Surface markings on hydrodynamically drawn wires

Scripta MIETALLURGICA

Vol. 18, pp. 1375-1378, 1984 Printed in the U.S.A.

bUi.P:~Ci] Ik\RKII GS Oi

Pergamon Press Ltd. All rights reserved

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J. Sickc:
(Revised September

Introduction irosress im the understamdi,' S of matorJa! flow during deforration processing depends e s s e n t i a l l y on the experimental methods which enable one to analyse flow patterns. There are first of all the :~ethod of v i s i o p l a s t i c i t y and the finite element methods, the latter can be applied only with the aid of computer services. However, the sensitivity of these methods to frictional effects on flow pattern is a difficult problem (I). C o n c e r n i n g the near-surface flow of a work piece, the observation of fIow characteristics on the surface can Give valuable information on the local states of strain and stress, respectively. However, the surfc~ces of ',ferked m a t e r i a l s reflect above all the frictional conditions between work piece and tool which are in general of a mixed type and include substantial portions of boundary friction (2). This means that the surfaces are ~moothed to a great extent, because of the thin lubricant layers, without leaving traces behind from plastic processes of the material below it. Yet the a p p l i c a t i o n of thick film l u b r i c a t i o n which prevents direct contacts and diminishes boundary contacts between the deformed material and the tool can reveal the flow pattern of near-surface material. Indications for this are giw~n in the case of wire drawing by means of hydrostatic lubrication(3). A n o t h e r procedure which has been developed in recemt years (4) is the application of a solid film l u b r i c a t i o n i:m combination with hydrodynamic double dies. This technique has been used in solving wire drawing problems which need a highly effective lubrication, as in m a n u f a c t u r i n g stainless steel wires, wires for heating elements, spring wires and wires of composite materials (4). In this case characteristic deformation markings can be observed on initially smooth wire surfaces, tlhe development of which is described in this paper. Experimental A heat r e s i s t i n g steel of the type X 1 5 C r N i S i 2 5 . 2 0 (similar to AISI Type 314) was shaved by drawing through a shaving ring tool from 7.70 rmm in diameter to 7.40 mm. The shaved wire was coated w i t h the liquid lubricant Hydrodyn I, the main constituents of w h i c h are calcium stearate and calcium oxide dispersed in an organic solvent (5). S u b s e q u e n t l y the wire was dried and drav~ from coil to coil by means of a double die similar to that described by Kolmogorow (6). Two conventional tungsten carbide dies were used as drawing and pressure ~ t r ~ p s c ~ e ~ ~h~sacCs~O~to~ew~tT~ ~ ~g~ of the dies was w p i any lubricant carrier at a drawing speed of 1.1 m/s. The wire diameter was redu-

1375 0036-9748/84 $3,00 + .00 Copyright (c) 198.4 Pergamon Press Ltd.

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SURFACE MARKINGS ON DRAWN WIRES

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ced according to the sequence 7.40/6.59/5.34/4.32/3.80/3.30 r~m. Correspondinoly,~ the increase of the total reduction in area was E g = 21/48/66/74/80 %. The first reduction in area was accomplished with the initial lubricant film present and the following steps were performed with a film remaining from the proceding deformation steps. Results In Fig. I the increase of the ultimate tensile strength Rm of the drav~ steel X15CrNiSi25.20 is shov~ as a function of the total reduction in area gg. There is a steady increase of Rm from Rm = 833 ~ a for the predrawn and shaved wire (~g = 0 %) up to Rm = 1481 ~ a for the final wire after five reduction steps (~g = 80 %). Deformation markings on the peripheral surface became visible after the third reduction, i. e. at E g = 66 ~~. The development of a characteristic deformation structure on the wire surface is evident in Figs. 2a-o. The scanning electron mierograph of Fig. 2a shows markings perpendicular to the drawing direction at £g = 66 %, the appearance of which is slightly wavy. H~wever, there are also de±ormation markings which are oriented at about 45 to the drawing direction, particularly in the vicinity of local stress concentrations at small particles in the surface region (Fig. 2a). These inclined characteristics become stronger with increasing total reduction in area, as evident from Fig. 2b (gg = 74 %) and Fig. 2c (~g = 80 %), respectively. At E g = 80 % the surface appearance is rather rough with predominant inclined traces of the plastic deformation. The resulting rhombic cells on the wire surface have an average magnitude of about (100 ... 150) ~m. It should be mentioned that the angle of inclination of the d~formation markings decreases with increasing reduction in area, from about 45 ~ at Eg = 66 % (Fig. 2a) to about 30 at

~g = so % ( F i g .

2c). Discussion

The thick film strength level appreciable die face roughening

lubrication technique was highly effective up to the ultimate of the deformed material of Rm ~ 1500 ~ a . In spite of the pressure the lubricant film permits a certain degree of surby plasticity (Figs. 2a-c).

However, the resulting deformation markings are not generally expected for axisymmetric flow through conical converging dies (1,7). Only the markings perpendicular to the wire axis, as partially obvious from Fig. 2a, would correspond to this type of deformation. This means that the perpendicular markings should be the traces of material flow that is controlled by the longitudinal and radial components of stress (maximum and minimum normal stresses). Yet the inclined characteristics (Figs. 2a-c), which are developed with increasing reduction in area, indicate a predominating plane strain deformation parallel to the surface. This is, however, controlled by the longitudinal and circumferential stress components. Thus, the circumferential compressive stress and the longitudinal stress control the direction of material flow at the wire surface, leading to the observed type of shear banding (Figs. 2a-c). This result contradicts most of the flow pattern models in wire drawing. However, some authors consider the circumferential stress as important for the deformation conditions inside the die (8,9). The pronounced shear banding (Figs. 2b and c) should be connected with the promoting effect of plane strain deformation with respect to flow localization (10,11). On the other hand it is difficult to accumulate shear in narrow bands under axisymmetric deformation (12). Nevertheless, axisymmetric flow might contribute to the surface appearance because of a component of inward flow which should support the grooving of deformation markings. Yet this would provide that only the near-surface material would be allowed to deform under plane strain, while the bulk material should flow axisymmetrically. Metallographic on the surface

examinations have shown that the rhombic deformation markings are well developed 10/um below it but are not apparent at a

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SURFAC~

MARKINGS

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depth of 140A~m (13). This fact manifests bands (Figs.'2a-c) can indeed be regarded plasticity.

