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Surface integrity and fatigue performance of 17-4PH stainless steel after cutting operations
Surface & Coatings Technology 307 (2016) 182–189
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Surface integrity and fatigue performance of 17-4PH stainless steel after cutting operations Guoliang Liu a,b, Chuanzhen Huang a,b,⁎, Bin Zou a,b, Xiangyu Wang a,b, Zhanqiang Liu a,b a b
Center for Advanced Jet Engineering Technologies (CaJET), School of Mechanical Engineering, Shandong University, 17923 Jingshi Road, Jinan, PR China Key Laboratory of High-efficiency and Clean Mechanical Manufacture (Shandong University), Ministry of Education, PR China
a r t i c l e
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Article history: Received 5 June 2016 Revised 26 August 2016 Accepted in revised form 29 August 2016 Available online 30 August 2016 Keywords: 17-4PH stainless steel Cutting operations Surface integrity Fatigue life
a b s t r a c t The 17-4PH stainless steel has been applied as a substitute of titanium to make jet engine parts, and fatigue life of the machined surfaces are very important due to its high reliability and safety demand. In this work, a series of experiments were conducted to investigate the impact of the cutting operations on the surface integrity, and its further influence on the fatigue life. It was confirmed that the fatigue performance of the machined surface was determined by the interactions of the surface integrity characteristics, including the work hardening, surface roughness and residual stress field. The softened layer under the machined surface and the compressive residual stress field were generated under all cutting conditions, resulting in the improved fatigue life when compared to the polished specimens. The plastic deformation and the resulting work hardening on the machined surface were enhanced continuously with an increase in each of the cutting parameters, which could cause significant decrease of the fatigue performance of the workpiece. The influence of surface roughness on the fatigue performance could be overshadowed by other surface integrity characteristics, since its effect was weakened by the curved feed marks generated on the face-milled surfaces. The cutting parameters could influence the fatigue performance of the machined components significantly by changing their surface integrity. © 2016 Elsevier B.V. All rights reserved.
1. Introduction 17-4PH is a typical kind of martensitic precipitation hardening stainless steel, consisting of martensite and a small amount of retained austenite. It is usually used to produce pump shafts, chemical process equipment, and nuclear reactor components because of its high strength and corrosion resistance [1]. These days, it has been used as a substitute of titanium in the aircraft engine. In these applications, the components are subjected to cyclical mechanical and thermal loads, and high reliability are extremely needed, thus good surface integrity and fatigue performance are required. It has been recognized that the fatigue performance relied on the surface integrity produced by cutting operations, for that the fatigue cracks could initiate from the machined surfaces [2]. However, the influence rule of the cutting operations on the fatigue performance has not been well-studied, since grinding or polishing was usually employed to finish the components after cutting operations. But today, the newly developed tools and the high speed machining technology has made it possible to use the components without grinding or polishing after cutting operations [3]. In such
⁎ Corresponding author at: Center for Advanced Jet Engineering Technologies (CaJET), School of Mechanical Engineering, Shandong University, 17923 Jingshi Road, Jinan, PR China. E-mail address: [email protected] (C. Huang).
http://dx.doi.org/10.1016/j.surfcoat.2016.08.086 0257-8972/© 2016 Elsevier B.V. All rights reserved.
cases, it is necessary to research the impact of the cutting operations on the fatigue performance of the machined components. Although in recent years some researchers started to pay attention to the fatigue performance of the machined specimens, the direct tests for fatigue life of the workpieces are still relatively rare. In addition, most of the fatigue performance tests were conducted to compare the two different machining processes, for example, turning vs. grinding [4–6] and electro-discharge machining (EDM) [7], milling vs. grinding [8] and EDM [9]. The studies on the effect of cutting conditions from Jeelani and Musial [10], Javidi et al. [11], Sasahara [3] and Choi [12] were limited to few types of fatigue performance tests, workpiece materials and cutting parameters. It is obvious that the role of cutting operations in determining the fatigue performance needs more attention. Another weakness in the existing fatigue life research is that the influence mechanism between the surface integrity characteristics and fatigue performances is not understood deeply enough. In many early published works, the effect of surface integrity on fatigue performance was often entirely characterized by the amplitude surface roughness parameters, such as Ra or Rt [2]. The fatigue stress concentration factor was calculated by several models based on the two parameters to predict the fatigue life [13,14]. However, because the surface roughness value was usually comparatively small in the finishing process, the effect of these parameters on the fatigue performance was not significant, especially in the high cycle conditions. In other researches, the residual stress was gradually regarded as the determinant factor to change the fatigue
G. Liu et al. / Surface & Coatings Technology 307 (2016) 182–189
signally promote the understanding of the generation mechanism of the fatigue performance of machined surfaces.
