Materials Characterization 114 (2016) 122–135

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Materials Characterization

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An investigation on effect of heating mode and temperature on sinteringof Fe-P alloys

A. Muthuchamy a, Rajiv Kumar b, A. Raja Annamalai a,⁎, Dinesh K. Agrawal c, Anish Upadhyaya b

a Department of Manufacturing Engineering, School of Mechanical Engineering, VIT University, Vellore 632 014, Tamil Nadu, Indiab Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, UP 208016, Indiac Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA

⁎ Corresponding author.E-mail address: raja.anna@gmail.com (A. Raja Annam

http://dx.doi.org/10.1016/j.matchar.2016.02.0151044-5803/© 2016 Elsevier Inc. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 April 2015Received in revised form 20 February 2016Accepted 22 February 2016Available online 24 February 2016

The present study examines the effect of sintering temperature and heating mode on densification, microstruc-ture andmechanical properties of Fe-P alloys. The green compactswere heated conventionally and inmicrowaveunder 95%N2–5%H2 (forming gas) at 1120 °C and 1250 °C for 60min. Both the compositions (Fe-1.5P and Fe-3P inwt%) were found to couple well inmicrowave field with rapid heating (~40 °C min−1) and resulted in reductionof overall processing time by about 90% in comparison of conventional heating. Microwave sintering resulted inhigher densification in case of Fe-P alloys. Moreover, an important feature of microwave and conventionallysintered samples was that their microstructures exhibited distinctly rounded pores. This resulted into improvedcorrosion and mechanical properties in general.

© 2016 Elsevier Inc. All rights reserved.

Keywords:Fe-P alloysMicrowaveSinteringDensificationMicrostructurePhase composition

1. Introduction

In recent years, ferrous alloys processed through powdermetallurgy(P/M) route have been used extensively in automobile applications. P/Mhas become a preferred route as compared to other manufacturing pro-cesses for a variety of reasons. It offers economic advantage, ease in themanufacturing of small-sized pieces of complicated shapes, high dimen-sional accuracy, greater material utilization (N95%), flexibility to tailorthe composition and engineered microstructure [1]. In case of ferrousalloys, certain chemical elements play important role in P/Mproduction.These elements are named as sintering activators, as they enhance thesintering processes. They enable the final compact to achieve sameme-chanical properties even at lower sintering temperatures with shortersintering time. One such element is phosphorus, which is added in theform of Fe3P, as pure phosphorus may lead to ignition. In conventionalsteel making, phosphorus is one of the most undesirable elementssince it provokes irreparable segregation during solidification and thiswould make steel more brittle. However, in P/M of Fe-based systems,phosphorus has proven to be a potential strength increasing alloying el-ement [2–5]. The most favourable rounded pores in the sintered prod-ucts can easily be obtained after addition of phosphorus [6–7]. It alsoincreases the sinter density and strength of P/M steels. These improve-ments have been attributed to liquid phase sintering and high diffusion

alai).

rate in ferrite [8]. In the last two decades, a number of studies demon-strated the potential application and advantages of microwave energyin sintering of powder metals over conventional sintering [9–11]. Mi-crowave energy is defined as part of electro-magnetic spectrum havinga wavelength typically ranging from about 1 mm to 1 m in free space,and the frequency ranging from about 300 MHz to 300 GHz. However,only a few narrow frequency bands centred at around 915 MHz,2.45 GHz, 28 GHz and 80GHz are actually permitted for research and in-dustrial use to avoid any interference with the communication deviceswhere microwaves are universally used. Microwave heating offersrapid heating rates and shorter sintering times as compared to the con-ventional routes and still maintaining microstructural homogeneity[12]. Microwave heating is recognized for its various advantages, suchas time and energy saving, very rapid heating rates, significantly re-duced processing time and improved mechanical properties [13]. In1999, Roy et al successfully consolidated the metal powders consistingof Fe, Cu, Ni and carbon in 2.45 GHz multimode cavity MW furnace[5]. After that a wide variety of metallic powders including Fe, Cu, W,Mg, Al and their alloys have been successfully sintered [12–21]. Tilldate, to the best knowledge of authors there is no sufficient literatureavailable for Fe-P steels. The role of P addition has not been investigatedin detail in the literature. This can be attributed to the deleterious effectof phosphorus during traditional steel making. In modern steel making,it is treated as an impurity and its content is restricted to below0.1%. Ac-cording to the binary Fe-P phase diagrammore than 2.7% phosphorus isknown to form a liquid phase with Fe during sintering at temperatures

Fig. 1. Scanning electron micrographs of as-received (a) sponge Fe and (b) Fe3P powder.

