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The effects of cryogenic cooling on surface integrity in hard machining: A comparison with dry machining
CIRP Annals - Manufacturing Technology 61 (2012) 103–106
Contents lists available at SciVerse ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
The effects of cryogenic cooling on surface integrity in hard machining: A comparison with dry machining Domenico Umbrello a,c,*, Fabrizio Micari (1)b, I.S. Jawahir (1)c a
Department of Mechanical Engineering, University of Calabria, Rende, CS 87036, Italy Department of Chemical, Management, Computer Science and Mechanical Engineering, University of Palermo, Palermo, PA 90128, Italy c Institute for Sustainable Manufacturing (ISM), University of Kentucky, Lexington, KY 40506, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Cutting Surface integrity Cryogenic cooling
This paper presents results of an experimental study of cryogenic machining of hardened AISI 52100 steel, focusing on surface integrity. Experiments were performed under dry and cryogenic cooling conditions using CBN tools varying cutting speeds, workpiece hardness and tool geometry. Surface integrity parameters (surface roughness, white layer thickness, residual stresses, metallurgical conditions including grain size, phase transformation, etc.) were investigated to establish the effects of cryogenic cooling on the surface integrity of the machined component, and results were compared with those from dry hard machining. Overall, cryogenic cooling provides improved surface integrity leading to extended product life and performance. ß 2012 CIRP.
1. Introduction Traditionally and historically, manufacturing processes are planned and developed in order to achieve either maximum productivity or minimum cost. In contrast, present trends push manufacturers to develop new methodologies incorporating sustainability concepts [1]. Sustainable manufacturing processes are those which demonstrate improved environmental impact and energy and resource efficiency, generate minimum quantity of wastes, provide operational safety and personal health, while maintaining or improving the product quality [2]. In this context, hard machining has become a much more desirable finishmanufacturing process, compared to the traditional grinding process because of its ability to reduce production costs, increase productivity, and especially enhance product quality. However, there are several issues related to this process that require further investigation, and the major issue among these is the high temperatures at the tool-chip and tool-workpiece interfaces in conjunction with the plastic deformation, both strongly affecting the surface integrity and the quality of the machined product. In fact, the deformation process is concentrated in a very small zone, and the local high temperatures due to rapid heat generation have important consequences on the machined surface layer such as the microstructural alterations and white layer formation. Although, numerous studies have been conducted on the white layer formation in machining of hardened steels [3–7], only a few studies investigate the effects of cooling, particularly the minimum quantity of lubricant (MQL), on this affected layer [8,9]. The effects of cryogenic cooling on the surface integrity of the machined
* Corresponding author at: Department of Mechanical Engineering, University of Calabria, Rende, CS 87036, Italy. E-mail addresses: [email protected], [email protected] (D. Umbrello). 0007-8506/$ – see front matter ß 2012 CIRP. http://dx.doi.org/10.1016/j.cirp.2012.03.052
component have recently been shown [10], while no specific studies as yet reported the effectiveness of cryogenic cooling in hard machining. Therefore, the primary objective of this paper is to investigate the effects of cryogenic cooling on surface integrity (surface roughness, white layer thickness, residual stresses, grain size, etc.) in hard machining of AISI 52100 and compare the performance with dry machining at varying process parameters (cutting speed, workpiece hardness, tool edge conditions, etc.). 2. Experimental procedure 2.1. Cryogenic machining Cryogenic machining presents a method of cooling the cutting tool and/or workpiece during material removal processes. The coolant is usually nitrogen fluid (LN) that is liquefied by cooling to 196 8C. Nitrogen is a safe, non-combustible, and noncorrosive gas. In fact, 78% of the air we breathe is nitrogen. The liquid nitrogen in a cryogenic machining system quickly evaporates and returns to the atmosphere, leaving no residue to contaminate the workpiece, chips, machine tool, or operator, thus eliminating disposal costs. Additionally, cryogenic machining can be used to machine work materials at higher cutting speeds, and to achieve higher surface quality and better surface integrity, with increased machinability and reduced overall costs. Proven benefits of cryogenic machining are: improved process sustainability (cleaner and safer, environmentally-friendly processes providing no adverse health effects for personnel on the shop floor); increased material removal rate (MRR) with no increase in toolwear rates and tool change time, resulting in reduced over all costs via higher productivity;
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higher tool-life due to lower abrasion and chemical wear; improved product quality in machined parts through the elimination of mechanical and chemical degradation of machined surface; and enhanced surface integrity through controllable microstructural and phase changes with more favorable dynamic recrystallization and corrosion and wear resistance.
