Introduction

Advanced machining industries are primarily concentrating on the achievement of high material removal rate (MRR) and high quality of product [1]. Hardened steels have wide applications due to its favourable mechanical properties [2], and historically, processing of hardened steel is done using grinding. However, in the recent years, hard turning is being explored due to high MRR and the benefit of fewer process steps [3,4,5] and possibilities of generating complex geometry under dry cutting [6].

Turning of hardened materials is generally a high speed operation with as high as 250 m/min cutting velocity or more than this. For finishing operations, high speed, low depth of cut and low feed rate are required [7]. Various researchers reported to have performed machining at a range of cutting velocity between 100 and 250 m/min, and in some cases, higher than this [8, 9]. For this machining operation, high hardness tools like HPC (high performance ceramics), coated carbides, CBN, PCBN tools were used. Sahoo and Sahoo [10] used uncoated and multilayer TiN and ZrCN coated carbide inserts at 150 m/min cutting speed.

Tool wear and tool life play important roles for finish hard turning due to its effect on surface integrity, white layer formation and dimensional accuracy [11]. Since long, many researchers worked on these areas. Zhang, et al  [12] found that under ultrasonic vibration, spindle speed, tool amplitude and the frequency had significant effect on tool wear during turning of titanium alloy. Rate of diamond tool wearing and vibration amplitude were found to be related inversely [12]. Aslan, et al  [13] in another work observed that with increase in cutting velocity, tool wear rate came down when Al2O3 and TiCN mixed ceramic tool were used for turning of AISI 4340 steel (43 HRC). This might be due to high cutting temperature, lowering of yield strength resulting in lesser cutting force and wear.

Cutting conditions and tool condition are directly related to the cutting force during machining. When depth of cut was given lower than nose radius of the cutting tool in finish hard turning, radial components of tool force were found most dominant [14]. It was found that, cutting force increased at low cutting velocity and decreases when cutting speed increased. This might be due to thermal softening of the workpiece material because of high heat generation in high cutting velocity [7, 8, 15].

In the present work, AISI 4340 steels were chosen for turning tests under varying cutting conditions. Using these different conditions, surface roughness, types of chips formed, cutting force components, etc. were noted, and machinability of the tool-work combine under different conditions is explored.

Materials and Methods

For turning experiments, a Kiloskar lathe was used. The workpiece was held between a 3-jaw chuck and a revolving centre. Detail of the set up and experimental conditions are given in Table 1. Five levels of cutting velocity with three levels of feed were chosen maintaining depth of cut constant. Machining tests were carried out using sharp new edges of cutting inserts.

Table 1 Experimental set-up and conditions
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Three different types of TiC coated K15 grade carbide inserts (make: Sandvik Asia Ltd., India) with wide groove (specification: SNMG 12 04 08/315 K15 with width of groove: 1.4 mm, width of land from cutting edge: 0.6 mm), narrow groove (specification: SNMG 12 04 08/315 K15 with width of groove: 0.64 mm, width of land from cutting edge: 0.6 mm), and without any groove (specification: SNMA 12 04 08/315 K15) were used in this work. Machinability was tested by noting cutting force variation, chip form observed, value of chip reduction co-efficient, formation of BUE and surface roughness. Chips collected were photographed using a Canon, Japan made digital camera (model: A420). Vernier caliper (make: Mitutoyo, Japan) was utilized for measuring the chip thickness. The response variables noted were chip thickness, type of chip, chip reduction co-efficient (CRC), and build-up edge formation. Chip reduction co-efficient (CRC) is the ratio of chip thickness and uncut chip thickness, and reciprocal of cutting ratio. Uncut chip thickness, a1 is evaluated by the expression, a1 = f Sin κp, where, f is longitudinal feed and κp is principal cutting edge angle.

