The Relation between the Sliding Wear of Lead-Bronze and Its Casting Structure* By Teruji Nojiri**, Fusao Hayama***

Transcription

1 The Relation between the Sliding Wear of Lead-Bronze and Its Casting Structure* By Teruji Nojiri**, Fusao Hayama*** and Shigeo Oya*** The relation between the casting structure and sliding wear properties of lead and tin as the major components of leadbronze was investigated. The test pieces were cut out from the chilled, columnar and equi-axial structures of lead-bronze ingots of different composition and were slid on a hardened steel ring in air without any lubricant. The results obtained are as follows: (1) The equi-axial crystal part in the casting structure shows the best wear resistance properties, followed by the columnar crystal, and the chilled crystal is the worst generally. These trends are remarkable in the high lead and high tin alloy. (2) The worn surfaces of the chilled structure are always metallic and the wear phenomenon is severe. The surfaces of columnar crystals are metallic or oxidized. The degree of oxidation depends on the compositions and sliding conditions, and is not satisfactory as compared with the equi-axial crystals. The surfaces of the equi-axial crystals are easily oxidized and the amount of wear is very small, while the wear of the steel ring is often severe. (3) It is considered that the wear of lead-bronze is influenced by the distribution and shape of lead and the contents of lead and tin. The size of lead in the equi-axial crystal structure is relatively large and its distribution is relatively continuous each other. This form of lead is more preferable for wear resistance than the fine and isolated form of lead in the chilled structure. (4) The materials with stable oxidizing wear seize the mating steel and show a high coefficient of friction, and these phenomena are relaxed by a suitable addition of lead. (Received February 1, 1971) I. Introduction Table 1 Compositions and properties of specimens Although lead-bronze is extensively used for bearing metal at high pressure and medium speed, there are very few investigations on the influence of the casting structure, especially that of the size and shape of lead and tin on its wear characteristics. In general, it is said that the columnar crystal part is practically better suited problem, the variation of the casting structure with contents of lead and tin or the cooling condition has been investigated in regard to its effect on the sliding wear characteristics of lead-bronze. II. Specimens and Experimental Method Pb were used in the experiment. Table 1 shows the composition, micro-structures and hardnesses (HRH) for some of the specimens. Cu-Pb and Cu-Sn alloys were also tested to compare with lead-bronze. The dimensions of the cast block are shown in Fig. 1. A chill-plate was used at the bottom to solidify it directionally, with a shell-mould at the side. Chilled crystal (A), columnar crystal (B) and equi-axial crystal (C) parts were formed in the casting structure of lead-bronze starting from the chill plate as shown in Fig. 1. At first * This paper was originally in Japanese in J. Japan Inst. Metals, 34 (1970), 45. ** Graduate School, Waseda University, Tokyo. Present address: Fuji Heavy Industrial Go., Ltd., Tokyo, Japan. *** Department of Metallurgy, Faculty of Science and Engineering, Waseda University, Tokyo, Japan. (1) C. B. Duddley: J. Franklin Inst., 13 (1892), 81, 161. A: Chilled crystal B: Columnar crystal C: Equi-axial crystal the cast block was sectioned vertically at the center, polished and etched to investigate the macro-structure. Then the sampling position of specimens was decided. Specimens were carefully sampled out of the chilled, columnar and equi-axial crystal parts as shown in Fig. 1. about 15mm in length. The typical micro-structures of those lead-bronze are shown in Photo. 1. Very fine lead particles are densely (2) G. H. Clamer: J. Franklin Inst., 156 (1903), 49. (2) H. J. French, S. J. Rosenberg and W. Le. C. Harbugh: U. S. Bur. Stds. J. Research, 1 (1928), 343. (4) H. J. French and E. M. Staples: U. S. Bur. Stds. J. Research, 2 (1929), Trans. JIM 1971 Vol. 12

2 262 The Relation between the Sliding Wear of Lead-Bronze and Its Casting Structure distributed in the chilled crystal part, while in the equiaxial crystal part the particles are big and continuous with each other. The lead particles in the columnar of the equi-axial crystal containing 9% Sn. As the Sn Fig. 1 Dimension of cast block and place of specimen. Photo. 1 Microstructure of lead-bronze.

