Study of the recycling grey cast iron swarf by powder metallurgy: an alternative for the development of new materials M. L. Parucker 1, C. E. da Costa 1 1 Universidade do Estado de Santa Catarina UDESC Centro de Ciências Tecnológicas - /CCT/FEJ CEP 89223-1, Campus Universitário, Bom Retiro, Joinville, Santa Catarina, Brasil moises_parucker@superig.com.br Keywords: grey cast iron swarf, recycling, powder metallurgy. Abstract: This work is concerned with issues related to using grey cast iron swarf from dry machining in powder metallurgy. In a first stage, a complete study on the characteristics of the raw powder is performed, in which aspects related to the dilution in different proportion of pure iron were studied. That a low compacting pressure produces workpieces more resistant and that the ideal sintering temperature is 116 ºC. In a second stage, aiming at reduction of the powder, a heat treatment is performed. New dilutions using the descarbured powder were made and results showed better structural homogeneity and an improvement in the mechanical properties. The steel alloy 5/5% of swarf powder provided the best results. In a third stage, a cylindrical bearing was produced. New dilutions are made. Based on the control of the interconnected porosity, radial and wear resistance, the sample prepared with 1% swarf powder provided the best results. Introduction The first published papers on the use of grey cast iron as cheap and viable raw material by powder metallurgy technique are by Brown [1]. Grey cast iron has the major feature of easily breaking into fine particles by milling process, thanks to the presence of graphite. However, there is interest on eliminating or reducing the content of graphite through appropriate treatments. By modifying its morphology these treatments improve the compressibility of such powders, and it is recommended sintering at high temperatures (>12 o C), still being capable of post treatment by forging and chemical treatment in order to improve its mechanical properties [2-4]. Table 1 presents a summary of processing conditions and the properties of grey cast iron, carried through for Nakagawa [2]. This table shows that after of descarbured grey cast iron presents a greater resistance to grey cast iron without descarburing, being this raised enough resistance for A powders, what it is related to the lesser quantity of carbon and the variation of graphite in the form of flakes. Systematic works with different and well controlled additions of 2, 4, 6, 7 and 8% of iron powder to powder grey cast iron swarf (8 MPa, 115-12 C/3 min.), have shown a resistance increasing of iron powder substances, finding the best values between 5-6% of addition.
Table 1- Conditions of compacting-sintering and mechanical resistance [2]. Materials Compacting Pressure MPa Sintering Temperature C Sintering time (minutes) Tensile Strength MPa Dimensional Variation % - 4-8 175-1125 3 1-2 <1 Powder 6-8 1125-1175 6 35-5 <1 A Powder B 4-8 11-1125 3 15-25 <1 In another published paper by Zapata, mixtures of iron casting powder swarf/iron powder in proportions of 7/3, 5/5 and 3/7 have been carried out and compacted at 7 MPa. Mixture with 5% of atomized iron and 5% of grey cast iron powder presented a tensile strength about 24 MPa and the evaluated values of ductility through elongation, have been above (< 5%) [5]. Bose and Mukunda have investigated the possibility of manufacturing selflubrificating bearing using mixtures of grey cast iron swarf and pure iron, processed by powder metallurgy. Densities of 5,6 to 6, g/cm 3 were obtained sintering samples between 1 and 11 C/6 min. It is according to the recommendations for this type of material that demands a variation of 2% maximum of volumetric lubrication oil [6]. Brown explains that this process may be very attractive but its economic result depends on local conditions, availability of grey cast iron, possibility of forging, machining and position in enterprise market segment [1]. A real economic advantage of the powder swarf-forging process can be achieved if parts weight is greater than 2g [4]. Experimental Procedure The raw material used in this work were lamellar grey cast iron swarf of perliticferritic matrix 7/3% which were obtained from machining operations (Table 2). Table 2 - Grey cast iron chemical composition. Cast Iron Chemical composition (%) C Si Mn P S Cr 3,45 2,8,58,35,12,8 A three - stage program was proposed to optimize this process : Stage 1: study of powder grey cast in crude state (carbon 3,5 %: analysis of time and grey cast milling process profit, microstructural features of obtained metal powder, mixtures of powders, sinterizing and evaluation of properties gotten during this experience). Stage 2: study of descabured grey cast iron powder (carbon 1.2%: powder microstructural features, study of compressibility and sintering of mixtures, evaluation of the mechanical properties). Stage 3: bearing manufacturing. Tables 3, 4 and 5 show how the mixtures are classified for this work. Table 3 - Swarf powder without descarburizing treatment. STAGE 1 Chemical composition Sample Fe Powder Cu C total Swarf M1 1 - - - M2 7 3-1.5 M3 5 5-1,75 M4 3 7-2,4 M5-1 - 3,5 M6 5 5 3,5 1,8