Table 1Powder characteristics of the as received powders.

Characteristics Fe Fe3P

Apparent density (g/cm3) 3.10 2.03Tap density (g/cm3) 3.50 3.28Flow rate (s/50 g) 29.29 No flowParticle size (μm) D10 32.68 4.17

D50 90.12 9.5D90 179.80 20.16

Theoretical density (g/cm3) 7.86 6.92

Fig. 2. Comparison of the (a) heating profile of Fe-P powder compacts and the(b) corresponding power consumption in a conventional and microwave sinteringfurnace. Here, the thermal profiles are compared for compacts heated to 1250 °C.

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above 1040 °C. Few studies have reported that up to 2wt% P can be usedin conventional sintering without any deleterious effect [22]. Hence, inthis study the requisite content (1.5P, 3P) was achieved by proportion-ately adding Fe3P to sponge Iron. Fe3Pmelts congruently at 1166 °C andat 1120 °C, maximum solubility of phosphorus in γ-iron to be 3 wt%.Therefore, in the present work we have made an attempt to add equalamount of phosphorous (1.5 and 3wt%) in the form of Fe3P and studiedits effect on the sintering behaviour in conventional and microwaveheatingmethods. The sintered sampleswere characterized for their cor-rosion and mechanical properties using the standard methods.

2. Experimental procedure

SEM Images of as received Sponge iron and Fe3P powders (GKNHoeganaes, NJ, USA) are shown in Fig. 1 and their characteristics are

Fig. 3. Photograph of (a) microwave sintered 3-point bend samples and (b) MPIF tensilebars prepared using Fe-P steels. No gross distortion or cracking is evident in themicrowave sintered compacts.

Table 2Effect of varying P addition (1.5 and 3 wt%), heating mode (conventional versus micro-wave) and sintering temperature (1120 °C and 1250 °C) on the densification responseof sponge iron powder compacts.

Composition Sintering temperature, °C Sintered densityg/cm3 (% theoretical)

Heating mode → CON MWS

Fe 1120 °C 6.74 (85.7%) 6.83 (86.9%)1250 °C 6.86 (87.3%) 6.61 (84.0%)

Fe-1.5P 1120 °C 7.13 (92.1%) 6.70 (86.0%)1250 °C 7.15 (92.1%) 6.99 (90.2%)

Fe-3P 1120 °C 7.51 (98.8%) 7.01 (90.6%)1250 °C 7.50 (98.7%) 7.13 (92.0%)

CON: conventional sintering.MWS: microwave sintering.

Fig. 4.Microstructures of the as-sintered (a) Fe, (b) Fe-1.5P and (c) Fe-3P compacts consolidatedand 1250 °C(right) for 30 min.

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listed in Table 1. The elemental Fe and Fe3P powders were weighed andmixed in a Turbulamixer (T2CNr.921266, Bachofen, AG, Germany) in totwo different compositions (Fe-1.5P, Fe-3P) for 1 h. The alloy powderswere compacted at 600MPa into cylindrical pellets (16mm in diameterand 6 mm in height) using a uniaxial semiautomatic hydraulic press(model: CTM-10, supplier: Bluestar, New Delhi, India). A small quantityof zinc-stearate was used as a lubricant on the die wall before compac-tion. The compacts were sintered at 1120 °C and 1250 °C for 60 minusing conventional and microwave furnaces in forming gas (95%N2–5%H2) atmosphere. Microwave sintering of the green compacts werecarried out using amultimode cavity 2.45GHz, 6 kWcommercialmicro-wave furnace (Cober Electronics, Ct, USA). The temperature of the sam-ple was monitored using an infrared pyrometer (Raytek, MarathonSeries) with the circular cross-wire focused on the sample surface. Thepyrometer is emissivity based; direct temperature measurement was

in a conventional (radiatively-heated) furnace at two temperatures, namely, 1120 °C (left)

Fig. 5.Microstructures of (a) Fe, (b) Fe-1.5P and (c) Fe-3P steels microwave sintered at 1120 °C (left) and 1250 °C(right). The compacts were isothermally held for 30min at the sinteringtemperature.

