Analysis of the characteristics of metallic powders realised from the waste resulted from ball bearing manufacturing.

Petrescu, Valentin ; Bibu, Marius ; Nemes, Toderita 等

1. INTRODUCTION

After the finishing machining of bearing balls and ball tracks, important quantities of waste from RUL steel material may result, with a fine grain structure (Petrescu & Nemes, 2001; Amza, 2002). The reentry of the metallic powder into the economic circuit represents a superior exploitation of wastes of RUL steel, because the metallic powder prices at the global scale are higher than the prices of other types of metallic parts (obtained through casting, milling etc.

The studies presented in this paper represent a continuing preoccupation of the authors to recover the waste material as metallic powder (Urdas & Petrescu, 2001). Until the present time this technology was not exploited (Salak et al., 2001).

The waste resulted from the process contains, beside the RUL steel grains, particles of manganese, white cast iron detached from the finishing disks, emulsion oils, also iron oxides form the part's surface and from the hot pressing machining previously executed.

2. EXPERIMENTAL RESEARCHES

The experiments have been realized at temperature values of 800, 850 and 900[degrees]C, with time ranges of 30, 45 and 60 minutes.

The waste's purification can be realized by oxidizing burns (anneal), and the obtained reduced compounds have been treated to obtain some alloyed iron powders with low carbide ratio.

The metallic waste obtained in the above-mentioned conditions was annealed at 950[degrees]C, over a period of 60 minutes, was reduced at 950, 1000 and 1050[degrees]C temperatures with the reduction period of 60 minutes, in a dissociated ammonia atmosphere (160 l/h yield). The pressing curves, for the obtained powders in these three reduction regimes, have been raised for the compacting pressures of 2, 4, 6 and 8 [10.sup.3] daN/[cm.sup.2]. In the sintered form, the hardness values have been determined on cylindrical plates with diameter d = 11.24 mm and height h = 20 mm, the increase of density through the sintering process on cylindrical plates with d = 16 mm, also tensile strength tests were carried out on pressed plates according to the MPA standards.

The sintering process was realized in a tubular sintering furnace, at a temperature of 1200[degrees]C, during a 90 minutes period, in a dissociated ammonia atmosphere (yield 150 l/h).

3. RESULTS AND DISCUSSIONS

The chemical composition of the recovered scrap is presented in table 1, the content of alloying elements matching the standard prescriptions for the RUL steels. It was determined that the carbide content concentration is higher than the standard concentration (0.95-1.10 %C), the initial scrap being unpurified with cast iron particles detached from the finishing disks and also with oils contained in the working emulsion.

The purpose of the annealing treatment is to obtain reduction compounds through the oxidation process, a decrease of the carbide concentration and a decrease of organic substances and oily impurities.

According to table 1, the total volume of carbide decreases with the increase of the annealing temperature, the lowest level was obtained at an annealing temperature of 950[degrees]C (0.08%).

This content may decrease with the decrease to 950[degrees]C of the reducing temperature and the spectral analysis shows only carbide residues in the reduced powder.

The amounts of the other elements maintain their values after the reduction and annealing processes. The reduction process at 950[degrees]C satisfies the necessities of elaborating the powder with a low carbide proportion. The reductions have been executed at higher temperatures in order to obtain discontinuous distributions adequate to sintering process conditions.

Table 2 shows the density and the fluidity of the obtained powders in the three reducing conditions. Those powders belong to the low compactness powders category, the values of the powder density being lower than iron powders, usually utilized at the sintering process, with a value of 2400 kg/[m.sup.3].

Because of the reduced powder fineness at 950[degrees]C, it was not possible to determine fluidity unless using a calibrated hole with a 4 mm diameter (the other two values of fluidity were obtained using a calibrated hole with a 2.54 mm diameter).

Figure 1 presents the pressing curves for the obtained powders in the three reducing conditions. With an increasing reducing temperature, the powder density increases in the pressed conditions, the powder reduced at 1050[degrees]C showing the highest density.

The increase of density through the sintering process as a function of compacting pressure is shown in figure 2.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Analyzing the presented curves it can be concluded that the highest sintering density was obtained for the powder reduced at the temperature of 1000[degrees]C. The trend of the spheroid shape transformation of the powders, with the increase of the reducing temperature, explains the lower values for the density increasing for the sintering process of the powder reduced at 1050[degrees]C compared with the powder reduced at 1000[degrees]C. The mechanical characteristics of the sintered material using the three types of reduced powders are shown in fig. 3 and fig. 4.

Figure 3 presents the hardness variation as a function of the compacting pressure. The highest values of the hardness were obtained for the powder reduced at 1050[degrees]C with the trend of sintering hardness increasing with the increase of the reducing temperature.

Figure 4 presents the variation of the yield strength of the different sintered parts obtained from reduced RUL powders, in a close range, but superior to the commonly used iron powders (NC 100-24, Ph 160-27, FREM S 160-30).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

4. CONCLUSIONS

The results of the presented research certify the possibility of elaborating alloyed powders with a low fraction of carbide from wastes of RUL steel, with good resistance characteristics after the sintering process, allowing the direct use of the powder mix recipes for the manufacturing of sintered parts with superior resistance characteristics, used in the machine manufacturing industry.

5. REFERENCES

Amza, Gh. (2002). Tratat de Tehnologia Materialelor. (Materials Technology Compendium). Romanian Academy House, Bucharest.

Petrescu, V., Nemes, T. (2001). Tehnologia Materialelor 1. Elaborarea si procesarea materialelor metalice (Materials Technology--Elaboration and processing of metallic materials, Editura Universitatii "Lucian Blaga" din Sibiu, ISBN 973-651-250-9, Sibiu, 286 p.

Rud, V.D., Gal'chuk, T. & Povstyanoi, A.Y. (2005) Powder Metallurgy Use of Waste from Bearing Production. Powder Metallurgy and Metal Ceramics, Vol. 44, No.1-2, p. 106-112.

Salak, A., Selecka, M. & Danninger, H. (2001) Machinability of Powder Metallurgy Steels. Cambridge International Science Publishing, ISBN 1-8983268-2-7.

Urdas V. et al. (2001). The study of materials used for the forging of metallic powders preforms Acta Universitatis Cibiniensis, Vol. XLIV Technical Series, A. Materials Science and Technology, ISSN 1221-4949, pp. 51-54

Tab. 1. The chemical characteristics of the employed
materials under different conditions

Material Working regime Chemical composition
type
 Burning Reduction C Si Mn
 temp., temp.,
 [degrees]C [degrees]C

Initial -- -- 1.89 0.28 0.31
RUL steel
scrap

Annealed 800 -- 0.20 0.26 0.31
RUL steel 850 0.14 0.27 0.30
scrap 900 0.09 0.25 0.30
 950 0.08 0.26 0.30
RUL steel-- 950 950 0.25 0.30
reduced
powder

Material Chemical composition
type
 Ca Ni Cu Mo

Initial 1.51 0.14 0.18 0.06
RUL steel
scrap

Annealed 1.42 0.12 0.16 0.06
RUL steel 1.39 0.11 0.15 0.06
scrap 1.38 0.10 0.16 0.06
 1.35 0.10 0.15 0.06
RUL steel-- 1.35 0.10 0.15 0.06
reduced
powder

Tab. 2. Density and fluidity of
obtained powders

Reduced RUL Density, Fluidity,
powder kg/ S/50g.
 [m.sub.3]

950[degrees]C 1790 8.3 *
1000[degrees]C 1950 11.6
1050[degrees]C 1660 121.6