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The effect on fatigue life of residual stress and surface hardness resulting from different cutting conditions of 0.45%C steel
International Journal of Machine Tools & Manufacture 45 (2005) 131–136 www.elsevier.com/locate/ijmactool
The effect on fatigue life of residual stress and surface hardness resulting from different cutting conditions of 0.45%C steel Hiroyuki Sasahara* Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan Received 17 May 2004; accepted 3 August 2004 Available online 16 September 2004
Abstract The affected layer is generated within the machined surface layer through the cutting process. Cutting conditions such as the nose radius of the tool, feed rate and shape of cutting edge at the finishing operation affect the residual stress, surface hardness, and surface roughness. In this paper, it is shown that such machined surface property could be controlled by the setting of the cutting conditions to some extent. Then the effect of the machining conditions on the fatigue life was investigated through a fatigue test using the specimen finished under various cutting conditions. It was shown that it is possible to get longer fatigue life for machined parts than the virgin material or the carefully finished material without affected layer, only by setting the proper cutting conditions. Such a situation was realized when the generated residual stress was small and the induced surface hardness was high. A longer fatigue life for the machined components can be obtained by applying such cutting conditions as a low feed rate, a small corner radius and a chamfered cutting edge tool. q 2004 Elsevier Ltd. All rights reserved. Keywords: Fatigue life; Cutting; Residual stress; Surface hardness; Roughness
1. Introduction These days hard materials can be machined and finished with newly developed tools, eliminating the need for other finishing operations such as grinding or polishing, even for dies and molds. In such cases, it is necessary to satisfy given standards of geometrical accuracy and surface roughness. In addition, the reliability or operating life of the machined products must be ensured because the residual tensile stress accelerates the progress of fatigue cracks and the fatigue life of the product is reduced [1]. Thus, it is necessary to know the relationship between fatigue life and the residual stress or surface hardness, which are induced in the machined surface layer after the cutting operation. The residual stress in a machined layer has been studied by many researchers [2–4]. Recently the control of residual stress has become more important in hard turning [5] or in various materials [6,7]. A new type tool to generate
* Tel.: C81 42 388 7240; fax: C81 42 388 7219. E-mail address: [email protected]. 0890-6955/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2004.08.002
compressive residual stress was proposed [8]. For example, to machine aerospace parts, in which thinness and accuracy are required, distortion and fatigue strength are very important issues. On the other hand, the fatigue life of mechanical components finished by cutting process was studied by some researchers. Jeelani [9] showed the effect of cutting speed and tool rake angle on the fatigue life of aluminum alloy and compared with the case of the virgin material. But, they did not show the data of residual stress, which is one of the dominant factors on the fatigue life. Fleming [10] showed the surface integrity and fatigue test results of turned hardened steel with PCBN tool. As the cutting operation is not usually employed to finish the parts for which a long fatigue life is required, the effect of the cutting process on the fatigue life has not been studied as much as the effect of the grinding process [11,12]. But the cutting operation as the finishing process has become to play more important role in these days, and there is an urgent need to know the relationship between the cutting conditions and the fatigue life of the machined parts. There are few studies that have examined the relationship
