Wear patterns of polycrystalline cubic boron nitride cutting tools when machining hardened bearing steel.

Benga, Gabriel ; Stanimir, Alexandru

Abstract: The paper presents different types of wear and plastic deformation when continuous dry turning 100Cr6 hardened steel with 60-62 HRC hardness using CB7020 and DBN45 polycrystalline cubic boron nitride-cutting inserts. There were used different cutting conditions varying cutting speed and feed rate in order to determine the influence of each cutting parameter on the wear of the cutting tool. It was also studied the tool life/flank wear influence for all cutting tools used as a function of cutting speed and feed rate.

Key words: wear, hardened steel, dry turning, polycrystalline cubic boron nitride, cutting parameters.

1. INTRODUCTION

The vast majority of components in the metalworking industry are machined to their final geometrical form after hardening. Fully hardened steels as: hardened bearing steel, case hardened steel, hardened high speed steel, martensitic stainless, hardened cold and hot work tool steel, heat resistant superalloys, and bimetals are all gaining broader acceptance in industry [Klimenko et al]. While such materials deliver practically indestructible parts, they come with this difficulty: how to machine them to final shape at a reasonable cost per part. Changes in workpiece materials, manufacturing processes and even government regulations catalyze parallel advances in metal cutting tooling technology. Nowadays, machining of hardened components with polycrystalline cubic boron nitride cutting tools has become increasingly common and it represents an established alternative to grinding [Tonshoff, H.K.]. Reduced production time, leading to lower costs, has been major factor in this change. The flexibility of hard turning and the possibility of machining without coolant are further advantages, which have stimulated interest. The automotive industry is the major consumer of PCBN cutting tools, demanding more than 50% of the total. The next largest consumers of PCBN cutting tools are those industries manufacturing heavy machinery [Jennings, M].

Ranking next to diamond on the hardness scale, polycrystalline cubic boron nitride (PCBN) has proven to be a durable tool material for cutting hard-to-machine metals such as high-temperature and hardened ferrous alloys. The increasing pressure on manufacturers to reduce costs makes the continued development of the hard turning process with PCBN inevitable. This paper presents the wear of some PCBN grades (Sandvik Coromant-CB7020 and De Beers-DBN45) when hard turning 100Cr6 bearing steel with different cutting conditions. There was also studied the tool life variation with the flank wear [V.sub.B]. The flank wear criterion was set at [V.sub.B]=0.2 mm according to the literature [Chou, Y.K.; Kishawi, H.A.].

2. EXPERIMENTAL PROCEDURE

The tests were performed on a CNC lathe (5.5 Kw and 3600rpm) using triangle inserts of Sandvik Coromant CB7020 polycrystalline cubic boron nitride and De Beers triangle inserts DBN 45. The geometry for all inserts used was TNGN 160408 T01020 attached to a toolholder coded CTGNR 2525 MID. The workpiece material consisted in tubes of DIN 100Cr6 bearing steel hardened and tempered to 60-62 HRC. There were taken some pictures using a SEM in order to present some specific features of the wear types. There were analyzed the wear patterns when machining 100Cr6 bearing steel hardened to 60-62 HRC varying one by one the cutting speed and feed rate in order to determine which of them has a significant influence on the tool wear. Depth of cut was maintained constant at [a.sub.p]=0.25 mm because in a previous experiment was showed that the influence of depth of cut on the tool life was far lower than cutting speed and feed rate. There were used three cutting conditions as follows:

1. Vc=100m/min, [f.sub.n]=0.06 mm/rev, [a.sub.p]=0.25 mm;

2. Vc=180m/min, [f.sub.n]=0.06 mm/rev, [a.sub.p]=0.25 mm;

3. Vc=180m/min, [f.sub.n]=0.22 mm/rev, [a.sub.p]=0.25 mm.

3. RESULTS AND DISCUSSIONS

In figure 1, is presented the wear pattern for the first cutting regime employed using CB7020, Sandvik Coromant's insert.

The SEM picture shows that the wear has a regular shape both on the rake face and the clearance face. The width of the wear crater on the rake face does not surpass the width of the negative land. On the clearance, face there were some wear grooves, which confirm that the abrasion wear was the main mechanism of wear. As others specialists have concluded, it seems that the crater wear has spread near the cutting edge without destroying it. The tool life for this cutting insert was remarkable, T=190 min.

In figure 2 is presented the wear pattern for DBN45 (De Beers) cutting tool when the same cutting regime was employed. It is obvious how regular is the wear on the rake face and on the clearance face. The width of the wear crater is not larger than the width of negative land. The crater wear starts very close to the cutting edge, causing a "re-sharpening" of the cutting edge. This was confirmed by the low value of the machined surface roughness (Ra=0.6 [there does not exist] m) even after the flank wear criterion ([V.sub.B]=0.2 mm) was reached. The flank wear presents the same pattern as for CB7020 with a lot of grooves oriented after the main cutting direction confirming the fact that the abrasion wear is the main wear mechanism on the clearance face. Abrasion wear requires the presence in the work material of hard particles (second phase particles, impurity particles, etc.) which are forced against the tool surface during machining, however abrasion can also result from hard particles present in the tool itself.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

When the next cutting regime (Vc=180m/min, [f.sub.n]=0.06 mm/rev, [a.sub.p]=0.25 mm) was employed, the wear patterns was the follows (see fig.3):

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Figure 3 shows the crater wear starting at the cutting edge without causing it a serious damage. This was confirmed by the low value of the machined surface roughness (Ra=0.75 [micro]m) for a [V.sub.B]=0.2 mm flank wear. In the crater wear are visible small particles of melted metal having a light gray color. This confirms that a high temperature was reached in that area. This fact can lead us to the idea that increasing the cutting speed from 100 m/min to 180 m/min results in an increasing of temperature. It is reasonable to assume that increasing the cutting temperature might promote the diffusion wear. The flank wear has a regular shape and presents some small metallic fragments bonded to the clearance face. The tool life three times lower than when a cutting speed of 100 m/min was used. It seems that the cutting speed has a very strong influence on the tool life of CB 7020 when turning 100Cr6-hardened steel. Figure 4 presents the wear pattern when turning 100Cr6 with DBN45 using the same cutting regime.

