Work hardening and tool surface damage in burnishing

Work hardening and tool surface damage in burnishing

Wear, 127 (1988) 149 149 - 159 WORK HARDENING BURNISHING AND TOOL SURFACE DAMAGE IN TOKIO MORIMOTO Faculty of Engineering, Osaka 558 (Japan) O...

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Wear, 127 (1988)

149

149 - 159

WORK HARDENING BURNISHING

AND TOOL SURFACE

DAMAGE

IN

TOKIO MORIMOTO Faculty of Engineering, Osaka 558 (Japan)

Osaka City University,

3-138, Sugimoto

(Received November 17, 1987; revised March 28,1988;

3-Chome,

Sumiyoshi-ku,

accepted May 31, 1988)

Summary Work hardening of the surface and subsurface of an annealed steel bar burnished using a simple newly designed tool system and also the reduction in dimensions of the workpiece accompanying burnishing are investigated in this paper. The influence of the burnishing force, number of passes of the burnishing tool across the surface and feed rate on work hardening are examined. Furthermore, the tool surface is inspected and the retainer surface is examined using an electron probe microanalyser to identify the particles that scratch the tool surface: some of which are embedded in the retainer surface. The hardness of the material at the surface and in the subsurface is materially increased by burnishing. In particular, the burnishing force has the greatest influence on work hardening the surface and subsurface. The work-hardened layer beneath the surface often attains a depth of more than 200 pm.

1. Introduction The burnishing process, which utilizes surface plastic deformation, easily produces a smooth surface and, moreover, can improve the fatigue strength and wear resistance of a workpiece by virtue of the residual compressive stress and the work hardening of the material at the surface. Previously, Hull [ 1] demonstrated that diamond burnishing can significantly work harden annealed aluminium and copper workpieces and also greatly extend the fatigue endurance of a die steel. Further, Ruseva and Fuks [2] investigated the properties of the surface layer, burnished using different methods, and showed that the compressive residual stress, induced by plastic deformation of the surface layer, depended on the burnishing method, either ball burnishing, roller burnishing or centrifugal ball burnishing. Further work is required to clarify the effect of burnishing on the work hardening of the surface and the improvement of the fatigue limit. 0043-1648/88/$3.50

@ EIsevier Sequoia/Printed in The Netherlands

150

I have previously reported [ 31 that a cylindrical surface of a mild steel bar can be successfully burnished using a simple newly designed tool system using a lathe. In this system the burnishing tool was a cemented carbide ball, elastically supported by a spring and rotated by the rotation of the workpiece. This paper studies the effect of the burnishing force, the number of passes of the tool across the surface and the feed rate on the work hardening of the material at the burnished surface and subsurface. The relationship between the reduction in dimensions of the workpiece and its surface roughness is also investigated. Moreover, the tool surface after burnishing and the retainer surface is examined using an electron probe micro analyser to identify the particles that scratch the tool surface and those which become embedded at the retainer surface.

2. Experimental method A cylindrical workpiece surface, turned just before working, was burnished with single and multiple passes of a newly designed burnishing tool attached to a lathe. The workpiece was mild steel (JIS S38C: 0.35% 0.41%, C; 0.15% - 0.35%, Si; 0.60% - 0.90%, Mn; less than 0.030%, l?; less than 0.035%, S). The workpiece was annealed at 850 “C for 1 h before turning. Turning was repeated three times (dry) to produce a uniform workpiece surface state. The burnishing tool system was similar to that proposed before (Fig. 1) [ 31. The burnishing tool was a cemented carbide ball 5 mm in diameter. The ball was so lightly held against the retainer, by means of a cap, that it was freely rotated by the rotation of the workpiece. The retainer and the cap

BURNISHING

TOOL

/

DYNAMOMETER /

I

\

SPRING

RETAINER ROTATION

OF

WORKPIECE

WORKPIECE DETAIL

OF TOOL HEAD

Fig. 1. Newly designed burnishing rotation of the workpiece.

tool;

the cemented

carbide

ball (tool)

is rotated

by the

151 were

made of brass (59.0% - 63.0%, Cu; less than 0.10% Pb; less than 0.07% Fe; balance, Zn). The burnishing force (normal force), with which the burnishing tool was pressed against the surface of the workpiece, was measured using a strain-gauge-type dynamometer. A straight mineral oil of low viscosity (8.8 mm2 s-’ at 40 “C) and containing 4 wt.% S (added as ditertiary nonylpolysulphide), was applied as a lubricant to assist smooth burnishing. The lubricant was fed into the contact point between the tool and the workpiece by an oil pump. The average rate of flow was 0.13 1 min-‘. The surface roughness of the workpiece was measured using a surface profilometer (Tokyo Seimitsu Surfcom 3B; diamond stylus type).