Table 1 Experimental test conditions for dry and cryogenic machining. Test number 1 vc [m/min] Hardness [HRC] Tool
75
2 150 61 1 Chamfered
3
4
5
250
250 54 1 Chamfered
250 54 1 Honed
2.2. Experimental set-up Dry and cryogenic orthogonal cutting tests were conducted on hardened 52100 steel disks using a MAZAK high speed CNC turning center, equipped with ICEFLYTM cryogenic fluid delivery system, with cubic boron nitride tools (Seco grade: CBN 100) with chamfered (ISO TNGN 110308S, chamfer = 208 0.1 mm) and honed geometries (ISO TNGN 110308E, hone radius = 0.015 mm), mounted on a CTFNR3225P11 tool holder (providing rake and clearance angles of 88 and 88, respectively). Disks were machined at varying cutting speeds, initial workpiece hardness and the tool shape, but at a fixed feed rate of 0.075 mm/rev, both in dry and cryogenic cooling conditions (Table 1); the cutting time for each test was 18–20 s to allow the machine to reach the mechanical and thermal steady-state conditions. In the machining tests, a very low, flank wear of 0.03–0.05 mm was observed on the utilized CBN tools, thus the influence of tool-wear was not investigated in this study. The cryogenic coolant was applied by a nozzle to the area of interest as shown in Fig. 1; it has been generally known that cryogenic cooling heavily influences the machining process along the primary, secondary and tertiary shear zones. After machining, samples of 5 mm 5 mm were sectioned by wire-EDM, then polished and etched for 5 s using 5% Nital solution to observe microstructural changes using a light optical microscope (1000) and a scanning electron microscope (SEM). Finally, major elements of surface integrity, such as surface roughness, residual stresses and grain size were taken into account. In particular, the surface roughness values of the machined workpieces were measured using a Zygo1 optical white light interferometry-based surface profilometer. The residual stress state in machined disks was analyzed by the X-ray diffraction technique (XRD) using the sin2c method [11]. The parameters used in the X-ray analysis are reported in [12]. Both axial and circumferential residual stresses were measured. To determine the in-depth residual stress profiles, successive layers of material were removed by electro-polishing, thus minimizing machining-induced residual stresses. Corrections to XRD data were made for the volume of material removed. XRD patterns of the processed surface were also collected on an X-ray diffractometer with Cu-Ka radiation (l = 1.54184 A˚, Ka1/ Ka2 = 0.5) from a source operated at 40 kV, and 40 mA for estimating the grain size in response to hard machining process by peak profile-fitting analysis [13,14]. This technique is a good alternative to transmission electron microscopy (TEM), and is based on peak broadening analysis, which is a consequence of high lattice strains and small grain size [15,16]. The 2u scans were carried out between 408 and 928 2u. The scan increment was 0.028; the corresponding acquisition time was varied.
also shows a mapped region called ‘‘turning replaces grinding’’ where cryogenic hard machining produced comparable Ra values with grinding. For both cooling conditions with chamfered tools (Tests 1–4), the mean surface roughness decreases with the increasing cutting speed and for samples at 54 HRC; in contrast, honed tools produce a worse surface roughness (Test 5), compared with chamfered tools (Test 4). 3.2. White layer Fig. 3 shows the experimental white layer thickness produced for all five tests. The white layer is less than 1 mm in cryogenic machining, while it ranges between 5 and 8 mm in dry machining as can be observed in optical micrographs and SEM images of the machined surfaces for Test 3 shown in Fig. 4. Also, the white layer observed in dry machining increases with increased cutting speed and the initial workpiece hardness, since the higher temperatures generated in the machining zone should lead to localized thermal softening of the workpiece material, as
Fig. 1. Experimental set-up for orthogonal cutting tests with cryogenic cooling system and nozzle position for cryogenic coolant delivery.
Fig. 2. Mean surface roughness on machined samples under dry and cryogenic cooling conditions for all five test conditions.
3. Experimental results and discussion 3.1. Surface roughness The surface roughness values, Ra, of the machined sample were measured three times for each set of cutting process parameters and cooling conditions to evaluate the characteristic of the machined surface (Fig. 2), and averaged to obtain mean values. The obtained Ra measurements reflect the surface quality in machining with cryogenic coolant, and were found to be consistently superior to that obtained in dry machining. Fig. 2
Fig. 3. Experimentally obtained white layer thickness.