Table 1 The mechanical properties of the workpiece. Reduction of area (%) Workpiece 45
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Yield strength (MPa)
Hardness Tensile strength (HRC) (MPa)
Elongation (%)
1235
43
12
1425
Table 2 The single factor experimental design. Parameters
Group I
Group II
Group III
V (m/min)
200
200
f (mm/tooth)
100, 150, 200, 250, 300 0.15
0.15
ap (mm)
0.6
0.05, 0.1, 0.15, 0.2, 0.25 0.6
0.3, 0.45, 0.6, 0.75, 0.9
performance of the machined surface [4,7,11,12], and it was widely acknowledged that the compressive residual stress could improve the fatigue performance. However, the influence of the surface and maximum compressive residual stress was still confusing [4]. As for the effect of work hardening and microstructure, there were more inconsistent results in existing research. Sasahara [3] believed that the fatigue life could be prolonged by the increased surface hardness, because of the increased yield stress of the machined surface. However, Mantle et al. [15, 16] claimed that the work hardening was beneficial to crack propagation, because it could reduce the material's ductility on the surface. Furthermore, the above mentioned sources mainly attributed the changed fatigue life to one of the surface integrity indicators, lacking the study on the combined effect of the plastic deformation, surface roughness and residual stress. Therefore, it is reasonable to deepen the study of the surface integrity and the fatigue performance of the machined workpiece after cutting operations. Although the fatigue life has serious effect on the application of 174PH stainless steel, there is almost no direct research on the effect of cutting operations on the fatigue life for 17-4PH material. In the present work, various cutting parameters were used in the face-milling experiments to reveal the relationships among the cutting parameters, surface integrity characteristics and the specimens' fatigue performance. The effect of the three cutting parameters on the machined surface integrity, and further on their fatigue performance was discussed separately. The combined effect of the surface integrity characteristics, including surface roughness, work hardening, residual stress and microstructure, on the fatigue life was revealed as well. It is undeniable that the presented test results were obtained with particular specific insert and cutting edge, and there may be some change in the effect of cutting parameter with different inserts. For example, if the inserts with larger nose radius were used, the turning point of surface roughness with the feed rate may be changed, and the optimized processing parameters may be different. Nonetheless, the influence mechanism of surface integrity characteristics on the fatigue life is universal. Thus, the present work can
2. Experimental procedures A vertical CNC machining center DMU-70V (Germany) was employed to do dry machining in all face-milling tests. The workpiece used in the experiments was a block of solution and aging treated 174PH stainless steel. The dimension of the workpiece was 110 mm × 50 mm × 110 mm, and a face of 110 mm × 50 mm was chosen as the machining surface. The symmetric down milling was adopted, resulting in a fixed cutting width (ae) of 50 mm. The mechanical properties of the workpiece are shown in Table 1. The TiAlN coated carbide inserts (DIJET, JC8015-ODHW606AEN) were mounted on a cutter (OCT-06100-32R) with a diameter of 100 mm to finish the experiments. The tool geometry after mounted was about 10° of rake angle, 5° of clearance angle, 1.2 mm of nose radius and 15–20 μm of hone size. The single factor experimental design adopted in the present work is shown in Table 2. The cutting parameters in Table 2, including the cutting speed (V), the feed rate (f) and the depth of cut (ap), were selected according to our previous work [17] and the recommended cutting parameters in the insert manual. The detrimental effect of tool wear and run-out of teeth was avoided by using a new insert each time. During the cutting process, a Kistler 9257B three component piezoelectric dynamometer (Switzerland) was adopted to record the cutting forces. After the milling process, a mobile surface roughness meter (China, TR200) was adopted to measure the roughness parameters (Ra and Rz) of the machined surface, and a KEYENCE VK-X200K laser microscope (Japan) was used to observe the surface morphology. Profiles of the residual stress in the workpiece were measured by an Xstress 3000 stress instrument (Stresstech Oy, Finland) using the XRD-sin2ψ technique [18]. The parameters of this technique adopted in the present work are showed in Table 3. The electro-polishing technique was used to measure the residual stresses field of the specimens. The 80% methanol and 20% perchloric acid solution was employed in an electropolishing machine to remove the surface material layer-by-layer. Every time the sample was polished for 15 s, and then the remaining sample thickness was measured by a screw micrometer to calculate the thickness of the removed layer. The micro-hardness measurement and the half-peak width (FWHM) from the XRD experiments are the common methods to test the work-hardening of the machined surface. However, the FWHM was caused by the stacking faults and structural disorder [19], and the definitive link between the hardness and the plastic strain was not found. Some researchers indicated that the FWHM was sensitive to the variation in microstructure and stress–strain accumulation in the material and the presence of tensile stress in the material caused increase in the FWHM [20]. Thus, the machined surface hardness was measured by the NanoTest Ventage (UK) with a load of 500 mN and Vickers hardness tester (China) with a load of 9.8 N separately. The full load has been dwelled for 15 s. The in-depth nanohardness measurements were conducted on the cross-section of the workpiece after mosaic, grinding and polishing. The first line of measuring points was located at a distance of 15 μm below the free surface. Other measuring points were located along a straight line in the indepth direction at 10-μm intervals until a stable hardness was obtained.
Table 3 The residual stress measurement parameters. Parameters
Value
Diffraction angle (2θ)/° Side inclination (ψ)/° Collimator diameter/mm Exposure time/S Operation voltage/kV Operation current/mA Target
156 0, ±10, ±20, ±30 3 15 29.3 6.76 Cr-Kα
Fig. 1. Schematic diagram of the three points bending fatigue life test.
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obvious that the feed rate was the only one parameter that affected the surface roughness significantly. When the feed rate was changed from 0.1 mm/tooth to 0.25 mm/tooth, the Ra and Rz both increased significantly. The relatively poor surface at the feed rate of 0.05 mm/tooth could be attributed to the excessive friction and squeezing of the cutting edge and workpiece, which would produce much more chip debris adhering on the machined surface. Compared to the feed rate, the variations of the Ra and Rz against the cutting speed or the depth of cut were pretty minute. The relatively high decrease of Ra (more than 0.05 μm) was recorded only when the cutting speed rose from 100 m/ min to 150 m/min and from 200 m/min to 250 m/min. 3.2. Hardness and microstructure
Fig. 2. Schematic diagram of the sample location on the workpiece.
Sub-surface microstructure was observed using the KEYENCE VKX200K 3D laser microscope on a cross-section of the workpiece, which was etched with 75% concentrated hydrochloric acid and 25% concentrated nitric acid after grinding and polishing. A servo-hydraulic fatigue testing machine (Instron 8801, USA) was adopted to perform the three-point bending fatigue life tests in this work, and the schematic diagram of the test is displayed in Fig. 1. The fatigue life test specimens were cut from the workpieces center using wire electrical discharge machining (WEDM), in the same size of 100 mm × 10 mm × 4 mm (seen in the “1”, “2” and “3” in Fig. 2). A dynamic sine wave load with the maximum tension stress of 1000 MPa was applied to the specimens in the three points bending fatigue tests. The loading frequency was 15 Hz, and the stress ratio was 0.1. The tests were running until the specimen fractured. For each cutting condition, at least three specimens were tested to assess the scatter. Three polished specimens were also tested to work as a reference group. 3. Results and discussion 3.1. Surface roughness The Ra (Arithmetic average surface roughness) and Rz (Maximum peak-to-valley roughness height) are the most commonly used surface roughness parameters in calculating the fatigue stress concentration factor, and they were both measured for each specimen. The average Ra and Rz values and their standard deviation for the three specimens are summarized in Fig. 3, and it can be seen that the Ra and Rz show a similar variation trend. Among the three cutting parameters, it is
The surface layer and in-depth nano-hardness distributions of the specimens are shown in Fig. 4. It can be seen that the similar spoonshaped curves were obtained in all cutting conditions. There was an obvious softened layer developed below the work-hardened surface, which agreed with the observations of Choi [12]. It is well acknowledged that the hardened surface is a result of the plastic deformation in the cutting processes. The microstructures in Figs. 6-8 proved the existence of the severe plastic deformation on the surface layer in all cutting conditions. However, it is also obvious that the thickness of the severe plastic deformation zone was no more than 10 μm for all specimens. Therefore, the materials below the plastic deformation layer were mainly influenced by the heat produced while machining, explaining the significantly reduced nano-hardness at the subsurfaces. Another obvious characteristic of the severe plastic deformation layers was their discontinuity, which could explain the large deviation of the nano-hardness shown in Fig. 4. To compare the work-hardening degree in different cutting conditions, the Vickers hardness tests with a load of 9.8 N was carried out, and the results are shown in Fig. 5. The diagonal length of each indentation was greater than 60 μm, which could overcome the adverse impact of the discontinuous plastic deformation effectively. It can be seen that the work-hardening was enhanced as a whole with an increase of each cutting parameter, and the feed rate displayed the most significant effect. It has been widely acknowledged that the work-hardening of the machined surface is mainly determined by the thermal and mechanical effects. The workpiece material has been treated by a solution heat treatment at 1030 °C and aging treatment at 480 °C, while the finite element analyses have shown that the temperatures in the cutting zone were no higher than 900 °C in all of the cutting conditions adopted in this work (The finite element analyses process has been described in our previous work [21]). In addition, the heating time was too short to produce severe structural transformation. As a result, the cutting heat could not harden the machined surfaces by the quenching effect, and the plastic hardening and thermal softening were the main cause of the work-hardening of the machined surface. Figs. 6-8 showed the microstructures of the most severe plastic deformation areas at the minimum and maximum cutting parameters. Some obvious grain
Fig. 3. Relationship of Ra and Rz with factors (three cutting parameters).
