EFFECT OF CARBON CONTENT AND SINTERING TEMPERATURE ON MECHANICAL PROPERTIES OF IRON- BASED PM PARTS PRODUCED BY WARM COMPACTION

Transcription

1 Powder Metallurgy Progress, Vol.8 (2008), No EFFECT OF CARBON CONTENT AND SINTERING TEMPERATURE ON MECHANICAL PROPERTIES OF IRON- BASED PM PARTS PRODUCED BY WARM COMPACTION A. Babakhani, A. Haerian Abstract The effect of carbon content varying from 0.15 to 0.45% and sintering temperature of 1120 C and 1250 C in Astaloy CrM powder was studied. It was found that both green density and green strength increase with reduced carbon. The results also showed that warm compaction at 135 C increases green density by around 0.18 g cm -3 and green strength by 15 MPa when compared to cold Increasing carbon content and sintering temperature increase tensile strength and impact strength. Compared to samples compacted at room temperature, warm compacted samples showed much smaller increase in mechanical properties. Impact resistance increased by up to 100% with increased carbon content and sintering at 1250 C. Keywords: green density, sintering, mechanical properties, warm compaction INTRODUCTION Powder metallurgy is a process used to produce many delicate and complicated parts to near net shape through compaction and sintering of metal powders. Reduced machining of parts is an attractive advantage of this process over competing methods [1,2]. Improving density has been a growing concern since the beginning of commercialization of the PM process in order to be able to compete with rival production processes. Various methods and techniques have been introduced for this purpose including hot isostatic pressing (HIP), double press-double sinter (DP-DS), powder forging (PF), etc. [3-8]. Warm compaction is a rather new technique for improving density. Here, the morphology of the product changes so as to give higher densities as well as more uniform density distribution throughout the workpiece. Warm compaction can be applied to most powder-material systems. Through warm compaction we can get higher strength, as well as better tolerances. It is also possible to do a good deal of green machining due to higher green strength [9]. The equipment and tooling used for warm compaction are those used for ordinary compaction and sintering, except for the need to preheat the powder and compaction in a die to C [10, 11]. Some of the advantages of using this technique are: increasing green density in the range of 0.1 to 0.3 g cm -3, increasing green strength from 50 up to 100% improved powder flow in the die, reduced scrap during compaction, improved machinability in green state, and Abolfazl Babakhani, Department of Materials Science and Engineering, Engineering Faculty, Ferdowsi University of Mashhad; Mashhad, Iran Ali Haerian, Engineering Faculty, Sadjad University of Technology; Mashhad, Iran

2 Powder Metallurgy Progress, Vol.8 (2008), No possibility of compaction under lower pressures, thereby increasing tool life. By this technique, densities as good as those obtained through double pressingdouble sintering methods can be achieved [11,12]. MATERIALS AND METHODS Powder preparation The starting powder used for this work was water-atomized Astaloy CrM made by Höganäs, AB. This powder contains around 3% Cr and 0.5% Mo. The powder was mixed with 0.15 to 0.45% UF4 graphite to adjust the carbon content, and 0.4% lithium stearate as lubricant. Compaction Tensile test samples (ASTM E8) shown in Fig.1 were compacted at 150 C and 650 MPa in a warm compaction equipment made at Ferdowsi University of Mashhad (Fig.2). Fig.1. Standard tensile test sample according to ASTM E8. Fig.2. Warm compaction test rig.

