Improvement of Machinability of Hardened Steel in Turning Operation Using Cryogenically Treated Cutting Tool

Hardened steels need special cutting tools like PCBN and ceramic to be machined. But these cutting tools aren’t cost-effective and require machine tool structures that are stiff and don’t let vibrations through. The current trend is to find other, less expensive ways to make these materials easier to work with. Cryo-treating cutting tools is an effective way to improve the way the tool materials work when they are cut. Cryogenically treated tungsten carbide inserts in dry turning operations were looked at in this study. For hard turning of hardened mild steel (48 HRC), the performance of cryogenically treated Tungsten Carbide (WC) inserts was compared with that of untreated inserts in terms of chip-tool interface temperature and surface roughness under dry cutting conditions. The cutting tool (Untreated and Treated), and cutting speed (375, 512, 706 rpm) were selected as experiment parameters at a constant feed rate of 0.0841mm/rev and depth of cut 1mm. The chip-tool interface temperature analysis results revealed that temperature increases with the increasing cutting velocities. A better surface finish can be found at a higher cutting speed. The lower value of Ra was found at 1.75 μm (without cryogenic treatment) and 1.05 μm (with cryogenic treatment) for cutting speed at 706 rpm.


Introduction
In recent years, hard turning has emerged as a popular alternative to grinding for the final stage in the machining of hardened steel.As a form of fine finishing, hard turning allows for the elimination of rough machining and grinding [1,2].Hard turning is the process of turning hard materials with hardness ranging from 40 HRC to 65 HRC [3].In modern high-speed machining, surface roughness, tool wear, cutting zone temperature, and chip formation are all crucial parameters for the final product's quality and overall production effectiveness [4,5].A large increase in cutting velocity was required to achieve a higher material removal rate (MRR) in machining, which in turn necessitated a rise in cooling efficiency to combat the rising cutting temperature [6].During hard turning, temperatures over 800 °C are created [7].For machining hardened steel, polycrystalline cubic boron nitride (PCBN) is the ideal tool material.However, the difficulty of compact CBN processing (high temperature and pressure) and the high cost of CBN tools have changed the obstacles for hard turning from technological feasibility to economic viability [8].In addition to CBN tools, investigations on the use of ceramics and carbide tools for finish hard turning under dry and wet conditions have been published [9][10][11][12].Cryogenic treatment, which involves cooling a material to temperatures as low as -196°C, is used to lengthen the useful life of tungsten carbide inserts and other tool steels [13,14].The hot hardness value and thermal conductivity of carbide inserts were improved by deep cryogenic treatment.In comparison to untreated inserts, it led to lower surface roughness, cutting forces, and tool wear [15].Compared to untreated tools, cryogenically treated tungsten carbide tools exhibited much higher chip resistance.Additionally, these tools outperformed untreated ones at higher cutting rates.The improvement in wear resistance was ultimately determined to be caused by an increase in Z-phase particles following cryogenic treatment, and this conclusion was supported by images captured by a scanning electron microscope (SEM) [16].The use of wear-resistant coatings, cryogenic treatment, and wiper geometry according to the literature, contribute to extending the tool life of carbide inserts.Additionally, the application of wiper geometry to carbide inserts improves surface finish.There is, however, no research comparing the performance of cryogenically treated tungsten carbide inserts to untreated tungsten carbide inserts in the dry turning of hardened steel.It is persuasive that using cryogenically treated uncoated or coated carbide tools under dry turning could provide a solution with a cheaper tool cost for the final turning of hardened steel when compared to pricey CBN tools.There is a considerable demand for inexpensive tool materials for hard turning.The purpose of this study is to compare the performance of cryogenically treated and untreated tungsten carbide inserts in dry finish turning of hardened steel.The tool life and surface characteristics produced by these tools in hard turning at varying cutting speeds while maintaining a consistent depth of cut and feed rate have been analysed critically.

