The productivity of any machining operation is tied to several interconnected factors. However, one factor that plays a critical role in defining productivity is the thermo-mechanical interaction between the tool and workpiece, which dictates tool change intervals and the duration of undesired machining downtime.
The tool-workpiece interaction can be studied from different standpoints. From the process point of view, measurable parameters such as force, power, vibration and tool wear can be analyzed to study the performance of a cutting tool and on the workpiece material. However, solely measuring these parameters may not provide enough information on the surface integrity characteristics of the workpiece. Surface roughness, dimensional tolerances, microstructural change and residual stress are some of the surface integrity parameters in machining operations that cannot be easily inferred from the measurable process parameters. Thus, an offline study is required to establish this relationship.
In the bearing manufacturing industry, hard turning is a material removal strategy employed as an alternative to grinding. It has gained significant attention over the past two decades due to its higher 
The productivity of any machining operation is tied to several interconnected factors. However, one factor that plays a critical role in defining productivity is the thermo-mechanical interaction between the tool and workpiece, which dictates tool change intervals and the duration of undesired machining downtime.
The tool-workpiece interaction can be studied from different standpoints. From the process point of view, measurable parameters such as force, power, vibration and tool wear can be analyzed to study the performance of a cutting tool and on the workpiece material. However, solely measuring these parameters may not provide enough information on the surface integrity characteristics of the workpiece. Surface roughness, dimensional tolerances, microstructural change and residual stress are some of the surface integrity parameters in machining operations that cannot be easily inferred from the measurable process parameters. Thus, an offline study is required to establish this relationship.
In the bearing manufacturing industry, hard turning is a material removal strategy employed as an alternative to grinding. It has gained significant attention over the past two decades due to its higher 
The productivity of any machining operation is tied to several interconnected factors. However, one factor that plays a critical role in defining productivity is the thermo-mechanical interaction between the tool and workpiece, which dictates tool change intervals and the duration of undesired machining downtime.
The tool-workpiece interaction can be studied from different standpoints. From the process point of view, measurable parameters such as force, power, vibration and tool wear can be analyzed to study the performance of a cutting tool and on the workpiece material. However, solely measuring these parameters may not provide enough information on the surface integrity characteristics of the workpiece. Surface roughness, dimensional tolerances, microstructural change and residual stress are some of the surface integrity parameters in machining operations that cannot be easily inferred from the measurable process parameters. Thus, an offline study is required to establish this relationship.
In the bearing manufacturing industry, hard turning is a material removal strategy employed as an alternative to grinding. It has gained significant attention over the past two decades due to its higher 
The thermomechanical interaction between the cutting tool and the workpiece in any material removal process is a key factor in defining the efficiency and productivity of the process. It is especially important in cutting hard-to-machine materials such as titanium-and nickel-based alloys or hardened steels. The productivity of the process can be defined in terms of machining time, intervals of tool change (due to tool wear) and surface integrity characteristics such as dimensional tolerances, surface roughness and the machining-affected zone. The objective of this work is to conduct a comprehensive study of the performance of three different industrial-grade CBN cutting inserts during hard turning of AISI 4320 case-carburized steel. The CBN inserts all had the same geometry but used different edge preparation and coating technologies. To quantify tool performance, the cutting forces and tool wear were measured at consistent intervals and the surface integrity of the workpiece was characterized in terms of surface roughness, white and dark layer depths, micro-hardness and residual stresses. Results show that the stability of the cutting edge on two of the tested inserts plays an important role in determining tool life (where 60% higher tool life was achieved compared to the other tested insert with unstable cutting edge), cutting force stability and the formation of compressive residual stresses on the surface of the turned workpiece
The productivity of any machining operation is tied to several interconnected factors. However, one factor that plays a critical role in defining productivity is the thermo-mechanical interaction between the tool and workpiece, which dictates tool change intervals and the duration of undesired machining downtime.
The tool-workpiece interaction can be studied from different standpoints. From the process point of view, measurable parameters such as force, power, vibration and tool wear can be analyzed to study the performance of a cutting tool and on the workpiece material. However, solely measuring these parameters may not provide enough information on the surface integrity characteristics of the workpiece. Surface roughness, dimensional tolerances, microstructural change and residual stress are some of the surface integrity parameters in machining operations that cannot be easily inferred from the measurable process parameters. Thus, an offline study is required to establish this relationship.