WIRES

1377

that the inclined families of shear as a phenomenon of near-surface

These types of shear bands obviously initiate at structural points where stress concentrations arise (Fig. 2a). The extreme sensitivity of shear band localization with respect to surface imperfections was emphasized recently (11) and is already known from the classic works of Nadai (14), who investigated flow patterns starting from accidental bubbles at the surface of paraffin cylinders. The angle of inclination of the observed shear bands depends on the total reduction in area. As already mentioned, the first shear bands appear at 45 o with respect to the wire axis (Fig. 2a), while in subsequent deformation steps the angle is diminished to about 30 o (Fig. 2c). Although the observed type of deformation banding can be regarded as a surface effect it should be emphasized that the surface to volume ratio is important. The higher this ratio, i. e. the thinner the wire, the more the banding may affect the whole volume during deformation. Some indications arise from the drawing of thin amorphous wires. Because of their ultra-high strength and poor strain hardening capability this group of materials is very sensitive to strain localization. After drawing, inclined flow patterns were also observed on the surface of amorphous wires and moreover an oblique type of fracture was exhibited (15,16). In such a case the surface effect may become a volume one. Another example is given by the drawing of thin-walled tubes. As of deformation markings run symrecently shown by Thomson (17~,±2tw 8 families the axis metrically at an angle of 35 to of a dravm Cu-O.04 % P tube, which had a wall thickness of only 91/um (17). In this case the shear bands were present on the outer and inner surface of the tube and their distance was about 10 r~. This magnitude of the rhombic cells differs, however, appreciably from that one in Fig. 2e which is two orders of magnitude lower. From the practical point of view the development of this type of shear band raises the question of how much factors like the frictional conditions at the tcol-workpiece interface, the amount of redundant deformation and the kind of local constraint forces may influence this phenomenon and the resulting properties of wire products. Further investigations are needed. Conclusions - Thick film lubrication reveals deformation markings on the surface of a dra~vn steel X15CrNiSi25.20 (similar to AISI Type 314) which are not consistent with axisymmetrie deformation, but rather with a plane strain one at the surface. - The

angle of inclination of the shear bands with respect to the wire axis emlounts to about 45 at theiroformation and is diminished by subsequent deformation steps to about 30 • Acknowledgement

The authors are very grateful to Dr. B. Pegel for critical manuscript and Dipl.-Ing. D. BSsel for his helpful support nation at the scanning electron microscope.

reading of the during the exami-

References I. 2. 3. 4.

B. Avitzur, la metallurgica italiana (1981) 385-410. O. Pawelski, Schmiertechnik und Tribologie 25, 137-140 (1978). J. Schiermeyer, in "Ziehen yon Drihten, Stangen und Rohren", pp. 131-138, Deutsche Gesellschaft fur ~etallkunde, Oberursel, (1981). J. Eickemeyer, B. Kurze, P. N~ller, H.-R. Vogel and H. Weinhold, Proceedings of Conference, Salgotarjan, Hungary, 11.-13. Oct. 1983.

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5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

ON DRAWN WIRES

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H. Weinh~Id, H.-R. Vogel and B. Kurze, DDR-Wirtschaftspatent WP C I0~i/205929, 12.6.1978. V. Kolmogorow, Neue H~tte 14, 404-409 (1969). P. van Houtte, J. G. Sevillano and E. Aernoudt, Z. lletallkunde, 70, 426432 (1979). A. A. Pozdeev, Ju. I. Nyashin, P. V. Trusov and A. A. Selyaninov, Izvest., Tschernaja L~etallurgija (1980) 67-70. O. Pawelski, Archiv EisenhGttenwesen 32, 607-616 (1961). D. P. Clausing, Intern. J. Fracture Eech. 6, 71 (1970). V. Tvergaard, A. Needleman and K. K. Lo, J. Idech. Phys. Solids 29, 11514-2 (1981). W. A. Baekofen, Deformation Processing, p. 254, Addison-Wesley Publ. Comp. (1972). J. Eickemeyer et all, to be published in 17eue Hitte. A. Eadai, Theory of Flow and Fracture of Solids, p. 289, ~Io Oraw-Hill, New York (1950). S. Takay~ma, ~Tater. Sci. Eng. 38, 41-48 (1979). H. Hagiwara, A. Inoue and T. Hasumoto, Ketall. Trans. 13A, 373-382 (1982) P. F. Thomson, Hater. Sci. Eng. 60, 25-30 (1983).

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TOTAL REDUCTION ~hi AREA gg, %

2(b)

FIG. 1 Ultimate tensile strength Rm of the steel X15CrNiSi25~20 as a function of the total reduction in area g after drawing; ~g = 0 % corresponds to the predrawn and shaved material.

FIG. 2 Development of deformation markings on the surface of the drawn steel X15CrNiSi25.20 after different total reductions in area Eg; magnification 150 X. (a) ~ g = 66 %, (b) gg = 74 %, (o) ~ g = 8o %.

2(c)