ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Regular Article
Machinability Improvement and Its Mechanism in SUS304 Austenitic Stainless Steel by Precipitated Hexagonal Boron Nitride
Satoshi EmuraMasahiro KawajiriXiaohua MinShigeo YamamotoKazuyuki SakurayaKaneaki Tsuzaki
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2013 Volume 53 Issue 10 Pages 1841-1849

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Abstract

To improve the machinability of SUS304 (Type 304) austenitic stainless steels, specimens were prepared containing 0.016 mass% boron and 0.2 mass% nitrogen, and hexagonal boron nitride (h-BN) particles with a diameter of 1 to 5 μm were precipitated. Precipitation of h-BN reduced the cutting force and tool wear during lathe turning with a cemented carbide tool insert, especially at cutting speeds of 40 m/min and higher. The reduction in cutting force appeared attributable to internal lubrication by h-BN in the chip shear region and the deformation flow layer, as well as to lubrication between the chip and carbide tool. Improved chip disposability and tool wear suppression were also achieved by h-BN precipitation. SUS304 steel with precipitated h-BN was found to exhibit good machinability in drilling and sawing operations with high-speed steel tools.

1. Introduction

Austenitic stainless steels, such as JIS SUS304 (AISI Type 304), are widely used due to their excellent mechanical properties, superior corrosion resistance, and good formability. However, they have poor machinability.1) The machinability ratio of austenitic stainless steel, which corresponds to tool life, is 40 or less, as opposed to a machinability index of 100 for a sulfur free-cutting steel (AISI B1112).2)

Some of the main reasons for the poor machinability of austenitic stainless steels are listed below.

(1) High work hardening

On lathe turning, the machined surface is work-hardened. This work-hardened surface is machined on the next lathe turning step, which accelerates tool wear. The degree of work hardening depends on factors such as the grain orientation at the surface. As the fluctuation of the cutting force increases, the surface finish becomes rougher.

(2) High adhesion to cutting tools

The newly formed chip is prone to adhere to the rake face of the cutting tool, causing hindrance of chip flow and increase of chip thickness. This accelerates tool wear and results in increased cutting force and a rough surface finish.

(3) Low thermal conductivity

Higher temperatures in the cutting region increase tool wear. A rise in cutting temperature is more severe when drilling, because heat generated in the drilling process is difficult to remove.

(4) High ductility

High ductility increases chip thickness and cutting force. Increased chip thickness results in the formation of continuous chips, which decreases chip disposability.

The addition of alloying elements that form free-cutting inclusions is widely used to improve the machinability of steels, including austenitic stainless steels.3,4) Lead (Pb), sulfur (S), bismuth (Bi), selenium (Se), and tellurium (Te) are known free-cutting additives. Lead free-cutting steels generally include 0.1 to 0.3 mass% Pb. Lead disperses in steels as pure metal particles with diameters in the micrometer range, and these particles melt at raised temperature during machining. Molten lead has positive effects on machinability: reduced tool wear due to lubrication both at the rake face and the flank face, reduced cutting force, and improved chip disposability due to liquid metal embrittlement in the chip shear region.5) The addition of lead has successfully improved SUS304 steel machinability without degrading the steel’s mechanical properties, formability, and corrosion resistance.6) However, environmental and recyclability concerns have made it difficult to continue using materials with lead additives. Adding sulfur to steel is one well-known method for producing a lead-free free-cutting steel. Sulfur reacts with the manganese (Mn) in steel to form MnS inclusions, which act as stress concentration points in the chip shear region that improve machinability,3) reduce chip thickness, reduce tool-chip contact length,7) reduce the friction coefficient between the tool and the chip,8) and provide internal lubrication.9) A drawback is that MnS inclusions create anisotropy in the mechanical properties because they are elongated along the rolling direction.10) SUS303, created by adding sulfur to SUS304 base steel (sulfur content: 0.15 mass% or higher), is a JIS standardized austenitic stainless steel widely used as a free-cutting steel. However, MnS inclusions worsen pitting corrosion, so the corrosion resistance of SUS303 steel is severely deteriorated.11)