3 Powder Metallurgy Progress, Vol.8 (2008), No Sintering and testing After dewaxing at 600 C for 45 minutes, samples were sintered at two different temperatures of 1120 C and 1250 C for 1 hour in pure nitrogen. Green density was determined using Archimedes method after covering their surface with impermeable material. Three point bend test was used for determination of green strength. Tensile strength was measured using 250 KN Zwick tension-compression testing machine, and a 50 J Charpy impact tester was used for impact toughness test. RESULTS AND DISCUSSION Green density It has been shown that in warm compaction, the dominant mechanism is rearrangement of particles during compaction, while plastic deformation mainly helps in providing free space (voids) for this rearrangement [9]. Therefore, plastic deformation does not greatly affect densification of the green compacts. Apparently, sliding and rotation of particles are the two main causes of increased green density, which with increasing compaction temperature and higher radial pressure, brings about higher green density. Variation of green density with carbon content is shown in Fig.3. Linear reduction of green density with carbon content can be attributed to the lower density of carbon as compared to that of iron. Figure 3 indicates a 0.2 g cm -3 increase in green density at almost all carbon levels as compared to cold compaction. 7.4 Green Density 7.3 Density (gr/cm3) cold compact Warm compact Fig.3. Variation of green density with carbon content for warm compacted and cold % C Sintered density Figure 4 shows changes in density after sintering. It is quite clear that increase in density after sintering at 1120 C for warm compaction and cold compacted samples are g cm -3 and g cm -3 respectively. Further increase in sintered density at 1250 C sintering temperature may be attributed to higher shrinkage of samples. Sintered density decreased with carbon content, as was the case for green density. These results indicate warm compaction can be preferred for compaction of Astaloy CrM which has low compressibility.

4 Powder Metallurgy Progress, Vol.8 (2008), No Sintered Density 7.4 Density(gr/cm3) C0ld (S.T=1120 C) Warm(S.T=1120 C) Cold (S.T=1250 C) Warm (S.T=1250 C) Fig.4. Variation of sintered density with carbon content for warm compacted and cold %C Green strength Green strength of samples increases with compaction temperature, Fog.5. This finding is in close agreement with those of Sajeev [9] and Höganäs [13]. In fact, increasing compaction temperature increases the fluidity of the lubricant. This increased fluidity, in turn, facilitates movement of lubricant out from interparticle space towards the die wall. Die wall lubrication thus provided, improves compressibility, and reduced amount of trapped lubricant between particles enhances metal-metal contact. Both these phenomena are in favour of increased green density. Maximum green density observed was 34 MPa for warm compacted samples containing 0.15% carbon. Increased green density eases manipulation and machinablity of green parts and reduces overall cost of production [14]. Green Strength (MPa) Green Strength cold compact warm compact % C Fig.5. Variation of green strength with carbon content for warm compacted and cold

5 Powder Metallurgy Progress, Vol.8 (2008), No Mechanical properties In warm compacted samples, high metal-to-metal contact renders higher mechanical properties. Increasing sintering temperature accelerates the reduction of surface oxides. This produces clean surfaces on the powder particles, which helps neck formation. Consequently, the effective load bearing surface area increases. This, in turn, improves mechanical properties. Figures 6 and 7 show increasing of hardness and tensile strength with increasing carbon content and sintering temperature. Maximum tensile strength of 938 MPa was observed for samples containing 0.45% carbon and sintered at 1250 C. It was also observed that warm compacted samples sintered at 1120 C had superior mechanical properties as compared to those compacted at room temperature and sintered at 1250 C Hardness Hardness (HV30) Cold (S.T=1120 C) Warm(S.T=1120 C) Cold (S.T=1250 C) Warm (S.T=1250 C) 0.3 %C 0.45 Fig.6. Variation of hardness with carbon content for warm compacted and cold compacted samples. Tensile Strength(MPa) Tensile Strength Cold (S.T=1120 C) Warm(S.T=1120 C) %C Cold (S.T=1250 C) Warm (S.T=1250 C) Fig.7. Variation of tensile strength with carbon content for warm compacted and cold