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monitored above 700 °C [23–24] by considering emissivity of steel as(0.35) [25]. Typically, emissivity varies with temperature. However, asvery little variation in the emissivity was reported in the temperaturerange used in the present study, hence, the effect of variation in emissiv-ity was ignored. Conventional sinteringwas carried out in aMoSi2 heat-ed tubular furnace (Bysakh & Co., Kolkata, India) in 95%N2–5%H2. Therewere intermediate holding times in conventional sintering for 15min inorder to remove lubricant and gases from the samples. The sintereddensity was obtained through dimensional measurements method.The densification parameter was calculated to determine the amount

of densification occurred during sintering. It is expressed as

Densification parameter ¼ Sintered Density−Green DensiyTheoretical‐Density−Green Density

:

An optical microscope with digital image acquisition capability(LEICA DM2500, Leica Microsystems GmbH, and Germany) was usedto obtain the micrographs of sintered samples. The samples werepolished in a series of SiC emery papers (paper grades 220, 320, 500,800 and 1000), followed by cloth polishing using a suspension of

Fig. 6. SEM micrographs comparing the microstructures of Fe, Fe-1.5P and Fe-3P steels conventionally sintered at (a) 1120 °C and (b) 1250 °C.

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0.05 μm alumina diluted with water. For each samples, the un-etchedsamples were acquired in order to perform the quantitative analysis ofthe pores. The pore size was estimated by measuring the pore area.The pore shape was characterized using a shape-form factor, F, whichis related to the pore surface area, A (in μm2), and its circumference inthe plane of analysis, P (in μm), as follows [26]:

F ¼ 4πAP2

:

The pore shape factor was directly measured using the simple %round standard in Q nodules licensed software from Leica. The poremeasurement was performed on the un-etched samples. For each sam-ple, 5 measurements were taken on each of the 10 micrographs cap-tured randomly at 200× magnification at different spots (5 on the topof the surface and the other 5 on the cross-sectioned surface) on thesame sample. Grain size measurement was done using the linear inter-ceptmethod. Themeasurements were done on 5micrographs capturedat different portions of each sample at 500×magnification and on eachmicrograph 7–10 lines (200 μm length) oriented in various directionswere considered. For electrochemical analyses, the sintered sampleswere polished on a series of SiC emery papers (paper grades 220, 320,500 and 1000), followed by cloth polishing using a suspension of 0.3and 0.05 μm alumina diluted with water. Manual polishing wasemployed in this study. Ultrasonic cleaning was performed for about5–7 min in acetone bath. The micro structural analysis of the sampleswas done on the sample surface using a Leica DM2500 optical micro-scope. The polished samples were etchedwith 3% nital and their opticalmicrographs were taken by using optical microscope (LEICA DM2500,LeicaMicrosystemsGmbH, andGermany). Bulk hardness of the sinteredsamples were measured on HRC scale by a semi-automatic Rockwell

hardness tester (4150AK, Indenter hardness testing machines Ltd, UK)at 150 kg load with a 16 in. ball indenter. The observed hardness valuesare the averages of ten readings taken at random spots throughout thesample. The load was applied for 5 s. The tensile properties were mea-sured using flat tensile bars pressed as perMetal Powder Industrial Fed-eration (MPIF) Standard 10 [27], with a gauge length 26 mm using anINSTRON universal testingmachine of full-load 20 kN at an initial strainrate of 3.3 × 10−4 s−1 (crosshead speed 0.5 mm/min). The tests wereperformed at room temperature. To ensure reproducibility, five sampleswere tested. The scanning electron micrographs of polished sampleswere obtained by Scanning Electron Microscope (Zeiss Evo 50, CarlZeiss SMT Ltd., UK) in both, Secondary Electron (SE) and Back ScatteredElectron (BSE) modes. The area mapping of the polished samples werealso performed by SEM in order to determine the distribution of phos-phorous. The electrochemical behaviour of the samples was studied ina freely aerated 0.1 N H2SO4 solution (pH 1.31 ± 0.4) at room tempera-ture using a Princeton Applied Research Versa STAT 3 electrochemicalsystem. Prior to polarization, the polished samples were allowed to sta-bilize for 3600 s in 0.1 N H2SO4 for obtaining a stable open circuit poten-tial (OCP) as per ASTM standard F2129. Electrochemical tests werecarried out in a flat corrosion cell using a standard three-electrode con-figuration with the sample as the working electrode, platinum mesh asthe counter electrode and Ag/AgCl (saturatedwith KCl) as the referenceelectrode (+197mVwith respect to the standard hydrogen electrode).The exposed area of the samplewas 1 cm2with a thickness of 6mm. TheTafel testswere carried out from−250mV to 250mVversusOCPwith ascan rate of 0.167 mV s−1. The potentiodynamic tests were carried outfrom the potential −250 mV versus OCP to +1600 mV versus the ref-erence electrode with the scan rate of 0.5 mV s−1. Other researchershave also chosen scan rates varying between 0.25 and 1 mV s−1 [28].The corrosion potential (Ecorr) and the Tafel slopes (βa and βc) were