Studies using hexagonal boron nitride (h-BN) in place of lead or sulfur as a free-cutting additive have mainly focused on carbon steels such as JIS S45C (AISI 1045), with the results showing improved machinability through h-BN addition.12,13,14,15) The crystal structure of h-BN is similar to that of graphite, and is easily shear-deformed at the layer boundary. Thus h-BN is expected to improve machinability by promoting deformation in the chip shear region and providing lubrication between the tool and the chip. Compared with sulfur free-cutting steel, h-BN free-cutting steel shows weaker anisotropy of mechanical properties as well as equal or improved cold forgeability.14)

One of the authors recently found that h-BN inclusions are precipitated in high-chromium ferritic heat-resistant steels for thermal power plants with addition of boron and nitrogen for the purpose of creep property improvement. Another finding was that these h-BN inclusions are re-dissolved during heat treatment at a temperature in excess of 1523 K, and re-precipitated during subsequent annealing treatment.16) On the basis of these findings, the authors have successfully developed an h-BN free-cutting SUS304 base austenitic stainless steel.17) A commercial SUS304 austenitic stainless steel and a predetermined amount of commercial ferroboron were melted under reduced pressure of a nitrogen atmosphere to produce SUS304 base steels containing up to 160 ppm (0.016 mass%) B and approximately 0.2 mass% N. After h-BN dissolution heat treatment at 1523 K followed by h-BN re-precipitation heat treatment at 1323 K, spherical h-BN particles with a diameter of 1 to 5 μm were homogeneously precipitated in the stainless steel matrix. This h-BN free-cutting austenitic stainless steel was found to have improved machinability, including a 20% reduction in cutting force during high-speed lathe turning (cutting speeds of 100 m/min and higher).17) Furthermore, this h-BN free-cutting austenitic stainless steel exhibits other advantages, such as weak anisotropy of mechanical properties due to the spherical shape of the h-BN particles, high corrosion resistance equivalent to that of SUS304 and superior to that of sulfur free-cutting steel (SUS303), and the availability of existing stainless steel production lines that use similar manufacturing processes.17)

As with other free-cutting steels, there are many possible mechanisms for the improved machinability of h-BN free-cutting austenitic stainless steel, such as embrittlement, internal lubrication, suppressed adhesion of the chip to the tool surface, and reduced tool wear; however, the details are still unknown. Figure 1 shows a schematic diagram of lathe turning. Here, the cross section at the center region of the chip is shown as the pseudo-orthogonal cutting model. The following parameters are thought to be affected by the precipitation of h-BN particles: (1) deformation resistance in the chip shear region; (2) shear angle of chip; (3) friction force between the tool and the chip; (4) adhesion of the chip to the tool surface; (5) deformation resistance at the deformation flow layer near the back surface of the chip, which is formed to cancel the difference in flow speed within chips; and (6) breakability of chips (chip disposability). It is important to understand how the h-BN particle precipitation affects these six parameters when investigating the mechanisms for improved machinability. In addition, because the machinability of a material depends on the machining method, it is also useful to evaluate the effects of h-BN particle precipitation when using machining methods other than turning. Therefore, this study targets the effects of h-BN particle precipitation on the machinability of h-BN free-cutting austenitic stainless steels. In particular, cutting force, chip disposability, and tool wear are evaluated, mostly with respect to the machining method of turning, but also with respect to drilling and sawing. Finally, the mechanism of improved machinability is discussed.

Fig. 1.

Schematic diagram of orthogonal cutting by lathe turning. ϕ: Shear angle of chip. L: Tool-chip contact length. Fμ: Width of deformation flow layer. F1: Principal force. F2: Feed force. F3: Thrust force.

