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Effect of Cryogenic Diamond Burnishing on Residual Stress and Microhardness of 17-4 PH Stainless Steel
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 18393–18399
www.materialstoday.com/proceedings
ICMPC_2018
Effect of Cryogenic Diamond Burnishing on Residual Stress and Microhardness of 17-4 PH Stainless Steel Sachin Ba*, Narendranath Sa, D Chakradharb A
department Of Mechanical Engineering, National Institute Of Technology, Karnataka, Surathkal, India B mechanical Engineering, Indian Institute Of Technology, Palakkad, Kerala, India
Abstract Diamond burnishing is a cold working process, which produces a work hardened and uniform surface by plastic deformation. The aim of the present work is to study the behavior of diamond burnishing on surface integrity of 17-4 precipitation hardenable stainless steel (PH-SS) under cryogenic, minimum quantity lubrication (MQL) and dry environments. Surface modification was achieved by the application of liquid nitrogen during diamond burnishing. The process parameters considered were speed, feed, burnishing depth and number of passes. Surface integrity characteristics such as microhardness and residual stresses were investigated after diamond burnishing under cryogenic, MQL and dry environments. In cryogenic diamond burnishing, the surface integrity characteristics of 17-4 PH stainless steel has been significantly improved when compared to MQL and dry environments. Maximum microhardness of 395 HV, 369 HV, and 357 HV respectively was observed under cryogenic, MQL and dry environment. The maximum residual stress of -352 MPa, -282 MPa and -195 MPa respectively were recorded for cryogenic, MQL and dry environment. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords: Diamond burnishing; 17-4 PH stainless steel; microhardness; residual stress.
1. Introduction Surface treatment techniques are one of the finest methods to improve the surface topography of the material by subjecting it to severe plastic deformation. Many secondary manufacturing techniques produce irregularities, voids,
* Corresponding author. Tel.: +91-990-052-9811 E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.
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flakes etc. To avoid these problems, chipless machining process such as burnishing is widely used in modern applications. In recent years, polymeric composite materials and titanium alloys have been replaced by 17-4 PH SS. It has an excellent corrosion resistance, ductility, high toughness, high impact and tensile strength properties thus it can be classified under difficult to cut materials [1]. One of the important application of 17-4 PH SS is in steam turbines. This material is also used in chemical industries, marine construction and power plants [2]. The high temperatures generated while finishing can be reduced with the application of a variety of coolants. Conventional coolants have been utilized by metal cutting industries to reduce the machining zone temperatures. Some of the coolants are hazardous to the health of the operator and also they cause environmental pollution when disposed of [3]. It has been revealed by Chomienne et al. [4] that the velocity is one of the parameters which will not affect the residual stresses profile of the work material. It has been suggested to use a single pass to get very good residual stress profile. Nestler and Schubert [5] reported that the surface roughness can be reduced along with the reduction in the voids formation. It was found that the burnishing feed is the parameter which has more influence on surface roughness. The mass of the component was reduced and performance was improved by using slide diamond burnishing process in AA2124. Babu et al. [6] performed experiments on three different steels with EN8, EN24, EN31 grades and found that as there is an increase in the magnitude of compressive residual stress and also burnishing depth was found to be more in EN31 when compared to other types of materials. El-Axir [7] proved that a better surface finish and hardness were observed at low burnishing feed. Improved micro-hardness has been observed when a high number of passes were used. For better surface finish it has been suggested to use a moderate number of passes and also maximum residual stress was observed at the surface and beneath the surface, it decreases with the increase in the depth. Yang et al. [8] have studied the effect of severe plastic deformation and cryogenic burnishing on the modifications of surface integrity of Co-Cr-Mo alloy by using a newly developed burnishing tool. It was observed that the temperature induced was significantly minimized by cryogenic treatment. Microstructural modifications and improved hardness have been observed after cryogenic burnishing. Pereira et al. [9] have performed turning operation on AISI 304 under the combined effect of minimum quantity lubrication and cryogenic environments. It has been realized that mixed effect of cryogenic and MQL was ecofriendly when compared to conventional wet machining. Caudill et al. [10] compared the effect of cryogenic, flood-cooled and dry burnishing on surface integrity of Ti-6Al-4V alloy by severe plastic deformation technique. It was revealed that in the severe plastic deformation layer which was produced by cryogenic environment showed improved grain structure, increased hardness, and improved surface finish. Yu and Wang [11] studied the effect of a polycrystalline diamond tool on the surface roughness of aluminium alloy. It was observed that the surface roughness