G. Liu et al. / Surface & Coatings Technology 307 (2016) 182–189
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Fig. 4. Nano-hardness distributions of the specimens.
boundary deflections (marked by the red dash line) were generated on the sub-surface for each cutting parameter, following the direction of the cutting speed. The deformed grain boundaries can help to identify the severe plastic deformation layer. It can be seen that the plastic deformation was enhanced with an increase in each of the three cutting parameters, which was the main cause of the increased hardness. However, it is obvious by comparing the Panels (b) in Figs. 6-8 that the cutting speed owned a smaller effect on the plastic deformation than the feed rate and depth of cut, which could explain the smaller increase of hardness. The cutting force is a main cause of the plastic flow of the material on the finished surfaces [22]. As shown in Fig. 9 the resultant cutting forces increased significantly when the feed rate or the depth of cut was increased, but remained unaffected by the cutting speed. The high cutting forces caused by the increased feed rate or depth of cut could reinforce the plastic flow. In addition to the plastic
deformation, the thermal softening was another reason of the various work-hardening degrees. Although more cutting heat would be generated in the cutting zone when each of the three cutting parameters was increased, the heat source movement would be speed up by the higher cutting speed or feed rate, and thus weaken the influence of thermal softening. As a result, the work hardening of the machined surface was decreased when the depth of cut was increased. 3.3. Residual stress Fig. 10 presents the distribution of the residual stress field within the machined surface layer after face-milled at the cutting speed of 200 m/ min, feed rate of 0.15 mm/tooth and depth of cut of 0.6 mm. σx stands for the residual stresses parallel to the feed direction, while σy means that the residual stresses were measured in the direction perpendicular
Fig. 5. Relationship of Vickers hardness with factors (three cutting parameters).
Fig. 6. Sub-surface microstructures of the specimens face-milled with the cutting speed of (a) 100 m/min and (b) 300 m/min. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Fig. 7. Sub-surface microstructures of the specimens face-milled at the feed rate of (a) 0.05 mm/tooth and (b) 0.25 mm/tooth. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 8. Sub-surface microstructures of the specimens face-milled at the depth of cut of (a) 0.3 mm and (b) 0.9 mm. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
to the σx. It can be seen that the most common spoon-shaped form of the residual stress distribution was generated. The residual stress first falls off to a maximum compressive value from the smaller ones on the machined surface layer with an increased in-depth. After a maximum compressive residual stress value is reached, the residual stress transforms to a steady value of the workpiece substrate. It must be noted that this residual stress distribution form was obtained in all cutting conditions, although the values of the residual stress field were diverse in different cutting conditions. It has been well- acknowledged that the spoon-shaped residual stress distribution was generated by the coupling effect of the thermal and mechanical load [23]. According to the previous research, the residual stresses on the free surface and the maximum compressive residual stresses were the main factors affecting the fatigue life [4]. In order to facilitate the comparison of the influence of different cutting parameters on the residual stresses, only the two residual stresses indexes and the standard deviations in all cutting conditions were extracted and displayed in Figs. 11-12. It is
obvious that both of the residual stresses on the free surface and the maximum compressive residual stresses were most severely impacted by the feed rate. The residual stresses in two directions were both transformed to higher compressive ones significantly when the feed rate was increased. When the cutting speed or the depth of cut increased, the larger compressive residual stresses were generated as well, but the changes were relatively slight. The only relatively large increase of the surface and maximum compressive residual stress was recorded when the depth of cut was increased to 0.9 mm, because of the significantly increased cutting force. 3.4. Fatigue life After the specimens were fractured in the three-point bending fatigue tests, the fracture surfaces of specimens were examined using SEM to eliminate the ones in which the crack initiation was located in the material, instead of initiating from the machined surface. The fatigue
Fig. 9. Relationship of resultant cutting force with factors (three cutting parameters).