Table 4 - Swarf powder with descarburizing treatement. STAGE 2 Sample Chemical composition Fe Powder Cu C total Swarf M2R 7 3 -,36 5 5 -,6 M4R 3 7 -,84 M5R - 1-1,2 M6R 5 5 3,5,62 M7R 3 7 3,5,85 M8R - 1 3,5 1,24 Table 5 Chemical composition of a typical commercial bearing Fe-Cu. The powder was gotten by mechanical milling by 2, 5, 1, 15, 25, 35, 45 and 7 hours. The descarburation of the powder cast iron was carried out using Fe 2 O 3. The composition of mixtures is presented in tables 3, 4 and 5. The uniaxial compacting was carried out under pressures of 1, 2, 3, 4, 5, 6 and 7 MPa, and pressure of 7 MPa has been chosen. The sintering was carried out in a period of 6 minutes under varied relations of temperatures like 16, 18, 11, 112, 114, 116 and 118 C under green gas atmosphere had been distinguished (N 2-8%H 2 ) for mixtures M1, M2, M3, M4, M5. The gotten samples have been characterized and new sintering under 116 C was carried out with mixtures M1, M2, M2R, M3, M3R, M4, M4R, M5, M5R, M6, M6R, M7, M7R, M8 and M8R. The temperature of 116 C was selected. The microstructural characterization was done by optical microscopy (OM), scanning eletctron microscopy (SEM) and the mechanical properties through hardness Vickers HV5, tensile strength and wear test pin-on-disc [7-9]. Manufacture of the bearing was carried at pressure of 25 MPa, and the radial resistance and the resistance constant (K-factor) were evaluated [1]. Results and Discussion STAGE 3 Bearing Fe-Cu (Sample MB) Raw material Composition (%) Fe Cu Graphite Lubricant (Zinc estearate) 95,8 3, 1,2,75 Stage 1: the production of powder resulted in particles with total rerycling of grey cast iron swarf and presented grain sized distribution as showed in figure 1. These studies indicate excellent conditions, 25 hours of milling. The grain sized distribution indicates that more than 9% of powder presents size lesser than,1 mm. The morphologic analysis of powder have been presented irregular and the microstructure presented a continuous welding and breaking because of mill balls action (See Fig. 2). The material presented low compressibility and it did not compact adequately, resulting in fissuration of the compact green, proceeding from the extreme amount of present free graphite. It was observed however, that lower pressures of compacting, produced samples with little fictions. Results (%) 1 8 6 4 2 15 25 35 45 7 Milling time (h) <1 mesh Size Grain sized distribution Percentage Mesh mm (%) +14 +2 +23-23 +,15 +,74 +,63 -,63 Aparent Density (g/cm 3 ) Green Density (g/cm 3 ) Compressibility at 7 MPa (g/cm 3 ),68 13,55 29,92 54,47 1,6 2,96 5,53 Fig. 1 Results grey cast iron swarf milling.
1 µm a 1 µm Fig. 2 - Morphology of powder cast iron swarf with no type of treatment: a) Individual particle, b) Structure of powder cast iron swarf (SEM). The conditions of sintering under low temperatures have not propitiated the sintering of test bodies. The sinterings carried out at 112, 114, 116 and 118 C have provided structural variations and it was possible to determine the ideal temperature for sintering at 116 C. However, the dimensional variation in b samples with greater amount of powder grey cast iron swarf (M3, M4 and M5) suggested the copper addition to amounts of 3,5% [11]. One of the first evaluated properties was the apparent hardness of sintering materials in several tested temperatures and due to perlitic structure presence it was observed an increase of content of powder swarf in the samples. The tribological study showed an increase of wear associated with materials with greater amount of powder grey cast added to mixtures. It is important to detach a substantial improvement with the presence of 3,5% of copper in the M6 mixture (5/5%). The wear zones correspond precisely to mixtures where the carbon content is greater, due to the increasing of hardness. Stage 2: the main point was to improve the powder quality. Table 6 shows some descabured powder physical characteristics. Table 6 Physical characteristics of descarbured powders. Aparent Densidaty (g/cm 3 ) 1,7 Green Density (g/cm 3 ) 2,86 Compressibility at 7 MPa (g/cm 3 ) 5,1 The different compact mixtures at 7 MPa and sintering at 116 C have showed homogeneous microstructure, due to an increasing powder swarf content. The analysis of these samples presented predominantly ferritic structures with ironsilicate composite presence, oxygen and free carbon in the porosities left for the sintering (SiO 2, MnFeSiOx, FeSiOx). These abrasive composites can influence the properties of the material (Fig. 3). 1 µm 2 µm 5 µm Fig. 3 - Sintering structures: M5R (Attack: Nital 1%, OM and SEM). The dimensional variation of samples M3R, M4R and M5R, was controlled by addition 3,5% of copper in weight. After conforming, the characterization of the distinct materials gotten through its main properties was related: hardness, resistance to tensile strength and elongation. The values of tensile strenght have been limited due to bad compatibility of mixtures with greater content 7% of powder swarf. The samples M1, M2R, M3R and M6R have been assayed. The elongation obtained was between 1 and 1,5%. The maximum tensile strength gotten