In the bearing manufacturing industry, hard turning is a material removal strategy employed as an alternative to grinding. It has gained significant attention over the past two decades due to its higher 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to COVID-19) material removal rate, ease of setup for different part geometries, and good surface finish quality compared to grinding [1] . Several studies have been conducted on the performance of different cutting tools and their effect on the efficiency of the hard-turning process.
Among all the cutting tools tested, cubic boron nitride (CBN) inserts are the most widely used in hard turning, thanks to their high hardness and good wear resistivity of the CBN material [2] .
A review of wear failure mechanisms and wear models in hard turning with CBN tools was conducted by Huang et al. in which abrasion, adhesion and diffusion were found to be the most dominant wear mechanisms contributing to the wear pattern [3] . But due to the high cost of CBN inserts, there also have been several attempts to study ceramic and carbide tools as alternatives. For example, Dogra et al. compared the performance of coated carbide tools in hard turning to that of a CBN tool and tried to optimize the cutting conditions of the carbide tool in order to make it perform similarly to the CBN tool in terms of tool wear and white layer formation depth [4] . They concluded that in interrupted cutting, the carbide tool did not perform as well as a CBN tool. However, they were able to optimize the cutting conditions for continuous cutting to get results similar to those of CBN tools.
Shalaby et al. conducted a comparative study on wear mechanisms and tool life between polycrystalline CBN, coated CBN and ceramic tools in cutting D2 tool steel and found that better tool life results were achieved with ceramic tools [5] . A similar study was conducted by Sobiyi et al. where the performance of ceramic and CBN tools was compared in terms of surface topography and wear mechanisms [6] . Finally, a review of different cutting tool materials, respective wear mechanisms and force generation can be found in the work of Shihab et al. [7] .
White layer formation on the surface of the turned workpiece is one of the most important aspects to control in hard turning. It is believed that both phase transformation due to high temperatures at the cutting zone and plastic deformation due to frictional force between the tool tip and the workpiece contribute to white layer formation. However, despite several proposed theories about the formation of the white layer, its true nature and properties are not very well understood. In a study by Bedekar et al., different inserts were used to produce white layers on the surface of a bearing steel. Transmission electron microscopy and glancing angle x-ray diffraction were utilized to study the white layer zone in nano-scale [8] . Ramesh et al. studied white layer formation in 52100 steel and showed that at higher surface speeds, a martensite phase transformation occurs at the white layer [9] . Zhang et al. studied white layer formation and the amount of retained austenite content in sequential cuts and concluded that with an increase in cutting speed, phase transformationrather than plastic deformationcontributes most significantly to white layer formation [10] . Recent work on white layer formation can be found in [11] [12] [13] [14] . A comprehensive review of the state of the art in hard turning, including force and temperature modeling, lubrication and cooling and surface integrity, can be found in reference [15] .
While hard turning has been the subject of many studies, very little of the published literature has considered this process from the industrial point of view. It is known that the use of coolant can reduce tool life by increasing the possibility of microchipping due to temperature shock at the CBN cutting edge. It is also known that a significant temperature rise at the cutting zone in dry cutting conditions can increase the depth of the white layer, leave undesired residual stress on the surface and distort the workpiece. Therefore, wet cutting conditions may be desirable from the industrial standpoint to improve surface integrity and dimensional accuracy.
This work seeks to conduct a comprehensive study on measurable process outputs (i.e., tool wear and cutting forces) and surface integrity parameters (i.e., white layer, surface roughness, micro-hardness and residual stress) using several CBN inserts in a wet turning operation, which is a feasible strategy in the bearing manufacturing industry.
The organization of this paper is as follows: In section 2, the experimental setup and cutting conditions are explained. Results are given in section 3, followed by detailed discussion. Conclusions and future directions are discussed in section 4, followed by acknowledgment and references sections.
