Effect of heating mode on sintering of ferrous compacts through powder metallurgy route

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1 Effect of heating mode on sintering of ferrous compacts through powder metallurgy route A. Raja Annamalai* 1, A. Upadhaya 1 and D. Agarwal 2 Microwave heating is recognised for its various advantages, such as time and energy saving, very rapid heating rates, considerably reduced processing cycle time and temperature, fine microstructures and improved properties. The present paper focuses on preliminary work carried out with the use of microwave radiation applied to sintering of ferrous compacts. The ferrous alloy compacts were sintered in a multimode microwave furnace of 2?45 GHz and 6 kw nominal power at 1120uC for 60 min in forming gas. Results of densification, mechanical properties and microstructural evaluation of the microwave sintered samples are reported and compared with conventionally sintered ones. In general, it is observed that the microwave radiation generally enhances the properties of the sintered material when compared with conventionally sintered material. Keywords: Ferrous compacts, Microwave sintering, Mechanical properties, Densification Introduction Powder metallurgical (PM) steel components are increasingly being utilised for automotive and structural applications. 1 Traditional sintering of ferrous alloy compacts is carried out in conventional furnaces using a slow heating rate in order to prevent thermal gradients in the powder compacts. This not only increases the process time but also results in significant microstructural coarsening during sintering, leading to degradation of the mechanical properties. 2 The application of microwave sintering has received growing attention during the past decade, due to its tremendous advantages over conventional sintering techniques. Microwave sintering is fundamentally different from conventional furnace sintering 3 as outlined in the first report on microwave sintering of metallic parts in A microwave is a part of electromagnetic spectrum having typical wavelengths ranging from y1 mmto1m in free space, and corresponding frequencies ranging from y300 MHz to 300 GHz. However, only narrow frequency bands centred at y915 MHz, 2?45 GHz, 28 GHz and 80 GHz are actually used for microwave heating and sintering purposes. 5 The mechanical properties of microwave sintered samples have been shown to be superior to those of conventionally sintered samples. 6 Ferrous PM alloys are widely used in a variety of structural applications, predominantly in the automotive industry. Iron nickel steel PM parts are used extensively for their properties such as high mechanical strength and excellent dimensional control. The main 1 Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur , India 2 Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA *Corresponding author, araja@iitk.ac.in objective of the present study is to understand the effect of microwave sintering of ferrous alloys upon the sintered density, pore shape factor and microstructure. Experimental For the present study, sponge iron powder manufactured by Höganäs, (Cinnaminson, PA, USA), nickel powder (carbonyl) manufactured by the Metal Powder Co., Ltd (Tamil Nadu, India) and lorenz graphite powder (K5,15) manufactured by Lonza (Basle, Switzerland) were used as starting material. The characteristics of the as received powders are detailed in Table 1. The powders were characterised as per Metal Powder Industries Federation standards. 7 9 Figure 1 shows scanning electron micrographs of the as received powder. The elemental powders were weighed and mixed in a Turbula mixer (T2CNr ; Bachofen AG, Nidderau-Heldenbergen, Germany) for 1 h. The powders were uniaxially pressed and made into cylindrical pellets (16 mm diameter and 6 mm height) with a floating die in a 100 ton hydraulic press (CTM-100; Blue star, Delhi, India) using zinc stearate as a die wall lubricant. To investigate the tensile properties and transverse rupture strength, flat tensile bars and transverse rupture test bars were pressed as per Metal Powder Industries Federation standards. 10,11 All the samples were compacted at 600 MPa. To study the density behaviour and the microstructural and mechanical properties, the green compacts (as pressed) were sintered using both conventional and microwave furnaces. The conventional sintering of the green compacts was carried out in a MoSi 2 radiant heated horizontal tubular sintering furnace (OKAY 70T-7; Bysakh, Kolkata, India). The compacts were heated at a constant heating rate of 5 K min 21.To ensure a uniform temperature distribution during heating, intermediate isothermal holds for 15 min were provided at 550 and 1000uC, with the final sintering being carried out ß 2011 IHTSE Partnership Published by Maney on behalf of the Partnership DOI / X International Heat Treatment and Surface Engineering 2011 VOL 5 NO 4 155