i.exe was M6R (5/5% in the presence of copper) reaching values of 13 MPa and the increasing of hardness was observed in the samples with greater powder swarf content. k(mm 3 /N.m),3,25,2,15,1,5 M1 M2R M3R Fig. 4 - Wear Constant k of these materials (Conditions: sintering at 116 C/6 min.). M6R Mixture M4R M7R M5R M8R The tribological study is showed in figure 4. The strong reduction can be verified that was produced in the wear of materials with copper presence (M6R, M7R and M8R). It had a substantial improvement in different mixtures constantly wear, which reached 5% reductions, comparing to conditions of first stage. However, it is observed that mixture M6R presented a lesser wear and that the same composition (M3R) presented an increasing of wear, but without copper presence. Stage 3: The figure 5 shows how the different mixtures, besides the standard bearing, have presented some mechanical properties as specified. The bearing MB presented greater radial resistance and the other mixtures presented a lower resistance. The sample M8R which is closer to the composition of bearing MB, showed an excellent result. The reason to have a better compaction of powder is to apply lower pressures (25 MPa). Analyzing the constant resistance K of these materials, all obtained values are in the minimum limit of resistance indicated in literature: 14 N/mm 2 for densities between 5,6/6, g/cm 3 [1], exception to the samples M4R and M7R. In this cases the increasing of powder swarf content reduces the density of these materials what is reflected in the amount of porosity. In addition, despite sample M8R has lower density of all the samples studied, it presented an interlinked homogeneous porosity and an excellent value K (17 N/mm 2 ). The Sample M6R (5/5%) presented the best radial resistance and a constant K greater than 19 N/mm 2 which would be according to literature references. The figure shows a strong resistance fall to samples with powder swarf content. This fact is confirmed by increasing of hardness and the structural homogeneity provided by of low pression of compacting. Observing the MB and the sample M8R hardness are really similar, but the wear resistance of M8R performed a better result. Fig. 6 shows the results gotten in the tribological study. Radial Resistence(N) 1 9 8 7 6 5 4 3 2 1 MB M2R M3R M6R M4R M7R M8R 35 3 25 2 15 1 5 K (N/mm 2 ) k(mm 3 /N.m),8,7,6,5,4,3,2,1 MB M2R M3R M6R M4R M7R M8R 8 7 6 5 4 3 2 1 Hardness HV(5) Mixture Fig. 5 Radial resistence and constant resistence K of these materials. Mixture Fig. 6- Materials consuming Constant. (Conditions: 25 MPa). Figure 7 shows that a resulted in heterogeneous distribution of porosity with a relative density of 75 %. The wear in b presents itself very large because of material loss and abrasive wear process is easily seen.
5µm a 5 µm Fig. 7 - Characteristics of material MB: a) Structure, b) Wear (SE SEM). b 1 µm a 2 µm Fig. 8 - Characteristics of material M8R: a) Structure, b) Wear (SE SEM). b A porosity homogeneous distribution and the presence of compounds of iron-silicate in material porosities are detailed in Fig. 8. In b it is shown the wear to a relative density of 6%. Conclusions The results of this research have shown that it is impossible to obtain a complete optimization of all properties at the same time, but the optimization for materials characteristics was performed satisfactorily. Materials with unique properties can be obtained through machining manufacturing using a single mechanical milling to get grey cast iron powder. The analysis of third stage results have shown that there is a need of a process optimization for each kind of material. In closing, the properties values obtained have shown these materials can be a viable alternative to apply in bearing and but deeply studies are necessary before giving a reliable and definite conclusion about the utilization and viability of recycling swarf. References [1] G.T. Brown, On the grey cast iron powder of making low-cost P/M forging. Progress in Powder Metallurgy, MPIF Princeton, NY, v. 28, p. 243-258, 1972. [2] T. Nakagawa, F. S. Dai, Sintering and forging of decarbonized cast iron powder. Modern Developments in Powder Metallurgy, v. 12, p. 723-743, 1981. [3] P. Ramakrishnan, S. L. Palkar, K. Gururaj, Preparation and evaluation of P/M grade iron powder from cast iron swarf. Modern Developments in Powder Metallurgy, v. 15, p. 73-84, 1985. [4] Y. Takeda et al., Process and properties of newly developed pulverized swarf forging. Modern Developments in Powder Metallurgy, v. 12, p. 745-755, 1981. [5] W. C. Zapata et al., Reciclagem de cavacos de usinagem por metalurgia do pó. Parte I. 5º. Congresso ABM, São Paulo, p. 431-448, Ago. 1995. [6] A. Bose, P. G. mukunda, Bearings from machined cast iron swarf powder. Powder Metallurgy International, v. 18, n. 5, p. 333-337, 1986. [7] Determination of hardness of powder metallurgy products. MPIF. n. 43, 1998. [8] Tension test specimens for pressed and sintered metal powders. MPIF. n. 1, 1998. [9] Standart Test Method for: Wear Testing with pin-on-disc Apparatus. ASTM G 99. [1] Equipe Metalpó, Mancais auto-lubrificantes. São Paulo, 1987. [11] T. Nakagawa, C. S. Sharma, P/M forging and sintering for the recycling of machining swarf. Modern Developments in Powder metallurgy, v. 9, p. 347-388, 1977.