Table 3Effect of varying P addition, heatingmode and sintering temperature on the averageα-fer-rite grain size in Fe-P steels.

Composition Sintering mode → Conventional Microwave

Sintering temperature, °C Grain size, μm

Fe 1120 °C * *1250 °C * *

Fe-1.5P 1120 °C 36 441250 °C 52 65

Fe-3P 1120 °C 82 631250 °C 86 76

‘*’: grain size could not be determined.

Fig. 7. Effect of varying P content on the microstructure of iron compacts sintered in a microwave furnace at (a) 1120 °C and (b) 1250 °C with 30 min isothermal hold.

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determined from the polarization curves. The corrosion current (Icorr)and the corrosion ratewere determined geometrically by using the cor-rosion potential and the Tafel slopes. The corrosion ratewas determinedusing the 1st-Stern method and is expressed as follows [29]:

Corrosion rate mmpyð Þ ¼ 3268 e=ρð Þ Icorr:

where, e is the equivalent weight (g); ρ is the density of the material(g/cm3) and Icorr is the corrosion current (A/cm2).

3. Results and discussion

Fig. 2a compares the typical temperature-time profile for Fe-P com-pacts during conventional and microwave heating. The correspondingpower consumption during two processing conditions has beenshown in Fig. 2b. The P addition did not influence the microwaveheating of iron powder compacts and they could be consolidated in mi-crowave furnace at very rapid heating rates. In conventional processingthe heating ratewas restricted to 5 °C/min and the thermal cycle includ-ed intermittent isothermal holds (or temperature homogenization;while in the microwave no intermittent holding steps were neededand much higher heating rates (~40 °C/min) could be achieved. FromFig. 2a, it is evident that there is about 90% reduction in the sinteringtime in microwave as compared with conventional heating. The sametrend has been reported by other researchers for different metallic ma-terials [19–21]. A comparison of the power consumption indicates thatthe average power consumed to sinter the pellets in conventional fur-nacewas ~1.5 kWwith peak power reaching2.5 kW. In comparison,mi-crowave sintering could be achieved at much lower power (b0.5 kW).

Thus, microwave processing has clear advantage in terms of cycletime and power consumption. It is also to be noted that due to thelower thermal mass, the cooling rate in microwave furnace is relativelyhigher. Despite such a fast heating and cooling rates, no drastic dimen-sional changes were observed (Fig. 3).

3.1. Densification response

Table 2 compares the densification response of pure iron and iron-phosphorus (Fe-1.5P and Fe-3P) alloys sintered in conventional andmi-crowave furnaces at two temperatures (1120 °C and 1250 °C). Irrespec-tive of the sintering temperature, the Fe-P alloys resulted in poor

Fig. 8. Pore shape form factor of (a) Fe, (b) Fe-1.5P and (c) Fe-3P steels sintered at 1120 °C (left) and 1250 °C (right).

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densification response as compared to their conventionally processedcounterparts. Even for pure Fe, compacts sintered in microwave at ele-vated temperature (1250 °C) had a relatively lower density. FromTable 2, it is rather evident that phosphorus acts as an effective Sintering

promotor. The sintered density increases with increasing phosphoruscontent and Fe-3P alloys attain nearly full density. For both Fe-1.5Pand Fe-3P alloys, sintering temperature does not significantly influencethe final density.

Fig. 9. SEM microstructure and the corresponding EDS area mapping of phosphorus in Fe-1.5P steels consolidated in a conventional furnace at (a) 1120 °C and (b) 1250 °C.