2. Experimental Procedure

2.1. Work Sample Production Process

A commercial SUS304 round bar with a diameter of 55 mm and a commercial ferroboron (Fe–19.2 mass% B) were used as raw materials. Two different lots of SUS304 were used in this study (Lot A and Lot B). The chemical compositions of these lots are shown in Table 1. The table also lists the measured chemical composition of the sulfur free-cutting steel SUS303. Approximately 18 kg of raw material was melted in a vacuum induction melting furnace. During the melting process, nitrogen gas with a partial pressure of 0.07 MPa was introduced into the furnace in order to adjust the concentration of nitrogen in the molten steel to a target value of 0.2 mass%. After removing the top of the ingot, cast ingots (top end: 95 mm square; bottom end: 85 mm square; height: 285 mm) were hot forged. The ingot made from Lot A was forged at 1473 K into a round bar with a diameter of 55 mm, and the ingot made from Lot B was forged at 1523 K into a round bar with a diameter of 60 mm. First, h-BN solution treatment was performed on the forged bars, consisting of heat treatment in air for 30 min at 1523 K, followed by water quenching. Then, h-BN precipitation treatment was performed on the water-quenched bars, consisting of heat treatment in air for 1 h at 1323 K, followed by water quenching. The conditions of the four different work samples in this study are listed in Table 2. Work samples from Lot A are identified as Sample A, and work samples from Lot B are identified as Sample B. To investigate the effect of boron content on machinability, Sample A0 contained no ferroboron and Sample A2 was prepared with ferroboron at a target boron content of 0.016 mass%. Both Sample A0 and A2 were hot forged and subjected to heat treatment. To further investigate the effect of boron addition on machinability, two Sample B ingots were prepared with 0.016 mass% boron. Sample B1 was subjected to only h-BN solution treatment after forging. Sample B2 was subjected to both h-BN solution treatment and h-BN precipitation treatment. The nitrogen content in the top of the ingot was determined by the inert gas fusion thermal conductivity method. The boron content of the heat-treated samples was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). In addition to the total boron content, the soluble boron content was also ascertained by subtracting the insoluble boron content (determined through a sequence of dissolution in aqua regia, filtration, and ICP-OES) from the total boron content.

Table 1. Chemical compositions of commercial SUS304 and SUS303 (sulfur free-cutting stainless steel) (mass%).
TypeLotCSiMnPSNiCrFe
304A0.070.331.150.0360.0228.0718.03bal.
B0.070.491.120.0250.0248.0618.05bal.
3030.050.331.790.0280.299.0217.9bal.
Table 2. Condition of work samples.
SampleUsed Lot of SUS304Nominal Compositions (mass%)Heat treatment
BN
A0A00.2ST1*+ST2**
A2A0.0160.2ST1+ST2
B1B0.0160.2ST1
B2B0.0160.2ST1+ST2
*  ST1: Heat treatment at 1523 K for 30 min followed by water quenching

**  ST2: Heat treatment at 1323 K for 1 h followed by water quenching

2.2. Observation of h-BN Particles

Precipitated h-BN particles in the samples were observed by scanning electron microscopy (SEM). In this study, fracture surfaces were observed to avoid the dropout of h-BN particles during normal SEM sample preparation sequence, namely cutting, mounting and polishing,16) Round bars with a diameter of 3.6 mm and a length of approximately 50 mm were cut out from heat-treated samples, where the longitudinal direction of the bars was normal to the forging direction. A circumferential notch 2 mm depth was formed at the position 7 mm from the end of the bar. Just prior to observation, the bar was bent and broken at the notch, and the fracture surface was observed by SEM as well as analyzed by energy-dispersive X-ray spectroscopy (EDS) to identify the inclusion particles. The observed surface was parallel to the forging direction.

2.3. Evaluation of Machinability

Machinability was evaluated mainly by lathe turning, but other machining methods, such as drilling and sawing, were performed on Sample A.

2.3.1. Lathe Turning

After removing the mill scale, the heat-treated samples were lathe turned. The cutting forces were measured, tool wear was evaluated, and the chips were observed to determine their form, disposability, and internal microstructure. As a comparative material, commercial SUS303 round bars with a diameter of 55 mm were also lathe turned. WC–TiC–TaC–Co cemented carbide tool inserts (M30) were used in the turning process. Inserts without a chip breaker were used, except in the evaluation of tool wear using chip breaker inserts. Using chip breaker inserts reduced cutting forces and stabilized the cutting state, which resulted in a reduced tool wear disturbance factor. The shape of the insert was as follows: front top rake: –6°; side rake: –6°; front clearance: 6°; side clearance: 6°; end cutting edge angle: 45°; side cutting edge angle: 15°; and tool edge radius: 0.4 mm. Figure 2 shows the schematic view of the turning process. Turning was performed dry with a cutting depth (Dc) of 1.0 mm and a feed rate (f) of 0.1 mm/rev. The cutting speed (v) was between 12 and 200 m/min. Three components of cutting force (principal force F1, feed force F2, and thrust force F3) were measured on a high-speed lathe that included a tool dynamometer. The resultant cutting force (R) was calculated using Eq. (1).   