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between tool geometry, cutting conditions and the fatigue life. In fact, the machined surface layer deforms threedimensionally through the chip forming process by the effect of the corner of the tool in common turning or milling. The tool’s edge roundness and the corner radius may cause deformation in the direction vertical to the cutting velocity. The tool’s corner radius and feed rate affects residual stress [4,13]. These mechanics of the generation of residual stress were explained by using the combination of two orthogonal plane models [13]. This model consists of orthogonal cutting model [14–17] and the corner effect model. In this paper, the relationship between the cutting conditions such as feed rate, corner radius and the tool edge status, which means sharp edge tool or chamfered edge tool, are examined. Then the effect of the machining conditions on the fatigue life was investigated through a fatigue test. 2. Mechanical properties in the machined surface layer As the test material 0.45%C steel is used for the cutting test and the fatigue life test because steel is very commonly used as the mechanical parts with balanced strength/weight ratio and the cost. The fatigue life test specimens were machined by turning and the effect of the finishing cutting condition on the residual stress or fatigue life was studied. Before the machining, the test specimens were annealed for 1 h at 1123 K, and then machined by an NC lathe. Cutting conditions employed are shown in Table 1. Nose radius of the tool and feed rate were varied, and two kinds of tool edges, sharp and chamfered, were employed. Then, the surface hardness, roughness, and residual stress of the machined surfaces were measured before the fatigue life test. During the machining of the test work piece, work hardening and residual stress caused by the pre-cutting process within the machined surface layer affect the cutting process and machined surface conditions [15]. Even if the employed cutting conditions were the same, the cutting forces or shear angle may be different according to the state of the affected layer’s thickness or degree of work hardening and residual stress within the pre-machined surface layer. To avoid such situations the machining conditions before the finishing path were carefully selected. The pre-finishing surface was made through the turning gradually decreasing the depth of cut in each path as 0.25, 0.1, 0.05, 0.03, 0.02, 0.01, 0.01 and 0.01 mm. Table 1 Cutting conditions Feed rate (mm/rev.) Corner radius (mm) Tool edge Depth of cut (mm) Cutting speed (m/min)
0.05, 0.1, 0.2, 0.4 0.2, 0,8 With chamfer, sharp edge 0.2 100
Fig. 1. Specimen configuration.
Fig. 1 shows the configuration of the fatigue test specimen for the rotating bending fatigue test. Cutting tools with 0.2 and 0.8 mm corner radius as shown Fig. 2 were employed. The effect of the chamfer of the cutting edge upon the machined surface was also studied by comparing with the sharp edge tool, which has no chamfer. Fig. 3 shows the residual stress of the machined surface measured by X-ray diffraction method. Measurements were performed along the circumference and axial directions. Residual stress along the axial direction was expected to affect the rotating bending fatigue life of the shaft. It must be noted that each measured data of residual stress has an uncertainty within 50 MPa. This uncertainty is larger than that on the polished metal surface, because the texture is developed within the machined surface by the cutting. From these measured results it can be seen that tensile residual stress increases as the feed rate increases in general. In Fig. 3(a), it can be seen that the residual stress in the circumference direction machined with the chamfered edge tool seemed more compressive than that machined with sharp edge tool. Residual stress in axial direction is rather different from that in circumference direction depending on the corner radius as shown in Fig. 3(b). When 0.8 mm corner radius tool is used, the tendency of residual stress is similar to that in circumference direction. The tensile residual stress increases as the feed rate increases. On the other hand, when 0.2 mm corner radius tool is used, the residual stress stays around zero or week compression even when the feed rate changes. Also there is little difference in residual stress between the results machined by the chamfered and sharp edge tools. We postulate the reasons for these tendencies as follows. In the case of orthogonal cutting, as plane strain conditions are assumed, the material flows only in the plane perpendicular to the cutting edge. Thus, the rake angle and wear of the cutting edge, which have an influence on the plastic flow of the machined surface, directly impact its residual stress. On the other hand, in the common cutting situation using the corner of the tool as shown in Fig. 4, the chip and the finished surface are generated through three-dimensional deformation since undeformed chip thickness along the cutting edge is not constant. We guess that deformation in the vicinity of the corner, where the corner thrusts at the finished surface in a crosswise direction, would yield a residual stress difference
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Fig. 2. Tool edge configuration with chamfer.