Fig. 10. Elemental mapping of phosphorus (determined using EDS) for Fe-3P steels sintered at (a) 1120 °C and (b) 1250 °C using a radiatively-heated, conventional resistance furnace.

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Fig. 11. SEM microstructure and the corresponding EDS area mapping of iron and phosphorus in Fe-1.5P steels consolidated in a 2.45 GHz microwave furnace at (a) 1120 °C and(b) 1250 °C.

Fig. 12. EDS elemental mapping of iron and phosphorus in Fe-3P steels sintered at (a) 1120 °C and (b) 1250 °C using a microwave furnace.

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3.2. Microstructure and phase analysis

Figs. 4 to 7 exhibit the effect of varying P content and temperature onthe representative microstructures of conventionally (Figs. 4 & 6) andmicrowave sintered (Figs. 5 & 7) compacts. Few observations are obvi-ous from these optical and SEMphotomicrographs. Themicrostructuresreveal the positive role of phosphorus on the densification enhance-ment in both conventional and microwave sintered sample. However,

besides improving densification, phosphorus also promotes coarseningof iron grains. Table 3 summarizes the effect of phosphorus addition andsintering temperature on the grain size of iron compacts. The averagegrain size for Fe-P compact increases with increasing temperature andphosphorus addition. In general, microwave sintered Fe-P alloys exhibitless microstructural coarsening (Table 3).

The sintered microstructures (Figs. 4 and 7) indicate that the poresseem to become more rounded and isolated with increasing

Fig. 13. Phase analysis of the sintered compacts a) 1120 °C, b) 1250 °C.

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phosphorus content. In Figs. 4 and 5, the areas with low P content ap-pear as a light phase with a heterogeneous surface structure and clearboundaries between the ferrite grains, while the areas with high P con-tent appear dark and flat. The black spots within the grain in Fig. 4 andFig. 5 are isolated pores. Fig. 8.a to .c compare the effect of heatingmodeand sintering temperature on the pore shape factor of Fe, Fe-1.5P andFe-3P alloys, respectively. Addition of phosphorus and microwave pro-cessing results in more rounded pore morphology compared with theconventional sintered samples and are in close conformity with the ob-servedmicrostructures. Analysis of distinction betweenpores andphos-phorus was difficult in optical micrograph. Hence, the sinteredcompacts were examined under a scanning electron microscope (Figs.6 and 7). From the SEM micrographs, one can infer that for somesintering conditions, and there is a second phase at the grain boundariesin Fe-P alloys. The conventionally sintered Fe-3P alloys shown in (Figs.4.c and 5) exhibit typical liquid phasewithwell-roundedα grains inter-spersed in a P-rich solidified phase. Elementalmapping of the represen-tative microstructures was done using energy dispersive spectroscopy(EDS) and was found to be in close conformity with the observed SEM

results. Figs. 9 and 10 compare the effect of sintering temperature onthe elemental distribution in conventionally sintered Fe-1.5P and Fe-3P alloy, respectively. In case of Fe-1.5P, phosphorus is distributed even-ly throughout the iron grains. In case of conventionally sintered Fe-3Palloys (Fig. 9), the grain boundaries are demarcated by P-rich region.The thickness of this rim in case of alloys sintered at 1250 °C (Fig. 9.b)is more than that observed in 1120 °C sintered samples (Fig. 9a). InFigs. 9 and 10 the grains are quite angular, and large Fe-3P phase eitherexists at the grain boundaries or within iron grains and the grain shapebecomes more spheroidal with an increasing phosphorus addition. Incase of microwave sintered samples, both Fe-1.5P and Fe-3P exhibitnon-uniform, P-rich phases which are unaffected by the sintering tem-perature (Figs. 11 and 12). This is due to the formation of eutectic phaseat 1050 °C with iron and preferentially alloy of inter-particle region. ForP-rich regions in microwaves the time was insufficient to cause diffu-sion, dissolution and dissemination of phosphorus within the iron ma-trix and across the grain boundaries. The elemental mappings confirmthat irrespective of the processing temperature, phosphorus hardlygets homogenized within or around the iron grains (Figs. 10 and 11)for compacts consolidated in microwave furnace. Elsewhere,Upadhyaya and Sethi [30] also confirmed similar inhomogeneity inthe distribution of tin in premixed Cu-Sn bronze consolidated in shorttime through microwave sintering. Fig. 13 compares the effect ofheating mode and sintering temperature and holding time on thephase evolution for compacts using X-ray diffraction (XRD) studies.Both conventional and microwave sintered compacts resulted only fer-rite phase under all sintering temperatures, i.e. complete absence ofinter-metallic phases.