of a burnished aluminium alloy has been gradually decreased. Hassan et al. [12] developed a mathematical model to correlate the effect of process parameters such as burnishing force and number of tool pass on ball burnishing of brass components. Optimization of process parameters was carried out by using response surface method. Many researchers have concentrated on the study of ball burnishing and roller burnishing to analyze the effect of process parameters on a variety of work materials. Till date, few researchers have concentrated on analyzing the surface integrity characteristics of diamond burnishing under different lubricants. This study mainly aims at analyzing the effect of process parameters such as speed, feed, burnishing depth and number of passes on the surface integrity characteristics such as residual stress and microhardness of a 17-4 PH SS under cryogenic, MQL and dry environments. The results reveal that after diamond burnishing, compressive residual stresses were formed under all the three environments and microhardness was observed to be maximum at the diamond burnished surface. 2. Materials and method The workpiece used for the present study is 17-4 PH SS. It is a martensitic stainless steel and categorized under difficult machine materials. Before diamond burnishing, 0.5 mm top surface layer of the workpiece was removed by turning process. Diamond burnishing was carried out under cryogenic, MQL and dry environments. In cryogenic environment liquid nitrogen (LN2) was sprayed at the tool and the workpiece interface. The experimental details are as presented in Table 1. The cryogenic and MQL setups are as depicted in Figure 1 and Figure 2 respectively. Figure 3 (a-c) shows the diamond burnishing zone of all the three environments. In the present investigation process parameters used are speed, feed, number of tool passes and burnishing depth. Burnishing depth was varied by
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keeping all other parameters constant as shown in Table 1. Vickers microhardness tester has been used to measure the microhardness at different depths from the burnished surface. X-Ray diffraction (XRD) has been used to study the residual stresses induced after diamond burnishing process. Table 1. Experimental information Workpiece material and dimensions Chemical composition of 17-4 PH stainless steel
17-4 PH SS round bar, (⌀ 32 mm x 150 mm)
Tool used Burnishing process parameters
Diamond burnishing tool (DB-200) Burnishing speed: 47 m/min Burnishing feed: 0.096 mm/rev Burnishing depth: 0.2, 0.4, 0.6 mm Number of passes: 2
Environments and coolants used Burnishing fluid supply
Cryogenic (LN2), MQL (Castrol oil), Dry (No coolant) Cryogenic cooling - compressed air: 4 kg/cm2, flow rate: 0.45 kg/min (through external nozzle)
(Ni-3.546 %, Cr-16.179 %, Cu-3.177 %, Mn-0.744 %, Si-0.360 %, C-0.042 %, P-0.028 %, S-0.011 %, Nb+Ta-0.356 % and Fe-Balance)
MQL cooling - compressed air: 4 kg/cm2, flow rate: 70 ml/h (through external nozzle) Nozzle diameter
cryogenic and MQL – ⌀ 1 mm
Fig. 1. Cryogenic diamond burnishing
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Fig. 2. MQL diamond burnishing
Fig. 3. Diamond burnishing zone at (a) cryogenic, (b) MQL and (c) Dry environments
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3. Results and Discussion 3.1. Effect of process parameters on microhardness It was revealed by [4,14] that the microhardness of the specimen decreases as the depth from the top surface increases. In the present study, similar results have been attained. The micro-hardness measurement was carried out from the top surface towards the bulk material for varying burnishing depth which is as shown in Figure 4. The measurement was carried out up to a depth of 160 µm from the burnished surface. It was observed that just beneath the burnished surface, the hardness will be more and as the depth from the surface increases the micro hardness reduces. Furthermore decrease in the depth from the surface causes reduction in the micro hardness and finally, it reaches the micro hardness of the bulk material. The average microhardness of the as-received material was found to be 340 HV. The reason for the reduction in the hardness can be explained by the fact that, less shearing observed between the tip of the tool and the workpiece surface. The surface just beneath the diamond burnished layer will have more hardness because of the work hardening process [7]. The highest micro hardness of 395 HV was observed for the cryogenic environment at a burnishing depth of 0.6 mm. Compared to dry diamond burnishing, MQL environment has provided a better micro-hardness result. The maximum value of 369 HV and 357 HV have been recorded for MQL and dry environments respectively at a burnishing depth of 0.6 mm. Therefore the percentage of microhardness improvement attained in the cryogenic environment was 7% and 11% respectively in contrast to MQL and dry environment. Similarly, in cryogenic environment percentage of improvement observed was 4%, 6% respectively at burnishing depth of 0.2 mm and 5%, 8% respectively at burnishing depth of 0.4 mm in contrast to MQL and dry environments. The improvement observed in the microhardness of cryogenic diamond burnished sample is due to the rapid cooling effect of the liquid nitrogen which minimizes the thermal softening effect. However, in MQL condition the thermal softening effect was minimized because of the presence of lubrication and in a dry environment because of the thermal softening effect the microhardness was observed to be decreased [18].