D. Umbrello et al. / CIRP Annals - Manufacturing Technology 61 (2012) 103–106
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well as a greater depth below the machined surface, where the temperature exceeds the austenitizing temperature for subsequent quenching through self-cooling. In contrast, white layer thickness decreases when honed tools were utilized for dry machining instead of chamfered tools. Similar trends are also seen for cryogenic machining, although the differences on white layer thickness are less noticeable, since the white layer depth in cryogenic cooling is much smaller compared with dry machining.
particular, it can be noted, as general trend, that the compressive areas obtained in dry machining are higher than those observed in cryogenic machining. Specifically, under dry conditions, it becomes larger (i.e., deeper profile) along the axial direction and when cutting speed increases. In contrast, the compressive area measured under cryogenic cooling machining conditions is slightly affected by the cutting speed (it increases with increasing of the cutting speed), while it does not show any significant difference with regard to the directions. Finally, a combination of lower workpiece initial hardness and the use of honed tools strongly affect the compressive area (i.e., smaller compressive area). However, since the application of cryogenic coolant provides the benefit of smaller white layer thickness, the consequence is that compressive area of components machined under cryogenic condition can be higher than the one obtained in dry machining after the post removal operation.
3.3. Residual stresses
3.4. Grain size
Fig. 5 shows the values of both the surface and the maximum compressive residual stresses along the axial direction for all the investigated cases. These stresses are always compressive in both, dry and cryogenic machining. Also, they becomes thicker (i.e., deeper profile) as cutting speed increases, although cryogenic cooling produces smaller values, in magnitude, than what is observed in dry condition. In contrast, the axial residual stresses become smaller with lower initial workpiece hardness and when using a honed tool. Similar trends are also noted for residual stresses along the hoop direction (Fig. 6). The reason for the higher compressive values for surface and the maximum compressive residual stresses in dry machining can be attributed to the higher hardness values observed on the machined surface and the sub-surface due to higher generation of white layer characterized by an untempered martensitic structure [12]. Another factor of cardinal interest, related to the fatigue life of any machined component, is the compressive stress area, which represents the compressive portion of the residual stress profile. Fig. 7 shows the effects of the cooling conditions and process parameters on both, the axial and hoop compressive areas. In
Finally, the grain size of each machined surface has also been estimated (Fig. 8) by peak profile-fitting technique applied on acquired XRD patterns in all investigated cases, where the recrystallization occurs. More precisely, all the examined samples presented a refinement of size, measured according to the mean grain diameter on the machined surface, and compared with the initial grain size the bulk (which averages at 15 mm). This clearly shows that the cooling conditions can influence the final microstructure of the machined product. In fact, the grain size becomes smaller when higher cutting speeds are utilized, while it increases using honed tools and samples with lower hardness. However, what is more remarkable is that cryogenic conditions help to keep the superficial grain smaller after the recrystallization phase.
Fig. 4. White layer in dry (a) an cryogenic machining (b): Test 3.
Fig. 5. Effect of dry and cryogenic cooling conditions on surface and compressive residual stresses along the axial direction.
Fig. 6. Effect of dry and cryogenic cooling conditions on surface and compressive residual stresses along the hoop direction.
4. Evaluation of hard machining performance Fig. 9 shows the overall evaluation of hard machining performance carried out under dry and cryogenic cooling conditions. In these charts, each parameter has been equally
Fig. 7. Compressive area at varying initial workpiece hardness, cutting speed, tool shape and cooling condition.
Fig. 8. Experimental grain size in all test conditions.
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simple method based on the total area under the curve for each test was considered. Based on this criterion, and the consideration of the desired product performance, the best results for surface integrity are achieved for Test 3 in both, dry and cryogenic cooling conditions. The differences can become more relevant after post removal operation of the white layer where the residual stress profiles and the compressive area can be similar or, in some circumstances, better in hard machining assisted by cryogenic coolant. Therefore, overall, it is evident as cryogenic machining offers potential benefit for surface integrity enhancement for improved product life. 5. Conclusions Experimental observations reported in this study suggest that the use of cryogenic coolant in machining of hardened AISI 52100 steels significantly affects the surface integrity. In particular, cryogenic cooling conditions limit the white layer thickness and offer better surface roughness. In contrast, dry machining offers better performance on residual stress profiles and, therefore would contribute to improved fatigue life, although it produces a thicker white layer which is detrimental for the product’s performance and relative cost (necessity of secondary removal operation). Overall, this study demonstrates that hard machining under cryogenic cooling has the potential for surface integrity enhancement for improved product life.