Hard-turning tests were conducted on a DMG MORI horizontal lathe machine in flood cooling conditions with a ~7% coolant concentration. The experimental setup is shown in Fig. 1 . The cutting conditions in all the experiments were kept constant with a surface speed of 180 m/min, feed of 0.06 mm/rev and depth of cut of 0.1 mm. AISI 4320 steel samples were chosen with an initial diameter of 76 mm and were turned for the length of 205 mm at each pass. All the samples were case-carburized to a 2.5mm depth with 58-62 Rockwell hardness. Both the microstructure (see Fig. 1 ) and the micro-hardness (see Fig. 3 ) of the heat-treated samples were examined before conducting the experiments to ensure testing was stopped before reaching the soft core of the material. Three different CBN inserts were used from two different manufacturers, referred to as Grade A, Grade B and Grade C in this paper. The major coating component of all three inserts was titanium aluminum nitride (TiAlN). All the inserts were DNGA432 grade with a 55° tip and 0.8 mm tool nose radius, with chamfered and honed (S-land) edge preparation. The edge specifications of each grade are given in Table 1 . Each test per insert was repeated three times to ensure the repeatability of the process, with a total of 9 tests. The forces in the tangential and feed directions were captured using a Kistler force dynamometer with sampling rate of 200 Hz. Tool wear was measured using a KEYENCE VH-Z500R optical microscope at constant intervals. Testing was stopped before flank wear reached the width of 150 µm. All the samples were cleaned and trued with sacrificial inserts after being clamped to the machine to remove any out of roundness, followed by replacement with a new sharp insert to initiate each replication. Surface roughness was measured in both average roughness (Ra) and peak-to-valley roughness (Rz) using a portable surface profilometer Hommel T1000. 
The cutting force in both the tangential (FT) and feed (FF) directions was captured at constant intervals, and the resultant force at the end of the cutting duration was calculated and reported with respect to the cutting time, as shown in Fig. 3 . The resultant cutting force generally trends in a linear manner with the increase in cutting time (i.e., increase in tool wear). Among the three insert grades, the Grade B tool withstood the lowest degree of cutting forces (lower than either the Grade A or Grade C insert), and the Grade A insert withstood higher forces with larger force variations between replications in comparison to Grade C.
It should be noted that even though the cutting forces were higher with Grade A than with Grade C, the Grade A insert demonstrated a higher wear resistivity and therefore higher cutting time (~60% more cutting time with respect to Grade C). Therefore, cutting forces should not be considered as the sole parameter in defining the performance of a cutting tool. Other factors, such as cutting time before Fig. 1 : Schematic of experimental setup reaching wear limit, should be considered simultaneously (refer to section 3.2). Also, as shown in Fig. 4 , the cutting edge of the Grade C insert became highly unstable and remained unstable after only 35 minutes of cutting time. Better edge stability (and, consequently, cutting force stability) was observed in both the Grade A and Grade B inserts. 
Scanning electron microscopy (SEM) was used to capture images of the insert wear on the flank and crater faces, as shown in Fig. 6 . After comparing the cutting edges between inserts, Grade B had the best stability, while Grade C had the lowest edge stability. Despite the newly generated cutting edge of the Grade C insert in Fig. 6(c) , there appears to be a buildup edge created out of the workpiece material. But the energy dispersive x-ray spectroscopy (EDS) and elemental analysis shown in Fig. 7 prove that the new cutting edge contains mainly the coating and CBN elemental contents, so the hypothesis of buildup edge generation on the Grade C insert can be rejected. Considering the EDS results, it can be concluded that the binding of the coating and CBN in the Grade C insert begins to weaken earlier than in the other two due to the high temperature and pressure at the cutting zone, which shift the cutting edge and flank wear land as tool wear increases. This theory also explains the unstable cutting force of the Grade C insert discussed in section 3.1.
The average tool life at the limit of 100 µm of flank wear was calculated for each insert and is given in Fig  8. An insignificant difference between Grade A and Grade B was observed; however, the Grade B insert exhibited less variation in the test results. Therefore, the wear behavior of Grade B is more predictable than Grade A. On the other hand, the Grade C insert showed very good repeatability but exhibited 60% shorter tool life than the other two grades. Combining the results shown in Fig. 6 and Fig 7 and the force comparative study in section 3.1, it can be concluded that even though lower forces are required to turn the material with the Grade C insert, its wear resistivity (and wear rate) is much lower than that of the other two grades. This is of critical importance, since the wear rate of an insert determines the tool replacement intervals and directly influences machine downtime and productivity. 