3. Hardness of the burnished

surface and subsurface

Roughness traces of a cylindrical steel surface turned by a cemented carbide throw-away insert and then repeatedly burnished with a constant burnishing force are shown in Fig. 2. The cutting speed, the depth of cut and the feed rate during turning were 156 m min-‘, 0.2 mm and 0.1 mm rev-’ respectively. The frequency of the peaks (or valleys) generated on the turned surface (Fig. 2(a)) correlated with the feed rate of the tool.

0.4

mm

Ra=4.01

pm

R~0.46

pm

Rm~~2.76

pm

Raz0.10

pm

Rmax= 0.78

pm

Rmaxz14.4

pm

(a)

0.4

mm

(b)

5 .s 0.4 mm Cc) ;;

5

-f

lI----0.4

mm

Ra=0.09

: MATERIAL p,m

Rmax=0.60

pm

(d) Fig. 2. Roughness of surface: (a) turned and then burnished with (b) the fist, (c) the . . , .I . _. .. second ana (a) the thud pass of the tool. Burnishing force, 60 N; burnishing speed, 98 m min-‘; feed rate, 0.05 mm rev-‘.

152

In contrast, the burnishing force, the burnishing speed and the feed of the burnishing tool were 60 N, 98 m min-’ and 0.05 mm rev’-’ respectively. Although the roughness on the machined surface was not completely eliminated by one pass of the tool ( the first pass, n = l), it was significantly smoothed by further passes of the tool (the second (n = 2) and the third pass (n = 3)). The hardness of the burnished surface was measured using a Vickers microhardness test under a load of 0.98 N. Scanning electron micrographs of indentations made by the diamond pyramid indenter on the machined and burnished surfaces are shown in Figs. 3(a) and 3(b). The roughness of the machined surface was so great that the indentation became distorted and difficult to measure (Fig. 3(a)). The formation of an abnormal indentation on the machined surface is shown in Fig. 3(c), where the vertically opposite angle of the diamond pyramid indenter 8 (148” 7’) has become modified by the ratio of the vertical to horizontal profile of the machined surface. To overcome distorted indentations the machined surface was lightly ground with silicon carbide abrasive paper number 1000 and then buff polished with alumina powder. This allowed exact hardness determination. The hardness of the material in the subsurface was measured in a cross-section of the workpiece under a load of 0.49 N. The hardness Hv (N mm--2) at each position was represented by an average value of five or more measurements. The effects of the burnishing force on the hardness of the surface and subsurface, burnished by a single pass of the tool, are shown in Figs. 4 and 5. Both the hardness of the surface and subsurface considerably increases with increasing burnishing force. For instance, the surface microhardness burnished with a force of 140 N is 390 HV: this corresponds to a 50% increase over the hardness of the machined surface. The depth of the workhardened layer, burnished with a force of 140 N, extends more than 200 pm beneath the surface. In contrast, Figs. 4 and 5 show that the microhardness

n=O (a)

8 =148'7'

100 pm (cl

Fig. 3. Vickers microhardness indentation of (a) turned and (b) burnished surface and (c) profile of diamond pyramid indenter on turned surface: n, number of tool passes; W, test load; t, depth of cut; V, cutting or burnishing speed; P burnishing force; f, feed iate; 8, vertically opposite angles of diamond pyramid indenter. (a) W = 4.9N, t = 0.1 mm, V= (b) W=0.98N,P=60N,V=98mmin-‘,f=0.05mm 156 m mine’, f= 0.1 mm rev-‘.

rev-‘.

153

*so~Tu’i_t’j / / j 0

50

BURNISHING

100 FORCE

150 P (N 1

0 DEPTH

100 BENEATH

200 SURFACE t(Wm)

Fig. 4. Effect of burn~~mg force on hardness of surface burnished with a single pass of the tool (test load, 0.98 N). Fig. 5. Effect of burnishing force on microhardness of subsurface burnished with a single pass of the tool (test load, 0.49 N).

of the turned surface (255 HV) has only been raised by 50% over the hardness of the bulk material (170 HV). The influence of the number of burnishing (tool pass) passes across the surface E (with constant burnishing force) on the surface and subsurface microhardness is shown in Figs. 6 and 7. The surface microhardnesses of each surface, burnished by the first, the second and third passes of the tool (n = 1, 2 and 3) were about 345 HV, 360 HV and 370 HV, repre~nt~g a 35%, 40% and 45% increase in hardness over that of the machined surface respectively. The extent of work hardening of the workpiece surface gradually decreases as the number of b~nish~g (tool pass) passes under a constant burnishing force is increased. Material in the subsurface, particularly deeper than about 60 pm, was not hardened very much by the first pass of the tool fn = 1) (Fig. 7). However, it was noticeably hardened by

In 2 =, 300 ii! 250 0 NUMBER

1 OF

2 TOOL-PASS

3 n

0 DEPTH

100 200 BENEATH SURFACE t ( pm)

Fig. 6. Effect of number of burnishing (tool pass) passes on microhardness of surface burnished with repeated passes of the tool (test load, 0.98 N). Fig. 7. Effect of number of burnishing (tool pass) passes on microhardness of subsurface burnished with repeated passes of the tool (test load, 0.49 N).