Fig. 4. Micro-hardness test result obtained at speed = 47 m/min, feed = 0.096 mm/rev, number of pass = 2 and burnishing depth (BD) = 0.2, 0.4, 0.6 mm
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3.2. Effect of process parameters on surface residual stress A study on residual stresses induced in diamond burnishing process plays a crucial role. After burnishing process, compressive residual stresses will be induced because of the plastic deformation of the material [5,13,15,16,17]. A similar trend has been observed in this study. In the present research work, the effect of diamond burnishing on residual stress was studied under cryogenic, MQL and dry environments. It was noticed that a large number of compressive residual stresses has been formed on the top surface layer after diamond burnishing. Compressive residual stresses are formed because of the plastic deformation produced on the surface layer after diamond burnishing [13]. Figure 5 shows the residual stress measurement carried out at speed = 47 m/min, feed = 0.096 mm/rev, number of pass = 2 and burnishing depth = 0.2, 0.4, 0.6 mm, under all the three environments. After diamond burnishing maximum compressive residual stress of -352 MPa, -282 MPa and -195 MPa respectively has been observed for cryogenic, MQL and dry environments on the burnished surface at burnishing depth of 0.6 mm. In a cryogenic environment, the percentage of improvement observed was 20% and 44% respectively in contrast to MQL and dry environments. Burnishing depth will have more impact in inducing the compressive residual stresses on the top surface layer. Increase in the burnishing depth causes more plastic deformation which results in improved surface residual stresses [16]. Cryogenic diamond burnishing reduces thermal stress inducement on the surface layer because of the cooling effect [19] when compared to MQL and dry environment. Hence an improved surface residual stress was observed in the cryogenic environment. Cryogenic MQL Dry
Residual stress (MPa)
-150 -200 -250 -300 -350 -400
0.2
0.3
0.4
0.5
0.6
Burnishing depth (mm)
Fig. 5. Residual stress obtained at speed = 47 m/min, feed = 0.096 mm/rev, number of pass = 2 and burnishing depth = 0.2, 0.4, 0.6 mm
4. Conclusions The present research work focuses on the study of microhardness and residual stresses obtained after diamond burnishing of 17-4 PH SS under cryogenic, MQL and dry environments. The results of these studies are summarized as follows: Maximum microhardness was formed just beneath the diamond burnished surface. As the depth from the surface increases, the microhardness gradually decreases. The percentage of improvement observed in the cryogenic environment was 4%, 6% at a corresponding burnishing depth of 0.2 mm. Similarly, an improvement 5%, 8% respectively was observed at a burnishing depth of 0.4 mm and 7% and 11% respectively was recorded at a burnishing depth of 0.6 mm compared to MQL and dry environments.
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Compressive residual stresses were formed on the surface layer after diamond burnishing. Maximum of -352 MPa, -282 MPa and -195 MPa were recorded for cryogenic, MQL and dry environments respectively at a burnishing depth of 0.6 mm. It has been experimentally proved that cryogenic environment is better for diamond burnishing of 17-4 PH SS in order to obtain improved surface integrity.
