Experimental investigation of the low-temperature oil-on-water cooling and lubrication in turning the hardened AISI D2 steel

In this paper, the influence of the cutting speed, feed, and the depth of cut on the cutting force, surface roughness, cutting temperature, and tool wear were experimentally investigated under the low-temperature oil-on-water (LTOoW) cooling and lubrication condition in turning the hardened tool steel AISI D2 (60 ± 1HRC) with the PCBN cutting tool. The results showed that the three-component cutting forces are FY > FZ > FX. The influence of the cutting speed on the cutting temperature is slightly more visible compared to the feed and depth of cut. In this experiment, a satisfactory surface roughness value of 0.54 µm can be obtained, gaining the effect of the turning instead of the grinding. The flank wear values of the PCBN tool are 142 µm and 148 µm at the cutting speeds of 55 and 140 m/min, respectively; however, the flank wear abruptly increases to 668 µm at a 495 m/min, which has a very serious impact on the tool life. The abrasive wear is considered to be a predominant wear mechanism on the flank wear of the PCBN tool. The rake face is dominated by crater wear due to the high temperature, high pressure, high stress, and high friction at the chip-tool interface. Compared with dry hard turning (DHT) condition, the lower surface roughness value, lower cutting temperature, and longer tool life can be obtained at LTOoW.