R= ( F 1 ) 2 + ( F 2 ) 2 + ( F 3 ) 2 (1)
Shear angle of chip (ϕ in Fig. 1) was calculated from the thickness of the chip,18) and chip disposability was evaluated from the chip’s form. A quick-stop cutting device was used to obtain specimens at the chip generation points.19) To observe the deformed inclusions inside the chips, chips with a width of 5 mm were created by cutting-off at a feed rate of 0.1 mm/rev and a cutting speed of 50 m/min. The chip specimens were then bend-fractured using a special device (Fig. 3), and the fracture surfaces were observed by SEM. To examine the effect of h-BN particles on lubrication in the early stage of cutting, samples were turned at a cutting speed between 50 and 200 m/min and a cutting distance of 10 m with inserts that had a mirror-polished rake face, and the tool-chip contact length was determined to measure the length of the scratch on the rake face. To evaluate the tool wear on the flank face of the inserts, flank wear (VB) and notch wear (VN) were measured on chip breaker inserts after turning at a cutting speed of 150 m/min and a cutting distance of 750 m.
Fig. 2.

Schematic view of lathe turning. F1: Principal force. F2: Feed force. F3: Thrust force. Dc: Cutting depth. f: Feed rate. v: Cutting speed.

Fig. 3.

Collection method of fracture section for interior observation of chip.

2.3.2. Drilling

A drilling machine with a tool dynamometer was used to measure torque and thrust during drilling. The tip of the drill was observed in order to evaluate tool wear. Drilling was performed at a drilling speed between 11.8 and 21 m/min, and a feed rate between 0.07 and 0.13 mm/rev using high-speed steel drills with a diameter of 10 mm, tip angle of 130°, clearance angle of 6°, and helix angle of 30°. Chlorine-free cutting oil was poured during drilling operation.

2.3.3. Sawing

A high-speed steel saw blade for a hand hacksaw (length: 250 mm; height: 12 mm; thickness: 0.64 mm; teeth: 14/inch) was attached to an innovated sawing machine20) in order to determine the number of necessary cutting strokes to saw through a 25 mm wide, 10 mm thick sample with a load of 196 N and a cutting speed of 40 m/min. Water-soluble cutting fluid was used in the operation. To minimize the effects of individual differences and the degradation of the saws, the reference material (sulfur free-cutting steel JIS SUM24L (AISI 12L14)) was cut using a new saw blade before cutting each sample. Each sample was cut 10 times. The necessary cutting strokes were counted for each cut, and normalized by the cutting strokes for the reference material.

3. Results and Discussion

3.1. h-BN Particle Precipitation

Table 3 shows the total boron, soluble boron, and nitrogen contents of the samples. In all samples, total boron content and nitrogen content were close to their target values (boron: 0.016 mass%; nitrogen: 0.2 mass%). Sample B1, the sample subjected to only h-BN solution treatment, had soluble boron content of 0.006 mass%. In contrast, Samples A2 and B2, which were subjected to both h-BN solution treatment and h-BN precipitation treatment, had lower soluble boron contents of 0.003 mass% and 0.002 mass%, respectively. Most insoluble boron was present as h-BN, so Samples A2 and B2 contained more h-BN particles than Sample B1 did.

Table 3. Measured chemical compositions of samples.
SampleTotal B (mass%)Soluble B (mass%)Total N (mass%)
A00.0000.0000.20
A20.0160.0030.18
B10.0170.0060.20
B20.0160.0020.20

Figure 4 shows the SEM micrographs of h-BN inclusions on the fracture surfaces of Samples B1 and B2. The types of inclusions, as identified by EDS, are also indicated in the figure. In both samples, spherical h-BN particles with a diameter of 1 to 5 μm were observed (indicated by white arrows); elongated MnS inclusions 10 to 20 μm in length were also observed (indicated by black arrows), which were derived from manganese and sulfur in the SUS304 raw material. Some MnS inclusions were combined with h-BN. Inclusion precipitation behavior was similar in both samples. It is difficult to quantify the amount of h-BN particles from Fig. 4, because fracture tended to occur more easily at a position with more inclusions.

Fig. 4.

SEM images of inclusions in (a) Sample B1 and (b) Sample B2.