Average roughness (Ra) and peak-to-valley roughness (Rz) were measured and plotted with respect to cutting time, as shown in Fig. 9 and Fig. 10 . In addition, Ra and Rz values at the tool flank wear width below 150 µm are compared in Fig. 11 . According to this figure, an insignificant difference between roughness values was observed. For Grade B insert the surface roughness began to improve near the end of its life. This improvement can be attributed to several factors, including the change in the tool's contact length and nose radius as flank wear increased, better chip breakage due to changes in tool geometry, or the higher edge stability shown in Fig. 6(b) . Understanding and identifying these effects requires further testing and were outside the scope of the current work. 
The workpiece microstructure analysis for white layer detection is shown in Fig. 12 . A small sample was obtained from each workpiece using wire-EDM machine and mounted in resin and etched with nital (a combination of nitric acid and alcohol). The samples were then polished and were studied under optical microscope with 500x magnification to observe any micro-structural changes after hard-turning process. A white layer was observed in all the experiments, with maximum depths shown for Grades A, B and C, respectively. The depth of the dark layer (heat affected zone) below the white layer zone of each grade is also quantified according to the depth of the deformed grains, and is shown in Fig. 12 as well.
The micro-hardness results are shown in Fig. 13 . The hardness readings were performed with an indentation force of 0.5 kgf and dwell time of 10 seconds. Considering the white layer depth of less than 5 µm identified in Fig. 12 , the micro-indenter was not able to provide readings at nano-scale depth. Examining hardness at the white layer zone would have required a nano-indenter that was not available at the time of writing this article. The residual stress results at the tool flank wear depth of 100 µm in both the circumferential and axial directions are shown in Fig. 14 and Fig. 15 . The initiation of micro-cracks in the early stages of product lifecycles and reduced fatigue life are known to be caused by tensile surface residual stresses. Conversely, surface compressive stresses are considered desirable since they improve surface integrity and fatigue life.
Tensile residual stress is the result of excessive heat generation at the cutting zone due to the extreme rubbing action of the tool flank area on the workpiece material. Excessive heat leads to expansion of the surface, thereby creating tensile stresses. On the other hand, mechanical shearing and plastic deformation at the cutting zone compress the material, which results in the generation of compressive residual stresses. By studying the surface residual stresses, it can be determined whether the cutting occurs as the result of excessive heat generation or plastic deformation of the material.
The surface residual stress results in Fig. 14 and Fig. 15 show that both the Grade A and B inserts produced tensile stress at the surface in the circumferential direction (cutting direction), whereas the Grade A insert produced compressive surface stress in the axial direction (feed direction). The residual stress results for the Grade C insert are especially interesting because compressive stresses were observed in both the circumferential and axial directions, indicating that plastic deformation was more dominant than heat generation.
It is our conjecture that the instability of the Grade C cutting edge shown in both the force graph Fig. 5(c) ) and SEM image (Fig. 6(c) ) exposes new areas of the flank and crater faces of the insert to the cutting zone. These new areas originally acted as the secondary contact area for the material removal process, but now act as the primary contact areas on the cutting tool responsible for chip formation. Therefore, these new areas are more capable of chip generation by means of plastic deformation rather than heat generation.
This work was intended to study the machinability and characterize the surface integrity of three different CBN insert grades in wet hard turning of casecarburized 4320 bearing steel. The metrics chosen for evaluating machinability performance were force and tool wear. The surface integrity parameters were characterized by surface roughness, white/dark layer depths, micro-hardness and residual stresses. Testing with each insert grade was repeated three times to ensure the repeatability of the experiments.
It was concluded that the stability of the CBN cutting edge is the primary factor in determination of the tool's life. This edge stability was detectable in the measured cutting forces and consequently resulted in 60% improvement of tool life in two of the tested inserts compared to the third one. Also, the results of residual stress measurement showed that the CBN grades with higher tool life and stable cutting edges produced tensile surface stresses at the limit of tool wear, while the insert grade with the unstable cutting edge was able to produce compressive residual stress on the surface of the workpiece. This work can be further expanded to identify the cutting condition at which the surface residual stress starts to shift from undesirable tensile stresses to desirable compressive stresses. Also, investigation of other unconventional cooling methods and the possibility of eliminating white layer formation by lowering the temperature at the cutting zone area could be other future directions of study.