6 Powder Metallurgy Progress, Vol.8 (2008), No An important observation here is the pronounced improvement in impact resistance of warm compacted samples sintered at 1250 C over the cold compacted material (Fig.8). This improvement may be attributed to the elimination of oxide layers from the surface of the grains due to interaction with carbon, producing more metal-tometal contacts and hence increased effective load bearing surface area. 60 Impact Energy Impact energy (J) C0ld (S.T=1120 C) Warm(S.T=1120 C) Cold (S.T=1250 C) Warm (S.T=1250 C) Fig.8. Variation of impact strength with carbon content for warm compacted and cold %C Fractographic observations Figure 9 shows SEM micrographs of fractured surfaces. In warm compacted samples, metal-to-metal contact has increased. Consequently, the effective load bearing surface area increases. Fracture here is of ductile type. However, cleavage fracture was also observed in some portions of the cross section. Ductile fracture areas have open pores. The free surfaces of the grains have little or no dislocations causing fracture accompanied by plastic deformation at low temperature. On the other hand, areas with cleavage indicate development of sinter necks, which results in closed pores [15].

7 Powder Metallurgy Progress, Vol.8 (2008), No a) Warm compacted and sintered at 1120 C. b) Cold compacted and sintered at 1120 C. c) Warm compacted and sintered at 1250 C. d) Cold compacted and sintered at 1250 C. Fig.9. Fractured surfaces of warm compacted and cold compacted samples containing 0.3% C. CONCLUSION In this work, mechanical properties of warm compacted iron-base powders containing 3% chromium and 0.5% molybdenum with addition of 0.15 to 0.45 carbons were investigated. Following results were observed: 1. green density and green strength increased with reduction of carbon content; 2. compared to samples compacted at room temperature, warm compacted samples showed 0.18 g cm-3 in green density and around 10 MPa increase in green strength; 3. compared to samples compacted at room temperature, warm compacted samples showed much smaller increase in mechanical properties over as-sintered samples; 4. tensile strength and impact resistance both increased with increased carbon content and sintering temperature; and 5. impact resistance increased up to 100% with increased carbon content and sintering at 1250 C. REFERENCES [1] Wojciechowski, S.: J. Mater. Process. Technol., vol. 35, 2000, p. 230 [2] Froes, FH., Eylon, D.: Int. Mater. Rev., vol. 35, 1990, p. 162

8 Powder Metallurgy Progress, Vol.8 (2008), No [3] Engström, U. High Performance PM Materials by Warm Compaction, PM98 [4] Engström, U., Johansson, B. In: 2000 Powder Metallurgy World Congress. PM Part 1. Kyoto, Ed. K.Kosuge, H.Nagai, p. 536 [5] Bengtsson, S., Fordén, L., Dizdar, S., Johansson, P. In: MPIF 2001 Int. Conf. on Power Transmission Components, October 16-17, 2001, Ypsilanti, MI, USA [6] Jones, PK., Buckley-Golder, K., Sarafinchan, D. In: Advances in Powder Metallurgy and Particulate Materials, vol. 1, 1997, p. 33 [7] Skoglund, P., Engström, U., Dizdar, S. In: SAE2002 World Congress, March 2002, Detroit, USA [8] Skoglund, P. In: MPIF Int. Conf. on Power Transmission Components, October 16-17, 2001, Ypsilanti, MI, USA [9] Höganäs AB: Höganäs handbook for sintered components. No. 4, Warm Compaction, 1998 [10] Ozaki, Y. et al.: Kawasaki Steel Giho, vol. 33, 2001, no. 4, p. 170 (in Japanese) [11] Luk, SH., Rutz, HG., Lutz, M. In: Advances in Powder Metallurgy and Particulate Materials, MPIF, Princeton, USA, 5, 1994, p. 135 [12] Rutz, HG., Hanejko, FG. In: Advances in Powder Metallurgy and Particulate Materials, MPIF, Princeton, USA, 5, 1994, p. 117 [13] Sajeev, VC., Sivaprahasam, D., Sivakumar, A., Sundaresan, R. In: Powder Metallurgy in Automotive Applications II, 2002, p. 143 [14] Engström, U., Johansson, B. In: 2002 International Conference on Powder Metallurgy for Automotive Parts, April , Isfahan, Iran, 2002 [15] Šlesár, M. In: Proc. of the Conf. Deformation and Fracture in Structural PM Materials, IMR SAS Košice. Vol. 1, 1996, p. 85