Results and Discussion

Experiment Set Ia

In Experiment Set Ia, plain TiC coated carbide inserts were used in dry environment for turning AISI 4340 steel rods. Results obtained are shown in Table 2, Figs. 1 and 2. Chip reduction coefficient (CRC) is found to be less when irregular flat continuous chips are mostly produced at cutting velocities of 172 and 272 m/min. This, in general, indicates desirable machining operation involving less shear deformation of chips consuming less force. Coiled type chips are mostly formed at lower cutting velocities of 44, 68 and 109 m/min and those conditions show higher CRC. This is expected as coiled type chip formation needs more deformation sideway to facilitate curling. At 109 m/min cutting velocity and 0.1 mm/rev feed along with open coiled chips, few flat continuous chips are also seen. Fractured chips of half and full round types and coiled with few turns are seen [Fig. 1j–o; Table 2 (sl. no. 10–15)] at 68 and 44 m/min Vc. CRC observed at this low velocity condition is quite high. This may be due to somewhat high degree of curling over a narrow radius resulting in their breakage. Also, presence of unstable built-up-edge at this low Vc may have caused variable rake angles resulting in cutting force variation, thereby causing broken chip formation. At a low cutting velocity of 43 and 67 m/min, expectedly, tiny loose built-up-edges are seen. At higher cutting velocities of 109, 172, and 272 m/min, no built-up-edge is detected as the tendency of BUE formation is generally detected at low cutting velocities.

Table 2 Experimental data obtained with plain insert under dry condition in Experiment Set Ia
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Fig. 1
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Photographs of observed chips for Experiment Set Ia with plain TiC coated insert under dry condition for AISI 4340 steel, a at experiment sl. no. 1, b at experiment sl. no. 2, c at experiment sl. no. 3, d at experiment sl. no. 4, e at experiment sl. no. 5, f at experiment sl. no. 6, g at experiment sl. no. 7, h at experiment sl. no. 8, i at experiment sl. no. 9, j at experiment sl. no. 10, k at experiment sl. no. 11, l at experiment sl. no. 12, m at experiment sl. no. 13, n at experiment sl. no. 14, o at experiment sl. no. 15

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Fig. 2
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Plot of variation of forces (Fc, Fcn) with cutting velocity (Vc) at different feed (So) at dry condition for Experiment Set Ia with plain insert without any groove

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Figure 2 shows variation of cutting force components (Fc: main cutting force component and Fcn: cutting normal force) with cutting velocity (Vc) and feed (f). Expectedly, force values are increased with an increase in feed due to high shear area. Change in force shows a decreasing trend up to a cutting velocity of 107 m/min. There is no appreciable change in force at the higher velocity. BUE formation as detected in 44 and 68 m/min cutting velocity and formation of coiled and broken chips needing higher deformation may have resulted in higher force values. At higher Vc, there is no BUE formed, and force values are less and do not show remarkable variation expectedly. Formation of flat to irregular continuous chips needing less deformation supports less value of cutting force components. At 272 m/min cutting velocity and 0.1 mm/rev feed, the least force requirement is experienced with mostly flat continuous chip formation indicating favourable machining condition. Thermal softening at high cutting velocity may have caused lowering of force values.

From the above discussion, it can be stated that fairly good machinability is obtained when plain carbide insert is employed at higher cutting velocities of 109, 172, 272 m/min at the chosen feed even in dry condition. At 272 m/min cutting velocity and 0.1 mm/rev feed, better machinability is obtained than that at other machining conditions.

Experiment Set Ib

Using inserts with a narrow chip breaking groove, experiment set Ib was performed in dry condition. Groove width was 0.64 mm and width of land from cutting edge was 0.6 mm. Results obtained from Experiment Set Ib are shown in Table 3, Figs. 3 and 4.

Table 3 Experimental data obtained with narrow groove insert under dry condition in Experiment Set Ib
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Fig. 3
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Photographs of observed chips for Experiment Set Ib with narrow groove tool insert with TiC coated insert under dry condition for AISI 4340 steel, a at experiment sl. no. 1, b at experiment sl. no. 2, c at experiment sl. no. 3, d at experiment sl. no. 4, e at experiment sl. no. 5, f at experiment sl. no. 6, g at experiment sl. no. 7, h at experiment sl. no. 8, i at experiment sl. no. 9, j at experiment sl. no. 10, k at experiment sl. no. 11, l at experiment sl. no. 12, m at experiment sl. no. 13, n at experiment sl. no. 14, o at experiment sl. no. 15

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Fig. 4
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Plot of variation of forces (Fc, Fcn) with cutting velocity (Vc) at different feed (So) at dry condition for Experiment Set Ib with narrow groove insert

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Chip reduction co-efficient is found to be within 1.78 and 2.32. As there is a narrow groove type chip breaker present in the insert, chips are curled over a small radius of curvature. Chip forms observed are shown in Fig. 3. Chips are mostly flat irregular long continuous at 172–272 m/min cutting velocity. Fractured chips with few turns, or half and full round chips, are formed at 44–109 m/min cutting velocity (Vc). With flat or irregular type chip, deformation is less and chip reduction coefficient (CRC) becomes less. At the cutting velocity of 68 and 44 m/min, tiny hard build-up-edges are found. As a narrow groove type chip breaker is used, formation of coiled chips with few turns is natural.

