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Dry Sliding Wear of Fe Based Powder Processed Through Hot Powder Forging Technique
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 17170–17179
www.materialstoday.com/proceedings
AMPCO-2017
Dry Sliding Wear of Fe Based Powder Processed Through Hot Powder Forging Technique S. K. Chaurasiaa*, Ujjwal Prakasha, Vikram Dabhadea, S. K. Natha a
Department of Metallurgical and Materials Engineering Indian Institute of Technology Roorkee, Uttarakhand-247667, India
Abstract This research investigates the effect of normal load, alloy composition and hardness on the coefficient of friction and wear rate of Iron-Phosphorus Powder metallurgy (Fe-P P/M) alloys. The specimens were prepared from hot forged alloys (Fe-1.3wt.%P, Fe2wt.%P and Fe-3wt.%P) were tested using Pin on disc apparatus. The wear rate increases as increasing in the normal loads. The coefficient of friction was inversely proportional to hardness. Since the phosphorus addition increases from 1.3 to 3 wt.% the volume percentage of phosphide (Fe3P) increased from 21 to 34vol. % which improves the hardness. The wear mechanism of the alloys is presented and discussed. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Advances in Materials & Processing: Challenges & Opportunities (AMPCO-2017). Keywords: Contact; Removal; Adhesive wear; Wear rate; Powder metallurgy.
1. Introduction In recent years there has been a considerable amount of work directed towards the development of high performance (improved mechanical properties) steels for various refineries, structural and ships etc. As automotive manufacturing is an area under high-cost pressure, less expensive and more easily processed materials are needed [1]. There are various technological methods to develop the steel alloys for the critical applications. One of the technological process that is being used to replace traditional metal-forming operations because of its low relative energy consumption, high material utilization and the low capital cost is powder metallurgy (P/M). Therefore, P/M
* Corresponding author. Tel.: +91-9412489182. E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Advances in Materials & Processing: Challenges & Opportunities (AMPCO-2017).
S.K.Chaurasia et.al / Materials Today: Proceedings 5 (2018) 17170–17179
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steels have been found wide application in the automotive industry, in engine and transmission systems. However, in the present research work, a process has been developed to achieve high performance (nearly full density) alloys for the tribological study. In this process, phosphorus (in the form of ortho-phosphoric acid) was mixed with an appropriate amount of water-atomized iron powder to prepare a master alloy of the required amount of phosphorus content for developing further Fe-P alloys. Phosphorus is normally considered an undesirable impurity in steels. It causes cold and hot shortness in ferrous alloys. It may also lead to grain boundary failure due to segregation of phosphorus at grain boundaries [2-3]. For these reasons, the amount of phosphorus is limited to 0.04% in the majority of steels. Ironphosphorus (Fe-P) P/M alloys show better mechanical properties in terms of high hardness, high strength. They show low friction coefficient and excellent wear resistance [4-5], enhanced corrosion resistance [6-7] and improved magnetic properties [8-11]. Phosphorus provides solid solution strengthening to the ferrite matrix [12-14]. Phosphorus addition may lead to increase in the density of P/M iron-based alloys [8, 15-16]. In Fe-P alloys prepared by melting and casting, the last liquid to solidify is rich in phosphorus and this can lead to embrittlement. In Fe-P P/M alloys developed by conventional pressing and sintering route, heavy volume shrinkage is experienced. Also, it takes a long time to complete the process. This problem has been overcome by using powder forging. The excess amount of phosphorus forms an iron phosphide which increases the tribological behavior of the alloy [17]. Phosphoric iron shows high wear resistance. The hard parts of phosphide favor an increase in the hardness and wear resistance of the alloy [18]. The role of phosphide in the wear behavior of iron has generally been suggested, to be due to one or more of the following. (i) The phosphide may stand proud of the surface and carry the applied load, (ii) The phosphide