154

the second and the third passes of the tool. The final depth of the workhardened layer beneath the surface burnished by the third pass of the tool (P = 60 N, n = 3, t = 200 pm) is almost the same as that of surfaces burnished by a single-tool pass using a large burnishing force, greater than or equal to 107 N (compare Fig. 5 with Fig. 7). The hardness of the surface increases as the feed rate is reduced (Fig. 8). The work hardening of the material in the subsurface shows a similar dependence (Fig. 9). Thus the effect of reducing the feed rate on work hardening is very similar to that of raising the number of passes of the tool. Usually the tool under a force of 100 N deforms the workpiece material in the area extending over the feed rate; producing a groove about 0.4 mm in width. As the feed rate is reduced, therefore, the workpiece material in neighbourhood of the tool is repeatedly subjected to plastic deformation. The hardness of the material at about 30 pm beneath the surface is about 205 HV for 12= 1 in Fig. 7: this hardness is considerably lower than that of the surface of hardness 350 HV (see Fig. 6). This means that the hardness of the material at the subsurface reduces sharply immediately below the surface. This confirms the findings of Kaczmarek and Polowski [4] which reported that the greatest gradient of microhardness occurred just beneath the surface. Figure 10(a) shows a scanning electron micrograph of two indentations formed at about 15 and 180 pm beneath the surface in the cross-section; the cross-section was lightly etched with 3% Nital solution after indenting. The dimensions of a and b of the indentation A in Fig. 10(b) are 8.5 pm and 11.8 E.crnrespectively. The inclination of the surface at the edge of the cross-section was too small to produce such a difference in dimensions (a[< 0.3” in Fig. 10(d)). In contrast, there is no difference between a and b for indentation B (Fig. 10(c)). If the hardness is simply estimated by multiplying a and b of indentation A by 2, although indentation A is too close to the edge of the cross-section, it gives 321 and

0

0.05

BURNISHING

0.10 FEED

f

0.15

‘401/ 0

(mm.reJ’)

DEPTH

Fig. 8. Effect of feed rate on microhardness tool (load, 0.98 N). Fig. 9. Effect of feed rate on hardness tool (test load, 0.49 N).

100

BENEATH

of surface

of subsurface

200 SURFACE

burnished burnished

t( km)

with a single pass of the with a single pass of the

155

20 w-n

(a)

(b)

BURNISHED

(cl

SURFACE

(d)

Fig. 10. Appearance of indentations just beneath burnished surface. (a) Indentations A and B; (b) indentation A, a = 8.5 pm, b = 11.8 pm; (c) indentation B, a = b = 10.5 pm; (d) schematic of cross-section. Burnishing force, 60 N; burnishing speed, 98 m min-‘; feed rate, 0.05 mm rev-‘; n = 3; test load, 0.49 N.

167 HV. The great difference in these two values of hardness suggests a large variation in hardness just beneath the surface. 4. Reduction in dimensions It is to be expected that the burnishing process causes a reduction in dimensions related to surface roughness because topographical peaks are flattened by the tool. In one study [ 51, the reduction in diameter of a cylinder, achieved by burnishing, was approximately equal to the magnitude of the initial roughness. However, the surface roughness after burnishing and turning achieved in this study showed that the reduction in radius was slightly larger than half the magnitude of the surface roughness (Fig. 11). The relationship between the reduction in dimensions and the surface roughness is shown in Fig. 12. The diameters were measured using a micrometer to an accuracy of 1 pm. The reduction in radius by the burnishing process is directly proportional to the magnitude of the initial roughness. Furthermore, the reduction in radius is larger than half the magnitude of the initial surface roughness R,,,,within a few micrometres. This is ascribed to the topographical ridges of the surface which are not always uniformly continuous in the machined direction, even for the turned surfaces.

156

BURNISHED

Rmax

I I 1.6

br-

74

TURNED

SUR.

( urn !

br>Rmax 2

(&l-n)

E 3. In

SUR.

L 0.5pm

Fig. 11. Surface roughness of burnished (burnishing load, 100 N; feed rate, 0.05 mm rev -‘; burnishing speed, 103 m min-‘) and t urned surface (cutting speed, 205 m min-‘; feed rate, 0.1 mm rev-‘).

;1

tm36

a; k” 2 L

4

6 ROUGHNESS

8

10

12

R,..(vm)

Fig. 12. Reduction in radius vs. surface roughness (after burnishing with a single pass of the tool): burnishing force, 100 N; feed rate, 0.05 mm rev-‘; burnishing speed, 100 - 130 m min-‘. Straight line fitted using least-squares method.