Fig. 3. Residual stress on machined surface. (a) Circumference direction; (b) axial direction.
according to inversely proportional to the tool’s corner radius [13]. The cutting sequence effect [15] also affects residual stress. In the turning operation, the layer affected by the previous process or previous revolution is removed as a chip. The affected layer is harder than the raw material and has residual stress, which changes the chip forming process and the resulting residual stress. Also the residual stress state largely depends on the stress–strain relationship of the work material. Fig. 5 shows the results of Vickers hardness test on a machined surface under various cutting conditions. The test weight was 4.9 N. Measured hardness means the average hardness limited to the surface layer; it is measured through plastic deformation around the indenter. Usually the hardness distribution along the depth direction caused by cutting is higher as it comes nearer to the surface. The hardness of the base metal is 230 HV. As shown in this figure, the corner radius of the tool and the chamfer have an
Fig. 4. Schematic view of cutting with tool corner and generation of affected layer.
important influence on machined surface hardness, which becomes higher when a smaller corner radius tool with chamfer is used, which means the plastic deformation within the machined surface layer becomes greater. The feed rate did not affect surface hardness so much. Fig. 6 shows the roughness in feed direction. As is well known, the roughness increases as the feed rate increases and corner radius becomes smaller. A dashed line shows the theoretically obtained roughness; the measured roughness was larger than that in each case.
3. Influences on fatigue life The rotating bending fatigue test was performed on the test specimens machined as mentioned above. Rotation speed was 3000 minK1, and the stress amplitude at
Fig. 5. Effect of cutting conditions on machined surface hardness.
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Fig. 6. Effect of cutting conditions on surface roughness.
the surface of the specimen was set to 271.8 MPa, which is very close to the fatigue limit of the 0.45%C steel we used. At least five specimens were used to verify the fatigue life for one cutting condition. When the number of rotation exceeded 107, the test was stopped, which means the test specimen will not break under that condition. The results are shown below. Fig. 7 shows the relationship between surface roughness and fatigue life. Generally, it is said that roughness decreases the fatigue life because roughness tends to induce
the initiation of a crack. In this case, however, the effect of roughness on fatigue life is not clear here. The roughness shown here is larger than 20 mm and the wavelength is as long as the tool feed rate. It is thought that the size of roughness and wavelength are so large that it will not lead to crack initiation directly. It is also seen that the fatigue life of the specimens show a wide range of variety, even if the specimens tested for roughness were close. Considering that stress amplitude is close to fatigue limit of the specimen, the fatigue life will be shortened by cutting operations in almost all cases. But under certain conditions, the specimen did not break, even after 107 cycles. This result means that it is possible to get longer fatigue life for machined parts than the virgin material or the carefully finished material without affected layer, only by setting the proper cutting conditions. It also clearly shows that the fatigue life of machined materials cannot be determined or explained solely by surface roughness. Fig. 8 shows the relationship between the axial residual stress of the machined specimen and the fatigue life. When the residual stress is tensile, every specimen breaks under 106 cycles. This tendency is the same whether the tool edge is chamfered or sharp. When the residual stress is compression or weak tension less than 100 MPa,
Fig. 7. Relationship between machined surface roughness and fatigue life. (a) Chamfered edge; (b) sharp edge.
Fig. 8. Relationship between axial residual stress and fatigue life. (a) Chamfered edge; (b) sharp edge.
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Fig. 9. Relationship between surface hardness and fatigue life. (a) Chamfered edge; (b) sharp edge.