References
Fig. 9. Overall evaluation of surface integrity in dry (a) and (b) cryogenic hard machining at varying process parameters.
weighted, and its disposition was made by considering that the values on the outer layers offer improved and better surface integrity. In dry machining (Fig. 9(a)), higher cutting speeds and initial workpiece hardness provide major benefits in term of the parameters related to the fatigue life (i.e., deeper surface residual stresses and higher compressive area) and productivity (material removal rate). Similar tendency is observed when machining operation is carried out with cryogenic machining too (Fig. 9(b)) although, the values related to the residual stress profile are less deeper than those observed in dry machining. Also, the grain size becomes smaller for the cryogenic machining, which makes it more desirable for use than dry machining. In contrast, for above cutting process parameters, other factors related to the surface integrity (white layer thickness and mean surface roughness) are not so good as they are for other cutting speeds, initial workpiece hardness and tool edge preparation. Once again, this trend is similar for both dry machining and cryogenic machining, although both white layer thickness and surface roughness are much better for cryogenic machining. Moreover, when parameters related specific targets are considered, other cutting process variables offer the best trade-offs. Thus, in order to evaluate the performance, it is necessary to define a criterion, which can consider all the aspects and, consequently, would optimize the benefits and the drawbacks for different combinations of the process parameters. Even though, several optimization algorithms are available for comprehensive analysis, in this work, by assuming that each factor is equally important, a
[1] Jovane F, Yoshikawa H, Alting L, Boer CR, Westkamper E, Williams D, Tseng M, Seliger G, Paci AM (2008) The Incoming Global Technological and Industrial Revolution Towards Competitive Sustainable Manufacturing. CIRP Annals – Manufacturing Technology 57(2):641–659. [2] Jayal AD, Badurdeen F, Dillon Jr OW, Jawahir IS (2010) Sustainable Manufacturing: Modeling and Optimization Challenges at the Product, Process and System Levels. CIRP Journal of Manufacturing Science and Technology 2(3):144– 152. [3] Matsumoto Y, Hashimoto F, Lahoti G (1999) Surface Integrity Generated by Precision Hard Turning. CIRP Annals – Manufacturing Technology 48(1):59–62. [4] Barry J, Byrne G (2002) TEM study on the surface white layer in two turned hardened steels. Journal of Materials Science and Engineering A 25:356–364. [5] Ramesh A, Melkote SN, Allard LF, Riester L, Watkins TR (2005) Analysis of White Layers Formed in Hard Turning of AISI 52100 Steel. Materials Science and Engineering A 390:88–97. [6] Umbrello D, Filice L (2009) Improving Surface Integrity in Orthogonal Machining of Hardened AISI 52100 Steel by Modelling White and Dark Layers Formation. CIRP Annals – Manufacturing Technology 58(1):73–76. [7] Ko¨nig W, Klinger M, Link R (1990) Machining Hard Materials with Geometrically Defined Cutting Edges – Field of Applications and Limitations. CIRP Annals – Manufacturing Technology 57(1):61–64. [8] Varadarajan AS, Philip PK, Ramamoorthy B (2002) Investigations on Hard Turning with Minimal Cutting Fluid Application (HTMF) and its Comparison with Dry and Wet Turning. International Journal of Machine Tools & Manufacture 42:193–200. [9] Vikram Kumar CHR, Ramamoorthy B (2007) Performance of Coated Tools during Hard Turning under Minimum Fluid Application. Journal of Materials Processing Technology 185:210–216. [10] Jawahir IS, Brinksmeier E, M’Saoubi R, Aspinwall DK, Outeiro JC, Meyer D, Umbrello D, Jayal AD (2011) Surface Integrity in Material Removal Processes: Recent Advances. CIRP Annals – Manufacturing Technology 60(2):603–626. [11] Noyan IC, Cohen JB (1987) Residual Stress – Measurement by Diffraction and Interpretation, Springer, New York. [12] Umbrello D (2011) Influence of Material Microstructure Changes on Surface Integrity in Hard Machining of AISI 52100 Steel. International Journal of Advanced Manufacturing Technology 54(9):887–898. [13] Warren BE (1990) X-ray Diffraction, Dover Pub, New York. [14] Klug HP, Alexander LE (1974) X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed. John Wiley & Sons. [15] Unga´r T (2004) Microstructural parameters from X-ray diffraction peak broadening. Scripta Materialia 51:777–781. [16] Akcan S, Shah S, Moylan SP, Chhabra PN, Chandrasekar S, Yang HTY (2002) Formation of White Layers in Steels by Machining and their Characteristics. Metallurgical and Materials Transactions A 33A:1245–1254.