G. Liu et al. / Surface & Coatings Technology 307 (2016) 182–189
Fig. 10. In-depth residual stresses after face-milled at v = 200 m/min, f = 0.15 mm/tooth and ap = 0.6 mm.
lives of the reference group and all cutting conditions were recorded, and the average fatigue life and its deviation for each condition are shown in Fig. 13. It can be seen that all the face-milled specimens had a higher fatigue life than the polished ones, and the longest fatigue life was about 1.5 times that of the polished specimens. According to the measured results of the surface integrity characteristics, mainly two reasons were considered to result in the improved fatigue performance of the face-milled specimens. Firstly, the compressive residual stress fields generated on the surface layer were considered. It is believed that the compressive residual stresses generated on the free surface could prevent the initiation of the fatigue cracks, while the maximum compressive residual stresses at the subsurface could prevent their propagation. Thus, both of them could weaken the adverse impact of the applied loadings and improve the fatigue performance. This agreed with the results of Jeelani and Musial [10] and Smith et al. [4]. Secondly, the softened layer below the machined surface was considered as the other reason to prolong the fatigue life. Contrary to the work hardening, the softened layer is of benefit to the material's ductility, and thus can prevent the propagation of cracks. As for the plastic deformation layer and the resulting work hardening, they were regarded as unfavorable factors for the fatigue performance. The first reason was pointed out by Mantle et al. [15,16] that the work hardening could reduce the ductility of the machined surface, and thus was detrimental to crack propagation. In addition, it can be seen from the sub-surface microstructures in Figs. 6-8 that the plastically deforming areas were not
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continuous. The plastic deformation could change the material properties of the surface, ductility, strength, hardness or grain size, and some of the changes may be beneficial for the fatigue life. However, the differences of the material properties would make the cracks much more easily to generate at the boundaries of undeformed-deformed areas. The changes of fatigue life against the three cutting parameters in Fig. 13 could be explained using the combined effect of the abovementioned surface integrity characteristics. It is easy to conclude from Fig. 13(a) that the fatigue life decreased on the whole with an increase in the cutting speed. The two singular points occurred when the cutting speed reached 150 m/min or 250 m/min. The surface integrity measurements have shown that with the increased cutting speed, the plastic deformation and work hardening which were detrimental to the fatigue performance were both enhanced significantly, while the decreased surface roughness and the increased surface and maximum compressive residual stresses could prolong the fatigue life. Therefore, the decreased fatigue life can mainly be attributed to the increased plastic deformation and work hardening. The residual stress analyses in Figs. 11-12 have shown that the residual stress field was just slightly affected by the cutting speed, and the small increase of the compressive residual stresses in the machined surface layer was not enough to offset the detrimental effect of the increased plastic deformation. As for the surface roughness, Taylor and Clancy [24] have revealed that the milling could cause curved feed marks, which may be perpendicular to the early initiation directions of cracks, and thus prevent the propagation of the cracks. Therefore, when the changes of the surface roughness were small, their effect on the fatigue life could be overshadowed by other surface integrity characteristics. Only when the aforementioned two relatively big downside of surface roughness were generated under the cutting speed of 150 m/min and 250 m/min, the surface roughness showed its effect, and led to the increased fatigue life combined with the compressive residual stresses. Fig. 13(b) displays the variation trend of the fatigue life with the feed rate. As recorded in Figs. 11-12, the residual stress was significantly determined by the feed rate, and both of the surface and maximum compressive residual stresses increased rapidly with the increased feed rate. Therefore, although the surface roughness, plastic deformation and work hardening all increased significantly when the feed rate was changed from 0.1 mm/tooth to 0.25 mm/tooth, the fatigue life reduced slightly, but had increased finally. In addition, the poor fatigue performance of the specimens produced under the feed rate of 0.05 mm/ tooth could be ascribed to their rough surface (seen in Fig. 3) and the decreased surface and maximum compressive residual stresses. The analysis in Fig. 3 has confirmed that the changes of the Ra and Rz with the changed depth of cut were no larger than that against the cutting speed. However, the sub-surface microstructures in Figs. 6 and 8 showed the much more significant increase of plastic deformation against the increased depth of cut. Therefore, the small effect of surface
Fig. 11. Relationship of surface residual stress with factors (three cutting parameters).
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Fig. 12. Relationship of maximum compressive residual stress with factors (three cutting parameters).
Fig. 13. Fatigue life of the specimens after milled at various (a) cutting speed, (b) feed rate and (c) depth of cut.
roughness on the fatigue performance was completely covered up. The changes of fatigue life in Fig. 13(c) were mainly caused by the combined effect of the plastic deformation and the residual stress field. When the depth of cut was increased from 0.3 mm to 0.75 mm, the plastic deformation and work hardening were enhanced continuously, while the compressive residual stress field changed slightly. The effect of the plastic deformation and work hardening played a dominant role, and led to the decreased fatigue life. The residual stresses shown in Figs. 11-12 recorded the only relatively large transform to the higher compressive residual stresses when the depth of cut of 0.9 mm was reached, which could be used to explain the increased fatigue life. 4. Conclusions In the present work, the fatigue performance of the 17-4PH stainless steel workpieces was measured using the three-point bending fatigue life tests. The effects of cutting operations on the surface integrity, and its further influence on the fatigue performance were discussed. The following conclusions were derived according to the researches: 1. Compared with the polishing operation, all of the cutting operations with different cutting parameters improved the fatigue performance of the machined surface, since the generation of the compressive residual stress fields on the machined surface and the softened layer below the machined surface. All of the three cutting parameters (cutting speed, feed rate and depth of cut) can influence the machined surface integrity significantly, and further influence the fatigue performance of the workpiece. 2. The fatigue performance of the machined surface was affected by the three cutting parameters differently. The fatigue performance presented a decreasing tendency on the whole with the increased cutting speed, because of the enhanced plastic deformation and work hardening. While when the feed rate was increased from 0.1 mm/ tooth, although the Ra, Rz, plastic deformation and work hardening