References [1] Mohanty, A., Gangopadhyay, S. and Thakur, A., 2016. On applicability of multilayer coated tool in dry machining of aerospace grade stainless steel. Materials and Manufacturing Processes, 31(7), pp.869-879. [2] Wang, Z., Jiang, C., Gan, X., Chen, Y. and Ji, V., 2011. Influence of shot peening on the fatigue life of laser hardened 17-4PH steel. International Journal of Fatigue, 33(4), pp.549-556. [3] Shaw, M. C. 1984, Metal Cutting Principles, Clarendon Press, Oxford Science Publication, UK. [4] Chomienne, V., Valiorgue, F., Rech, J. and Verdu, C., 2016. Influence of ball burnishing on residual stress profile of a 15-5PH stainless steel. CIRP Journal of Manufacturing Science and Technology, 13, pp.90-96. [5] Nestler, A. and Schubert, A., 2015. Effect of machining parameters on surface properties in slide diamond burnishing of aluminium matrix composites. Materials Today: Proceedings, 2, pp.S156-S161. [6] Babu, P.R., Ankamma, K., Prasad, T.S., Raju, A.V.S. and Prasad, N.E., 2011. Effects of Burnishing Parameters on the Surface Characteristics, Microstructure and Microhardness in EN Series Steels. Transactions of the Indian Institute of Metals, 64(6), pp.565-573. [7] El-Axir, M.H., 2000. An investigation into roller burnishing. International Journal of Machine Tools and Manufacture, 40(11), pp.16031617. [8] Yang, S., Umbrello, D., Dillon, O.W., Puleo, D.A. and Jawahir, I.S., 2015. Cryogenic cooling effect on surface and subsurface microstructural modifications in burnishing of Co–Cr–Mo biomaterial. Journal of Materials Processing Technology, 217, pp.211-221. [9] Pereira, O., Rodríguez, A., Fernández-Abia, A.I., Barreiro, J. and de Lacalle, L.L., 2016. Cryogenic and minimum quantity lubrication for an eco-efficiency turning of AISI 304. Journal of Cleaner Production, 139, pp.440-449. [10] Caudill, J., Huang, B., Arvin, C., Schoop, J., Meyer, K. and Jawahir, I.S., 2014. Enhancing the surface integrity of Ti-6Al-4V alloy through cryogenic burnishing. Procedia CIRP, 13, pp.243-248. [11] Yu, X. and Wang, L., 1999. Effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool. International Journal of Machine Tools and Manufacture, 39(3), pp.459-469. [12] Hassan, A.M., Al-Jalil, H.F. and Ebied, A.A., 1998. Burnishing force and number of ball passes for the optimum surface finish of brass components. Journal of Materials Processing Technology, 83(1), pp.176-179. [13] Rodríguez, A., de Lacalle, L.L., Celaya, A., Lamikiz, A. and Albizuri, J., 2012. Surface improvement of shafts by the deep ball-burnishing technique. Surface and Coatings Technology, 206(11), pp.2817-2824. [14] Mhaede, M., 2012. Influence of surface treatments on surface layer properties, fatigue and corrosion fatigue performance of AA7075 T73. Materials & Design, 41, pp.61-66. [15] Salahshoor, M. and Guo, Y.B., 2011. Surface integrity of biodegradable Magnesium–Calcium orthopedic implant by burnishing. Journal of the Mechanical Behavior of Biomedical Materials, 4(8), pp.1888-1904. [16] Zhang, T., Bugtai, N. and Marinescu, I.D., 2015. Burnishing of aerospace alloy: a theoretical–experimental approach. Journal of Manufacturing Systems, 37, pp.472-478. [17] Yuan, X., Sun, Y., Li, C. and Liu, W., 2017. Experimental investigation into the effect of low plasticity burnishing parameters on the surface integrity of TA2. International Journal of Advanced Manufacturing Technology, 88(1-4), pp.1089-1099. [18] Sun, Y., Huang, B., Puleo, D.A., Schoop, J. and Jawahir, I.S., 2016. Improved Surface Integrity from Cryogenic Machining of Ti-6Al-7Nb Alloy for Biomedical Applications. Procedia CIRP, 45, pp.63-66. [19] Bicek, M., Dumont, F., Courbon, C., Pusavec, F., Rech, J. and Kopac, J., 2012. Cryogenic machining as an alternative turning process of normalized and hardened AISI 52100 bearing steel. Journal of Materials Processing Technology, 212(12), pp.2609-2618.