3.2. Lathe Turning

The effect of h-BN particles on the resultant cutting force of SUS304 base samples is shown in Fig. 5 as a function of cutting speed. Lathe turning was performed using inserts without a chip breaker. In the cutting speed range of 12 to 30 m/min, all samples exhibited similar cutting forces. In contrast, at cutting speeds of 40 m/min and higher, and especially at the cutting speed of 60 m/min, Sample A0, the sample with no boron addition, showed a higher resultant cutting force than the other samples that contained added boron. Sample B1, which was subjected to only h-BN solution treatment, exhibited a higher cutting force than Samples A2 and B2, which were subjected to both h-BN solution treatment and h-BN precipitation treatment. As was mentioned in Section 3.1, Sample B1 contained fewer h-BN particles than Samples A2 and B2, based on the total boron and soluble boron content. Thus the difference in cutting force seems to be due to the difference in the amount of h-BN particles. Furthermore, the h-BN particles in Sample B1 were already present when the ingots solidified, so they might have had a different precipitation state than the re-precipitated particles, which would affect the cutting force. The effects of the amount, shape, and precipitation state of the h-BN particle on machinability are now under investigation. Samples A2 and B2 showed similar cutting force values at the cutting speed range used in this study, so the effects of lot-to-lot variation in the raw material were small.

Fig. 5.

Effect of h-BN precipitation on the cutting force of SUS304 steel samples. Cutting force of SUS303 is shown for comparison.

Figure 6 shows the shear angle of chip of Samples A0 and A2, calculated based on measurements of the thickness of the chip generated during a turning operation of each sample; a thicker chip corresponds to a smaller shear angle of chip. For Sample A0, the sample with no boron addition, the shear angle of chip was smallest at the cutting speed of 60 m/min. At this cutting speed, the formation of thicker chips resulted in the increased cutting force indicated in Fig. 5. The effects of h-BN precipitation and cutting speed on the appearance of chips during lathe turning of Samples A0 and A2 are shown in Fig. 7. For Sample A0, continuous chips were formed at cutting speeds of 23 m/min and 44 m/min, which were artificially divided as shown in Fig. 7. In Sample A2, continuous chips were formed at a cutting speed of 23 m/min. Although the size and shape of chips varied depending on the cutting speed, Sample A2, which contained boron, shows better chip disposability. Chip breakage appeared to be enhanced by h-BN precipitation.

Fig. 6.

Effects of h-BN precipitation and cutting speed on shear angle of chip during lathe turning of SUS304 steel samples.

Fig. 7.

Effects of h-BN precipitation and cutting speed on the appearance of chips during lathe turning of SUS304 steel samples.

As shown in Fig. 5, the effect of h-BN particles on the resultant cutting force differed according to cutting speed. To further investigate, quick-stop tests were performed during turning of Sample A0 and A2 at cutting speeds of 20, 40, and 60 m/min. Figure 8 shows the chip generation points obtained during the quick-stop tests. At lower cutting speeds, build up edges (B. U. E.) were formed at the tip of the inserts due to the adhesion and pileup of heavily deformed work samples, as shown in Figs. 8(a), 8(b), 8(d), and 8(e). The B. U. E. shape and hardness depended on various factors such as cutting speed and work materials, and considerably affected the cutting force. Thus at lower cutting speeds, the effects of B. U. E. on cutting force made the effects of h-BN precipitation unclear. B. U. E. also increased the shear angle of chip, which reduced the area fraction of the chip shear region, and suppressed tool-chip contact. These effects also reduced the effects of h-BN particles. In contrast, at cutting speeds of 60 m/min and higher, B. U. E. disappeared because work softening due to deformation heat surpassed work hardening due to machining. In this cutting speed range, the reduction of deformation resistance in both the chip shear region and the deformation flow layer due to internal lubrication by h-BN particles appeared to reduce cutting force. Figure 9 shows SEM micrographs of bend-fracture surfaces of chips obtained from Sample A2 by cutting-off. Figure 9(a) shows a low magnification view, and Figs. 9(b), 9(c) and 9(d) show the magnified views of positions B, C, and D in Fig. 9(a), respectively. As can be seen in Fig. 9(c), h-BN particles elongated along the chip shear direction (indicated in Fig. 9(c) by a white arrow) were observed in the chip shear region; additionally, MnS inclusions can be seen in Fig. 9(b). Such elongated h-BN particles were not observed in work samples prior to turning, so they are thought to be elongated along the chip shear direction at the chip generation points. Internal lubrication due to h-BN particle deformation in the chip shear region appeared to contribute to reducing cutting force. Thinly spread h-BN particles were also observed near the back surface of the chip, as shown in Fig. 9(d). The observation area is thought to have been dragged onto the fracture surface side during the bending fracture. This spread of h-BN particles suggests internal lubrication due to the heavy deformation of h-BN particles inside the deformation flow layer.