the probability of exceeding 106 cycles increases. And in some cases, fatigue life completed over 107 cycles. Fig. 9 shows the relationship between the surface hardness and the fatigue life. The fatigue life becomes longer as the surface hardness becomes higher because the yield stress of the surface layer increases by the work hardening. The fatigue life of the specimen machined with the large nose radius tool, indicated here by the solid symbol, is less than 106 cycles because the degree of the work hardening is small. As shown above, the fatigue life of the machined work piece largely depends on the work hardening state and the residual stress induced within the machined surface. But the work hardening and the residual stress are not independently generated, but are interdependent through the cutting process. Thus, the multivariate relationship should be shown to evaluate the effective cutting condition. Fatigue life against the surface hardness and the residual stress is plotted in Fig. 10. Each plotted point shows the central value of one combination of the cutting conditions. As a general trend, it is obvious that the fatigue life around
the region A, where the axial residual stress is tensile, is short, and surface hardness is low. Additionally the surface hardness is not high in this region. On the other hand, the fatigue life around the region B is very long. Around this region, the axial residual stress is around zero and the surface hardness is over 290 HV. Comparing the region C and A, where the region C shows higher compressive residual stress but the surface hardness is almost same as region A, the region C apparently shows a higher fatigue life than the region A. In fact, the residual stress and hardness distributes along the depth direction, not only the surface. And it is considered that these distribution profiles also affect the fatigue life. But, it can be seen that the surface residual stress and hardness are enough to grasp the fatigue life of the machined parts. If we mention about the relationship against the cutting conditions, the residual stress and the surface hardness can be controlled by selecting the corner radius, tool edge sharpness and feed rate in the machining operation. To obtain a longer fatigue life, the region D, where the high compressive residual
Fig. 10. Interaction of axial residual stress and hardness on fatigue life.
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stress and high surface hardness is realized, would be the best. But it has not realized under the tested cutting conditions. Additionally, when a small corner radius tool was used, the residual stress was kept around zero and the surface hardness varies to some extent. A slow feed rate is preferable in the finishing operation to generate good surface roughness and high geometrical accuracy in general. It is also possible to increase the fatigue life by using smaller corner radius tool at the finishing operation with low feed rate. In this study, a 0.2 mm corner radius tool with chamfer edge achieved the longest fatigue life.
4. Conclusions (1) The circumference residual stress induced by turning process tends to become more compressive as the feed rate decreases and when a chamfered tool is used. As for axial residual stress, it stays around 0 by using a 0.2 mm corner radius tool regardless of the feed rate or the type of tool edge. (2) Surface hardness on machined surface becomes high when a small radius tool is used. Surface hardness becomes also higher when a chamfered tool is used rather than sharp edge tool. (3) It is possible to give higher fatigue life to the machined components comparing with the virgin material if compressive residual stress and high hardness within surface layer can be induced by cutting process. This situation can be realized by applying such cutting conditions as a low feed rate, a small corner radius and a chamfered cutting edge tool.
References [1] Y. Matsumoto, D. Magda, D.W. Hoeppner, T.Y. Kim, Effect of machining processes on the fatigue strength of hardened AISI 4340 steel, Trans. ASME J. Eng. Ind. 113 (1991) 154–159.