Introduction
In recent years, with the increasing demands on the mechanical and other properties of the parts, some difficult-tomachine materials with hardness as high as 50-70 HRC, such as the hardened tool steel AISI D2, have emerged. It is widely used in industrial fields such as bearings and molds due to its mechanical strength, fatigue resistance, and wear resistance. It is accompanied by waste liquid pollution, low processing efficiency, and difficult to guarantee product quality as machining these materials in traditional processing technology [1]. In the past 30 years, many researchers have been focusing on the dry hard cutting technology in order to avoid its substantial disadvantages. Although dry hard cutting technology has the advantages of high processing accuracy, high production efficiency, and less environmental pollution [2], it also has disadvantages such as the high cutting temperature, large cutting force, and short tool life [3]. Nevertheless, these green machining technologies, such as lowtemperature cold air (LTCA), minimal quantity lubrication (MQL), and oil on water (OoW), have always been focused by various researchers in recent years [4] because they have been considered to be the replacement for dry machining.
The LTCA cutting technology is a cooling and lubrication technique that sprays low-temperature compressed air into the cutting zone to reduce the temperature of the cutting zone. In 2010, Sun et al. [5] have conducted an experimental study on the LTCA in the turning of titanium alloys and confirmed that it has a longer tool life and higher machined surface quality than the dry cutting. Furthermore, Su et al. [6] also have come to the conclusion that the LTCA cutting can significantly prolong the tool life and improve the quality of the machined surface. Next year, Boswell et al. [7] from Australia have conducted cutting tests by using the LTCA + MQL technology. According to the findings of the research, the LTCA + vegetable oil MQL technology is the most promising cooling and lubrication technology. The machining performance of Ti-6Al-4 V titanium alloy under the condition of LTCA has been studied by Liu et al. in 2013 [8]. They have pointed out that, compared with dry cutting, LTCA can significantly reduce the cutting temperature, prolong tool life, as well as obtain the best-machined surface. Subsequently, Jozic et al. [9] have optimized the cutting parameters in milling 42CrMo4 under the conditions of the cutting fluid, LTCA, and dry cutting by using the orthogonal experiment in 2015. It has been discovered by Kopac et al. [10] that the residual compressive stress conducive to improving the machined surface quality can be obtained after the low temperature machining. Moreover, Arruda et al. [11] have considered that a significantly longer tool life can be obtained in cutting API 5L X70 steel under LTCA condition.
The MQL cutting technology is also a cooling and lubrication technique that sprays the minimum quantity of lubricant oil mist mixed up with compressed air into the cutting zone to decrease temperature and friction in a chip-toolworkpiece interface, reduce tool wear, and improve the quality of the machined surface. Anthony-Xavior et al. [12] have investigated the influence of the coconut oil, emulsion, and oily cutting fluid on the surface roughness and tool wear in turning AISI 304 steel. It has a great impact on improving the surface roughness and extending the tool life. Besides, Saini et al. [13] have experimentally investigated the influence of variables such as cutting speed, feed, and depth of cut on the cutting force and tool tip temperature in turning AISI 4340 steel at different environmental conditions of dry and MQL cutting. The results obtained in this paper showed that the main cutting force is largest among the three-component cutting forces, and that MQL cutting has an excellent lubricating effect when compared with dry cutting. In 2017, Pervaiz et al. [14] have experimentally evaluated such important indicators as the surface roughness, cutting force, tool life, and other important indicators focused by the machinists in turning titanium alloy Ti-6Al-4 V at the condition of low-temperature MQL. Masoudi et al. [15] have considered that the MQL system significantly increases the cutting efficiency after researched the influences of different parameters on the tool wear, cutting force, and surface roughness in machining AISl 1045 steel. Currently, Makhesana et al. [16] have further investigated the surface roughness, chip-tool interface temperature, and tool life in turning AISI 4140 steel with coated carbide tools under the cooling and lubrication conditions of vegetable oil-based MQL (VMQL) and minimum quantity solid lubricant (MQSL). The results showed that VMQL and MQSL have significant advantages compared to dry cutting.
The OoW droplet cutting technology refers to the processing technology that sprays a small amount of compressed and atomized vegetable oil and water into the cutting zone to reduce the cutting temperature and lubricate the chip-tool-workpiece interfaces. In 2015, Lin et al. [17] have analyzed and compared the tool wear at two cooling and lubrication conditions of the OoW droplets and LTCA in turning of titanium alloy Ti-6Al-4 V. They have considered that it can obtain lower surface roughness values and lower tool wear by using the OoW cooling lubrication technology. Then, Wang et al. [18] have studied the tool wear mechanism in turning compacted graphite cast iron at different OoW droplets cooling conditions in 2017. The results showed that it can prolong the life of the cutting tool. Moreover, the influence law and internal mechanism of the nozzle position and diameter on surface roughness and tool wear have been investigated by Yao et al. [19] under the condition of OoW droplets cooling and lubrication.
According to a large number of literatures, cooling and lubricating techniques such as the LTCA, MQL, and OoW can achieve satisfactory results, and each has its own advantages. Such difficult-to-machine materials as the hardened AISI D2 tool steel necessitates the use of an innovative cooling and lubrication technology to minimize the cutting temperature at the cutting zone. In this paper, an innovative low-temperature oil-on-water (LTOoW) mist cooling and lubrication technology were proposed by combining the three cooling and lubrication technologies of MQL + OoW + (LTCA). And the influence law and mechanism of the cutting speed, feed, and depth of cut on the cutting force, surface roughness, cutting temperature, as well as tool wear, will be experimentally investigated by using the PCBN cutting tool in turning the hardened tool steel AISI D2 (60 ± 1HRC) at the condition of LTOoW. And in order to show the advantages of using this cooling and lubrication condition, the experimental results were compared with the dry hard turning (DHT) condition.

Mechanism of the LTOoW cooling and lubrication technology
LTOoW cooling and lubrication technology is the lowtemperature compressed air (− 20 to − 30 ℃) mixed with the minimum vegetable oil and water does converge at the atomization nozzle to form a low-temperature oil-on-water mist jet for cooling and lubrication effect within chip-toolworkpiece interfaces. As shown in Fig. 1, the oil in the low-temperature medium is attached to the surface of the water drops and is sprayed into the high-temperature cutting zone at the chiptool-workpiece interfaces by the nozzle at high speed, high pressure, and high kinetic energy to form a lubricating film. It plays the cooling and lubrication of the chip-tool-workpiece interfaces, thus reducing the friction at the chip-toolworkpiece interfaces and cutting force.
The low-temperature water droplets sprayed into the high-temperature cutting zone are instantly vaporized in the high-temperature environment at the chip-tool-workpiece interfaces, taking away a large amount of cutting heat and reducing the cutting temperature. The formed boundary lubrication film not only reduces the cutting force, but also prolongs the tool life, and even improves the machined surface quality. On the one hand, the high pressure, high speed, and low-temperature airflow in the cutting zone has the cooling effect due to forced convection and an increase in temperature difference. On the other hand, it is conducive to the chip removal, reducing the contact area and friction at the tool chip, thereby reducing the tool wear of the rake face.
These combined effects avoid the loss of the lubricating effect of the lubrication film at the chip-tool-workpiece interfaces due to too high temperature in the cutting zone, which has obvious advantages compared to the normal temperature MQL technology.