ScienceDirect Materials Today: Proceedings 5 (2018) 18393–18399
www.materialstoday.com/proceedings
ICMPC_2018
Effect of Cryogenic Diamond Burnishing on Residual Stress and Microhardness of 17-4 PH Stainless Steel Sachin Ba*, Narendranath Sa, D Chakradharb A
department Of Mechanical Engineering, National Institute Of Technology, Karnataka, Surathkal, India B mechanical Engineering, Indian Institute Of Technology, Palakkad, Kerala, India
Abstract Diamond burnishing is a cold working process, which produces a work hardened and uniform surface by plastic deformation. The aim of the present work is to study the behavior of diamond burnishing on surface integrity of 17-4 precipitation hardenable stainless steel (PH-SS) under cryogenic, minimum quantity lubrication (MQL) and dry environments. Surface modification was achieved by the application of liquid nitrogen during diamond burnishing. The process parameters considered were speed, feed, burnishing depth and number of passes. Surface integrity characteristics such as microhardness and residual stresses were investigated after diamond burnishing under cryogenic, MQL and dry environments. In cryogenic diamond burnishing, the surface integrity characteristics of 17-4 PH stainless steel has been significantly improved when compared to MQL and dry environments. Maximum microhardness of 395 HV, 369 HV, and 357 HV respectively was observed under cryogenic, MQL and dry environment. The maximum residual stress of -352 MPa, -282 MPa and -195 MPa respectively were recorded for cryogenic, MQL and dry environment. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords: Diamond burnishing; 17-4 PH stainless steel; microhardness; residual stress.
1. Introduction Surface treatment techniques are one of the finest methods to improve the surface topography of the material by subjecting it to severe plastic deformation. Many secondary manufacturing techniques produce irregularities, voids,
* Corresponding author. Tel.: +91-990-052-9811 E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.
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flakes etc. To avoid these problems, chipless machining process such as burnishing is widely used in modern applications. In recent years, polymeric composite materials and titanium alloys have been replaced by 17-4 PH SS. It has an excellent corrosion resistance, ductility, high toughness, high impact and tensile strength properties thus it can be classified under difficult to cut materials [1]. One of the important application of 17-4 PH SS is in steam turbines. This material is also used in chemical industries, marine construction and power plants [2]. The high temperatures generated while finishing can be reduced with the application of a variety of coolants. Conventional coolants have been utilized by metal cutting industries to reduce the machining zone temperatures. Some of the coolants are hazardous to the health of the operator and also they cause environmental pollution when disposed of [3]. It has been revealed by Chomienne et al. [4] that the velocity is one of the parameters which will not affect the residual stresses profile of the work material. It has been suggested to use a single pass to get very good residual stress profile. Nestler and Schubert [5] reported that the surface roughness can be reduced along with the reduction in the voids formation. It was found that the burnishing feed is the parameter which has more influence on surface roughness. The mass of the component was reduced and performance was improved by using slide diamond burnishing process in AA2124. Babu et al. [6] performed experiments on three different steels with EN8, EN24, EN31 grades and found that as there is an increase in the magnitude of compressive residual stress and also burnishing depth was found to be more in EN31 when compared to other types of materials. El-Axir [7] proved that a better surface finish and hardness were observed at low burnishing feed. Improved micro-hardness has been observed when a high number of passes were used. For better surface finish it has been suggested to use a moderate number of passes and also maximum residual stress was observed at the surface and beneath the surface, it decreases with the increase in the depth. Yang et al. [8] have studied the effect of severe plastic deformation and cryogenic burnishing on the modifications of surface integrity of Co-Cr-Mo alloy by using a newly developed burnishing tool. It was observed that the temperature induced was significantly minimized by cryogenic treatment. Microstructural modifications and improved hardness have been observed after cryogenic burnishing. Pereira et al. [9] have performed turning operation on AISI 304 under the combined effect of minimum quantity lubrication and cryogenic environments. It has been realized that mixed effect of cryogenic and MQL was ecofriendly when compared to conventional wet machining. Caudill et al. [10] compared the effect of cryogenic, flood-cooled and dry burnishing on surface integrity of Ti-6Al-4V alloy by severe plastic deformation technique. It was revealed that in the severe plastic deformation layer which was produced by cryogenic environment showed improved grain structure, increased hardness, and improved surface finish. Yu and Wang [11] studied the effect of a polycrystalline diamond tool on the surface roughness of aluminium alloy. It was observed that the surface roughness of a burnished aluminium alloy has been gradually decreased. Hassan et