3 Teruji Nojiri, Fusao Hayama and Shigeo Oya 263 content becomes 12%, its precipitation is observed in large quantity in the equi-axial crystal part and in small quantity in the columnar crystal part, but it is not found in the chilled crystal part at all. Then the segregation of lead can hardly be found in the alloys containing less than 15% Pb by a quantitative analysis of compositions. When the lead content is more than 20%, the segregation of lead increases in the chilled crystal. The segregation of tin, however, is not found generally. The test piece is slid on the mating hardened steel ring (SUJ 2) 75mm in outside diameter, 10mm in width, and HRC 63 in hardness. The wear test has been done by using the Hayamatype wear test apparatus, and the test piece is slid on the outer circumference of the rotating hardened steel ring under a constant load as shown in Fig. 2. The weight of wear loss is measured at intervals of 1km in sliding distance, and the coefficient of friction is obtained from the strain value in the plate spring made of phosphorbronze with a strain gauge as shown in Fig. 2. The experiment was carried out under a constant pressure of 5kg/cm2 at a sliding speed between 0.5 and 6m/sec in air without any lubricant. Using the contact electric resistance measuring apparatus(5), the variation in contact resistance was recorded continuously to read off the change of the sliding surface condition between the test piece and the mating steel ring. The apparatus consists of two electrodes, one of which is connected to the specimen holder and the other to the turning steel ring through a mercury bath, and the current variation is amplified and recorded by the pen-oscillograph. crystal parts, respectively. The sliding surface of specimens was prepared as shown in Fig. 1. As for the columnar crystal part, the effect of the sampling direction on the wear rate in the cast block was examined at first as shown in Fig. 3. The results on two kinds of specimens indicate very little effect of the sliding direction on the wear rate. Figures 4 and 5 show the wear characteristics of Cu- Pb alloys. The wear of an alloy containing about 5% Pb is severe in general and depends definitely on the casting structure. The coefficient of friction is higher in the equi-axial specimen showing a small wear rate. However, when the Pb content is about 24%, there is almost no difference between the effects of casting structure on wear and friction, and the wear rate is in general reduced, as shown in Fig. 5. In this case, the wear rate increases at a sliding speed higher than 4m/sec, and the coefficient of friction decreases. In this condition, it is considered that adhesive failure is easy to occur on the sliding surface, because of the oxide film being thinner than that at a medium speed, and the alloy base is softened with frictional heat. In Fig. 6 the wear of the Cu-Sn alloy is very small and its coefficient of friction is high, with an oxide layer on the worn surface of all the specimens. The result Fig. 3 Relation between wear rate and sliding speed in the columnar-structure. Fig. 2 Apparatus for wear test. III. Experimental Results and Discussion 1. Wear rate and coefficient of friction The experiment was done with the alloys containing various amounts of tin and lead as shown in Table 1, because it was considered that the wear characteristics of alloys having different casting structures depend largely on the amounts of tin and lead. The specimens A, B and C indicate the chilled, columnar and equi-axial (5) N. Yamazaki and F. Hayama: J. Japan Inst. Metals, 31 (1967), Fig. 4 Relation between wear rate or coefficient of friction and sliding speed (Sn 0%, Pb 5%, Pressure 5kg/cm2).

