Basic research on mechanism of BN inclusion in improving the machinability of steel

Boron nitride-added eco-friendly free cutting steel has recently drawn more and more attention. But, the mechanisms explaining the role of BN inclusions improving the machinability of steels is not very clear. In this investigation, the material removal mechanism for cutting of BN inclusions in steels is explored, using a combination of theoretical analysis and a series of experiments. First, the actual shape of BN inclusions is observed and the amount and distribution of BN inclusions is quantitatively analyzed. Subsequently, the cutting performance of the steel is determined by cutting experimental tests. Moreover, the micro mechanical properties and the material removal mechanisms for cutting of BN inclusions are investigated by means of nanoindentation. The results revealed that the BN inclusions are hexagonal and are uniformly distributed, their average content is 23.2 per unit area and their volume fraction is 0.51% in the steel with 74 ppm B and 180 ppm N. It is shown that BN inclusions can improve the cutting performance of steel significantly, and a model describing the material removal mechanism for cutting of BN inclusions is proposed. BN inclusions act as stress concentration source, lubrication and wrappage of hard particles.


INTRODUCTION
As an important material for national economy, free cutting steel is very useful for machining, household appliance and automobile.At present, free cutting steel is mainly divided into two categories: lead series and sulfur series (Yoshimura, 1985;Katayama et al., 1988).Non-metal elements added deliberately in those series which can decrease steel quality, pollute the environment and cause serious injuries to the health of operators (Liu and Chen, 2014;Spitzig and Sober, 1981;Yamamoto et al., 2011;Chen et al., 2007;Spitzig, 1983).Thereby it is urgently required to develop a new generation of environment-friendly free cutting steel and make innovations on cutting mechanisms.BN free cutting steel, as an important new free cutting steel, has drawn more and more attention because of its advantages in environmental protection and good mechanical properties.
In recent years, the possibility of adding BN to improve the machinability of steel was explored (Tanaka et al., 2007;Tanaka et al., 2009).The influences of steel composition and tool material on the machinability have been investigated and confirmed that BN inclusions could improve the machinability of steel significantly.The precipitation behavior of BN type inclusions in steel was studied and the relationship between the size, distribution of BN inclusions and the cooling rate of the steel was explored (Wang et al., 2013a;Wang et al., 2013b).Then a control model of BN inclusions was proposed (Wang et al., 2013c).The influence of heat treatment on formation behavior of boron nitride inclusions in heat resistant steel was discussed (Sakuraya et al., 2006).Besides, BN free-cutting stainless steel for precision machining capable of attaining all of excellent cutting accuracy, machinability, corrosion resistance, and environmental friendliness at the same time was developed successfully by the method of heat treatment (Emura et al., 2012;Emura et al., 2013).In order to optimize cutting performance of BN added steel, cutting mechanism of BN inclusions is required to be investigated.In this paper, different modes and materials of cutting are chosen to investigate the cutting mechanism of BN inclusions.By combination with nanoindentation, the cutting mechanism model of BN inclusions is proposed.Owing to the great wealth of BN-type environmentfriendly free cutting steel applications, the study will have significant practical implication.

EXPERIMENT
The samples were obtained by crucible melting and the experiments were proceeded in vacuum induction furnace.A crucible with a capacity of 200 kg was used.The whole process of the experiments was carried out with argon protection.The industrial pure iron was adopted as the basic material in the experiment.Ferro boron (boron content is 20 wt%) and MnN (purity is 99.99%) were added into steel to obtain B and N. The influence of free B on the hardenability of the steel could be eliminated for the N/B ratio in steel higher than 1.7, so N/B ratio is controlled higher than 1.7 in the research.The chemical composition of the samples obtained by crucible melting is shown in Table 1.
Cutting experiments were performed to investigate the machinability of different samples.Experimental materials were BN free cutting steel sample and comparison sample.Experimental conditions are shown in Table 2. Experimental processes were shown as follows: high-speed drills were used to cut experimental materials, and cutting was interrupted to measure the average width of flank wear by an optical microscope.To obtain a statistically valid conclusion, each sample undertook three tests and the average value were adopted.
In order to obtain the original BN inclusions in steel without loss, the samples were electrolyzed by organic electrolyte solution (Bao et al., 2012) (methanol: triethanolamine: tetramethylammonium chloride is 71:11:2 by mass ratio) and the inclusions were extracted by high-speed centrifuge (TG-16) and glass capillary (inside diameter is 1 mm) after electrolysis.The rotation speed was set to 9000 r min −1 and the centrifugal time was set to 3 minutes.
Nanoindentation method is used to investigate the micro mechanical properties of the matrix and the inclusions in steel, including BN, MnS, Al 2 O 3 and TiN, so that the effect of BN inclusion on improving machinability of steel can be explored further.

The morphology of BN inclusions
The analysis results of the BN inclusion in experimental samples which extracted by electrolysis shows that BN is cluster-like inclusion, ranged from 5~30 μm, shown in Figure 1.The BN participated in the steel is analyzed by Raman spectroscopy.The result (Fig. 2) shows that Ramanactive high-energy phonon is at 1367 cm −1 , which agrees well with the pure hexagonal boron nitride (Reich et al., 2005), which has the same hexagonal crystal structure and lubrication action as graphite, and is opposite with super hard cubic BN.