Workpiece material and heat treatment
The workpiece material was the hardened tool steel AISI D2 (Cr12MoV, China), and the chemical composition and content are displayed in Table 1. The specimen is 128 mm in diameter, 210 mm in length, 6 mm in trough width, and 80 mm in ring width, as shown in Fig. 2. After heat treatment at 1040 °C and low-temperature tempering, the hard layer on the workpiece surface was removed to obtain a workpiece with a hardness of 60 ± 1 HRC. The quenching hardness of the specimen was determined by a digital Rockwell hardness tester (type: HRS-150), as shown in Fig. 3.

Cutting tool
According to the literature [20], a polycrystalline cubic boron nitride (PCBN) tools with a CBN content of 50% were selected. The composition and properties of CBN are  shown in Table 2. The geometric angle with a negative rake angle and chamfer was selected to improve the strength of the somewhat brittle PCBN composite tool. The effective geometric parameters are shown in Table 3. The PCBN composite tool used in this experiment is shown in Fig. 4. Figure 5 shows the schematic diagram of the turning experiment system, which consists of a turning experiment test system, a LTOoW cooling and lubrication system, and other measuring instruments such as a surface roughness tester. The turning experiment system consists of a common lathe (type: CA6140), a Kistler dynamometer (type: 9257C), and a high-speed infrared temperature measurement intelligent acquisition system (type: DM63-II), as shown in Fig. 6.

Turning experiment system
The average values of the cutting forces data collected in the steady-state cutting stage are taken as the three-component cutting forces. The directions of X, Y, and Z are the directions of the feed force F X , radial cutting force F Y , and main cutting force F Z , respectively. And the obtained highest temperature during the period is the cutting temperature.

LTOoW mist cooling and lubrication system
The LTOoW mist cooling and lubrication system consists of the air compressor, air storage tank, low-temperature compound spray cooling equipment (type: AR-800-OoW-55), as well as an atomizing nozzle with a diameter of 0.8 mm, as shown in Fig. 7a and b. The compressed air from the screw air compressor (type: LDB-10A/8) is stored in the air storage tank in order to obtain a stable air source and enters the low-temperature composite cooling equipment; and then the low-temperature airflow is generated after heat exchange.
The low-temperature airflow, as well as the low-temperature gas, oil, and water mixture pressed from the oil and water tanks, is converged at the atomizing nozzle and sprayed a LTOoW mist at a low temperature of − 30 °C after repeatedly adjusting the low-temperature cold air control system. Figure 6c presents the jet temperature field at the nozzle collected by the thermal imager after repeated debugging. The vegetable oil-based micro-cutting oil (type: MIRCOLUBE 2000-30) was selected to be the cooling and lubricant. The detailed parameters of the cooling and lubrication system used in this experiment are shown in Table 4.

Measurement of surface roughness
The surface roughness values were measured by utilizing the SJ-210 portable surface roughness tester. It was measured five times along the circumference of each turning ring surface, and then the average value was taken as the final surface roughness.

Experimental parameters
Experiments under cutting parameters listed in Table 5 were conducted in order to investigate the influences of cutting speed, feed, and depth of cut on cutting force, surface roughness, cutting temperature, and tool wear at LTOoW condition.