Experimentation
The experimental study aimed to improve the machinability of using cryo-treated tungsten carbide inserts in terms of Chip-Tool Interface temperature and surface roughness (Ra) in turning hardened mild steel.Experiments are conducted to study the effects of turning hardened mild steel [48 HRC] shafts using cryogenically treated coated Tungsten carbide inserts.

Tool Preparation
In this experiment, coated and uncoated tungsten carbide were used.The cutting tool inserts were mounted to the dipping rod of a Dewar and it was submerged into the liquid nitrogen for cryogenic treatment.The tool inserts remained submerged in the liquid nitrogen for about an hour.Then inserts were taken out and mounted to a tool holder and a turning operation was performed.The following are the steps involved in the cryogenic treatment process: • Lowering the temperature by 2⁰C per minute from air temperature to -196⁰C.
• Keeping the temperature constant at -196⁰C over a period of twenty-four hours • Increasing the temperature gradually from -196⁰C to the ambient temperature at a rate of 2⁰C per minute • Bringing the temperature from the ambient temperature up to 200⁰C at a tweaked rate of 2⁰C per minute and maintaining that temperature for two hours.• Every moment, the temperature is gradually decreased by 2⁰C to return it to atmospheric conditions.This method was used to prepare the cryogenic-treated insert and carry out the experiments.

Cutting Conditions
According to the insert grade for machining steels, cutting tool manufacturers' recommendations were used to determine the cutting conditions for each material.The cutting conditions used for the material of the workpiece are shown in Table 2 below.In each experiment, the feed rate and depth of cut remained constant.

Result and Discussion
Experimental results for both dry and cryogenic machining are recorded and compared.The temperature of the chip tool interface and surface roughness are two main parameters that are compared to find out the best practice in this case.

Temperature of Chip-Tool Interface
In this investigation, the work-tool interface temperature was measured by using an Infra-Red Pyrometer keeping a minimal distance from the chip-tool interface.IR Beam length was kept short for better accuracy in temperature measurement.At first dry machining operations were conducted and was observed that the temperature at the work-tool interface was significantly high.With the increase in cutting speed and feed rate, the temperature at the work-tool interface increases significantly.When the hardness of the material was increased, the temperature at the work-tool interface also increased due to the high amount of friction.After performing cryogenic treatment on the tungsten carbide cutting inserts, all these operations were performed again.Temperatures at each operation were measured in the same process.The change in temperature concerning cutting velocity is shown in Figure 1.It was discovered that dry machining cannot properly cool and lubricate at the chip-tool interface, where the temperature is highest.This is mostly due to the flowing chips making bulk contact with the tool rake surface, which may be followed by elastic contact right before leaving the tool contact.Bulk contact prevents cutting fluid from penetrating the interface.In such case, treating the carbide insert cryogenically can yield satisfactory results as seen from this experiment.

Surface roughness (Ra)
It plays a key role in determining manufacturing quality as well as tribological phenomena like wear resistance, corrosion, creep and fatigue, and life.This behaviour is attributed to wear, lubrication, heat conductivity, refraction, reflection, and transmission of light.Ra of the machined samples was measured using a surface tester (Mitutoyo SJ-210) with a cut-off length of 0.25 mm throughout three different sampling lengths.The average of these three surface roughness values Ra was utilized to calculate the roughness of the machined surfaces.Surface roughness is a key measure of machined part surface integrity [17].Eq. 1 can be used to calculate theoretical surface roughness, where fr is the feed rate and rn is the tool's nose radius [18].
Table 3 below lists the surface roughness values of Ra, Rq, and Rz for a feed rate of 0.081 mm/rev and a DOC of 1 mm with various cutting velocities.Lower Ra, Rq, and Rz values were seen while using cryogenically treated tools during machining.The most favorable surface roughness results were achieved when tools underwent deep cryogenic treatment for 24 hours.In general, surface roughness (Ra) values were found to vary between 3.45 µm and 1.75 µm for the untreated tool and between 2.36 to 1.05 for the treated one.These values showed a consistent decline as the cutting speed increased across various cutting parameter values for both tools.By raising the cutting speed, the contact area between the tool and the chip decreased, leading to a reduction in friction, which, in turn, enabled the attainment of improved surface quality.For the cutting speed of 512 rpm, SEM images were taken to look at the flank and rake area after machining.Figure 2 below shows the pictures.It was seen that the insert that had not been treated had broken off completely where the rake and flank met.The insert that had been treated with cryogenics had a better flank area.