2 1 Scanning electron micrographs of as received a Fe, b Ni and c graphite powder at 1120uC for 30 min. The microwave sintering of the green compacts was carried out in a multimode microwave furnace 2?45 GHz, 6 kw (RC/20SE; Amana Radarange, Benton Harbour, MI, USA). Unlike a conventional furnace, the temperature of the samples inside a microwave furnace cannot be monitored using a thermocouple. 12,13 Therefore, the temperature of the sample was monitored using an infrared pyrometer (Raytek, Marathon Series) with the circular cross-wire focused on the sample crosssection. The pyrometer is Emissivity based and could not measure temperatures accurately below 700uC. 14 Both conventional and microwave sintering was carried out in a 95%N 2 5%H 2 atmosphere at 1120uC witha30minsoak. The density of the sintered compacts was measured using the Archimedes principle. The sintered density was expressed in grams per cubic centimetre and also normalised with respect to the theoretical density based on the weight fraction w of the respective component. The theoretical density for a given composition r th was calculated using inverse rule of mixture and is expressed as 1 r T ~ XN i~1 w i r i The sinterability of the compact was also determined through a densification parameter which is expressed as Densification parameter~ sintered density{green density theoretical density{green density The sintered samples were polished using SiC emery paper followed by cloth polishing using a suspension of 0?05 mm alumina diluted with water and then the polished samples were etched with 3% nital to examine the microstructures. The microstructural analyses of the samples were carried out using an optical microscope (DM2500; Leica Imaging System Ltd, Cambridge, UK). The pore size was estimated by measuring the pore area, while the pore shape was characterised using a shape form factor F, which is related to the pore surface area A (mm 2 ), and its circumference in the plane of analysis P (mm), as follows F~ 4pA P 2 Both pore area distribution and shape factor were directly measured using an image analyser. The pore measurement was performed on unetched samples. For each sample, pore shape quantification was performed on 10 photomicrographs captured randomly. On the whole, between 3500 and 5700 pores were quantified for statistical accuracy. The shape factor of a feature is inversely proportional to its roundness. A shape factor of 1 Table 1 Characteristics of experimental powder Characteristics Fe Ni Graphite Apparent density/g cc Tap density/g cm Flowrate, 50 g s No flow Particle size/mm D D D Theoretical density/g cm Thermal profile of ferrous compact heated in conventional radiant furnace and 2?45 GHz microwave furnace 156 International Heat Treatment and Surface Engineering 2011 VOL 5 NO 4

3 3 Effect of heating mode on a sintered density and b densification parameter of ferrous alloy compacts represents a circular pore in the plane of analysis and as it reduces the pores tend to become more irregular. The hardness of the samples was measured using a Vickers hardness tester (a Leco V-100-C1 hardness tester) at a 0?05 kg load. Results and discussion Heating profiles and densification response The thermal profiles of the ferrous compacts in conventional and microwave furnaces are shown in Fig. 2. It a Fe; b Fe 2Ni; c Fe 2Ni 0?8C 4 Optical micrographs of unetched steel compacts sintered in conventional furnace (left) and microwave furnace (right) International Heat Treatment and Surface Engineering 2011 VOL 5 NO 4 157

4 5 Comparison of a shape factor distribution and b pore area distribution of ferrous compacts sintered in conventional and microwave furnaces: C: conventional sintered samples; M: microwave sintered samples was observed that all of the ferrous compacts coupled with the microwave very rapidly, without a susceptor, and resulted in higher heating rates without any intermediate holds. The overall heating rate in the microwave furnace was y35 K min 21, whereas in the conventional furnace the heating rate is 5 K min 21 maximum. Also, in order to avoid thermal shock and maintain the temperature uniformity during the sintering, intermediate holds at different temperatures are required with conventional furnaces. Another interesting feature noted with the microwaved samples is that the transformation of one phase to another phase was indicated by a small spike in the heating profile curve. The first event is the ferrite to austenite transformation which results in a depression on the plot at y910uc, whereas the second event is the phase transformation from austenite to ferrite. Effect of heating mode on sintered density and densification parameter of ferrous compacts is given in Fig. 3. It was noted that on the whole microwave sintered compacts showed a marginal improvement in sintered density compared with conventional counterparts. Since the microwave heating rate is faster than conventional heating, it reduces the total processing time as well as enhancing the sintering kinetics and leads to the increase in the sintered density. All the compacts were pressed at the same pressure and to the same green a Fe; b Fe 2Ni; c Fe 2Ni 0?8C 6 Scanning electron micrographs of steel compacts sintered in conventional furnace (left) and microwave furnace (right) 158 International Heat Treatment and Surface Engineering 2011 VOL 5 NO 4