4 264 The Relation between the Sliding Wear of Lead-Bronze and Its Casting Structure indicates the oxidation wear and the effect of casting structure on wear does not appear at all. In this case, Fig. 5 Relation between wear rate or coefficient of friction and sliding speed (Sn 0%, Pb 24%, Pressure 5kg/cm2). Fig. 8 Relation between wear rate, coefficient of friction or wear of the mate ring and sliding speed (Sn 11%, Fig. 6 Relation between wear rate or coefficient of friction and sliding speed (Sn 11%, Pb 0%, Pressure 5kg/cm2). Fig. 7 Relation between wear rate or coefficient of friction and sliding speed (Sn 9%, Pb 19%, Pressure 5kg/cm2). the surfaces of mating steel rings are damaged and worn considerably. The results on lead-bronze are shown in Figs. 7 and 8. The wear characteristics depend definitely on the casting structure, and the wear rate is largest in the chilled part, followed in order by the columnar part and the equi-axial crystal part. The chilled crystal part indicates metallic wear having a metallic glossy surface, and the columnar crystal part shows the oxidation wear at a low sliding speed. In the equi-axial crystal part, oxidation wear easily appears in general, and the tendency is more remarkable with increasing tin content (Fig. 8). The variation in the coefficient of friction is also observed; the value is higher when wear is less in oxidation wear, while it is lower and the wear rate is higher in metallic wear. In addition, the wear of the mating steel ring can be seen in Fig. 8, and it indicates a high wear rate in the equi-axial crystal part with oxidation wear. Though it is said that lead-bronze gives less wear to mating steel generally(6), but when the tin content is high and the oxide layer in oxidation wear is stable, the results indicate that mating steel is worn considerably. 2. Relation between lead content and wear The relation between lead content and wear rate is investigated. Figure 9 indicates the effect of lead content on wear rate and coefficient of friction at the (6) F. Hayama: J. Japan Mech. Engrs., 66 (1963), 1652.

5 Teruji Nojiri, Fusao Hayama and Shigeo Oya 265 Fig. 9 Effect of Pb in the chill, columar and equi-axial structures on the wear rate and the coefficient of This tendency is remarkable in the equi-axial crystal specimen, facilitating the oxidation wear. The specimen containing more than 15% Pb in the chilled crystal part is hardly obtainable because of the segregation of lead, and similarly the result of specimen containing more than 20% Pb cannot be obtained in the columnar crystal part. The wear rates in the chilled, columnar and equiaxial crystal parts differ remarkably, and metallic wear is observed in the whole range of lead content for the chilled crystal except bronze with no lead. In the columnar crystal, metallic wear is observed in almost all cases and the worn surface of specimens containing above 20% Pb is slightly covered with an oxide layer. On the contrary, in the equi-axial crystal part, metallic tion wear is observed in those with more than 15% Pb and the wear rate decreases remarkably. In the alloys containing a large amount of lead (more than 20% Pb), the coefficient of friction decreases gradually with increasing lead content, which might be due to a pronounced lubricating effect of lead on the sliding surfaces. 3. Relation between tin content and wear The variation in wear characteristics of the alloys in Fig. 10. In the alloy with no tin, the effect of casting structure on the wear rate is unclear, but in the alloy with more than 4% Sn, the wear rate is most remarkable in the chilled, and decreases in the order of the columnar crystal and the equi-axial crystal. Following this order, the wear rate decreases with increasing tin content up to 11%. Especially, this tendency is remarkable in the equi-axial crystal specimen, and the phase exhibit complete oxidation wear and are hard to wear, though the coefficient of friction is high. When metallic wear occurs, the coefficient of friction is almost Fig. 10 Effect of Sn in the chill, columnar and equi-axial structures on the wear rate and the coefficient of independent of sliding speed and casting structure. After all, in the alloy containing less tin, metallic wear readily occurs by the weakness of materials due to the lead content, and in this case the coefficient of friction is low and the wear of the mating steel ring is ing tin content, oxidation wear appears easily. With increasing oxidation wear, the coefficient of friction becomes higher and the wear of specimens is less in contrast to the mating steel which is worn severely. 4. The contact electric resistance on the sliding surface The contact electric resistance changes remarkably in general as shown in Fig. 11. An example of the results of the contact resistance test is shown in Fig. 12, in regard to the relation between the sliding speed and the mean value of resistance in the most normal range for the same specimen as shown in Fig. 8. The contact electric resistance is higher when the thickness of the oxide film on the sliding surface increases. In the case of Fig. 12, the very small resistance in the chilled crystal indicates the occurrence of metallic wear. In the columnar crystal part, the resistance is found in some extent at a lower sliding speed, and it is shown that there is an oxide layer on the sliding surface. While in the equi-axial crystal specimen, the resistance Fig. 11 Variation of contact electric resistance (Sn 11%, Pb 21%, Sliding speed 0.5m/sec, Pressure 5kg/cm2).