Eventually, the reduction in radius varies with surface topography, assuming no volume change of the material under plastic deformation. If the asperities on the surface are approximated to cones or hemispheres, the reduction in radius is smaller or larger than half the roughness.

5. Tool surface damage The smoothness of the tool surface is essential to success in the burnishing process because the burnishing mechanism relates to plastic deformation of surface irregularities through rolling or sliding contact. If the surface of the used tool is worn or smeared with material transferred from the workpiece during burnishing, that tool should be changed for a new one. Therefore it is necessary to inspect periodically the tool surface after use. Figures 13(a), 13(b) and 13(d) show scanning electron micrographs of the surfaces of two different tools that were used for a series of experiments. Despite a number of small defects being generated on both tool surfaces, all burnishing was successful because there was little tool surface damage. The surface damage usually appeared as adherent workpiece material (Figs. l3( a) and 13(b)). Figure 13(c) shows the image from Fe Ka radiation obtained by X-ray microanalysis using an electron probe microanalyser (Shimadzu EPM810, wavelength dispersive type) and reveals the presence of iron transferred

(b)

(cl

(d)

Fig. 13. Two different types of tool surface damage: (a) adherent workpiece material; (b) as (a) but higher magnification; (c) iron X-ray image; (d) scratching damage. Time used for burnishing, 1 h.

to the tool surface from the workpiece. However, scratches, about 5 pm in width, also sometimes appeared at the tool surface (Fig. 13(d)). The scratches were probably formed during burnishing by particles which were harder than the cemented carbide tool. It was suspected that some of these particles were embedded in the retainer of brass. Therefore the surface of the used retainer was investigated using the electron probe microanalyser to detect and identify the particles. Figure 14(a) shows one such particle, about 30 ,um across, embedded in the retainer surface. Furthermore, wavelength-dispersive X-ray analysis showed that the particle contained silicon and carbon. A state analysis was tried using the electron probe microanalyser to identify the particle; the characteristic spectral patterns of elements in the particle and silicon carbide grain are compared. The conformities in the characteristic X-ray spectra of the particle and silicon carbide (Fig. 14(b)) as well as in the characteristic X-ray spectra of carbon (Fig. 14(c)) reveal that the particle is silicon carbide. The silicon carbide particles probably attached themselves to the turned surface of the workpiece or were transferred to the lubricant during the bur-

SEAM

7.0850

A

7.1225

h

4i.OA

WAVELENGTH

(a)

(b)

DIA.

: 20pm

5d.OA

WAVELENGTH (cl

Fig. 14. Analysis of retainer surface by electron probe microanalyser: (a) secondary electron micrograph of retainer surface; (b) characteristic X-ray spectra of silicon of the particle and silicon carbide; (c) characteristic X-ray spectra of the particle and silicon carbide.

nishing process. Such hard particles must be excluded from the interface between the tool and the workpiece. Other elements such as iron, sulphur and aluminium were also detected around the particle. 6. Conclusions The work hardening response of steel workpiece burnished using a newly designed burnishing tool and the associated tool surface damage have been investigated. The following findings were obtained. (1) The hardness of the surface is raised by 50% by a single burnishing pass with the large burnishing force of 140 N. (2) The work-hardened layer extends more than 200 E.trnbeneath the surface during a single burnishing pass with the large burnishing force of 140 N or repeated burnishing with a force of 60 N. (3) The extent of work hardening of the workpiece surface gradually decreases as the number of burnishing (tool passes) passes is increased. (4) The effect of feed rate on work hardening of the burnished surface and subsurface has a similar effect as that of raising the number of burnishing passes (tool passes). (5) The reduction in radius by burnishing is greater than half the magnitude of the surface roughness, within a few micrometres. (6) Close attention must be paid to preventing hard particles entering the interface between the tool and workpiece. Acknowledgment The author would like to thank Mr. K. Kawano for his help in operating the electron probe microanalyser.

159

References 1 E. H. Hull, Diamond burnishing, Machinery, 68 (1962) 92 - 97. 2 E. V. Ruseva and M. Ya. Fuks, Surface layer properties after burnishing by different methods, Ruse. Eng. J., 58 (1978) 28 - 29. 3 T. Morimoto, Examination of the burnishing process using a newly designed tool, J. Mech. Work. Tech&., 13 (1986) 257 - 272. 4 J. Kaczmarek and W. Polowski, The state of the surface layer of constructional steels after burnishing with a multi-roll head, Ann. CZRP, 23 (1974) 195 - 196. 5 G. SchSn, Oberflfchenbearbeitung durch Glattwalzen Steigerung der Verschleiasfestigkeit, Znd. Am., 108 (50) (1986) 20 - 21.