all increased significantly, the fatigue life decreased slightly, and even had an increase finally because of the rapidly increased compressive residual stresses. When the workpiece was machined with the feed rate of 0.05 mm/tooth, the increased surface roughness and the decreased compressive residual stresses led to a short fatigue life. The fatigue performance of the machined specimens first declined rapidly when the depth of cut got bigger, because of the continuously increased plastic deformation. A relatively good fatigue performance was recorded only when the depth of cut of 0.9 mm was reached, because of the large increase of the compressive residual stresses. 3. The fatigue performance of the machined workpiece was determined by the interactions of the above-mentioned surface integrity characteristics. The softened layer below the work-hardened surface and the compressive residual stress field were regarded as the two reasons to prolong the fatigue life of the machined surface, since they could prevent the initiation or propagation of the cracks. Because of the curved feed marks, when the changes of the surface roughness were small, their effect on the fatigue life was overshadowed. The plastic deformation layer and resulting work hardening were confirmed to be detrimental for the fatigue life. Acknowledgement This work was supported by Key Special Project of Numerical Control Machine Tool (2015ZX04003006) and Taishan Scholars Program (TS20130922). References [1] D. Karthik, S. Kalainathan, S. Swaroop, Surface modification of 17-4PH stainless steel by laser peening without protective coating process, Surf. Coat. Technol. 278 (2015) 138–145. [2] D. Novovic, R.C. Dewes, D.K. Aspinwall, et al., The effect of machined topography and integrity on fatigue life, Int. J. Mach. Tools Manuf. 44 (2004) 125–134.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Surface integrity and fatigue performance of 17-4PH stainless steel after cutting operations Guoliang Liu a,b, Chuanzhen Huang a,b,⁎, Bin Zou a,b, Xiangyu Wang a,b, Zhanqiang Liu a,b a b
Center for Advanced Jet Engineering Technologies (CaJET), School of Mechanical Engineering, Shandong University, 17923 Jingshi Road, Jinan, PR China Key Laboratory of High-efficiency and Clean Mechanical Manufacture (Shandong University), Ministry of Education, PR China
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Article history: Received 5 June 2016 Revised 26 August 2016 Accepted in revised form 29 August 2016 Available online 30 August 2016 Keywords: 17-4PH stainless steel Cutting operations Surface integrity Fatigue life
a b s t r a c t The 17-4PH stainless steel has been applied as a substitute of titanium to make jet engine parts, and fatigue life of the machined surfaces are very important due to its high reliability and safety demand. In this work, a series of experiments were conducted to investigate the impact of the cutting operations on the surface integrity, and its further influence on the fatigue life. It was confirmed that the fatigue performance of the machined surface was determined by the interactions of the surface integrity characteristics, including the work hardening, surface roughness and residual stress field. The softened layer under the machined surface and the compressive residual stress field were generated under all cutting conditions, resulting in the improved fatigue life when compared to the polished specimens. The plastic deformation and the resulting work hardening on the machined surface were enhanced continuously with an increase in each of the cutting parameters, which could cause significant decrease of the fatigue performance of the workpiece. The influence of surface roughness on the fatigue performance could be overshadowed by other surface integrity characteristics, since its effect was weakened by the curved feed marks generated on the face-milled surfaces. The cutting parameters could influence the fatigue performance of the machined components significantly by changing their surface integrity. © 2016 Elsevier B.V. All rights reserved.
1. Introduction 17-4PH is a typical kind of martensitic precipitation hardening stainless steel, consisting of martensite and a small amount of retained austenite. It is usually used to produce pump shafts, chemical process equipment, and nuclear reactor components because of its high strength and corrosion resistance [1]. These days, it has been used as a substitute of titanium in the aircraft engine. In these applications, the components are subjected to cyclical mechanical and thermal loads, and high reliability are extremely needed, thus good surface integrity and fatigue performance are required. It has been recognized that the fatigue performance relied on the surface integrity produced by cutting operations, for that the fatigue cracks could initiate from the machined surfaces [2]. However, the influence rule of the cutting operations on the fatigue performance has not been well-studied, since grinding or polishing was usually employed to finish the components after cutting operations. But today, the newly developed tools and the high speed machining technology has made it possible to use the components without grinding or polishing after cutting operations [3]. In such
⁎ Corresponding author at: Center for Advanced Jet Engineering Technologies (CaJET), School of Mechanical Engineering, Shandong University, 17923 Jingshi Road, Jinan, PR China. E-mail address: [email protected] (C. Huang).
http://dx.doi.org/10.1016/j.surfcoat.2016.08.086 0257-8972/© 2016 Elsevier B.V. All rights reserved.
cases, it is necessary to research the impact of the cutting operations on the fatigue performance of the machined components. Although in recent years some researchers started to pay attention to the fatigue performance of the machined specimens, the direct tests for fatigue life of the workpieces are still relatively rare. In addition, most of the fatigue performance tests were conducted to compare the two different machining processes, for example, turning vs. grinding [4–6] and electro-discharge machining (EDM) [7], milling vs. grinding [8] and EDM [9]. The studies on the effect of cutting conditions from Jeelani and Musial [10], Javidi et al. [11], Sasahara [3] and Choi [12] were limited to few types of fatigue performance tests, workpiece materials and cutting parameters. It is obvious that the role of cutting operations in determining the fatigue performance needs more attention. Another weakness in the existing fatigue life research is that the influence mechanism between the surface integrity characteristics and fatigue performances is not understood deeply enough. In many early published works, the effect of surface integrity on fatigue performance was often entirely characterized by the amplitude surface roughness parameters, such as Ra or Rt [2]. The fatigue stress concentration factor was calculated by several models based on the two parameters to predict the fatigue life [13,14]. However, because the surface roughness value was usually comparatively small in the finishing process, the effect of these parameters on the fatigue performance was not significant, especially in the high cycle conditions. In other researches, the residual stress was gradually regarded as the determinant factor to change the fatigue
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signally promote the understanding of the generation mechanism of the fatigue performance of machined surfaces.