Fig. 8.

Optical micrographs of chip generation points obtained using a quick-stop device during lathe turning of SUS304 steel samples at various cutting speeds. B. U. E.: Build up edge.

Fig. 9.

SEM images of bend-fracture section of chip obtained from Sample A2. (a) Low magnification. (b)–(d) High magnification of the regions B to D in (a). Cutting speed: 50 m/min. Feed rate: 0.1 mm/rev.

Lubrication between the tool and the chip due to the h-BN particle precipitation should also reduce the cutting force. To confirm this, tool-chip contact lengths (L) were measured after lathe turning with a cutting distance of 10 m. As shown in Fig. 10, L was calculated as a function of L1, the length of the area where the tool and the chip were in constant contact, and L2, the length of the area where the tool and the chip were in intermittent contact.   

L=L1+ L2 2 (2)
Figure 11 shows the effects of h-BN precipitation and cutting speed on L. Sample A2, which contained h-BN, exhibited smaller L values than did Sample A0, the sample containing no h-BN, regardless of the cutting speed. This result suggests that lubrication due to h-BN precipitation was present from the early stage of cutting. The precipitation of h-BN particles also enhanced chip disposability because a smaller tool-chip contact length reduced the chip curl radius, causing the chip to impact the tool and work sample more frequently and thus enhancing chip breakage. We attribute the improved chip disposability to primarily the reduced chip curl radius, although h-BN precipitation might also increase the breakability of chips.
Fig. 10.

Measurement of tool-chip contact length.

Fig. 11.

Effects of h-BN precipitation and cutting speed on tool-chip contact length during lathe turning of SUS304 steel samples.

As shown in Fig. 5, there was a marked difference in resultant cutting force at a cutting speed of 60 m/min. As the cutting speed increased to 80 m/min and higher, chip temperature increased especially at the area contacting the rake face of tool. This temperature rise increased the deformability of the deformation flow layer, and reduced the cutting force, which made the effect of h-BN particle precipitation relatively small. The change in the cutting force corresponded to the change in the shear angle of chip shown in Fig. 6.

As shown in Fig. 5, SUS303, the sulfur free-cutting stainless steel, exhibited the lowest resultant cutting force of all the investigated SUS304 steels containing h-BN precipitates. However, SUS303 contained approximately 0.3 mass% sulfur, which was 10-fold the amount of boron added to the h-BN free-cutting austenitic stainless steels. This study revealed that even a small addition of boron was effective in improving the machinability of austenitic stainless steels.

Figure 12 shows tool wear on the flank faces after lathe turning of Sample A0, Sample A2, and SUS303 steel at a cutting speed of 150 m/min and a cutting distance of 750 m. After turning, flank wear (VB) and notch wear (VN) were the lowest in SUS303, while Sample A2, the sample with boron addition, had lower VB and VN values than did Sample A0, the sample with no boron addition. As with the rake face, internal lubrication and lubrication between the tool and the chip are thought to be effective on the flank surface, which suppressed tool wear. Because the work-hardened surface of the work sample was turned at the position of notch wear, notch wear values were higher than flank wear values after turning steels with added h-BN. On the other hand, the notch wear was not significant after turning of SUS303; it has been reported that after turning, the work-hardened layer is smaller in SUS303 than in SUS304.21) Another possibility is that notch wear was suppressed during lathe turning of SUS303 due to the formation of upwardly curled chips with smaller curl radius.

Fig. 12.

Effects of h-BN precipitation on tool wear after lathe turning of SUS304 steel samples. Tool wear after lathe turning of SUS303 steel sample is also shown. Cutting speed: 150 m/min. Cutting distance: 750 m.