increases the strength of the iron under compressive conditions and this increases the resistance to plastic flow, (iii) Phosphide may spread over the surface and form a continuous film, (iv) The phosphide may melt and cause a redistribution of stress [19]. However, a few works have been reported on hot forging route of Fe-P alloys (less amount of P) in the liquid phase (above 1100̊C) region [16], But there is also need to go for the high P content alloys. In the present investigation Fe-P alloys were developed by forging of hot solid coated powder which may also allow us to go to the high phosphorus level without the problem of the segregation of phosphorus and shrinkage as observed in convention powder metallurgy process and melting and casting route. Hence, the wear and friction behavior research has been relatively scarce compared to steels developed by another route. Most of the mechanical components are made by powder metallurgy technique which exhibits good wear resistance [20]. Phosphorus is used as an alloying element in the iron-based alloy for special applications. Due to its high wear resistance and lack of sparking, high content of phosphorus in the iron would best material suitable for the brake. The role Fe-P alloys also exhibit improved corrosion resistance and magnetic permeability. 2. Experimental Procedure Fe-P with 1.3%, 2% and 3% phosphorus (all compositions in weight %) P/M alloys were developed by hot solid powder forging technique in the present investigation. In this process, water-atomized Fe powder (~8 microns) supplied by M/S HOGANAS AB, Sweden was used in the present study. An Fe-P master alloy was prepared by mixing the iron powder with the required amount of laboratory grade of ortho-phosphoric acid, H3PO4 with purity 98%. The ortho-phosphoric acid was diluted by slowly mixing it into an adequate amount of distilled water so that the volume of the diluted ortho-phosphoric acid was sufficient to completely cover the iron powder, which was laid in a tray. For the preparation of 1 kg master alloy with 5wt. % P, 950 gm atomized iron powder was mixed with 100 ml ortho-phosphoric acid and left for two hours. The mixture was dried in air for 24 hours. The ortho-phosphoric acid would react with iron particle and form a surface coated iron powder as Fe3 (PO4)2 after the chemical reaction such as:
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3Fe + 2H3PO4 Fe3 (PO4) + 3H2 (1) The coated (Fe3 (PO4)2) powder so produced is poured out and dried in open atmosphere. By using atomized Fe powder and master alloy (Fe3 (PO4)2), the powder blends were used for making the different composition of Fe-P alloys. Mixing carried out mechanically in a laboratory ball mill for 2-3 hours. The total weight of the mixture was taken 1 Kg. The distribution of P in a typical Fe-P powder mixture is shown in Fig. 1.
Figure 1: Elemental mapping of powder mixture alloy.
This M.S (Fig.2) capsule was a hollow tube of 63.5 mm outer diameter (OD), 2 mm wall thickness and 80 mm length fabricated with hemispherical end caps at both sides. A hole of 10mm was provided at the center of end caps. The end caps were joined by electric arc welding with the hollow can as well as with 12mm diameter hollow pipe of length 1000 mm for the passing of hydrogen gas during heating. The mixed powder is encapsulated in an M.S capsule (Fig.4a).
Figure 2: Hollow mild steel capsule.
The encapsulated powders were thus heated below the melting point (liquid phase) of the alloy (upto1048̊C) in the presence of hydrogen gas (H2 gas) at the rate of 18̊C/min and held for 30 minutes in the tubular furnace prior to forging. The following reaction [16] takes place during the heating of encapsulated powder: 3Fe + Fe3(PO4)2 + 8H2 → 2Fe3P + 8H2O
(2)
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The Fe-P phase diagram is shown in the Fig.3 [14]. The cleaning of the powder during heating provides the good bonding between the particles which helps to get the nearly full density.
Figure 3: Fe-P phase diagram [14]. During heating coated iron powder with iron-phosphate converted into iron-phosphide prior to the forging. The powders lead to a solid solution of phosphorus in the iron-based matrix. The hot capsule was taken out from the furnace and placed in the channel die (220 × 75 × 25 mm3) for hot forging in a 100 metric ton friction screw driven forge press (Fig. 4b). The schematic forging process is shown in Figure 4.