Contents lists available at SciVerse ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
The effects of cryogenic cooling on surface integrity in hard machining: A comparison with dry machining Domenico Umbrello a,c,*, Fabrizio Micari (1)b, I.S. Jawahir (1)c a
Department of Mechanical Engineering, University of Calabria, Rende, CS 87036, Italy Department of Chemical, Management, Computer Science and Mechanical Engineering, University of Palermo, Palermo, PA 90128, Italy c Institute for Sustainable Manufacturing (ISM), University of Kentucky, Lexington, KY 40506, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Cutting Surface integrity Cryogenic cooling
This paper presents results of an experimental study of cryogenic machining of hardened AISI 52100 steel, focusing on surface integrity. Experiments were performed under dry and cryogenic cooling conditions using CBN tools varying cutting speeds, workpiece hardness and tool geometry. Surface integrity parameters (surface roughness, white layer thickness, residual stresses, metallurgical conditions including grain size, phase transformation, etc.) were investigated to establish the effects of cryogenic cooling on the surface integrity of the machined component, and results were compared with those from dry hard machining. Overall, cryogenic cooling provides improved surface integrity leading to extended product life and performance. ß 2012 CIRP.
1. Introduction Traditionally and historically, manufacturing processes are planned and developed in order to achieve either maximum productivity or minimum cost. In contrast, present trends push manufacturers to develop new methodologies incorporating sustainability concepts [1]. Sustainable manufacturing processes are those which demonstrate improved environmental impact and energy and resource efficiency, generate minimum quantity of wastes, provide operational safety and personal health, while maintaining or improving the product quality [2]. In this context, hard machining has become a much more desirable finishmanufacturing process, compared to the traditional grinding process because of its ability to reduce production costs, increase productivity, and especially enhance product quality. However, there are several issues related to this process that require further investigation, and the major issue among these is the high temperatures at the tool-chip and tool-workpiece interfaces in conjunction with the plastic deformation, both strongly affecting the surface integrity and the quality of the machined product. In fact, the deformation process is concentrated in a very small zone, and the local high temperatures due to rapid heat generation have important consequences on the machined surface layer such as the microstructural alterations and white layer formation. Although, numerous studies have been conducted on the white layer formation in machining of hardened steels [3–7], only a few studies investigate the effects of cooling, particularly the minimum quantity of lubricant (MQL), on this affected layer [8,9]. The effects of cryogenic cooling on the surface integrity of the machined
* Corresponding author at: Department of Mechanical Engineering, University of Calabria, Rende, CS 87036, Italy. E-mail addresses: [email protected], [email protected] (D. Umbrello). 0007-8506/$ – see front matter ß 2012 CIRP. http://dx.doi.org/10.1016/j.cirp.2012.03.052
component have recently been shown [10], while no specific studies as yet reported the effectiveness of cryogenic cooling in hard machining. Therefore, the primary objective of this paper is to investigate the effects of cryogenic cooling on surface integrity (surface roughness, white layer thickness, residual stresses, grain size, etc.) in hard machining of AISI 52100 and compare the performance with dry machining at varying process parameters (cutting speed, workpiece hardness, tool edge conditions, etc.). 2. Experimental procedure 2.1. Cryogenic machining Cryogenic machining presents a method of cooling the cutting tool and/or workpiece during material removal processes. The coolant is usually nitrogen fluid (LN) that is liquefied by cooling to 196 8C. Nitrogen is a safe, non-combustible, and noncorrosive gas. In fact, 78% of the air we breathe is nitrogen. The liquid nitrogen in a cryogenic machining system quickly evaporates and returns to the atmosphere, leaving no residue to contaminate the workpiece, chips, machine tool, or operator, thus eliminating disposal costs. Additionally, cryogenic machining can be used to machine work materials at higher cutting speeds, and to achieve higher surface quality and better surface integrity, with increased machinability and reduced overall costs. Proven benefits of cryogenic machining are: improved process sustainability (cleaner and safer, environmentally-friendly processes providing no adverse health effects for personnel on the shop floor); increased material removal rate (MRR) with no increase in toolwear rates and tool change time, resulting in reduced over all costs via higher productivity;
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higher tool-life due to lower abrasion and chemical wear; improved product quality in machined parts through the elimination of mechanical and chemical degradation of machined surface; and enhanced surface integrity through controllable microstructural and phase changes with more favorable dynamic recrystallization and corrosion and wear resistance.