6 266 The Relation between the Sliding Wear of Lead-Bronze and Its Casting Structure back-transfer are repeated between the sliding surfaces of the specimen and the mating steel(8). Especially, this tendency is found definitely in the specimens containing more tin and lead, and this is the reason why the wear of this group is less than that of the chilled crystal part. Further, the equi-axial crystal specimen tends to exhibit the state of oxidation wear that forms an oxide layer preventing metallic contact each other in the whole range of sliding speed in this experiment. In this case, it has fine wear debris and oxidation readily occurs during the repetition of transfer and back-transfer phenomena, and these oxides cover uniformly both sliding surfaces. Bronze without any lead is liable to exhibit oxidation wear as shown in Fig. 6(9). So, the trend that the is higher in the whole range of sliding speed, indicating the occurrence of oxidation wear. 5. Wear debris Photo. 2 is a micrograph of the wear debris detached from the specimen under the condition exhibiting metallic wear in each casting structure. The wear debris is the largest in the chilled crystal, fairly small in the columnar crystal, and the least in the equi-axial crystal. These results are considered to be connected with the size and distribution of lead as shown in Photo. 1. In relation to lead, the chilled crystal part may be rather brittle and the equi-axial one may be ductile. Generally, the metallic wear is found in the brittle alloy with larger wear debris. 6. Discussion As considered above, in the alloy containing a little tin and having tough structure, the wear is most severe in the chilled crystal, followed by the columnar crystal, and the least in equi-axial crystal. The chilled crystal specimens containing lead present metallic wear, and adhesion to the mating steel ring is very little. Almost all the columnar crystal parts present metallic wear too, but oxidation wear may be found in the alloy without any lead (bronze) or containing more tin and lead at a lower sliding speed, and then the adhesion or transfer to the mating steel is more than in the chilled crystal part. Then the adhesive metal on the surface of mating steel transfers again to the surface of the specimen, that is back-transfer(7), and the phenomena of transfer and (7) M. Kerridge and T. K. Lancaster: Pros. Roy. Soc. A, 236 (1956), 255. oxidation-wear of lead bronze is smaller than that in bronze is considered due to the effect of lead addition. It has been reported that brass containing lead prevents readily metallic wear and this is caused mainly by the decrease of impact strength(10). The same reason holds for to lead-bronze too, and it is considered that the metallic wear in lead-bronze occurs with the decrease of impact strength due to the presence of lead(11) or with the increased adhesive capacity due to the microstructural distribution of lead particles. In this case, the difference in wear characteristics among the chilled, columnar and equi-axial crystal parts is caused by the variation of impact strength or the adhesive capacity due to the size and the distribution of lead particles. The metallic wear is liable to occur in the materials having a fine distribution of lead as in the chilled crystal part. The reason is considered that it will be pulled off with a smaller shearing force, because the area having no defects against the frictional shearing stress is narrower at the adhesive contacting point when lead particle points in the structure are regarded as defects. On the contrary, when the lead particles are larger and are sparsely distributed, the distance between the lead particles becomes larger, and the area of bronze without lead increases. Therefore, it is difficult to be pulled off at a small contact point adhered to. In like manner, the fact that the chilled crystal part exhibits metallic wear easily (8) F. Hayama: Trans. Castings Res. Lab., Waseda Univ., 23 (1968), 37. (9) F. Hayama: Trans. Castings Res. Lab., Waseda Univ., 22 (1968), 19. (10) F. Hayama: J. Japan Soc. Pres. Engng., 21 (1955), 427. (11) F. Hayama: Rep. Castings Res. Lab., Waseda Univ., 12 (1961), 13.