Table 1 The mechanical properties of the workpiece. Reduction of area (%) Workpiece 45
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Yield strength (MPa)
Hardness Tensile strength (HRC) (MPa)
Elongation (%)
1235
43
12
1425
Table 2 The single factor experimental design. Parameters
Group I
Group II
Group III
V (m/min)
200
200
f (mm/tooth)
100, 150, 200, 250, 300 0.15
0.15
ap (mm)
0.6
0.05, 0.1, 0.15, 0.2, 0.25 0.6
0.3, 0.45, 0.6, 0.75, 0.9
performance of the machined surface [4,7,11,12], and it was widely acknowledged that the compressive residual stress could improve the fatigue performance. However, the influence of the surface and maximum compressive residual stress was still confusing [4]. As for the effect of work hardening and microstructure, there were more inconsistent results in existing research. Sasahara [3] believed that the fatigue life could be prolonged by the increased surface hardness, because of the increased yield stress of the machined surface. However, Mantle et al. [15, 16] claimed that the work hardening was beneficial to crack propagation, because it could reduce the material's ductility on the surface. Furthermore, the above mentioned sources mainly attributed the changed fatigue life to one of the surface integrity indicators, lacking the study on the combined effect of the plastic deformation, surface roughness and residual stress. Therefore, it is reasonable to deepen the study of the surface integrity and the fatigue performance of the machined workpiece after cutting operations. Although the fatigue life has serious effect on the application of 174PH stainless steel, there is almost no direct research on the effect of cutting operations on the fatigue life for 17-4PH material. In the present work, various cutting parameters were used in the face-milling experiments to reveal the relationships among the cutting parameters, surface integrity characteristics and the specimens' fatigue performance. The effect of the three cutting parameters on the machined surface integrity, and further on their fatigue performance was discussed separately. The combined effect of the surface integrity characteristics, including surface roughness, work hardening, residual stress and microstructure, on the fatigue life was revealed as well. It is undeniable that the presented test results were obtained with particular specific insert and cutting edge, and there may be some change in the effect of cutting parameter with different inserts. For example, if the inserts with larger nose radius were used, the turning point of surface roughness with the feed rate may be changed, and the optimized processing parameters may be different. Nonetheless, the influence mechanism of surface integrity characteristics on the fatigue life is universal. Thus, the present work can
2. Experimental procedures A vertical CNC machining center DMU-70V (Germany) was employed to do dry machining in all face-milling tests. The workpiece used in the experiments was a block of solution and aging treated 174PH stainless steel. The dimension of the workpiece was 110 mm × 50 mm × 110 mm, and a face of 110 mm × 50 mm was chosen as the machining surface. The symmetric down milling was adopted, resulting in a fixed cutting width (ae) of 50 mm. The mechanical properties of the workpiece are shown in Table 1. The TiAlN coated carbide inserts (DIJET, JC8015-ODHW606AEN) were mounted on a cutter (OCT-06100-32R) with a diameter of 100 mm to finish the experiments. The tool geometry after mounted was about 10° of rake angle, 5° of clearance angle, 1.2 mm of nose radius and 15–20 μm of hone size. The single factor experimental design adopted in the present work is shown in Table 2. The cutting parameters in Table 2, including the cutting speed (V), the feed rate (f) and the depth of cut (ap), were selected according to our previous work [17] and the recommended cutting parameters in the insert manual. The detrimental effect of tool wear and run-out of teeth was avoided by using a new insert each time. During the cutting process, a Kistler 9257B three component piezoelectric dynamometer (Switzerland) was adopted to record the cutting forces. After the milling process, a mobile surface roughness meter (China, TR200) was adopted to measure the roughness parameters (Ra and Rz) of the machined surface, and a KEYENCE VK-X200K laser microscope (Japan) was used to observe the surface morphology. Profiles of the residual stress in the workpiece were measured by an Xstress 3000 stress instrument (Stresstech Oy, Finland) using the XRD-sin2ψ technique [18]. The parameters of this technique adopted in the present work are showed in Table 3. The electro-polishing technique was used to measure the residual stresses field of the specimens. The 80% methanol and 20% perchloric acid solution was employed in an electropolishing machine to remove the surface material layer-by-layer. Every time the sample was polished for 15 s, and then the remaining sample thickness was measured by a screw micrometer to calculate the thickness of the removed layer. The micro-hardness measurement and the half-peak width (FWHM) from the XRD experiments are the common methods to test the work-hardening of the machined surface. However, the FWHM was caused by the stacking faults and structural disorder [19], and the definitive link between the hardness and the plastic strain was not found. Some researchers indicated that the FWHM was sensitive to the variation in microstructure and stress–strain accumulation in the material and the presence of tensile stress in the material caused increase in the FWHM [20]. Thus, the machined surface hardness was measured by the NanoTest Ventage (UK) with a load of 500 mN and Vickers hardness tester (China) with a load of 9.8 N separately. The full load has been dwelled for 15 s. The in-depth nanohardness measurements were conducted on the cross-section of the workpiece after mosaic, grinding and polishing. The first line of measuring points was located at a distance of 15 μm below the free surface. Other measuring points were located along a straight line in the indepth direction at 10-μm intervals until a stable hardness was obtained.
Table 3 The residual stress measurement parameters. Parameters
Value
Diffraction angle (2θ)/° Side inclination (ψ)/° Collimator diameter/mm Exposure time/S Operation voltage/kV Operation current/mA Target
156 0, ±10, ±20, ±30 3 15 29.3 6.76 Cr-Kα
Fig. 1. Schematic diagram of the three points bending fatigue life test.
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obvious that the feed rate was the only one parameter that affected the surface roughness significantly. When the feed rate was changed from 0.1 mm/tooth to 0.25 mm/tooth, the Ra and Rz both increased significantly. The relatively poor surface at the feed rate of 0.05 mm/tooth could be attributed to the excessive friction and squeezing of the cutting edge and workpiece, which would produce much more chip debris adhering on the machined surface. Compared to the feed rate, the variations of the Ra and Rz against the cutting speed or the depth of cut were pretty minute. The relatively high decrease of Ra (more than 0.05 μm) was recorded only when the cutting speed rose from 100 m/ min to 150 m/min and from 200 m/min to 250 m/min. 3.2. Hardness and microstructure
Fig. 2. Schematic diagram of the sample location on the workpiece.
Sub-surface microstructure was observed using the KEYENCE VKX200K 3D laser microscope on a cross-section of the workpiece, which was etched with 75% concentrated hydrochloric acid and 25% concentrated nitric acid after grinding and polishing. A servo-hydraulic fatigue testing machine (Instron 8801, USA) was adopted to perform the three-point bending fatigue life tests in this work, and the schematic diagram of the test is displayed in Fig. 1. The fatigue life test specimens were cut from the workpieces center using wire electrical discharge machining (WEDM), in the same size of 100 mm × 10 mm × 4 mm (seen in the “1”, “2” and “3” in Fig. 2). A dynamic sine wave load with the maximum tension stress of 1000 MPa was applied to the specimens in the three points bending fatigue tests. The loading frequency was 15 Hz, and the stress ratio was 0.1. The tests were running until the specimen fractured. For each cutting condition, at least three specimens were tested to assess the scatter. Three polished specimens were also tested to work as a reference group. 3. Results and discussion 3.1. Surface roughness The Ra (Arithmetic average surface roughness) and Rz (Maximum peak-to-valley roughness height) are the most commonly used surface roughness parameters in calculating the fatigue stress concentration factor, and they were both measured for each specimen. The average Ra and Rz values and their standard deviation for the three specimens are summarized in Fig. 3, and it can be seen that the Ra and Rz show a similar variation trend. Among the three cutting parameters, it is
The surface layer and in-depth nano-hardness distributions of the specimens are shown in Fig. 4. It can be seen that the similar spoonshaped curves were obtained in all cutting conditions. There was an obvious softened layer developed below the work-hardened surface, which agreed with the observations of Choi [12]. It is well acknowledged that the hardened surface is a result of the plastic deformation in the cutting processes. The microstructures in Figs. 6-8 proved the existence of the severe plastic deformation on the surface layer in all cutting conditions. However, it is also obvious that the thickness of the severe plastic deformation zone was no more than 10 μm for all specimens. Therefore, the materials below the plastic deformation layer were mainly influenced by the heat produced while machining, explaining the significantly reduced nano-hardness at the subsurfaces. Another obvious characteristic of the severe plastic deformation layers was their discontinuity, which could explain the large deviation of the nano-hardness shown in Fig. 4. To compare the work-hardening degree in different cutting conditions, the Vickers hardness tests with a load of 9.8 N was carried out, and the results are shown in Fig. 5. The diagonal length of each indentation was greater than 60 μm, which could overcome the adverse impact of the discontinuous plastic deformation effectively. It can be seen that the work-hardening was enhanced as a whole with an increase of each cutting parameter, and the feed rate displayed the most significant effect. It has been widely acknowledged that the work-hardening of the machined surface is mainly determined by the thermal and mechanical effects. The workpiece material has been treated by a solution heat treatment at 1030 °C and aging treatment at 480 °C, while the finite element analyses have shown that the temperatures in the cutting zone were no higher than 900 °C in all of the cutting conditions adopted in this work (The finite element analyses process has been described in our previous work [21]). In addition, the heating time was too short to produce severe structural transformation. As a result, the cutting heat could not harden the machined surfaces by the quenching effect, and the plastic hardening and thermal softening were the main cause of the work-hardening of the machined surface. Figs. 6-8 showed the microstructures of the most severe plastic deformation areas at the minimum and maximum cutting parameters. Some obvious grain
Fig. 3. Relationship of Ra and Rz with factors (three cutting parameters).