Figure 4: Schematic diagram of a process for development of Fe-P alloys.
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After cutting the end gas flow pipes, re-forging of the slab was carried out in a 100 metric ton friction screw driven forge press to improve the density level nearly full density. The forged slabs were then homogenized at 1100̊C for 2 h in a muffle furnace followed by air cooling to ensure uniform distribution of alloying elements. After homogenization, the end hemispherical caps of mild steel capsule were cut off and the mild steel encapsulation was also removed by machining. A forged plate measuring 120 × 73 × 10 mm was thus obtained (Fig.4d). Samples were cut off from the plates for further characterization. Samples measuring 10x10x10 mm3 were cut for SEM, microstructure studies, hardness measurement and density measurement. All the samples were annealed by keeping them in muffle furnace at 600̊C for 1hour to relieve the residual stresses and then cooling in the furnace. The annealed samples were mechanically polished to 1 micron grade diamond powder finish and etched with 2% nital solution for metallographic examination. The metallographic structures were analyzed to determine the volume fraction of iron-phosphide in iron-phosphorus alloys by Radical Meta Check 5.0 phase analyzer software from polished samples in the optical microscope. Hardness of the annealed alloys was measured with Vickers’s Hardness tester using 5kg load. The hardness of matrix and precipitates was measured in a UHL VHMT micro-hardness testing machine on polished samples using a 50g load. The samples were prepared for SEM examination on Zeiss EVO18 Special Edition and micro-structural studies on optical microscope (Leica). The density of the alloys was determined by Archimedes principle and it was in the range of 7.84-7.87 g/cc (Table 1). Cylindrical pin samples (40mm x 6mmø) of the developed Fe-P alloy were used for the current investigation. All the specimens were kept in a muffle furnace at 600̊C for an one hour. The wear tests were conducted using Pin samples having flat surfaces in the contact regions and rounded corners, polished up to 4/0 grade (~38µm) emery paper and cleaned with acetone to remove the dust particles from the surface of the pin. Dry sliding wear tests were carried out against the counter face of a hardened and polished disc made up of EN-31 steel having HRC 63 hardness at atmospheric temperature. A pin-on-disc wear testing apparatus, made by Magnum Engineers, Bangalore, was used to carry out the wear tests. Pin weight losses were measured at different intervals of time. Weight loss data were converted to volume loss using alloy density of 7760 kg/m3. Each test at a given load and sliding velocity was repeated three times with identical new samples on a fresh disk surface, and the average data for volume loss after each interval of time was used for the analysis of wear rate. Samples of all alloys were tested at stresses of 0.17, 0.26 and 0.35MPa and at a fixed sliding speed of 3 m/s. The variation in frictional force was continuously displayed by a digital electronic sensor installed in the equipment to calculate the average coefficient of friction (COF). The wear rate was calculated by dividing the volume of worn-out material by sliding distance traveled. The pin wear surfaces were cleaned by ultrasonic cleaning of wear debris and virtually examined. The wear surfaces, as well as the sub-surfaces of the pin specimens, were examined to identify the various mechanisms of material loss using scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDS) (ULTRA plus, Carl Zeiss, Germany). 3. Results and discussion 3.1. Characterization of forged Fe-P Samples The microstructure of the Fe-P alloys revealed dark and bright areas when etched with 2 pct Nital, as shown in Fig. 5(a) and (b). The structure of alloys has dark etching iron-phosphide and bright regions of ferrite as shown in Figure 5. The microstructure of Fe-2wt. %P indicates the phosphide in rounded shape and size. The specimen Fe3wt. %P shows the network of the phosphide of irregular shape.
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Figure 5.Optical micrograph of the etched microstructures of P/M alloy steels. (a) Fe-2wt.%P (b) Fe-3wt.%P.