Investigation of Surface Texture
SEM images of the machined surface were taken of 48 HRC steel for further observation and investigation.TESCAN-Vega was used to obtain the images of the machined surface of the specimen.
In the case of using treated and untreated tungsten carbide inserts the surface of the workpiece shows different textures.The images for the machined surface obtained while turning with a treated cutting insert are presented below in Figure 3. Using untreated tungsten carbide produced a lot of heat at the work-tool interface thus the property of the machined surface suffered.There was uneven surface integrity and a rougher surface was observed.When cryogenically treated tungsten carbide insert was used, it produced a lesser heat at the work-tool interface thus the property of the machined surface was retained at a better state.There was less uneven surface integrity and uniform rough surfaces were observed (in Figure 4).

Conclusion
In this study, the turning of 48 HRC mild steel shafts was carried out in a dry machining environment with the aid of a tungsten carbide cutting insert both in cryogenic treated and untreated state.Cryogenically treated tools produced a better surface finish and higher surface integrity of the machined surface than the standard untreated tools.The following conclusion can be drawn from it: • It was noticed that temperature increased with the increasing cutting velocities.Higher cutting velocities gave lower time for heat transfer and consequently, chip tool interface temperature increased.High temperature was induced during the Dry machining process and sometimes the chips were burnt.Temperature was significantly reduced using a cryogenically treated cutting tool.• Surface roughness has a proportional relationship with feed rate and an inverse relationship with cutting speed.A better surface finish can be found at higher cutting speeds.The lower value of Ra was found for a cutting speed of 706 RPM and a feed rate of 0.108 mm/rev.Surface integrity is drastically reduced during machining harder materials.• Though the tools became a little bit brittle their lifetime increased after this treatment.
Cryogenically treated tools are less worn than normal ones.Surface roughness improved when machining with cryogenically treated tools which is a remark of a good machining process.It signifies lower friction between chip and tool which results in better tool life.• It is an environment-friendly machining process as there is no need for coolant.As a result, fewer pollutants were produced here which in turn also minimizes the need for physical labor.
The limitations and recommendations made for future research are as follows: • Due to some limitations sufficient data collection wasn't possible.If more data were collected, the effect of the cryogenically treated cutting tool could be more visible.• As the temperature was measured here by using the optical pyrometer, its accuracy wasn't at a satisfactory level.If the temperature was measured by using a thermos couple, a more accurate result could be found.• Liquid nitrogen had to be stored in Dewar which is very expensive.Handling this type of equipment and liquid is a very difficult task because a little bit of carelessness can cause a massive accident.• Future studies can be pursued using other tools like CBN tools, and coated Carbide tools for cryogenic treatment on other work materials like Stainless steel, Alloy etc. Response modeling of the performance should be accomplished to effectively control the machining process before the start of the actual machining.

Figure 1 .
Figure 1.Cutting velocity vs Temperature of chip-tool interface graph

Figure 2 .
Figure 2. SEM images of (a) untreated insert (b) cryogenic treated insert

Figure 3 .
Figure 3. SEM images of the machined surface at (a) 375, (b) 512, and (c) 706 RPM while using an untreated tungsten carbide insert

Table 1
displays the chemical compositions of the hardened steel shaft.The dry turning is conducted for 180 seconds.

Table 3 .
Surface Roughness Value