Effect of Sulfide Inclusion Morphology on Surface Roughness in Free Cutting Steel

Naoki MATSUI1 and Junsuke FUJIWARA2

1 Corporate R&D Laboratories, Sumitomo Metal Industries, Japan, matsui-nok@sumitomometals.co.jp 2 Graduate School of Engineering, Osaka University, Japan, fujiwara@mech.eng.osaka-u.ac.jp

Abstract: In order to develop a new eco-friendly non-leaded free cutting steel, it is important to get good machinability by the use of manganese sulfide (MnS) inclusions in place of lead. In this study, the effect of MnS morphology on the surface roughness was assessed in a turning test used two steels containing different type of MnS, and dominant factors controlling the surface roughness were investigated. The distinct type of MnS affected the formation of built-up edge, and it accordingly caused the difference in the surface roughness. This would be based on difference in deformation characteristics of distinct type of MnS. Keywords: Free cutting steel, Manganese Sulfide, Inclusion, Built-up edge, Surface Roughness

1. Introduction

Low carbon leaded free cutting steel as typified by AISI 12L14 is commonly used for machinery components used in automotive, electrical goods and other industrial fields in manufacturing industry. Among required machinability factors, the surface roughness is emphasized and that of the leaded free cutting steel is particularly superior. However, the use of lead has become restrictive from the environmental point of view, and new eco-friendly steel containing no lead has been required. To comply with this demand, it was considered that an efficient use of manganese sulfide (MnS) inclusions in place of lead is important to get good machinability. It has been widely known that the surface roughness of free cutting steel is affected by MnS morphology, and it has been considered to be associated with the formation of a built-up edge [1-2]. However, the factors controlling the surface roughness of free cutting steels in turning operation or the effect of steel chemistries on the formation of built-up edge is considered to be still unclear.

In this study, a turning test with a carbide tool was performed for two steels containing different type of MnS inclusions. The machined surface and the carbide tool were assessed in detail to investigate factors controlling the surface roughness in the turning operation. Then, a quick stop test (QST) with plunge cutting was performed for these steels, and the deformation behavior of MnS inclusions in chip shear zone was microscopically observed. Microscopic mechanism on the difference in machining performance between the steels containing different type of MnS was also investigated. 2. Experimental methodology 2.1 Work Materials

Chemical compositions of tested steels are shown in Table 1. Both steels were designed to compositions of

0.02wt.%-carbon, 0.4wt.%-sulfur and 1.3wt.%-manganese based on the low carbon free cutting steel in AISI 1215. The steels were obtained from ingots cast through a small vacuum malting furnace. MnS morphologies were changed by controlling the oxygen contents in the steel making process. That is, Steel G was cast as an ingot with higher oxygen content without the addition of any deoxidized elements like silicon or aluminum, and globular shaped MnS were expected to be crystallized in the ingot. In the other Steel F, a small amount of aluminum was added to obtain fine and spindle shaped MnS by reducing the oxygen content.

Table 1 Chemical compositions of tested materials (mass%)

C Si Mn S Al O Steel G 0.02 <0.01 1.3 0.4 0.001 0.018Steel F 0.02 <0.01 1.3 0.4 0.034 0.002

a) Steel G

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Figure 1: Comparison of optical micrographs and size distribution of MnS inclusions between a) Steel G and b) Steel F.

The ingots were forged to bars of 40mm in diameter. Then, the forged bars were annealed at 1173K for an hour, and they were cooled in the air. After that, the bars were peeled to 31mm in diameter, and they were drawn to 28mm diameter bright bars. Vickers hardness in bright bar was almost equivalent between both steels, which were Hv193 in Steel G and Hv191 in Steel F.

Optical micrographs and size distribution of MnS inclusions in bright bars are compared between Steel G and Steel F in figure 1. An apparent difference in MnS morphology can be seen among them. Large globular MnS inclusions, which were elongated to forged direction, can be observed in Steel G. On the other hand, a lot of finer and thinner MnS were distributed throughout the cross section of Steel F. Thus, although MnS morphologies were considerably different between both steels, total amount of MnS inclusions can be regarded as about same because the sulfur contents are equivalent between both steels. 2.2 Methods of machining tests

Cutting conditions in the turning test are shown in Table 2. Non-coated carbide tools with 0.2mm in nose radius were used in the test. The bright bars were continuously machined for 200m in cutting distance at each pass, and it was machined up to 3000m in total. Averaged surface roughness values (Ra) of the machined surface were measured every 400m and at point of 3000m in cutting distance by a surface roughness meter. And, the worn tool surface was also assessed every 400m in cutting distance by a laser microscope. The machined surface was observed by using a scanning electron microscope (SEM) as necessary.