3.3. Drilling and Sawing

Figure 13 shows the effect of drilling speed on the torque and thrust during drilling of Samples A0 and A2. Two drilling feed rates were used: 0.07 mm/rev and 0.13 mm/rev. Severe wear of the cutting edges was not observed under these drilling conditions. Drills made of high-speed steels usually exhibit severe loss of hardness and cutting edge wear at temperatures between 773 K and 823 K, so the drilling temperature appeared to be less than 773 K under the drilling conditions in this study, which suggests the formation of B. U. E. Although the variation in torque was not large, the thrust was lower when drilling Sample A2, which contained h-BN, under all the drilling conditions in this study. Figure 14 shows the effects of h-BN particle precipitation on tool wear after drilling of Samples A0 and A2 at a drilling speed of 21 m/min, a feed rate of 0.13 mm/rev, and a total drilling distance of 440 mm. Figure 14(a) shows the notch wear at the chisel and cutting edges; Figure 14(b) provides a magnified view. Notch wear was suppressed in Sample A2, which contained h-BN, compared with that in Sample A0, which contained no h-BN. Figure 14(c) shows the flank wear at the cutting edge, which was also suppressed in Sample A2.

Fig. 13.

Effects of h-BN precipitation and drilling speed on thrust and torque during the drilling of SUS304 steel samples. (a) Feed rate: 0.07 mm/rev. (b) Feed rate: 0.13 mm/rev.

Fig. 14.

Effects of h-BN precipitation on tool wear observed after drilling of SUS304 steel samples. (a), (b) Notch wear at chisel edge and cutting edge. (c) Flank wear at cutting edge. Drilling speed: 21 m/min. Feed rate: 0.13 mm/rev. Total drilling distance: 440 mm.

Figure 15 shows the results of the sawing tests. Each sample was cut 10 times, and the number of cutting strokes for each cut is plotted with respect to cutting order (shown on the horizontal axis). As the sawing test proceeded, the number of cutting strokes necessary to make a cut increased because the cutting edge of the saw became duller. As in the drilling test, Sample A2, which contained h-BN, required fewer cutting strokes than Sample A0, which contained no h-BN.

Fig. 15.

Effects of h-BN precipitation on the number of cutting strokes while sawing SUS304 steel samples. Load: 196 N. Cutting speed: 40 m/min. Sample size: 25 mm × 10 mm.

Under the drilling and sawing conditions in this study, high-speed steels were used for machining, and the formation of B. U. E. was expected. The effect of the adhesion of the chip to the tool surfaces was negligible because cutting oil or cutting fluid was used. Therefore the main reason for the reduction of thrust during drilling (Fig. 13), the suppression of tool wear after drilling (Fig. 14), and improved sawability (Fig. 15) can be attributed to internal lubrication by h-BN particles in front of the B. U. E. However, the effect of h-BN particle precipitation was not clear in low-speed lathe turning, where the formation of B. U. E. was observed. Further investigation considering the shape and hardness of the B. U. E. is necessary. For example, on the lathe turning at a cutting speed of 12 m/min, Samples A2 and B2, which contained h-BN, exhibited a lower resultant cutting force than Sample A0, which contained no h-BN, whereas no significant difference was observed at a cutting speed of 30 m/min. This difference appeared to be due to variation of the B. U. E. shape.

4. Conclusions

Hexagonal boron nitride (h-BN) particles precipitated in SUS304 austenitic stainless steels improved their machinability. Lathe turning, drilling, and sawing were performed on samples containing different amounts of h-BN to investigate the effects of h-BN particle precipitation on machinability including cutting force, chip disposability, and tool wear. The study also considered the mechanism of the machinability improvement and led to the following conclusions:

(1) During lathe turning with a cemented carbide tool insert, h-BN precipitation reduced the resultant cutting force, especially at cutting speeds of 40 m/min and higher. Work samples with a larger amount of h-BN particles after h-BN precipitation treatment exhibited a lower resultant cutting force.

(2) The reduction in cutting force appeared to be due to internal lubrication by h-BN in the chip shear region and the deformation flow layer, as well as lubrication between the chip and the tool.

(3) The precipitation of h-BN particles also enhanced chip disposability by reducing the tool-chip contact length through lubrication, which reduced the curl radius of the chips and enhanced chip breakage.

(4) Both flank wear and notch wear during lathe turning were suppressed due to internal lubrication by h-BN in the deformation flow layer and lubrication between the chip and the tool.

(5) The precipitation of h-BN was also found to improve machinability in drilling and sawing operations using high-speed steel tools.

Acknowledgement

The authors are grateful to the Materials Manufacturing and Engineering Station of NIMS for their assistance in melting and forming the samples, and to the Materials Analysis Station of NIMS for supporting chemical analysis. The authors thank the Iketani Science and Technology Foundation for financial support.

References
 
© 2013 by The Iron and Steel Institute of Japan
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