Table 1 Experimental test conditions for dry and cryogenic machining. Test number 1 vc [m/min] Hardness [HRC] Tool
75
2 150 61 1 Chamfered
3
4
5
250
250 54 1 Chamfered
250 54 1 Honed
2.2. Experimental set-up Dry and cryogenic orthogonal cutting tests were conducted on hardened 52100 steel disks using a MAZAK high speed CNC turning center, equipped with ICEFLYTM cryogenic fluid delivery system, with cubic boron nitride tools (Seco grade: CBN 100) with chamfered (ISO TNGN 110308S, chamfer = 208 0.1 mm) and honed geometries (ISO TNGN 110308E, hone radius = 0.015 mm), mounted on a CTFNR3225P11 tool holder (providing rake and clearance angles of 88 and 88, respectively). Disks were machined at varying cutting speeds, initial workpiece hardness and the tool shape, but at a fixed feed rate of 0.075 mm/rev, both in dry and cryogenic cooling conditions (Table 1); the cutting time for each test was 18–20 s to allow the machine to reach the mechanical and thermal steady-state conditions. In the machining tests, a very low, flank wear of 0.03–0.05 mm was observed on the utilized CBN tools, thus the influence of tool-wear was not investigated in this study. The cryogenic coolant was applied by a nozzle to the area of interest as shown in Fig. 1; it has been generally known that cryogenic cooling heavily influences the machining process along the primary, secondary and tertiary shear zones. After machining, samples of 5 mm 5 mm were sectioned by wire-EDM, then polished and etched for 5 s using 5% Nital solution to observe microstructural changes using a light optical microscope (1000) and a scanning electron microscope (SEM). Finally, major elements of surface integrity, such as surface roughness, residual stresses and grain size were taken into account. In particular, the surface roughness values of the machined workpieces were measured using a Zygo1 optical white light interferometry-based surface profilometer. The residual stress state in machined disks was analyzed by the X-ray diffraction technique (XRD) using the sin2c method [11]. The parameters used in the X-ray analysis are reported in [12]. Both axial and circumferential residual stresses were measured. To determine the in-depth residual stress profiles, successive layers of material were removed by electro-polishing, thus minimizing machining-induced residual stresses. Corrections to XRD data were made for the volume of material removed. XRD patterns of the processed surface were also collected on an X-ray diffractometer with Cu-Ka radiation (l = 1.54184 A˚, Ka1/ Ka2 = 0.5) from a source operated at 40 kV, and 40 mA for estimating the grain size in response to hard machining process by peak profile-fitting analysis [13,14]. This technique is a good alternative to transmission electron microscopy (TEM), and is based on peak broadening analysis, which is a consequence of high lattice strains and small grain size [15,16]. The 2u scans were carried out between 408 and 928 2u. The scan increment was 0.028; the corresponding acquisition time was varied.
also shows a mapped region called ‘‘turning replaces grinding’’ where cryogenic hard machining produced comparable Ra values with grinding. For both cooling conditions with chamfered tools (Tests 1–4), the mean surface roughness decreases with the increasing cutting speed and for samples at 54 HRC; in contrast, honed tools produce a worse surface roughness (Test 5), compared with chamfered tools (Test 4). 3.2. White layer Fig. 3 shows the experimental white layer thickness produced for all five tests. The white layer is less than 1 mm in cryogenic machining, while it ranges between 5 and 8 mm in dry machining as can be observed in optical micrographs and SEM images of the machined surfaces for Test 3 shown in Fig. 4. Also, the white layer observed in dry machining increases with increased cutting speed and the initial workpiece hardness, since the higher temperatures generated in the machining zone should lead to localized thermal softening of the workpiece material, as
Fig. 1. Experimental set-up for orthogonal cutting tests with cryogenic cooling system and nozzle position for cryogenic coolant delivery.
Fig. 2. Mean surface roughness on machined samples under dry and cryogenic cooling conditions for all five test conditions.
3. Experimental results and discussion 3.1. Surface roughness The surface roughness values, Ra, of the machined sample were measured three times for each set of cutting process parameters and cooling conditions to evaluate the characteristic of the machined surface (Fig. 2), and averaged to obtain mean values. The obtained Ra measurements reflect the surface quality in machining with cryogenic coolant, and were found to be consistently superior to that obtained in dry machining. Fig. 2
Fig. 3. Experimentally obtained white layer thickness.
