depth of cut of 0.1 and 0.2 mm. But the magnitude of these residual tensile stresses is small near the state without the residual stress. These residual tensile stresses shifted to the residual compressive stress and reached the maximum compressive stress with the increase of the depth beneath the machined surface. The shape of the residual stress profile beneath the machined surface is similar to that of the nose radius of 0.4 mm. From above-mentioned results, it can be considered that the nose radius of CBN tool affects the residual stress greatly, especially the residual stress at the machined surface. There exists a tendency that the residual stresses induced at the machined surface shift from compressive stress to tensile stress as the nose radius increases in the cutting condition of this study.
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The residual stresses in a ground surface have been reported that it is primarily generated due to the thermal expansion and contraction, the phase transformations and the plastic deformation [26–29]. The residual stresses are mainly induced by the mechanical, thermal and chemical impacts during the cutting process as the same as the grinding process. These impacts are acting at the same time with overlapping effects. This leads to the residual stress distribution which cannot be correlated to one impact only. The existence of the tensile or compressive residual stress will depend to a great extent on the depth of the permanent plastic deformation zone which penetrates into the workpiece. This zone depends on the stress generated by the mechanical and thermal impact which exist during cutting process [1,2]. In hard turning, the cutting energy is mainly consumed for shearing to form chips and friction between tool–chips, tool–workpiece. This enables the produced heat, the cutting heat flows not only into the workpiece but also into the chips. The heat absorbed in the workpiece is generally small ratio of total cutting consumption energy. During cutting process, the thermal stress on the surface layer is capable of producing only tensile residual. This is because during cutting process, the material near the surface is heated and elongated more than the bulk material. Thus the surface layer expands and experiences a compressive stress resulting from the bulk material beneath. If this compressive stress exceeds the yield stress of the material, then the tensile stress will remain in the surface after cooling. The deformation of the surface layer due to the applied mechanical impact may produce both tensile and compressive residual stresses [1,4,6]. From above-mentioned results, the experimental results obtained in this study can be explained by the different effect of the mechanical and thermal impacts due to the difference of the nose radius of CBN tool. a larger nose radius
causes not only a higher thermal impact but also a higher mechanical impact. In hard turning, the machined surface is formed by the tool nose. It has been proved that the friction between tool and workpiece induces the increase of forces and the cutting temperature increase due to the tool nose. With a larger nose radius tool, this results in producing higher friction and thus the cutting temperature increases [23]. On the other hand, the cutting process using a tool with a larger nose radius, the cross-section region machined will be thinner and wider, and a smaller chip thickness will be produced. Thus, the tool with larger nose angle produces a larger thrust force (as shown in Figs. 2 and 3) and a higher degree of the plastic deformation which results in decreasing of surface tensile stress and increasing of the compressive stresses beneath the machined surface.
4. Effect of the tool wear on residual stress 4.1. Cutting forces and tool wear In order to evaluate the effect of the tool wear on the residual stress for various nose radius, cutting tests were carried out until cutting length reached 550 m in the condition which the depth of cut was 0.2 mm. The residual stress distributions for various nose radius were measured after each cutting test. Fig. 7 shows the photographs of the worn CBN tools after cutting tests. The flank wear VBmax is less than 0.2 mm at the cutting length of 550 m for three types of CBN tools. Fig. 8 shows the variation of the cutting forces with the cutting length for various nose radius. From Fig. 8, there exists a tendency that the cutting force and the thrust force, especially the thrust force, increase gradually with the increase of the tool wear, and this tendency is remarkable in the cutting process using a large nose radius especially.
Fig. 7. Photographs of worn CBN tools (depth of cut 0.2 mm, cutting length 550 m).
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Fig. 8. Variation of cutting forces (depth of cut 0.2 mm).
4.2. Residual stress Fig. 9 shows the comparison of the effect of tool wear on the residual stress for various nose radius. For various nose radius, as the tool wear increases, the residual stresses in-
duced at the machined surface show tensile stresses and the maximum residual compressive stress was induced deeply with a lager magnitude. On the other hand, as the tool wear increases, there exist no remarkable differences in the residual stress distribution among various nose radius. Namely,
Fig. 9. Results of residual stress distribution as a function of tool wear.
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the effect of the nose radius on the residual stress decreases greatly with the increase of the tool wear. In prior studies, about the effect of the tool wear on residual stress, it has been concluded that the tool wear leads to higher thermal process energy related to the increase of the friction. The residual tensile stresses at the machined surface are caused by the thermal impacts due to the friction and plastic deformation. And, the mechanical impacts lead to induce the compressive residual stress which admittedly influence deeper material regions. Therefore, the increase of the tool wear leads to the shift to tensile stress range at the machined surface, and the compressive residual stresses are induced beneath the machined surface [30,31]. The experimental results obtained in this study can been explained by the reason as stated above. In all experimental conditions, the tool wear leads to the increase of the friction between tool and workpiece. The results of increase of the thrust forces and the cutting forces (as shown in Fig. 8) indicate the increase of the friction. The friction between tool and workpiece causes an increase of the thermal process energy. On the other hand, the tool wear causes a larger thrust force, consequently, a larger mechanical impact is produced to the workpiece. Therefore, the residual stress distribution was produced due to the influence of the thermal impact and the mechanical impact.
5. Conclusions In this study, the effect of the tool nose radius and the tool wear on the residual stress induced in hard turning process was discussed experimentally. The main experimental results are as follows: (1) The increase of the tool nose radius leads to an increase of the thrust force greatly. The ratio of the thrust force to cutting force and the ratio of the thrust force to feed force increase with the increase of the tool nose radius. (2) The tool nose radius affects the residual stress at the machined surface significantly at early cutting stage. There exists a tendency that the residual stress at the machined surface shift to tensile range with the increase of the tool nose radius. (3) With the increase of the tool wear, the residual tensile stresses at the machined surface increases, and the residual compressive stress beneath the machined surface increases remarkably. The effect of the nose radius on the residual stress distribution decreases greatly with the increase of the tool wear.
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