The wear shape for DBN45 seems not to be different as the wear shape when the previous cutting regime was used. It appears that the increasing of cutting speed from 100m/min to 180 m/min affects more the tool life (which is now 8.6 times lower) than the wear pattern. It can be remarked the same regular shape of the wear and the fact that the width of the crater wear is even lower that the width of the negative land area. In the crater wear can also be observed few small particles of melted metal. We can conclude that the temperature was higher than in the previous case when a cutting speed of 100 m/min was used. This increasing in temperature can lead to the appearance of the diffusion wear, but it is obvious that it is not the dominant wear mechanism. The wear grooves on the clearance face confirms that the abrasion wear plays a significant role in this case.

In figures, 5 and 6 are presented the wear patterns when the third cutting regime was employed. Actually, comparing with the second cutting regime, only the feed rate was increased from 0.06 mm/rev to 0.22mm/rev, maintaining constant the cutting speed and the depth of cut. The wear presents a different shape when the feed rate was increased about four times. This cutting regime seems not to be very appropriate for the two brands of polycrystalline cubic boron nitride used taking in account the low values for tool life resulted.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

From figure 5, it can be seen that the wear crater has extended to the cutting edge causing the chipping of the cutting edge. This has not happened before, when the feed rate was lower ([f.sub.n]=0.06 mm/rev). This chipping might be the result of the vibrations generated by the higher feed rate (0.22 mm/rev). In the crater wear there is an amount of melted metal, which confirm the fact that the diffusion wear has played a significant role in the wear mechanism. The wear grooves on the rake face appear as result of the friction between the swarf and the rake face. The wear on the rake face started in the negative land area, then the wear has progressively increased leading to the extension of the crater on the rake face and its depth. Regarding the flank wear, it can be observed the exfoliation of a layer from the clearance face probably due to the high contact pressure and the vibrations produced during the cutting process. Figure 6 shows the wear shape of the DBN45 cutting insert, when the third cutting regime was used. On the rake face there is a crater wear, which is characterized by the missing of wear grooves. A possible explanation is that the wear crater does not appear as a result of the friction between the rake face and the swarf but due to the fracture of the cutting insert. The fracturing of cutting insert is also obvious on the clearance face where it cannot be seen wear grooves but chippings. The tool was fractured probably due to the mechanical shocks generated by the vibrations resulted from the increasing of feed rate from 0,06 mm/rev to 0,22 mm/rev. Under these circumstances it appears that the feed rate must be kept in reasonable limits in order to avoid the vibrations, which may generate the fracture of the tool.

4. CONCLUSIONS

I. When the first cutting regime ([V.sub.c]=100m/min, [f.sub.n]=0.06 mm/rev, [a.sub.p]=0.25 mm) was employed, the wear for the both cutting tool materials (CB7020 and DBN45) presented a relatively regular shape. The high hardness of these cutting tool materials do not allow the appearance of an irregular wear. The abrasion wear appears to be the dominant wear mechanism of these PCBN cutting tools. Abrasion wear is caused by the action of sliding chips in the shear zone, as well as by friction generated between the tool flank and workpiece material.

II. When the cutting speed was increased from 100m/min to 180 m/min in the crater wear have appeared fragments of melted metal, which confirm that the temperature in the cutting area was high enough to initiate the diffusion wear.

III. Using the third cutting regime (Vc=180m/min, [f.sub.n]=0.22 mm/rev, [a.sub.p]=0.25 mm) consisting in an increasing of feed rate from 0.06 to 0.22 mm/rev comparing with the second cutting regime. The tool life has dramatically decreased and the wear patterns shows, that the tools were fractured, probably due to the vibrations generated by the high feed rate. Especially the DBN45 seems to have a lower resistance to vibrations than CB7020.

5. REFERENCES

Chou, Y.K., Evans, C.J., Barash, M.M, (2002), Experimental investigation on CBN turning of hardened AISI 52100 steel Journal of Materials Proc. Techn., 124 (3), pp.274-283

Jennings, M. (1995) Amborite- the first 15 years, Industrial Diamond Review V/55, pp.151-153.

Kishawi, H.A., Elbestawi, M.A., (2001) Tool wear and surface integrity during high speed turning of hardened steel, Proc. Inst. Mech, Eng. B, 215, pp.755-767.

Klimenko, S.A. Mukovoz, Y.U., Polonsky, L. G. (1996) Advanced ceramic tools for machining applications, Key Engineering Materials, vol 114, Zurich.

Tonshoff, H.K., Arendt, C., Ben Armor, R. (2000) Cutting of hardened steel, CIRP Annals-Manufacturing Technology, 49 (2), pp.547-566.