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well as a greater depth below the machined surface, where the temperature exceeds the austenitizing temperature for subsequent quenching through self-cooling. In contrast, white layer thickness decreases when honed tools were utilized for dry machining instead of chamfered tools. Similar trends are also seen for cryogenic machining, although the differences on white layer thickness are less noticeable, since the white layer depth in cryogenic cooling is much smaller compared with dry machining.
particular, it can be noted, as general trend, that the compressive areas obtained in dry machining are higher than those observed in cryogenic machining. Specifically, under dry conditions, it becomes larger (i.e., deeper profile) along the axial direction and when cutting speed increases. In contrast, the compressive area measured under cryogenic cooling machining conditions is slightly affected by the cutting speed (it increases with increasing of the cutting speed), while it does not show any significant difference with regard to the directions. Finally, a combination of lower workpiece initial hardness and the use of honed tools strongly affect the compressive area (i.e., smaller compressive area). However, since the application of cryogenic coolant provides the benefit of smaller white layer thickness, the consequence is that compressive area of components machined under cryogenic condition can be higher than the one obtained in dry machining after the post removal operation.
3.3. Residual stresses
3.4. Grain size
Fig. 5 shows the values of both the surface and the maximum compressive residual stresses along the axial direction for all the investigated cases. These stresses are always compressive in both, dry and cryogenic machining. Also, they becomes thicker (i.e., deeper profile) as cutting speed increases, although cryogenic cooling produces smaller values, in magnitude, than what is observed in dry condition. In contrast, the axial residual stresses become smaller with lower initial workpiece hardness and when using a honed tool. Similar trends are also noted for residual stresses along the hoop direction (Fig. 6). The reason for the higher compressive values for surface and the maximum compressive residual stresses in dry machining can be attributed to the higher hardness values observed on the machined surface and the sub-surface due to higher generation of white layer characterized by an untempered martensitic structure [12]. Another factor of cardinal interest, related to the fatigue life of any machined component, is the compressive stress area, which represents the compressive portion of the residual stress profile. Fig. 7 shows the effects of the cooling conditions and process parameters on both, the axial and hoop compressive areas. In
Finally, the grain size of each machined surface has also been estimated (Fig. 8) by peak profile-fitting technique applied on acquired XRD patterns in all investigated cases, where the recrystallization occurs. More precisely, all the examined samples presented a refinement of size, measured according to the mean grain diameter on the machined surface, and compared with the initial grain size the bulk (which averages at 15 mm). This clearly shows that the cooling conditions can influence the final microstructure of the machined product. In fact, the grain size becomes smaller when higher cutting speeds are utilized, while it increases using honed tools and samples with lower hardness. However, what is more remarkable is that cryogenic conditions help to keep the superficial grain smaller after the recrystallization phase.
Fig. 4. White layer in dry (a) an cryogenic machining (b): Test 3.
Fig. 5. Effect of dry and cryogenic cooling conditions on surface and compressive residual stresses along the axial direction.
Fig. 6. Effect of dry and cryogenic cooling conditions on surface and compressive residual stresses along the hoop direction.
4. Evaluation of hard machining performance Fig. 9 shows the overall evaluation of hard machining performance carried out under dry and cryogenic cooling conditions. In these charts, each parameter has been equally
Fig. 7. Compressive area at varying initial workpiece hardness, cutting speed, tool shape and cooling condition.
Fig. 8. Experimental grain size in all test conditions.
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simple method based on the total area under the curve for each test was considered. Based on this criterion, and the consideration of the desired product performance, the best results for surface integrity are achieved for Test 3 in both, dry and cryogenic cooling conditions. The differences can become more relevant after post removal operation of the white layer where the residual stress profiles and the compressive area can be similar or, in some circumstances, better in hard machining assisted by cryogenic coolant. Therefore, overall, it is evident as cryogenic machining offers potential benefit for surface integrity enhancement for improved product life. 5. Conclusions Experimental observations reported in this study suggest that the use of cryogenic coolant in machining of hardened AISI 52100 steels significantly affects the surface integrity. In particular, cryogenic cooling conditions limit the white layer thickness and offer better surface roughness. In contrast, dry machining offers better performance on residual stress profiles and, therefore would contribute to improved fatigue life, although it produces a thicker white layer which is detrimental for the product’s performance and relative cost (necessity of secondary removal operation). Overall, this study demonstrates that hard machining under cryogenic cooling has the potential for surface integrity enhancement for improved product life.