5 a Fe; b Fe 2Ni; c Fe 2Ni 0?8C 7 Optical micrographs of etched steel compacts sintered in conventional furnace (left) and microwave furnace (right) density; hence, the densification parameter response is similar to sintered density. All the samples in both heating modes had undergone shrinkage. Pore shape factor The unetched optical micrographs which show the effect of heating mode on compacts are given in Fig. 4. They reveal the distinct differences in pore morphology between the microwave compacts and their conventional counterparts. It was observed that the pores have a more rounded appearance in microwave compacts than those in conventional ones. This was verified by quantifying and comparing the distribution of pore sizes (measured as area) and shape factors of conventional and microwave sintered compacts as shown in Fig. 5. The conventionally sintered samples which exhibit a broader distribution of shape factors than the microwave sintered samples are skewed towards lower shape factor value 1, which is indicative of irregular pores, as can be clearly observed in the unetched micrographs. The same features are confirmed by the scanning electron micrographs in Fig. 6. Microstructure and phase evaluation Etched optical micrographs showing the effect of heating mode on the compacts are given in Fig. 7. Nickel is a substitutional alloying element and its diffusion rate is very slow. Hence, in the solid state sintering which took place in the present study, the nickel remains concentrated in the areas around the original nickel particles, as can clearly be seen in the figure. Since nickel is an austenite stabiliser, these areas will tend to remain austenitic. The concentration of nickel also tends to be high in the boundaries between the iron particles, which can result in the formation of nickel rich martensite in localised areas. If carbon is present in the material, a small amount of pearlite will also be present. The pores are black and in many particles it is possible to observe areas of pearlite. The white areas are nickel rich retained austenite. Figure 8 compares the X-ray diffraction patterns of both conventionally and microwave sintered samples, showing that there is no observable difference as far as phase evolution is concerned. However, it is interesting to note that although the sample size and other parameters were constant during the experiment, nevertheless, the relative intensity of the X-ray peaks was slightly lower in the microwave sintered samples than the conventional ones. There is no accepted explanation for this observation yet, although it may be that the faster heating rate caused changes to occur on the surface of the material. Vickers microhardness Figure 9 shows the effect of heating mode on the hardness of ferrous compacts. The harness of the sintered iron is International Heat Treatment and Surface Engineering 2011 VOL 5 NO 4 159

6 8 X-ray diffraction pattern of conventionally and microwave sintered ferrous alloy compacts mainly affected by the porosity which exists in the material. The hardness was found to be increase with the increase in the green density of the sample. The samples were sintered in forming gas in which the major proportion is nitrogen. During sintering at 1120uC, nitrides may be formed which enhances the strength of the pure iron. Addition of nickel increases the hardness of the material marginally. The carbon addition greatly improves the strength and therefore the hardness of the nickel alloys. Conclusions Ferrous alloy powders were successfully consolidated at different temperatures through conventional and microwave sintering. Microwave sintering resulted in a significant (y60%) reduction in processing time compared with conventionally processed samples. No microor macrocracks were observed in the sintered samples, which essentially supports the volumetric heating nature of microwave sintering. The microstructure of the microwave sintered samples is finer and contains rounder pores than the conventionally sintered samples. Acknowledgement This collaborative research was carried out as a part of the Centre for Development of Metal Ceramic Composites 9 Effect of heating mode on Vickers hardness of ferrous alloy compacts through Microwave Processing which was funded by the Indo US Science and Technology Forum (IUSSTF), New Delhi, India. References 1. R. M. German: Powder metallurgy science, 2nd edn; 1994, Princeton, NJ, Metal Powder Industries Federation. 2. R. M. German: Sintering theory and practice ; 1996, New York, John Wiley. 3. R. Roy, D. Agrawal, J. Cheng and S. Gedevanishvili: Nature, 1999, 399, R. M. Anklekar, K. Bauer, D. K. Agrawal and R. Roy: Powder Metall., 2005, 48, R. Wroe: Microwave sintering coming of age, Met. Powder Rep., 1999, 54, S. S. Panda, V. Singh, A. Upadhyaya and D. Agrawal: Effect of conventional and microwave sintering on the properties of yttria alumina garnet-dispersed austenitic stainless steel, Metall. Mater. Trans. A, 2006, 37A, MPIF standard 48: determination of apparent density of metal powders using the Arnold Meter, Standard test methods for metal powders and powder metallurgy products, Metal Powder Industries Federation, Princeton, NJ, USA, MPIF standard 46: determination of tap density of metal powder, Standard test methods for metal powders and powder metallurgy products, Metal Powder Industries Federation, Princeton, NJ, USA, MPIF standard 3: determination of flow rate of free flowing metal powders using the hall apparatus, Standard test methods for metal powders and powder metallurgy products, Metal Powder Industries Federation, Princeton, NJ, USA, MPIF standard 10: tension test specimens for pressed and sinter metal powders, Standard test methods for metal powders and powder metallurgy products, Metal Powder Industries Federation, Princeton, NJ, USA, MPIF standard 41: determination of transverse rupture strength of sintered metal powder test specimen, Standard test methods for metal powders and powder metallurgy products, Metal Powder Industries Federation, Princeton, NJ, USA, A. Nayer (ed.): The metals data book ; 1997, New York, McGraw-Hill. 13. S. Gedevanishvili, D. Agrawal and R. Roy: Microwave combustion synthesis and sintering of intermetallics and alloy, J Mater. Sci. Lett., 1999, 18, R. M. Anklekar, D. K. Agrawal and R. Roy: Microwave sintering and mechanical properties of PM copper steel, Powder Metall., 2001, 44, (5), International Heat Treatment and Surface Engineering 2011 VOL 5 NO 4