[2] E.K. Henriksen, Residual stress in machined surface, Trans. ASME 1951; 73. [3] C.R. Liu, M.M. Barash, The mechanical state of the sublayer of a surface generated by chip-removal process, Trans. ASME J. Eng. Ind. 98 (1976) 1192–1201. [4] E. Brinksmeier, J.T. Cammett, W. Konig, P. Leskovar, J. Peters, H.K. Jonshoff, Residual stresses—measurement and causes in machining process, Ann. CIRP 31 (1982) 491–509. [5] J. Rech, A. Moisan, Surface integrity in finish hard turning of casehardened steels, Int. J. Mach. Tools Manuf. 43 (5) (2003) 543–550. [6] M.H. El-Axir, A method of modeling residual stress distribution in turning for different materials, Int. J. Mach. Tools Manuf. 42 (9) (2002) 1055–1063. [7] A.R.C. Sharman, D.K. Aspinwall, R.C. Dewes, D. Clifton, P. Bowen, The effects of machined workpiece surface integrity on the fatigue life of titanium aluminide, Int. J. Mach. Tools Manuf. 41 (11) (2001) 1681–1685. [8] T. Segawa, H. Sasahara, M. Tsutsumi, Development of a new tool to generate compressive residual stress within a machined surface, Int. J. Mach. Tools Manuf. 44 (11) (2004) 1215–1221. [9] S. Jeelani, M. Musial, Effect of cutting speed and tool rake angle on the fatigue life of 2024-T351 aluminum alloy, Int. J. Fatigue 6 (3) (1984) 169–172. [10] M.A. Fleming, C. Sweeney, T.J. Valentine, R. Simpkin, PCBN hard turning and work piece surface integrity, Ind. Diamond Rev. 58 (579) (1998) 128–133. [11] L.P. Taraov, et al., Effects of grinding direction and of abrasive tumbling on the fatigue limit of hardened steel, Am. Soc. Test. Mater. 58 (1958) 528–539. [12] E.C. Reed, J.A. Viens, The influence of surface residual stress on fatigue limit of titanium, Trans. ASME J. Eng. Ind. 2 (1960) 76–78. [13] H. Sasahara, T. Obikawa, T. Shirakashi, Prediction model of surface residual stress within a machined surface by combining two orthogonal plane models, Int. J. Mach. Tools Manuf. 44 (7/8) (2004) 815–822. [14] T. Shirakashi, H. Sasahara, T. Obikawa, T. Wada, The analytical prediction of residual stress within machined sublayer and its effect on accuracy, Int. J. Jpn Soc. Prec. Eng. 28-3 (1994) 200–205. [15] H. Sasahara, T. Obikawa, T. Shirakashi, FEM analysis of cutting sequence effect on mechanical characteristics in machined layer, J. Mater. Process. Technol. 62 (1996) 448–453. [16] T. Obikawa, H. Sasahara, T. Shirakashi, E. Usui, Application of computational machining method to discontinuous chip formation, Trans. ASME J. Manuf. Sci. Eng. 119 (1997) 667–674. [17] H. Sasahara, T. Obikawa, T. Shirakashi, Finite element modeling of residual stress control on machined surface, Trans. NAMRI/SME XXV (1997) 213–218.
The effect on fatigue life of residual stress and surface hardness resulting from different cutting conditions of 0.45%C steel Hiroyuki Sasahara* Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan Received 17 May 2004; accepted 3 August 2004 Available online 16 September 2004
Abstract The affected layer is generated within the machined surface layer through the cutting process. Cutting conditions such as the nose radius of the tool, feed rate and shape of cutting edge at the finishing operation affect the residual stress, surface hardness, and surface roughness. In this paper, it is shown that such machined surface property could be controlled by the setting of the cutting conditions to some extent. Then the effect of the machining conditions on the fatigue life was investigated through a fatigue test using the specimen finished under various cutting conditions. It was shown that it is possible to get longer fatigue life for machined parts than the virgin material or the carefully finished material without affected layer, only by setting the proper cutting conditions. Such a situation was realized when the generated residual stress was small and the induced surface hardness was high. A longer fatigue life for the machined components can be obtained by applying such cutting conditions as a low feed rate, a small corner radius and a chamfered cutting edge tool. q 2004 Elsevier Ltd. All rights reserved. Keywords: Fatigue life; Cutting; Residual stress; Surface hardness; Roughness
1. Introduction These days hard materials can be machined and finished with newly developed tools, eliminating the need for other finishing operations such as grinding or polishing, even for dies and molds. In such cases, it is necessary to satisfy given standards of geometrical accuracy and surface roughness. In addition, the reliability or operating life of the machined products must be ensured because the residual tensile stress accelerates the progress of fatigue cracks and the fatigue life of the product is reduced [1]. Thus, it is necessary to know the relationship between fatigue life and the residual stress or surface hardness, which are induced in the machined surface layer after the cutting operation. The residual stress in a machined layer has been studied by many researchers [2–4]. Recently the control of residual stress has become more important in hard turning [5] or in various materials [6,7]. A new type tool to generate