References
Fig. 9. Overall evaluation of surface integrity in dry (a) and (b) cryogenic hard machining at varying process parameters.
weighted, and its disposition was made by considering that the values on the outer layers offer improved and better surface integrity. In dry machining (Fig. 9(a)), higher cutting speeds and initial workpiece hardness provide major benefits in term of the parameters related to the fatigue life (i.e., deeper surface residual stresses and higher compressive area) and productivity (material removal rate). Similar tendency is observed when machining operation is carried out with cryogenic machining too (Fig. 9(b)) although, the values related to the residual stress profile are less deeper than those observed in dry machining. Also, the grain size becomes smaller for the cryogenic machining, which makes it more desirable for use than dry machining. In contrast, for above cutting process parameters, other factors related to the surface integrity (white layer thickness and mean surface roughness) are not so good as they are for other cutting speeds, initial workpiece hardness and tool edge preparation. Once again, this trend is similar for both dry machining and cryogenic machining, although both white layer thickness and surface roughness are much better for cryogenic machining. Moreover, when parameters related specific targets are considered, other cutting process variables offer the best trade-offs. Thus, in order to evaluate the performance, it is necessary to define a criterion, which can consider all the aspects and, consequently, would optimize the benefits and the drawbacks for different combinations of the process parameters. Even though, several optimization algorithms are available for comprehensive analysis, in this work, by assuming that each factor is equally important, a
[1] Jovane F, Yoshikawa H, Alting L, Boer CR, Westkamper E, Williams D, Tseng M, Seliger G, Paci AM (2008) The Incoming Global Technological and Industrial Revolution Towards Competitive Sustainable Manufacturing. CIRP Annals – Manufacturing Technology 57(2):641–659. [2] Jayal AD, Badurdeen F, Dillon Jr OW, Jawahir IS (2010) Sustainable Manufacturing: Modeling and Optimization Challenges at the Product, Process and System Levels. CIRP Journal of Manufacturing Science and Technology 2(3):144– 152. [3] Matsumoto Y, Hashimoto F, Lahoti G (1999) Surface Integrity Generated by Precision Hard Turning. CIRP Annals – Manufacturing Technology 48(1):59–62. [4] Barry J, Byrne G (2002) TEM study on the surface white layer in two turned hardened steels. Journal of Materials Science and Engineering A 25:356–364. [5] Ramesh A, Melkote SN, Allard LF, Riester L, Watkins TR (2005) Analysis of White Layers Formed in Hard Turning of AISI 52100 Steel. Materials Science and Engineering A 390:88–97. [6] Umbrello D, Filice L (2009) Improving Surface Integrity in Orthogonal Machining of Hardened AISI 52100 Steel by Modelling White and Dark Layers Formation. CIRP Annals – Manufacturing Technology 58(1):73–76. [7] Ko¨nig W, Klinger M, Link R (1990) Machining Hard Materials with Geometrically Defined Cutting Edges – Field of Applications and Limitations. CIRP Annals – Manufacturing Technology 57(1):61–64. [8] Varadarajan AS, Philip PK, Ramamoorthy B (2002) Investigations on Hard Turning with Minimal Cutting Fluid Application (HTMF) and its Comparison with Dry and Wet Turning. International Journal of Machine Tools & Manufacture 42:193–200. [9] Vikram Kumar CHR, Ramamoorthy B (2007) Performance of Coated Tools during Hard Turning under Minimum Fluid Application. Journal of Materials Processing Technology 185:210–216. [10] Jawahir IS, Brinksmeier E, M’Saoubi R, Aspinwall DK, Outeiro JC, Meyer D, Umbrello D, Jayal AD (2011) Surface Integrity in Material Removal Processes: Recent Advances. CIRP Annals – Manufacturing Technology 60(2):603–626. [11] Noyan IC, Cohen JB (1987) Residual Stress – Measurement by Diffraction and Interpretation, Springer, New York. [12] Umbrello D (2011) Influence of Material Microstructure Changes on Surface Integrity in Hard Machining of AISI 52100 Steel. International Journal of Advanced Manufacturing Technology 54(9):887–898. [13] Warren BE (1990) X-ray Diffraction, Dover Pub, New York. [14] Klug HP, Alexander LE (1974) X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed. John Wiley & Sons. [15] Unga´r T (2004) Microstructural parameters from X-ray diffraction peak broadening. Scripta Materialia 51:777–781. [16] Akcan S, Shah S, Moylan SP, Chhabra PN, Chandrasekar S, Yang HTY (2002) Formation of White Layers in Steels by Machining and their Characteristics. Metallurgical and Materials Transactions A 33A:1245–1254.