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Fig. 4. Nano-hardness distributions of the specimens.
boundary deflections (marked by the red dash line) were generated on the sub-surface for each cutting parameter, following the direction of the cutting speed. The deformed grain boundaries can help to identify the severe plastic deformation layer. It can be seen that the plastic deformation was enhanced with an increase in each of the three cutting parameters, which was the main cause of the increased hardness. However, it is obvious by comparing the Panels (b) in Figs. 6-8 that the cutting speed owned a smaller effect on the plastic deformation than the feed rate and depth of cut, which could explain the smaller increase of hardness. The cutting force is a main cause of the plastic flow of the material on the finished surfaces [22]. As shown in Fig. 9 the resultant cutting forces increased significantly when the feed rate or the depth of cut was increased, but remained unaffected by the cutting speed. The high cutting forces caused by the increased feed rate or depth of cut could reinforce the plastic flow. In addition to the plastic
deformation, the thermal softening was another reason of the various work-hardening degrees. Although more cutting heat would be generated in the cutting zone when each of the three cutting parameters was increased, the heat source movement would be speed up by the higher cutting speed or feed rate, and thus weaken the influence of thermal softening. As a result, the work hardening of the machined surface was decreased when the depth of cut was increased. 3.3. Residual stress Fig. 10 presents the distribution of the residual stress field within the machined surface layer after face-milled at the cutting speed of 200 m/ min, feed rate of 0.15 mm/tooth and depth of cut of 0.6 mm. σx stands for the residual stresses parallel to the feed direction, while σy means that the residual stresses were measured in the direction perpendicular
Fig. 5. Relationship of Vickers hardness with factors (three cutting parameters).
Fig. 6. Sub-surface microstructures of the specimens face-milled with the cutting speed of (a) 100 m/min and (b) 300 m/min. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Fig. 7. Sub-surface microstructures of the specimens face-milled at the feed rate of (a) 0.05 mm/tooth and (b) 0.25 mm/tooth. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 8. Sub-surface microstructures of the specimens face-milled at the depth of cut of (a) 0.3 mm and (b) 0.9 mm. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
to the σx. It can be seen that the most common spoon-shaped form of the residual stress distribution was generated. The residual stress first falls off to a maximum compressive value from the smaller ones on the machined surface layer with an increased in-depth. After a maximum compressive residual stress value is reached, the residual stress transforms to a steady value of the workpiece substrate. It must be noted that this residual stress distribution form was obtained in all cutting conditions, although the values of the residual stress field were diverse in different cutting conditions. It has been well- acknowledged that the spoon-shaped residual stress distribution was generated by the coupling effect of the thermal and mechanical load [23]. According to the previous research, the residual stresses on the free surface and the maximum compressive residual stresses were the main factors affecting the fatigue life [4]. In order to facilitate the comparison of the influence of different cutting parameters on the residual stresses, only the two residual stresses indexes and the standard deviations in all cutting conditions were extracted and displayed in Figs. 11-12. It is
obvious that both of the residual stresses on the free surface and the maximum compressive residual stresses were most severely impacted by the feed rate. The residual stresses in two directions were both transformed to higher compressive ones significantly when the feed rate was increased. When the cutting speed or the depth of cut increased, the larger compressive residual stresses were generated as well, but the changes were relatively slight. The only relatively large increase of the surface and maximum compressive residual stress was recorded when the depth of cut was increased to 0.9 mm, because of the significantly increased cutting force. 3.4. Fatigue life After the specimens were fractured in the three-point bending fatigue tests, the fracture surfaces of specimens were examined using SEM to eliminate the ones in which the crack initiation was located in the material, instead of initiating from the machined surface. The fatigue
Fig. 9. Relationship of resultant cutting force with factors (three cutting parameters).
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Fig. 10. In-depth residual stresses after face-milled at v = 200 m/min, f = 0.15 mm/tooth and ap = 0.6 mm.