Table 2: Cutting conditions in turning test

Tool Non-coated Carbide tool (JIS K10)(0º,0º,7º,7º,32º, -3º,0.2)

Cutting speed 100m/min Feed rate 0.03mm/rev

Depth of cut 1.0mm Lubrication oil Straight-type cutting oil

A quick stop test (QST) was demonstrated to assess the

deformation behavior of MnS in chip shear zone during machining. Figure 2 is a schematic diagram of the QST. The plunge cutting is frozen in a blink by swiftly releasing the cutting tool from the machined surface in the middle of operation. As specifically shown in figure 2, the work materials were preliminarily machined to leave the ridges of 2mm in width, and these were machined by an uncoated brazed carbide tool. The brazed tool was swiftly released without loading to the deformed chip by pulling out a bar, which has pinned the cutting tool during machining. The cutting condition of plunge cutting is shown in Table 3. A frozen deformed chip, which was obtained in the QST, was directly observed by the SEM. Then, the specimen was preliminarily deposited with nickel electrolytic plating for the observation, and it was mounted in resin and polished to observe microstructures

in cross section of the shear zone of the deformed chip. 2.0mm2.0mm

Figure 2: Schematic diagram of quick stop test in plunge cutting

Table 3: Cutting conditions in quick stop test

ToolUncoated brazed carbide tool (JIS K10)Rake angle 6° Clearance angle 6°

Cutting speed 50m/min Feed rate 0.2mm/rev

Lubrication oil Straight-type cutting oil 3. Results and Discussions 3.1 Turning test

The evolutions of surface roughness in tested steels are shown in figure 3. The surface roughness of Steel G was almost comparable to that of Steel F at initial 400m in cutting distance. But after 400m, it underwent a transition at lower level compared to that of Steel F. As the result, the averaged surface roughness at 3000m was 0.6µm in Steel G, and it was 0.8µm in Steel F.

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Figure 3: Comparison in surface roughness between Steel G and Steel F

SEM images of machined surface at 3000m are compared in figure 4. Machined surface was different in appearance between both Steel G and F. Smooth surface could be seen in Steel G, but the machined surface of Steel F was randomly indented. It could be seen that the steel G containing globular MnS gave better surface roughness than Steel F containing finer MnS in the turning used the carbide tool.

The surface of work material is finished by a minor cutting edge of cutting tool in turning operation. In order to investigate factors giving the difference in surface roughness, the tip of nose radius corresponding to the minor cutting edge in the cutting tool was particularly

assessed by the laser microscope. Results in the laser microscope assessment of the worn tool are shown in figure 5. Focusing images and three dimensional diagrams at the tip of nose radius in the carbide tools, which machined for 3000m in cutting distance, were compared.

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Figure 4: SEM images of machined surface at 3000m in a) Steel G and b) Steel F.

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Figure 5: Results in laser microscope assessment on minor cutting edge of carbide tools, which machined a) Steel G and b) Steel F for 3000m.

In Steel G, the tip of nose radius wore about 150µm in

length, and no built-up edge was formed at this portion. It could be seen from the three dimensional image that the tool surface was worn away with regularly-arranged groove-like depressions. The worn surface in each depression was smooth, and interval of each depression was about 30µm, which was equivalent to the feed rate (0.03mm/rev) applying in this test. This result would suggest that the surface of Steel G was finished by the worn depressions at the minor cutting edge. Therefore, the surface roughness in Steel G was expected to have some sort of correlation with undulation of the worn tool surface. To confirm this, the worn minor cutting edge was assessed by the laser microscope every 400m in cutting distance with same way as figure 5a), and the cross sectional diagram at top position of the minor cutting edge just like between point A and point B showing at figure 5a) was taken. Then the depth of last depression, which appeared at the position away from the major cutting edge as indicated by a star mark in figure 5a), was particularly measured, and it was correlated with the averaged value of surface roughness (Ra) at every 400m in cutting distance.

The surface roughness is plotted as a function of the depth of worn depression in figure 6. It was unclouded that the surface roughness had good correlation with the depth of worn depression at minor cutting edge in Steel G.

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Figure 6: Relationship between surface roughness and depth of worn depression at minor cutting edge in Steel G.

On the other hand, in Steel F, a large built-up edge was observed at tip of nose radius corresponding to the minor cutting edge, but the wear as seen in Steel G was not observed. The indented surface in Steel F would have been affected by the built-up edge at this portion.

Thus, it was clear that there was difference in surface roughness between both steels containing different type of MnS even in turning used carbide tools. And, this could be associated with the difference in status of minor cutting edge of the machined tool. At this point, the growth of built-up edge was suppressed in Steel G containing globular MnS, but the wear with groove-like depressions evolved larger at minor cutting edge instead. Consequently, the smoother surface could be obtained due to the trace of the worn smooth depressions. On the other hand, in Steel F containing fine MnS, the large built-up edge was formed at minor cutting edge, and the groove-like wear was not observed at this portion. From these points, the acquired results should be associated with difference in the formation of built-up edge through difference in MnS morphology.