Figure 4 shows variation of cutting force components (Fc, Fcn) at different cutting velocity (Vc) and feed (f). Fc force increases with increase in feed as usual due to large shear area. However, Fcn force shows comparable values at different feeds with varying cutting velocity. At low Vc of 44 and 68 m/min, forces are higher than that at 109, 172 and 272 m/min, indicating better machinability at higher cutting velocities taken in this investigation. A close look at Fig. 4 reveals that at 172 m/min under all feed values, minimum force components are needed. At these conditions, chips of flat continuous types are also observed. So, it can be stated that 172 m/min cutting velocity offers the suitable condition for machining to have good machinability.

Experiment Set Ic

Experiment Set Ic was conducted in dry condition using wide groove inserts (width: 1.4 mm, width of land from cutting edge: 0.6 mm). Results regarding chips and build-up edge formed as obtained from Experiment Set Ic are shown in Table 4, Figs. 5 and 6.

Table 4 Experimental data obtained with wide groove insert under dry condition in Experiment Set Ic
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Fig. 5
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Photographs of observed chips for Experiment Set Ic with wide groove tool insert with TiC coated insert under dry condition for AISI 4340 steel, a at experiment sl. no. 1, b at experiment sl. no. 2, c at experiment sl. no. 3, d at experiment sl. no. 4, e at experiment sl. no. 5, f at experiment sl. no. 6, g at experiment sl. no. 7, h at experiment sl. no. 8, i at experiment sl. no. 9, j at experiment sl. no. 10, k at experiment sl. no. 11, l at experiment sl. no. 12, m at experiment sl. no. 13, n at experiment sl. no. 14, o at experiment sl. no. 15

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Fig. 6
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Plot of variation of forces (Fc, Fcn) with cutting velocity (Vc) at different feed (So) at dry condition for Experiment Set Ic with wide groove insert

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At this set of experiments, no BUE is observed. Chip reduction coefficient (CRC) or chip ratio equivalent is found to be within 1.67–2.32. Chip forms observed as shown in Fig. 5 are mostly of flat, irregular long continuous types at a cutting velocity of 109 m/min or more. Fractured discontinuous chips are mostly obtained at a low cutting velocity of 44 and 68 m/min similar to the other two experiment sets. Although at 44 m/min cutting velocity and low feed of 0.063 and 0.08 mm/rev (experiment sl. no. 14 and 15), a number of close coiled long chip is also noticed. Therefore, it may be stated that no clear chip breaking effect of using the wide groove chip breaker is there at most of the experimental results obtained except that at few cases of low cutting velocity of 68 and 44 m/min. This wide groove results in the rake angle becoming effectively positive, thereby may need less cutting forces.

Figure 6 shows the variation of cutting force components (Fc, Fcn) at different cutting velocity (Vc) and feed (f). Expectedly, forces increase with increase in feed due to increasing shear area. In this experiment, no clear trend on variation of forces with cutting velocity is seen. However, slight decrease in force components at higher cutting velocity is noted at all feed. This may be due to thermal softening of workpiece at high cutting velocity.

Therefore, it can be stated that desirable machinability tool performance can be obtained when cutting velocity chosen is from 109 up to 272 m/min within the domain of experiments of the present investigation during turning of hardened AISI 4340 steel. However, at 272 m/min cutting velocity with 0.1 mm/rev feed, minimum force is needed than that of other cutting velocity indicating achievable high machinability. Chips are also favourable at this condition.

Conclusion

From turning experiments carried out on hardened AISI 4340 steel rods, following conclusions may be drawn:

  1. (i)

    Good machinability is observed while turning hardened AISI 4340 steel rods with all the three inserts when the cutting velocity is greater than 100 m/min.

  2. (ii)

    Not much chip breaking effect is seen even when narrow and wide groove tool inserts are used. Only at certain conditions with low cutting velocity, broken chips are noticed.

  3. (iii)

    Comparing the results obtained by employing all the three types of inserts, it can be stated that at a high cutting velocity of 272 m/min and feed of 0.1 mm/rev, wide groove type chip breaking insert is seen to turn the workpiece requiring low Fc, and yields favourable machining chip. Therefore, this machining condition may be recommended to adopt.