7 Teruji Nojiri, Fusao Hayama and Shigeo Oya 267 may be explained to some extent. Further, for the impact strength of lead bronze, it has been reported that the chilled casting alloy with fine lead particles has a lower Izod impact value than the casting alloy in a sand mold(12). Then, with increasing lead content to some extent, oxidation-wear is liable to occur. It is considered that the lubricity of lead works sufficiently. In this case, the lead exists among the sliding surfaces and becomes a lead oxide easily and facilitates the formation of a cuprous oxide film. In addition, Bowden et al. have reported that the wear resistance characteristics are better in copper alloys of the dendritic structure and contained a continuous lead phase between the dendritic copper structures because of the better lubricating action by lead(13). Considering these facts, the lubricating action by lead is better in the equi-axial crystal part with a continuous lead phase than in the chilled crystal part with fine lead particles. Besides, copper alloys containing a certain amount of tin have the tendency to increase the tear resistance and induce oxidation wear because of their low adhesive character and high shearing strength. When lead bronze wear is stabilized because the lubricating characteristics of lead makes up the brittleness. After all, the finely distributed lead particles as in the chilled crystal part reduce the frictional shearing resistance and tearing resistance, and simultaneously lead particles fall off with the copper base material as the wear debris, thus lead particles cannot exhibit the lubricating characteristics. On the contrary, when the lead particles are larger and the base structure of bronze is stronger, the base metal structure is difficult to be torn and the lubricity of lead can act sufficiently. As mentioned above, the equi-axial crystal part exhibits oxidation wear more easily and has wear resistance properties. As the columnar crystal part lies between the other parts in regard to the distribution of lead particles, its wear characteristics are intermediate between them. I V. Conclusions The relation between the sliding wear of lead-bronze and its casting structure was investigated experimentally. The wear tests without any lubricants were carried out in air with the test pieces cut off from the chilled, columnar and equi-axial crystal parts from casting ingots of lead-bronze. The results obtained are as follows. (1) The wear rate tends to increase in the order of (12) H. J. French, S. J. Rosenberg and W. Le. C. Harbugh: U. S. Bur. Stds. J. Research., 1 (1928), 343. (13) F. P. Bowden and D. Tabor: The Friction and Lubrication of Solids, Oxford at the Clarendon Press, (1950), p the equi-axial crystal part, the columnar crystal part and chilled crystal part. (2) Fine lead particles are densely distributed in the chilled crystal part, while a continuous precipitation of fairly large lead particles is observed in the equi-axial crystal part. The state of the columnar crystal lies between the other crystals. (3) In the chilled crystal part, the lead particles act as defects for the frictional force and consequently the worn surfaces tend to be metallic. In the equi-axial crystal part, however, the lead content acts as a lubricant to develop the oxidation wear on the worn surfaces and to decrease the wear rate. (4) At lower lead concentrations, the lead in leadbronze is a function to reduce the impact strength of metal, and the wear rate is very high because of the low lubricant effect of lead. (5) With increasing lead content, the wearing state of lead-bronze proceeds from metallic wear to oxidation wear and the lubricating effect of lead increases. The amount of Pb % for this transformation decreases as the amount of Sn % increases. (6) The oxidation wear is stabilized with the precipi- In the alloy exhibiting steady oxidation wear, the oxide film does not easily fall off and tends to wear the mating steel. But a suitable addition of lead relaxes the tendency by the lubricating action of lead. The essential point of lead-bronze is that it prevents damages on a steel shaft by the lubricating effect of lead, being different from bronze or phosphor-bronze of high wear resistance. In other words, it is possible to select the lead-bronze with a suitable strength of the oxide film by controlling the lead content and adjusting the lubricating effect of lead, thus minimizing damages on the steel shaft. Therefore, it cannot be said that the alloy of which the oxide film is strong and the wear rate is less is better. In this regard, further consideration should also be given to the above-mentioned casting structures, and a fuller account of the wear characteristics under oil lubrication is required. However, in this experiment, the effects of the amounts of tin and lead, the distribution of fine lead particles in the chilled crystal part and the distribution of large lead particles in the equi-axial crystal part on the wear characteristics are made clear. Thus, it is convinced that these results would be a useful guide in examining the adaptability of lead-bronze under applications. Acknowledgment This investigation was supported by the Grant-in-Aid for Scientific Research of the Ministry of Education.