* Tel.: C81 42 388 7240; fax: C81 42 388 7219. E-mail address: [email protected]. 0890-6955/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2004.08.002
compressive residual stress was proposed [8]. For example, to machine aerospace parts, in which thinness and accuracy are required, distortion and fatigue strength are very important issues. On the other hand, the fatigue life of mechanical components finished by cutting process was studied by some researchers. Jeelani [9] showed the effect of cutting speed and tool rake angle on the fatigue life of aluminum alloy and compared with the case of the virgin material. But, they did not show the data of residual stress, which is one of the dominant factors on the fatigue life. Fleming [10] showed the surface integrity and fatigue test results of turned hardened steel with PCBN tool. As the cutting operation is not usually employed to finish the parts for which a long fatigue life is required, the effect of the cutting process on the fatigue life has not been studied as much as the effect of the grinding process [11,12]. But the cutting operation as the finishing process has become to play more important role in these days, and there is an urgent need to know the relationship between the cutting conditions and the fatigue life of the machined parts. There are few studies that have examined the relationship
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between tool geometry, cutting conditions and the fatigue life. In fact, the machined surface layer deforms threedimensionally through the chip forming process by the effect of the corner of the tool in common turning or milling. The tool’s edge roundness and the corner radius may cause deformation in the direction vertical to the cutting velocity. The tool’s corner radius and feed rate affects residual stress [4,13]. These mechanics of the generation of residual stress were explained by using the combination of two orthogonal plane models [13]. This model consists of orthogonal cutting model [14–17] and the corner effect model. In this paper, the relationship between the cutting conditions such as feed rate, corner radius and the tool edge status, which means sharp edge tool or chamfered edge tool, are examined. Then the effect of the machining conditions on the fatigue life was investigated through a fatigue test. 2. Mechanical properties in the machined surface layer As the test material 0.45%C steel is used for the cutting test and the fatigue life test because steel is very commonly used as the mechanical parts with balanced strength/weight ratio and the cost. The fatigue life test specimens were machined by turning and the effect of the finishing cutting condition on the residual stress or fatigue life was studied. Before the machining, the test specimens were annealed for 1 h at 1123 K, and then machined by an NC lathe. Cutting conditions employed are shown in Table 1. Nose radius of the tool and feed rate were varied, and two kinds of tool edges, sharp and chamfered, were employed. Then, the surface hardness, roughness, and residual stress of the machined surfaces were measured before the fatigue life test. During the machining of the test work piece, work hardening and residual stress caused by the pre-cutting process within the machined surface layer affect the cutting process and machined surface conditions [15]. Even if the employed cutting conditions were the same, the cutting forces or shear angle may be different according to the state of the affected layer’s thickness or degree of work hardening and residual stress within the pre-machined surface layer. To avoid such situations the machining conditions before the finishing path were carefully selected. The pre-finishing surface was made through the turning gradually decreasing the depth of cut in each path as 0.25, 0.1, 0.05, 0.03, 0.02, 0.01, 0.01 and 0.01 mm. Table 1 Cutting conditions Feed rate (mm/rev.) Corner radius (mm) Tool edge Depth of cut (mm) Cutting speed (m/min)
0.05, 0.1, 0.2, 0.4 0.2, 0,8 With chamfer, sharp edge 0.2 100
Fig. 1. Specimen configuration.