Measurement and analysis of the tool wear
The rake face and flank wear and morphology of the PCBN cutting tools (Fig. 8) were measured by a Zeiss ultra-depth microscope (type: Smartzoom5), as shown in Fig. 9a. And the average value of the three measurement readings was taken as the evaluation value. The analysis and observation of the micro-morphology and tool wear mechanism were completed by a thermal field electron microscope (type: INSPECT F50, SEM) equipped with an energy dispersive X-ray spectrometer (EDS) detector, as shown in Fig. 9b. In this part of the experiment, the cutting parameters of the 55, 140, 249, 395, and 495-m/min cutting speed, a fixed 0.15mm/r feed, 0.15-mm depth of cut, and 215-m spiral cutting length were chosen to investigate the tool wear.  Figure 10a shows the variation of the three-component cutting forces with increments of the cutting time in a case of 140-m/min cutting speed in turning hardened tool steel AISI D2 (60 ± 1HRC). As shown in Fig. 10b, the three-component cutting forces are F Y > F Z > F X in the range of 55-495 m/min, which is a remarkable characteristic of the hard cutting. This is similar to the results of the literature [21]. It can be seen in  So what is the reason that the radial cutting force increases sharply at 495 m/min, while the other two-component cutting forces change less? It should be that the adhesion effect of the chips softened by high temperature at the cutting zone increases the frictional force at the chip-tool interface, resulting in an abrupt increase of the radial cutting force, which is similar to the results of the literature [2].   of 0.05-0.25-mm/r feed. Moreover, it increases rapidly and linearly, which has a greater influence on the radial cutting force compared to cutting speed. While the main cutting force gradually increases and reaches a peak of 106 N in a case of 0.20-mm/r feed, and then gradually decreases. Figure 12 illustrates the influence of the depth of cut on the three-component cutting forces in the range of 0.05-0.25-mm depth of cut at the fixed 249-m/min cutting speed and 0.15-mm/r feed. Figure 12a shows the variation of the three-component cutting force with increments of the cutting time in a case of 0.10-mm depth, 249-m/min cutting speed, and 0.15-mm feed. As shown in Fig. 12b, the variation law and characteristics of the three-component cutting force are very similar to those of the feed. It is not repeated here. Figures 13,14, and 15 describe the influence of the cutting speed, feed, and depth of cut on the surface roughness at conditions of LTOoW and DHT. On the whole, the surface roughness values obtained at LTOoW are lower than those obtained at DHT except for the cutting speed of 55 m/min, feed of 0.05 mm/r, and depth of cut of 0.05 mm. It can be observed in Fig. 13 that the minimum value of 0.90 µm is obtained in a case of 249 m/min at LTOoW; it slightly increases to 1.02 µm at 395 m/min, and then rapidly increases to 2.32 µm at 495 m/min. As observed in Fig. 19, it is the severe flank wear that results in deteriorating the machined surface quality. It can be clearly observed in Table 6 that the more serious serrated nose wear at the nose part of the PCBN inserts may explain why the roughness in cases of 55 and 495 m/min begins to deteriorate. And the sharply flattened nose radius induced by tool wear should contribute to the reduction of surface roughness at 249 and 395 m/min, as presented in this table.

Surface roughness
It can be clearly visible in Fig. 14 that with increments of the feed, the surface roughness value gradually increases, Water pressure/MPa 0.6 Oil-to-water flow ratio 1:100  As observed in Fig. 15, the effect of the depth of cut on the surface roughness is very little, but it is very interesting that the surface roughness value reaches the minimum value in a case of 0.15-mm depth of cut.
Combining with Fig. 10, and 13, it can be seen that not only the cutting force is smaller, but also the surface roughness value is the smallest as selecting the cutting parameter of 249-m/min cutting speed, 0.15-mm/r feed, and 0.15-mm depth of cut.