3.3. Properties of sintered alloys

Table 4 compares the effect of phosphorus addition and sinteringtemperature on the bulk hardness of sintered compacts. The results in-dicate that phosphorus addition leads to significant increase in thehardness of sintered iron. For both Fe-1.5P as well as Fe-3P alloys, hard-ness increases with increasing sintering temperature for conventionallysintered alloys. In contrast, for Fe-3P compacts high heating rate consol-idation (through microwaves) adversely affected the hardness. In viewof the poor densification response of microwave sintered Fe-P alloys(Table 1), the 3-point transverse rupture strength and tensile testingwere conducted only for conventionally sintered alloys. Note that allcompacts, including the ones containing 3 wt% P and sintered up to1250 °C show excellent shape retention and isotropic dimensionalchange during sintering. Table 5 compares the effect of sintering tem-perature on the bending strength, tensile strength and ductility ofpure Fe, Fe-1.5P and Fe-3P alloys. It is very clear that higher sinteringtemperature results in degradation of properties for both iron and Fe-P compacts. The only exception is the tensile strength which showsmarginal improvement at higher sintering temperatures. At 1120 °C,phosphorus addition results in drastic improvement in bend strength(more than two-fold) and tensile strength (55% increases). SinteredFe-3P alloy yielded the best combination of strength (TRS: 1050 MPa,UTS: 450MPa). Sahoo andBalasubramaniam [31] investigated the effectof varying P content on themechanical properties of pure iron preparedby casting and forging route for concrete reinforcement applications.They demonstrated that up to 0.32% addition of P results in improve-ment of strength without adversely affecting the ductility [31]. Howev-er, ≥0.49% P results in brittle failure mode. In P/M literature, the tensilestrength of Fe-0.45P alloys have been reported to vary between 350 and390 MPa [32–34]. In this study the tensile strength achieved in Fe-1.5Pand Fe-3P alloys is better than that of reported data [32–34]. In fact,themechanical property of Fe-3P alloys is even superior to that reportedfor the MPIF Standard 35 for grade FY-8000-16Y [35]. Another uniqueattribute of high phosphorus containing iron is significant improvement(~126%) in the bending strength. It is also interesting to note that thestrength improvement in Fe-3P alloy sintered at 1120 °C is not at the

Fig. 14. SEM fractographs of (a) Fe, (b) Fe-1.5P and (c) Fe-3P compacts sintered at 1120 °C (left) and 1250 °C (right) in a conventional furnace.

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expense of the ductility. In fact, theductility of Fe-3P seems tobe slightlyhigher than that of pure iron compacts when sintered at 1120 °C.

Fig. 14 shows the SEMmicrographs of the fractured tensile samplesof Fe and Fe-P alloys consolidated at both the sintering temperatures.Note that iron compacts alloyed with 1.5 and 3% phosphorus andsintered at 1250 °C exhibit transgranular and cleavage mode fracture(Fig. 13b, c) typical of brittle failure mode. In comparison, both pureFe as well as Fe-3P compacts exhibit dimpled fractured surface (Fig.13a, c) that is indicative of a ductile failure mode.

3.4. Corrosion behaviour of sintered Fe-P alloys

As indicated in the introduction, phosphorus addition has a benigneffect in imparting corrosion resistance to iron. However, so far no sys-tematic investigation has undertaken on the corrosion response ofsintered Fe-P alloys. In order to investigate the corrosion behaviour ofthis system, the open corrosion potential and potentiodynamic

polarization experiments were conducted in 0.1 N H2SO4 on compactsconsolidated using conventional and microwave heating methods attwo sintering temperatures (1120 °C and 1250 °C). Fig. 15a and b com-pare the open circuit potential (OCP) of Fe and Fe-P alloys sintered at1120 °C using conventional and microwave furnace, respectively. Thecorresponding OCP stabilization curves as a function of time for1250 °C sintered compacts are presented in Fig. 16a and b. From OCPstabilization trend in the figures, it can be observed that irrespective ofthe sintering mode and temperature, for pure iron compacts, the OCPvalue progressively decreases with time and tends towards more nega-tive values. This trend in the OCP is indicative of the compact surfacechanging to a more active potential, which is indicative of a poor corro-sion response. In contrast, the Fe-1.5P and Fe-3P compacts exhibit a re-verse trend with respect to the variation in OCP with time. This is anindicator of the sintered compact surface attaining a relatively morenoble behaviour upon exposure to 0.1 N sulphuric acid solution. Thequalitative estimate of the corrosion behaviour of the sintered alloys