3.2 Quick stop test

The formation behavior of built-up edge was different between the steels containing different type of MnS in the turning test. This appeared to affect the difference in surface roughness among them. In order to assess the effect of MnS morphologies on the formation of built-up edge, the QST was demonstrated for both Steel G and F. Figure 7 shows SEM images of the specimens obtained from the QST. These images were taken from the tool side of frozen deformed chip. A distinct built-up edge could not be found at root portion of the deformed chip in Steel G. But, a large built-up edge was observed in Steel F, and the machined surface of Steel F was much rougher than that of Steel G. This suggests that MnS morphologies affected the formation of built-up edge also in plunge cutting. And, it is supposed that the steel containing fine MnS like Steel F tends to promote the formation of large built-up edge with an associated rough surface.

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Figure 7: SEM images of frozen deformed chip in a)Steel G and b)Steel F Microstructures in cross section of deformed chip were

observed by the SEM. Figure 8 shows the microstructures at the secondary shear zone in the specimen from Steel G. Very small built-up edge could be seen at root portion of deformed chip. As indicated by triangular arrowed marks in figure 8a), some micro-cracks were observed in the vicinity of this small built-up edge. These micro-cracks seemed to generate from drastically elongated globular MnS by the severe shear deformation in the chip shear zone. And, it can be also seen that they attained to the boundary between the small built-up edge and the chip. Furthermore, as shown in figure 8b), many deformed MnS were observed in the secondary shear zone of Steel G, and almost of them were attended with micro-cracks similar to figure 8b). It can be considered that the micro-cracks generated from deformed globular MnS with shear deformation. They would have a role to separate off the built-up edge from the chip, and they would contribute to suppress the growth of built-up edge.

Figure 9 shows the microstructure at the vicinity of large built-up edge in the specimen from Steel F. As shown in figure 9a), the size of built-up edge was over 300µm in width, and it was much larger than that of Steel G. Figure 9b) shows an enlarged image of surrounded area by a dotted square in Figure 9a). A lot of fine MnS were observed in the built-up edge interior. They were severely deformed to longer shape, but the distinct micro-cracks were not observed around them as shown in figure 8.

It was clear from the QST that the size of built-up edge was totally different between the steels containing different type of MnS. And, it was suggested that the generation of micro-cracks from deformed MnS was associated with growth of the built-up edge. That is, it was supposed that the micro-cracks, which generated from deformed globular MnS, had a role to separate off the built-up edge from the chip and that they contributed to suppress the growth of built-up edge. But, fine MnS tended to be involved into the interior of built-up edge without the generation of distinct micro-cracks, and it consequently made the built-up edge larger. Thus, it was considered that the formation behavior of built-up edge was closely associated with the deformation characteristics of different types of MnS. Iwata et al. indicated that the strain field around MnS in the chip shear zone was different among different shaped MnS. And, they also indicated by micro-cutting in SEM that the globular MnS created bigger voids around them than the fine MnS [3]. It is supposed that the results in this study would be able to be explained by similar conceptions to

theirs, and it would be closely associated with deformation characteristics of distinct types of MnS. Also in turning test, it is considered that the difference in the formation of built-up edge between the tested steels is based on the similar theoretical concept.

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Figure 8: SEM images observed cross section of deformed chip in Steel G

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Figure 9: SEM images observed cross section of deformed chip in Steel F 4. Conclusion

Conclusions of this study can be summarized as follows; 1) The formation trend of built-up edge was different

among the steels containing different type of MnS in the turning test. It affected the surface roughness through the difference in tool wear among them.

2) Large built-up edge was hardly formed in the steel containing globular MnS, and the groove-like wear with regularly-arranged depressions consequently evolved at minor cutting edge in turning test. The surface roughness had good correlation with the depth of worn depression in the groove-like wear.

3) Globular MnS was beneficial to suppress the growth of built-up edge. The growth of built-up edge would be closely associated with the deformation characteristics of distinct type of MnS through the generation of micro-cracks around them.

References [1] Yaguchi, H., 1986, Effect of MnS Inclusion Size on

Machinability of Low-Carbon, Leaded Resulfurized Free-Machining Steel, J. Appl. Metalwork., Vol.4, No.3, pp.214

[2] Milovic, R. and Wallbank, J., 1983, The Machining of Low Carbon Free Cutting Steels with High Speed Steel Tools, J. Appl. Metalwork., Vol.2, No.4, pp.249

[3] Iwata, K., Ueda, K. and Shibasaka, T., 1977, Study on Micro-machining Mechanics Based on Direct SEM Observation (1st Report), J. Jpn. Soc. Precis. Eng. (In Japanese), Vol.43, No.3, pp.311