Cutting temperature
Here, the cutting temperature known as one of the crucial issues was investigated because it highly influences the tool life and machined workpiece surface quality. Since the chips cover the cutting edge during the turning process, (b) Thermal field electron microscope (a) Ultra depth of field microscope The changing laws of the three-component cutting forces Fig. 11 The influence of the feed on the three-component cutting forces the temperature collected online by the thermal imager can only be the temperature of the chips in the cutting area. In fact, this temperature is much lower than the cutting-edge temperature. Figures 16, 17, and 18 respectively present the temperature field of the cutting zone collected in real-time by the thermal imager and the variation law of the maximum temperature with increments of the cutting speed, feed, and depth of cut at conditions of LTOoW and DHT. It can be seen that the cutting temperatures at DHT are higher about 100 °C than those at LTOoW. As observed in Fig. 16b, that the cutting temperature gradually increases with increments of the cutting speed, and that it varies in the range of 230-340 °C. And it remains basically unchanged between 300 and 330 °C with increments of the feed, as seen in Fig. 17b. It also tends to increase with the increments of the depth of cut, and the maximum temperature in the cutting zone varies from 260 to 333 °C, as shown in Fig. 18b. Although the collected cutting temperature may be lower than that in the cutting zone due to the lag in the response time of the thermal imager and the rapid heat dissipation of the chip temperature, its variation law is real and credible.
The experimental results show that the maximum cutting temperature does reach the peak of 340 °C at the LTOoW mist cooling and lubrication condition, far being lower than the peak cutting temperature of 770 °C described in dry cutting in the literature [3], which indicates that the generated high heat is absorbed by the water mist during evaporation.

Wear law of the PCBN cutting tool
Since the influence of the cutting speed on the tool wear is the most important key one among three cutting variables, and the mechanism is also the most complicated, [22] in this experiment, the influence and mechanism of the cutting speed on the tool wear were investigated at cutting speeds of 55, 140, 249, 395, and 495 m/min at fixed 0.15-mm/r feed, 0.15-mm depth of cut, and 215-mm spiral cutting length. The rake face and flank wear of the PCBN tool at the five cutting speeds are shown in Table 5. Figure 19 presents the influence of the cutting speed on the tool life. At the condition of LTOoW, the flank wear V Bmax of the PCBN tool basically does not change and is 142 µm and 148 µm at the cutting speeds of 55 and 140 m/ min, respectively, which is in good agreement with the allowable wear criteria of [V B ] = 0.3 mm [23]. When the cutting speed reaches 249 m/min, the flank wear V Bmax suddenly increases, and its value runs up to 319 µm and then it slightly increases to 340 µm at a cutting speed of 495 m/ min. It is obvious that the flank wear meets the allowable wear criteria [V B ] = 0.4 mm [23] as the spiral cutting length reaches 215 m at the two cutting speeds. When the cutting speed reaches 495 m/min, the flank wear begins to deteriorate and drastically increases to 668 µm, with an increase of about 109%, which is in good agreement with the variation of radial cutting force as shown in Fig. 10. Obviously, if only the tool life factor is considered, the cutting speeds of 55 and 140 m/min are two more appropriate cutting speeds in turning hardened tool steel AISI D2 (60 ± 1 HRC). However, the cutting speed of 495 m/min has a deteriorative influence on the flank wear of the PCBN tool and is not suitable for cutting, as also confirmed in Table 6 and Fig. 20e.
In addition, the tool wear under the condition of LTOoW was compared with that of DHT. As shown in Table 7 and Fig. 19, except at 55 m/min, the tool life is longer under LTOoW compared with the DHT condition. When turning at higher cutting speeds of 395 and 495 m/min, the flank wear obtained at the DHT condition becomes worse than that under the condition of LTOoW.