al. [12] developed a mathematical model to correlate the effect of process parameters such as burnishing force and number of tool pass on ball burnishing of brass components. Optimization of process parameters was carried out by using response surface method. Many researchers have concentrated on the study of ball burnishing and roller burnishing to analyze the effect of process parameters on a variety of work materials. Till date, few researchers have concentrated on analyzing the surface integrity characteristics of diamond burnishing under different lubricants. This study mainly aims at analyzing the effect of process parameters such as speed, feed, burnishing depth and number of passes on the surface integrity characteristics such as residual stress and microhardness of a 17-4 PH SS under cryogenic, MQL and dry environments. The results reveal that after diamond burnishing, compressive residual stresses were formed under all the three environments and microhardness was observed to be maximum at the diamond burnished surface. 2. Materials and method The workpiece used for the present study is 17-4 PH SS. It is a martensitic stainless steel and categorized under difficult machine materials. Before diamond burnishing, 0.5 mm top surface layer of the workpiece was removed by turning process. Diamond burnishing was carried out under cryogenic, MQL and dry environments. In cryogenic environment liquid nitrogen (LN2) was sprayed at the tool and the workpiece interface. The experimental details are as presented in Table 1. The cryogenic and MQL setups are as depicted in Figure 1 and Figure 2 respectively. Figure 3 (a-c) shows the diamond burnishing zone of all the three environments. In the present investigation process parameters used are speed, feed, number of tool passes and burnishing depth. Burnishing depth was varied by
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keeping all other parameters constant as shown in Table 1. Vickers microhardness tester has been used to measure the microhardness at different depths from the burnished surface. X-Ray diffraction (XRD) has been used to study the residual stresses induced after diamond burnishing process. Table 1. Experimental information Workpiece material and dimensions Chemical composition of 17-4 PH stainless steel
17-4 PH SS round bar, (⌀ 32 mm x 150 mm)
Tool used Burnishing process parameters
Diamond burnishing tool (DB-200) Burnishing speed: 47 m/min Burnishing feed: 0.096 mm/rev Burnishing depth: 0.2, 0.4, 0.6 mm Number of passes: 2
Environments and coolants used Burnishing fluid supply
Cryogenic (LN2), MQL (Castrol oil), Dry (No coolant) Cryogenic cooling - compressed air: 4 kg/cm2, flow rate: 0.45 kg/min (through external nozzle)
(Ni-3.546 %, Cr-16.179 %, Cu-3.177 %, Mn-0.744 %, Si-0.360 %, C-0.042 %, P-0.028 %, S-0.011 %, Nb+Ta-0.356 % and Fe-Balance)
MQL cooling - compressed air: 4 kg/cm2, flow rate: 70 ml/h (through external nozzle) Nozzle diameter
cryogenic and MQL – ⌀ 1 mm
Fig. 1. Cryogenic diamond burnishing
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Fig. 2. MQL diamond burnishing
Fig. 3. Diamond burnishing zone at (a) cryogenic, (b) MQL and (c) Dry environments
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3. Results and Discussion 3.1. Effect of process parameters on microhardness It was revealed by [4,14] that the microhardness of the specimen decreases as the depth from the top surface increases. In the present study, similar results have been attained. The micro-hardness measurement was carried out from the top surface towards the bulk material for varying burnishing depth which is as shown in Figure 4. The measurement was carried out up to a depth of 160 µm from the burnished surface. It was observed that just beneath the burnished surface, the hardness will be more and as the depth from the surface increases the micro hardness reduces. Furthermore decrease in the depth from the surface causes reduction in the micro hardness and finally, it reaches the micro hardness of the bulk material. The average microhardness of the as-received material was found to be 340 HV. The reason for the reduction in the hardness can be explained by the fact that, less shearing observed between the tip of the tool and the workpiece surface. The surface just beneath the diamond burnished layer will have more hardness because of the work hardening process [7]. The highest micro hardness of 395 HV was observed for the cryogenic environment at a burnishing depth of 0.6 mm. Compared to dry diamond burnishing, MQL environment has provided a better micro-hardness result. The maximum value of 369 HV and 357 HV have been recorded for MQL and dry environments respectively at a burnishing depth of 0.6 mm. Therefore the percentage of microhardness improvement attained in the cryogenic environment was 7% and 11% respectively in contrast to MQL and dry environment. Similarly, in cryogenic environment percentage of improvement observed was 4%, 6% respectively at burnishing depth of 0.2 mm and 5%, 8% respectively at burnishing depth of 0.4 mm in contrast to MQL and dry environments. The improvement observed in the microhardness of cryogenic diamond burnished sample is due to the rapid cooling effect of the liquid nitrogen which minimizes the thermal softening effect. However, in MQL condition the thermal softening effect was minimized because of the presence of lubrication and in a dry environment because of the thermal softening effect the microhardness was observed to be decreased [18].