Fig. 1 shows the configuration of the fatigue test specimen for the rotating bending fatigue test. Cutting tools with 0.2 and 0.8 mm corner radius as shown Fig. 2 were employed. The effect of the chamfer of the cutting edge upon the machined surface was also studied by comparing with the sharp edge tool, which has no chamfer. Fig. 3 shows the residual stress of the machined surface measured by X-ray diffraction method. Measurements were performed along the circumference and axial directions. Residual stress along the axial direction was expected to affect the rotating bending fatigue life of the shaft. It must be noted that each measured data of residual stress has an uncertainty within 50 MPa. This uncertainty is larger than that on the polished metal surface, because the texture is developed within the machined surface by the cutting. From these measured results it can be seen that tensile residual stress increases as the feed rate increases in general. In Fig. 3(a), it can be seen that the residual stress in the circumference direction machined with the chamfered edge tool seemed more compressive than that machined with sharp edge tool. Residual stress in axial direction is rather different from that in circumference direction depending on the corner radius as shown in Fig. 3(b). When 0.8 mm corner radius tool is used, the tendency of residual stress is similar to that in circumference direction. The tensile residual stress increases as the feed rate increases. On the other hand, when 0.2 mm corner radius tool is used, the residual stress stays around zero or week compression even when the feed rate changes. Also there is little difference in residual stress between the results machined by the chamfered and sharp edge tools. We postulate the reasons for these tendencies as follows. In the case of orthogonal cutting, as plane strain conditions are assumed, the material flows only in the plane perpendicular to the cutting edge. Thus, the rake angle and wear of the cutting edge, which have an influence on the plastic flow of the machined surface, directly impact its residual stress. On the other hand, in the common cutting situation using the corner of the tool as shown in Fig. 4, the chip and the finished surface are generated through three-dimensional deformation since undeformed chip thickness along the cutting edge is not constant. We guess that deformation in the vicinity of the corner, where the corner thrusts at the finished surface in a crosswise direction, would yield a residual stress difference
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133
Fig. 2. Tool edge configuration with chamfer.
Fig. 3. Residual stress on machined surface. (a) Circumference direction; (b) axial direction.
according to inversely proportional to the tool’s corner radius [13]. The cutting sequence effect [15] also affects residual stress. In the turning operation, the layer affected by the previous process or previous revolution is removed as a chip. The affected layer is harder than the raw material and has residual stress, which changes the chip forming process and the resulting residual stress. Also the residual stress state largely depends on the stress–strain relationship of the work material. Fig. 5 shows the results of Vickers hardness test on a machined surface under various cutting conditions. The test weight was 4.9 N. Measured hardness means the average hardness limited to the surface layer; it is measured through plastic deformation around the indenter. Usually the hardness distribution along the depth direction caused by cutting is higher as it comes nearer to the surface. The hardness of the base metal is 230 HV. As shown in this figure, the corner radius of the tool and the chamfer have an
Fig. 4. Schematic view of cutting with tool corner and generation of affected layer.
important influence on machined surface hardness, which becomes higher when a smaller corner radius tool with chamfer is used, which means the plastic deformation within the machined surface layer becomes greater. The feed rate did not affect surface hardness so much. Fig. 6 shows the roughness in feed direction. As is well known, the roughness increases as the feed rate increases and corner radius becomes smaller. A dashed line shows the theoretically obtained roughness; the measured roughness was larger than that in each case.
3. Influences on fatigue life The rotating bending fatigue test was performed on the test specimens machined as mentioned above. Rotation speed was 3000 minK1, and the stress amplitude at
Fig. 5. Effect of cutting conditions on machined surface hardness.
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Fig. 6. Effect of cutting conditions on surface roughness.
the surface of the specimen was set to 271.8 MPa, which is very close to the fatigue limit of the 0.45%C steel we used. At least five specimens were used to verify the fatigue life for one cutting condition. When the number of rotation exceeded 107, the test was stopped, which means the test specimen will not break under that condition. The results are shown below. Fig. 7 shows the relationship between surface roughness and fatigue life. Generally, it is said that roughness decreases the fatigue life because roughness tends to induce
the initiation of a crack. In this case, however, the effect of roughness on fatigue life is not clear here. The roughness shown here is larger than 20 mm and the wavelength is as long as the tool feed rate. It is thought that the size of roughness and wavelength are so large that it will not lead to crack initiation directly. It is also seen that the fatigue life of the specimens show a wide range of variety, even if the specimens tested for roughness were close. Considering that stress amplitude is close to fatigue limit of the specimen, the fatigue life will be shortened by cutting operations in almost all cases. But under certain conditions, the specimen did not break, even after 107 cycles. This result means that it is possible to get longer fatigue life for machined parts than the virgin material or the carefully finished material without affected layer, only by setting the proper cutting conditions. It also clearly shows that the fatigue life of machined materials cannot be determined or explained solely by surface roughness. Fig. 8 shows the relationship between the axial residual stress of the machined specimen and the fatigue life. When the residual stress is tensile, every specimen breaks under 106 cycles. This tendency is the same whether the tool edge is chamfered or sharp. When the residual stress is compression or weak tension less than 100 MPa,
Fig. 7. Relationship between machined surface roughness and fatigue life. (a) Chamfered edge; (b) sharp edge.