Tool wear mechanism
(1) Flank wear The wear region of the flank and rake face was analyzed by scanning electron microscope in order to further study the flank wear mechanism in turning of AISI D2 hardened steel at the condition of the LTOoW.
It can be noticed in Figs. 20a-e and Table 6 that when the spiral cutting length runs up to 215 m, deep grooves occur on the flank, which indicates that the abrasive wear on the flank is the dominant wear mechanism. The binder is more prone to chemical, dissolution, and diffusion wear than the BN due to high temperature, high load, and high stress, resulting in the falling off of loose CBN particles.
The grooves on the flank should be caused by the rolling and sliding of these loose CBN particles or hard carbon particles at the tool-chip interface.
It must be noted that, as observed in Table 6 and Figs. 19 and 20, the chipping and delamination wear generate at the cutting speed of 495 m/min. Figure 10 can explain the internal mechanism inducing this phenomenon. The figure shows that the radial cutting force suddenly increases to 420 N at 495 m/min, which increases the friction at tool-workpiece interface, resulting in more serious abrading and tearing on the flank, which leads to more serious chipping and delamination wear on the flank. Moreover, the generation of higher temperature at 495 m/min shown in Fig. 16 should be responsible for faster growth of tool wear, too.
As can be seen from the figures in Tables 6 and 7 and Fig. 21 that, compared with the LTOoW, when the cutting speed reaches 249 m/min, the flank surface suffers severe delamination wear and tool breakage due to the loss of cooling and lubrication in turning of the hardened steel.
(2) Rake face wear It can be observed in Figs. 20 a-e that the crater wear is the most typical wear form on the rake face of the PCBN tool. The crater region is justly located in the high temperature and the larger mechanical load zone, which will result in high normal stress and high shear stress caused by the toolchip friction effect. The temperature field (Fig. 16) obtained by the infrared thermal imager shows that it reaches a peak of approximately 340 °C as the cutting speed reaches 495 m/ min. In fact, the temperature at the cutting edge must be far higher than this. Such high temperature must lead to adhesive wear, delamination wear, and diffusion wear, and then accelerates the crater wear and eventually induces serious damage to the rake face.
It can be seen in Fig. 20 that the adhesive wear and diffusion wear do occur at the rear of the crater region, and the cutting temperature, friction force, pressure, and compressive stress in this region are lower than those in the crater region. However, the friction at the tool-chip interface results in abrading and tearing effects, which must induce two cases: first, the chip material may remain in the crater; secondly, the material on the rake surface should be adhered away by the high-temperature chip to form adhesive wear, as shown in Figs. 20 a-e.
According to the literature [24], the calculation formula of the tool-chip interface friction coefficient is: Taking the main, radial, and feed forces shown in  Table 3 into Formula (1), it can be obtained that the friction coefficient at 495 m/min abruptly increases to 0.65. It can be observed that the friction force loaded at the crater region on the rake face of the PCBN tool becomes very large. As a result, a large plastic strain within the rake sub-surface leads to the thermal micro-crack initiation in this region, resulting in a large piece of delamination wear on the rake face of the tool as shown in Fig. 20e, which is similar to the earlier literature [25]. Furthermore, as clearly shown in Figs. 20a-b, workpiece material can be transferred to the tool surface. The deposition of workpiece material in the crater becomes much more at a lower cutting speed when compared to other higher cutting speeds. This may be the reason that  the back of the softened chip has sufficient time to adhere to the rake face due to the relatively lower speed of chips flying away from the rake face as turning at the lower cutting speeds of 55 and 140 m/min, which results in more softened chips staying in the crater; while the crater has very little adhered material at higher cutting speeds of 249, 395, and 495 m/min. Figure 22 presents the EDS analysis of adhered material in the crater on the rake face at 395 m/min. It can be seen in the figure that except for the chemical elements (B, N, Ti) of the PCBN tool and the main chemical elements (Fe, C, Cr, Mo, V, Si) of the workpiece material, the oxygen element was also detected. It indicates that these adhered materials should be metal and non-metal oxide.

Conclusions
The cutting force, surface roughness, cutting temperature, and tool wear were experimentally investigated and analyzed in turning hardened tool steel AISI D2 (60 ± 1 HRC) with the PCBN tool at the condition of the LTOoW mist cooling and lubrication. Then those experimental results were compared with the DHT condition. The conclusions are as follows: (1) The three-component cutting force becomes F Y > F Z > F X , which should be a significant characteristic of the hard cutting. (2) The maximum cutting temperature does reach the peak of 340 °C at the LTOoW mist cooling and lubrication condition, far being lower than the peak cutting temperature in dry cutting. Meanwhile, a large plastic strain within the rake sub-surface leads to the thermal micro-crack initiation in this region, resulting in a large piece of delamination wear on the rake face of the tool. The rake face wear of the PCBN is dominated by crater wear due to the high temperature, high pressure, high stress, and high friction at the chip-tool interface. (6) Compared to the DHT condition, the LTOoW cooling and lubrication can improve the machined workpiece surface quality, reduce the cutting temperature, as well as prolong the tool life.
Author contribution All authors contributed to the material preparation, data collection, and experimental study. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. Data availability Not applicable.
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The authors declare no competing interests.