Fig. 4. Micro-hardness test result obtained at speed = 47 m/min, feed = 0.096 mm/rev, number of pass = 2 and burnishing depth (BD) = 0.2, 0.4, 0.6 mm
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3.2. Effect of process parameters on surface residual stress A study on residual stresses induced in diamond burnishing process plays a crucial role. After burnishing process, compressive residual stresses will be induced because of the plastic deformation of the material [5,13,15,16,17]. A similar trend has been observed in this study. In the present research work, the effect of diamond burnishing on residual stress was studied under cryogenic, MQL and dry environments. It was noticed that a large number of compressive residual stresses has been formed on the top surface layer after diamond burnishing. Compressive residual stresses are formed because of the plastic deformation produced on the surface layer after diamond burnishing [13]. Figure 5 shows the residual stress measurement carried out at speed = 47 m/min, feed = 0.096 mm/rev, number of pass = 2 and burnishing depth = 0.2, 0.4, 0.6 mm, under all the three environments. After diamond burnishing maximum compressive residual stress of -352 MPa, -282 MPa and -195 MPa respectively has been observed for cryogenic, MQL and dry environments on the burnished surface at burnishing depth of 0.6 mm. In a cryogenic environment, the percentage of improvement observed was 20% and 44% respectively in contrast to MQL and dry environments. Burnishing depth will have more impact in inducing the compressive residual stresses on the top surface layer. Increase in the burnishing depth causes more plastic deformation which results in improved surface residual stresses [16]. Cryogenic diamond burnishing reduces thermal stress inducement on the surface layer because of the cooling effect [19] when compared to MQL and dry environment. Hence an improved surface residual stress was observed in the cryogenic environment. Cryogenic MQL Dry
Residual stress (MPa)
-150 -200 -250 -300 -350 -400
0.2
0.3
0.4
0.5
0.6
Burnishing depth (mm)
Fig. 5. Residual stress obtained at speed = 47 m/min, feed = 0.096 mm/rev, number of pass = 2 and burnishing depth = 0.2, 0.4, 0.6 mm
4. Conclusions The present research work focuses on the study of microhardness and residual stresses obtained after diamond burnishing of 17-4 PH SS under cryogenic, MQL and dry environments. The results of these studies are summarized as follows: Maximum microhardness was formed just beneath the diamond burnished surface. As the depth from the surface increases, the microhardness gradually decreases. The percentage of improvement observed in the cryogenic environment was 4%, 6% at a corresponding burnishing depth of 0.2 mm. Similarly, an improvement 5%, 8% respectively was observed at a burnishing depth of 0.4 mm and 7% and 11% respectively was recorded at a burnishing depth of 0.6 mm compared to MQL and dry environments.
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Compressive residual stresses were formed on the surface layer after diamond burnishing. Maximum of -352 MPa, -282 MPa and -195 MPa were recorded for cryogenic, MQL and dry environments respectively at a burnishing depth of 0.6 mm. It has been experimentally proved that cryogenic environment is better for diamond burnishing of 17-4 PH SS in order to obtain improved surface integrity.
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