Fig. 15.Open circuit potential (OCP) for Fe, Fe-1.5P and Fe-3P steels sintered at 1120 °C for30 min in a (a) conventional and (b) microwave furnace. The sintered alloys werestabilized in 0.1 N H2SO4.

Fig. 16.Open circuit potential (OCP) for Fe, Fe-1.5P and Fe-3P steels sintered at 1250 °C for30 min in a (a) conventional and (b) microwave furnace. The sintered alloys werestabilized in 0.1 N H2SO4.

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from the free corrosion potential assessment was further confirmedthrough potentiodynamic polarization studies. Fig. 17a and b comparesthe effect of P addition polarization behaviour of iron compacts sinteredat 1120 °C in a conventional and microwave furnace, respectively. Thecorresponding potentiodynamic polarization curves for Fe and Fe-Pcompacts sintered at 1250 °C are shown in Fig. 18a and b. FromTable 6, it is obvious that the corrosion current (icorr) and corrosionrate values exhibit self-similar trend relative to the sintering atmo-sphere and increase in the order: While the polarization curves ofsintered Fe and Fe-1.5P alloys are self-similar, there is a striking differ-ence in the anodic polarization response in all the Fe-3P compacts. Un-like the other compositions, the Fe-3P alloys show a transition fromactive to passive behaviour between 1.25 and 1.6 V. The onset of thistransition is reflected in sudden decrease in the current density by sev-eral orders of magnitude. Unfortunately, the potentiodynamic polariza-tion setup had an upper voltage restriction of 1.75 V, hence, thepolarization curves could not be evaluated over a much broader rangeto capture the breakdown voltage for this passive region in Fe-3P alloys.

From the polarization curves (Figs. 17 and 18), the corrosion poten-tial (Ecorr), corrosion current density (icorr), passive current density(ipass) and the corrosion rates were determined and are summarizedin Table 6. As indicated in the OCP curves (Figs. 15 and 16), the mea-sured corrosion parameters too show a relatively more negative Ecorrvalues in pure Fe compacts as compared to sintered Fe-P alloys. TheFe-3P compacts exhibited as active-passive transition and this wasreflected in a passivation current whichwas nearly orders of magnitude

lower that the corresponding icorr values obtained for the same alloy(Table 6). Irrespective of the sintering mode, the corrosion rate of Fe-1.5P is not much different from that obtained for sintered Fe compacts.However, the Fe-3P compacts show significantly lower corrosion ratewhich for some cases was two orders lower than those observed forFe powder compacts sintered under identical conditions.

Mukherjee and Upadhyaya [36] also reported that the presence ofphosphorus increases the corrosion potential of sintered ferritic stain-less steels towards a more noble direction. They also reported that thecorrosion resistance of ferritic stainless steels in 1 N H2SO4 improveswith up to 1wt% phosphorus addition. However, as stainless steel is cor-rosion resistant hence the improvement in the corrosion behaviourwasnot that marked. Furthermore, the authors [36] did not offer any expla-nation for their observations. To the best of our knowledge, the onlyother reference that investigated the influence of phosphorus on thecorrosion resistance in steel is the paper from Gabe [37], who men-tioned that the presence of relatively high phosphorus content in theFe results in formation of protective surface film that leads to good cor-rosion resistance.

4. Conclusions

In this study it is found that there is not much influence of sinteringtemperature on the sintered Iron compacts with the addition of P (Fe-1.5P and Fe-3P). The sintered density increases with increasing phos-phorus content and Fe-3P alloys attaining near full density. The pores

Fig. 17. DC Potentiodynamic polarization curves for iron compacts with varyingphosphorus contents and sintered in a (a) conventional and (b) microwave furnace at1120 °C for 30 min.