Fig. 8. Relationship between axial residual stress and fatigue life. (a) Chamfered edge; (b) sharp edge.
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Fig. 9. Relationship between surface hardness and fatigue life. (a) Chamfered edge; (b) sharp edge.
the probability of exceeding 106 cycles increases. And in some cases, fatigue life completed over 107 cycles. Fig. 9 shows the relationship between the surface hardness and the fatigue life. The fatigue life becomes longer as the surface hardness becomes higher because the yield stress of the surface layer increases by the work hardening. The fatigue life of the specimen machined with the large nose radius tool, indicated here by the solid symbol, is less than 106 cycles because the degree of the work hardening is small. As shown above, the fatigue life of the machined work piece largely depends on the work hardening state and the residual stress induced within the machined surface. But the work hardening and the residual stress are not independently generated, but are interdependent through the cutting process. Thus, the multivariate relationship should be shown to evaluate the effective cutting condition. Fatigue life against the surface hardness and the residual stress is plotted in Fig. 10. Each plotted point shows the central value of one combination of the cutting conditions. As a general trend, it is obvious that the fatigue life around
the region A, where the axial residual stress is tensile, is short, and surface hardness is low. Additionally the surface hardness is not high in this region. On the other hand, the fatigue life around the region B is very long. Around this region, the axial residual stress is around zero and the surface hardness is over 290 HV. Comparing the region C and A, where the region C shows higher compressive residual stress but the surface hardness is almost same as region A, the region C apparently shows a higher fatigue life than the region A. In fact, the residual stress and hardness distributes along the depth direction, not only the surface. And it is considered that these distribution profiles also affect the fatigue life. But, it can be seen that the surface residual stress and hardness are enough to grasp the fatigue life of the machined parts. If we mention about the relationship against the cutting conditions, the residual stress and the surface hardness can be controlled by selecting the corner radius, tool edge sharpness and feed rate in the machining operation. To obtain a longer fatigue life, the region D, where the high compressive residual
Fig. 10. Interaction of axial residual stress and hardness on fatigue life.
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stress and high surface hardness is realized, would be the best. But it has not realized under the tested cutting conditions. Additionally, when a small corner radius tool was used, the residual stress was kept around zero and the surface hardness varies to some extent. A slow feed rate is preferable in the finishing operation to generate good surface roughness and high geometrical accuracy in general. It is also possible to increase the fatigue life by using smaller corner radius tool at the finishing operation with low feed rate. In this study, a 0.2 mm corner radius tool with chamfer edge achieved the longest fatigue life.
4. Conclusions (1) The circumference residual stress induced by turning process tends to become more compressive as the feed rate decreases and when a chamfered tool is used. As for axial residual stress, it stays around 0 by using a 0.2 mm corner radius tool regardless of the feed rate or the type of tool edge. (2) Surface hardness on machined surface becomes high when a small radius tool is used. Surface hardness becomes also higher when a chamfered tool is used rather than sharp edge tool. (3) It is possible to give higher fatigue life to the machined components comparing with the virgin material if compressive residual stress and high hardness within surface layer can be induced by cutting process. This situation can be realized by applying such cutting conditions as a low feed rate, a small corner radius and a chamfered cutting edge tool.
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