lives of the reference group and all cutting conditions were recorded, and the average fatigue life and its deviation for each condition are shown in Fig. 13. It can be seen that all the face-milled specimens had a higher fatigue life than the polished ones, and the longest fatigue life was about 1.5 times that of the polished specimens. According to the measured results of the surface integrity characteristics, mainly two reasons were considered to result in the improved fatigue performance of the face-milled specimens. Firstly, the compressive residual stress fields generated on the surface layer were considered. It is believed that the compressive residual stresses generated on the free surface could prevent the initiation of the fatigue cracks, while the maximum compressive residual stresses at the subsurface could prevent their propagation. Thus, both of them could weaken the adverse impact of the applied loadings and improve the fatigue performance. This agreed with the results of Jeelani and Musial [10] and Smith et al. [4]. Secondly, the softened layer below the machined surface was considered as the other reason to prolong the fatigue life. Contrary to the work hardening, the softened layer is of benefit to the material's ductility, and thus can prevent the propagation of cracks. As for the plastic deformation layer and the resulting work hardening, they were regarded as unfavorable factors for the fatigue performance. The first reason was pointed out by Mantle et al. [15,16] that the work hardening could reduce the ductility of the machined surface, and thus was detrimental to crack propagation. In addition, it can be seen from the sub-surface microstructures in Figs. 6-8 that the plastically deforming areas were not
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continuous. The plastic deformation could change the material properties of the surface, ductility, strength, hardness or grain size, and some of the changes may be beneficial for the fatigue life. However, the differences of the material properties would make the cracks much more easily to generate at the boundaries of undeformed-deformed areas. The changes of fatigue life against the three cutting parameters in Fig. 13 could be explained using the combined effect of the abovementioned surface integrity characteristics. It is easy to conclude from Fig. 13(a) that the fatigue life decreased on the whole with an increase in the cutting speed. The two singular points occurred when the cutting speed reached 150 m/min or 250 m/min. The surface integrity measurements have shown that with the increased cutting speed, the plastic deformation and work hardening which were detrimental to the fatigue performance were both enhanced significantly, while the decreased surface roughness and the increased surface and maximum compressive residual stresses could prolong the fatigue life. Therefore, the decreased fatigue life can mainly be attributed to the increased plastic deformation and work hardening. The residual stress analyses in Figs. 11-12 have shown that the residual stress field was just slightly affected by the cutting speed, and the small increase of the compressive residual stresses in the machined surface layer was not enough to offset the detrimental effect of the increased plastic deformation. As for the surface roughness, Taylor and Clancy [24] have revealed that the milling could cause curved feed marks, which may be perpendicular to the early initiation directions of cracks, and thus prevent the propagation of the cracks. Therefore, when the changes of the surface roughness were small, their effect on the fatigue life could be overshadowed by other surface integrity characteristics. Only when the aforementioned two relatively big downside of surface roughness were generated under the cutting speed of 150 m/min and 250 m/min, the surface roughness showed its effect, and led to the increased fatigue life combined with the compressive residual stresses. Fig. 13(b) displays the variation trend of the fatigue life with the feed rate. As recorded in Figs. 11-12, the residual stress was significantly determined by the feed rate, and both of the surface and maximum compressive residual stresses increased rapidly with the increased feed rate. Therefore, although the surface roughness, plastic deformation and work hardening all increased significantly when the feed rate was changed from 0.1 mm/tooth to 0.25 mm/tooth, the fatigue life reduced slightly, but had increased finally. In addition, the poor fatigue performance of the specimens produced under the feed rate of 0.05 mm/ tooth could be ascribed to their rough surface (seen in Fig. 3) and the decreased surface and maximum compressive residual stresses. The analysis in Fig. 3 has confirmed that the changes of the Ra and Rz with the changed depth of cut were no larger than that against the cutting speed. However, the sub-surface microstructures in Figs. 6 and 8 showed the much more significant increase of plastic deformation against the increased depth of cut. Therefore, the small effect of surface
Fig. 11. Relationship of surface residual stress with factors (three cutting parameters).
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Fig. 12. Relationship of maximum compressive residual stress with factors (three cutting parameters).
Fig. 13. Fatigue life of the specimens after milled at various (a) cutting speed, (b) feed rate and (c) depth of cut.
roughness on the fatigue performance was completely covered up. The changes of fatigue life in Fig. 13(c) were mainly caused by the combined effect of the plastic deformation and the residual stress field. When the depth of cut was increased from 0.3 mm to 0.75 mm, the plastic deformation and work hardening were enhanced continuously, while the compressive residual stress field changed slightly. The effect of the plastic deformation and work hardening played a dominant role, and led to the decreased fatigue life. The residual stresses shown in Figs. 11-12 recorded the only relatively large transform to the higher compressive residual stresses when the depth of cut of 0.9 mm was reached, which could be used to explain the increased fatigue life. 4. Conclusions In the present work, the fatigue performance of the 17-4PH stainless steel workpieces was measured using the three-point bending fatigue life tests. The effects of cutting operations on the surface integrity, and its further influence on the fatigue performance were discussed. The following conclusions were derived according to the researches: 1. Compared with the polishing operation, all of the cutting operations with different cutting parameters improved the fatigue performance of the machined surface, since the generation of the compressive residual stress fields on the machined surface and the softened layer below the machined surface. All of the three cutting parameters (cutting speed, feed rate and depth of cut) can influence the machined surface integrity significantly, and further influence the fatigue performance of the workpiece. 2. The fatigue performance of the machined surface was affected by the three cutting parameters differently. The fatigue performance presented a decreasing tendency on the whole with the increased cutting speed, because of the enhanced plastic deformation and work hardening. While when the feed rate was increased from 0.1 mm/ tooth, although the Ra, Rz, plastic deformation and work hardening
all increased significantly, the fatigue life decreased slightly, and even had an increase finally because of the rapidly increased compressive residual stresses. When the workpiece was machined with the feed rate of 0.05 mm/tooth, the increased surface roughness and the decreased compressive residual stresses led to a short fatigue life. The fatigue performance of the machined specimens first declined rapidly when the depth of cut got bigger, because of the continuously increased plastic deformation. A relatively good fatigue performance was recorded only when the depth of cut of 0.9 mm was reached, because of the large increase of the compressive residual stresses. 3. The fatigue performance of the machined workpiece was determined by the interactions of the above-mentioned surface integrity characteristics. The softened layer below the work-hardened surface and the compressive residual stress field were regarded as the two reasons to prolong the fatigue life of the machined surface, since they could prevent the initiation or propagation of the cracks. Because of the curved feed marks, when the changes of the surface roughness were small, their effect on the fatigue life was overshadowed. The plastic deformation layer and resulting work hardening were confirmed to be detrimental for the fatigue life. Acknowledgement This work was supported by Key Special Project of Numerical Control Machine Tool (2015ZX04003006) and Taishan Scholars Program (TS20130922). References [1] D. Karthik, S. Kalainathan, S. Swaroop, Surface modification of 17-4PH stainless steel by laser peening without protective coating process, Surf. Coat. Technol. 278 (2015) 138–145. [2] D. Novovic, R.C. Dewes, D.K. Aspinwall, et al., The effect of machined topography and integrity on fatigue life, Int. J. Mach. Tools Manuf. 44 (2004) 125–134.
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