Fig. 18. DC potentiodynamic polarization curves for iron and iron-phosphorus alloys(having 1.5% P and 3% P) sintered at 1250 °C in (a) conventional and (b) microwavefurnace.

Table 4Rockwell hardness of Fe, Fe-1.5P and Fe-3P alloys consolidated at two sintering tempera-tures (1120 °C and 1250 °C) in a radiatively-heated resistance furnace (conventional) anda 2.45 GHz, multimode microwave furnace.

Composition Sintering mode → Conventional Microwave

Sintering temperature, °C Hardness

Fe 1120 °C 22HRB 32HRB1250 °C 52HRC 40HRC

Fe-1.5P 1120 °C 72HRC 52HRC1250 °C 82HRC 59HRC

Fe-3P 1120 °C 84HRC 83HRC1250 °C 94HRC 79HRC

Table 5Effect of P content and sintering temperature on the bend strength and tensile propertiesof iron compacts. All compacts were sintered in a radiatively-heated, resistance furnace in95N2–5H2 atmosphere.

Sample Temperature TRS, MPa UTS,MPa

Elongation%

Reduction inarea, %

Fe 1120 °C 471 ± 11 296 ± 3 5.5 ± 0.8 5.2 ± 0.71250 °C 358 ± 8 305 ± 5 2.8 ± 0.2 1.9 ± 0.2

Fe-1.5P 1120 °C 1015 ± 21 454 ± 7 2.0 ± 0.1 2.7 ± 0.11250 °C 519 ± 8 513 ± 4 0.4 ± 0.01 0.4 ± 0.01

Fe-3P 1120 °C 1067 ± 11 462 ± 2 7.1 ± 0.02 6.6 ± 0.021250 °C 715 ± 4 517 ± 4 0.8 ± 0.05 0.8 ± 0.04

134 A. Muthuchamy et al. / Materials Characterization 114 (2016) 122–135

seem to become more rounded and isolated with increasing phospho-rus content. In general, phosphorus addition, higher sintering tempera-ture and microwave processing results in more rounded poremorphology and are in conformity with the observed microstructures.The average grain size for Fe-P compact increases with increasing tem-perature and phosphorus addition. Phosphorus addition leads to signif-icant increase in the hardness of sintered iron. For phosphorus-addediron, high heating rate consolidation (through microwaves) adverselyaffects the compact hardness. Higher sintering temperature results indegradation of properties for both iron and Fe-P compacts. For pureiron compacts, the OCP value progressively decreases with time andtends towards more negative values. This trend in the OCP is indicativeof the compact surface changing from to amore passive to active poten-tial, which is indicative of a poor corrosion response. The Fe-3P com-pacts exhibited as active-passive transition and this was reflected in apassivation current which was nearly orders of magnitude lower thatthe corresponding icorr values obtained for the same alloy. Ecorr valuesin pure Fe compacts as compared to sintered Fe-P alloys.

Acknowledgements

The authors would like to acknowledge Indo-US Science and Tech-nology Forum (IUSSTF), New Delhi and seed fund from VIT University,Vellore for partial support of this research work. Also, the authors A.

Table 6Parameters determined from DC potentiodynamic polarization experiments for sintered Fe, Fe-1.5P and Fe-3P steels. Where (a) 1120 °C and (b) 1250 °C.

Composition Heating mode Icorr (mA/cm2) Ipass (mA/cm2) Ecorr (mV) Corrosionrate (mmpy)

(a)Fe CON 4.21 – −457 57.16

MW 3.80 – −350 51.68Fe-1.5P CON 3.77 – −371 48.38

MW 4.99 – −408 68.26Fe-3P CON 3.46 8.66 × 10−3 −375 10.63

MW 4.44 2.84 × 10−3 −370 3.70

(b)Fe CON 4.01 – −465 53.48

MW 6.28 – −409 86.20Fe-1.5P CON 3.57 – −353 45.68

MW 4.70 – −389 61.64Fe-3P CON 3.44 7.14 × 10−4 −374 0.93

MW 4.45 2.55 × 10−3 −348 3.27

135A. Muthuchamy et al. / Materials Characterization 114 (2016) 122–135

Muthuchamy & A. Raja Annamalai acknowledge the DST-FIST facilitiesat Department of Manufacturing Engineering, VIT University, Vellore.

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