The distribution of BN inclusions
Cluster-like BN inclusions distribute uniformly in experimental samples, as shown in Figure 3.The 3D sketch of BN inclusions is shown in Figure 4. Six metallographic specimens were taken from different positions of the ingot in order to count and analyze the inclusions in sample as accurate as possible.The amount of BN inclusions was counted and the distribution was quantitatively analyzed by optical microscope (400×) with 100 view fields of each specimen collected.According to statistic data, the average diameter of BN inclusions is 16.8 μm, the average number is 23.2 per unit area and the area ratio is 0.507% in C45BN sample with 74 ppm B and 180 ppm N, as shown in Table 3.

The improved machinability of BN added steel
The hardness of samples, including cast samples and forged samples, are compared in Table 4.The hardness of cast C45 and C45BN samples is 199.8 and 190.3 respectively, while the hardness of forged C45 and C45BN samples is 225 and 220 respectively, which shows that the hardness of samples in the same condition is almost the same level.Based on the hardness results, different modes and materials of cutting were chosen to investigate the effects of BN inclusions on machinability of steel.Figures 5-7 show the relationship between the average width of flank wear and cutting length in cast and forged samples, if the rotational speed (RS) is 2100 r min −1 and 560 r min −1 , separately.Table 5 shows the tool life during cutting different materials in different modes.It can be seen from above results that BN inclusions can improve the cutting performance of steel significantly.According to Figure 7, the average width of flank wear is only 154 μm with 2051 mm cutting length in forged C45BN sample, while the average width of flank wear is reach up to 354 μm with only 1053 mm cutting length in forged C45 sample.The tool wear in cutting BN added steel is less than half of that in cutting the compared steel while the cutting length of BN added steel is twice than that of compared steel.Especially, if forged samples were cut in high rotational speed,   C45BN 190.3 220 FIGURE 5. Relationship between the average width of flank wear and cutting length in cast samples (RS: 2100 r•min −1 ).BN inclusions have the most obvious effect on machinability of steel.The tool life of the cutting forged C45BN steel at 2100 r min −1 is about 19 minutes, which is almost 5.4 times as long as the other grade at the same condition.Generally speaking, the tool flank wear can be reduced remarkably at low speed cutting and the service life of a tool can be prolonged noticeably at high speed cutting of BN added steel.

The cutting mechanism model of BN inclusions
Based on the theory analysis and experiments, the cutting mechanism model of BN inclusions was proposed.

Stress concentration source
The micro mechanical properties of the matrix and the inclusions in sample were investigated by nanoindentation method.As can be seen from the nanoindentation characterization results (Table 6), the hardness of BN is the lowest in comparison to base metal, followed by MnS, Al 2 O 3 , and TiN.The hardness of TiN is greater than the base metal.Accordingly, BN inclusions can destroy the matrix continuity and play the role of internal gap in matrix due to its low hardness, just as the 3D sketch of BN inclusions in matrix shown in Figure 4.Because of this, stress concentration will be caused in the cutting process and then the cutting force will be reduced.

Lubrication
Table 7 shows the plastic deformation of inclusions in samples, which are obtained by the loaddisplacement curve (nanoindentation experiments results).The value of BN plastic deformation is the largest, and the second is MnS, both are larger than the base metal and Al 2 O 3 .And TiN has the smallest plastic deformation, relatively.Accordingly, BN inclusions, just like as MnS inclusions, have a good plastic deformation capacity.In addition, BN added steel was cut by high speed steel drill, elements B and N were found attached on the surface of the cutting tool, as shown in Figure 8.
The presented results show that BN inclusions soften and adhere the tool rake face in the cutting process, participating in the plastic flow of rheological layer in tool-chip contact region.The toolchip friction and the cutting force can be reduced.Consequently, BN inclusions have an effective internal lubrication in the chip shear region and the deformation flow layer, as well as lubrication between the chip and the tool.

Wrappage of hard particles
Some compound BN inclusions were observed in C45BN sample.As shown in Figure 9, hard particles, such as Al 2 O 3 , SiO 2 , were wrapped by BN.The direct contact of cutting tool and hard particles will be avoided in the cutting process, thus wear of hard inclusion on cutting tool can be reduced and then steel cutting performance can be improved.

CONCLUSIONS
The BN participated in steel is h-BN.The BN inclusions distribute uniformly, the average number is 23.2 per unit area and the area ratio is 0.507% in steel with 74 ppm B and 180 ppm N.
BN inclusions can improve the cutting performance of steel significantly.The flank wear can be reduced remarkably at low speed cutting and cutting tool life can be extended noticeably at high speed cutting in BN added steel.
The cutting mechanism model of BN inclusions is proposed in the paper.BN inclusions have the effects of stress concentration source, lubrication and wrappage of hard particles, which improve the cutting performance of steel greatly.

FIGURE 2 .
FIGURE 2. Raman spectra of BN inclusion participated in steel.

FIGURE 6 .
FIGURE 6. Relationship between the average width of flank wear and cutting length in forged samples (RS: 2100 r•min −1 ).

TABLE 1 .
Chemical compositions (in mass pct) of casted samples

TABLE 2 .
Cutting experiment conditions

Tool Bench drilling machine(LT-13), Optical microscope, High-speed drill
FIGURE 1. SEM image of actual BN cluster-like inclusions in steel.

TABLE 3 .
The average diameter, average number per unit area and the area ratio of BN inclusions in C45BN sample