Microhardness of the dark and bright regions in Fe-P alloys are given in Table 1. The volume fractions of phosphide in Fe-1.3wt. %P, Fe-2wt. %P and Fe-3wt. %P alloys, as determined by Radical Meta Check 5.0 phase analyzer software from polished samples in the optical microscope, are approximately 21, 25 and 34 vol. % respectively. Table 1. Microhardness of alloy phases. Alloy
Density (g/cc)
Regions
Phases identified
Av. Microhardness (HV) (Standard Deviation)
Fe-1.3wt.%P
7.84
Bright Dark
Ferrite Iron-phosphide
218 ( 7 480 ( 10
Fe-2wt.%P
7.87
Bright Dark
Ferrite Iron-phosphide
254 ( 8) 487 ( 9)
Fe-3wt.%P
7.76
Bright Dark
Ferrite Iron-phosphide
263 ( 7) 475 ( 10
The increase in hardness may be attributed to increasing volume fraction of iron-phosphide. 3.2. Friction and Wear characteristics
Figure 6: Variation of cumulative wear volume loss.
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The variation of cumulative wear volume loss with a sliding distance under three normal stresses (0.17, 0.26 and 0.35MPa) and at a sliding speed of 3 m/s for Fe-1.3wt. %P, Fe-2wt. %P and Fe-3wt. % P alloys are shown in Figs. 6(a-c) respectively. The variation of the wear rate with a normal load is shown in Fig. 7 for Fe-1.3wt. %P, Fe-2wt. %P and Fe-3wt. %P alloys. The wear rate, i.e., volume loss per unit sliding distance at a given load was determined from the slope of the lines by linear least-squares fit at different loads given in Figs. 6 (a, b and c).
Figure 7: Variation of the wear rate with normal stress at 3m/s.
Figure 8: Variation of wear rate with phosphorus content for three stresses.
The wear rate is observed to increase linearly with increasing stress, i.e. 0.17, 0.26 and 0.35MPa. For any given load wear rate is maximum for Fe-1.3wt. %P as compared to Fe-2wt. %P and Fe-3wt. %P alloys. It is further observed that wear rate for Fe-2wt. %P alloy is higher for all loads as compared to Fe-3wt. %P alloy. Wear rate is found to be constant for the alloys for all the three loads. The variation of wear rate with phosphorus content for three loads is shown in Figure 8. It shows that as phosphorus content increases in the ferrite, wear rate decreases. Other researchers have also found same trend [21-22]. Wear rate is maximum for 0.35MPa and minimum for 0.17MPa for Fe-1.3wt. %P. This higher wear rate is attributed to comparatively less formation of iron-phosphide. Iron phosphide is the load bearing phase in the alloy. Phosphorus addition to iron results in the formation of phosphide phase producing a dense structure with high hardness. Apart from that phosphorus also goes into iron and causes solid solution strengthening of ferrite matrix. In the present investigation, the average micro-hardness of iron-phosphide is 480VHN. Hardness affects significantly wear characteristic which is well known from Archard wear equation. The following is the Archard wear equation: Wear rate [
=
(3)
Where V is the cumulative volume loss, W a normal load, sliding distance S, and H is the hardness of softer material (Pin). K is Archard wear coefficient which is dimensionless. This equation states that as the hardness of the sample increases wear rate decreases for a given load in atmospheric conditions (at temperature: 22 ± 3̊ C, relative humidity: 50 ± 10%). The wear coefficient was calculated from the slope of linear variation of wear rate with load, by multiply with the hardness of the corresponding pin sample. For the Fe-1.3wt. %P, the wear coefficient is 1.3 x 10-7. For Fe-2wt. %P and Fe-3wt. %P, the wear coefficients are 1.1 x10-7and 0.9 x10-7 respectively. Figure 9 represents the variation of coefficient of friction (COF) with normal load. It is observed that as the load increases, the average coefficient of friction decreases linearly for all the alloys.
S.K.Chaurasia et.al / Materials Today: Proceedings 5 (2018) 17170–17179
Figure 9: Variation of coefficient of friction (COF) with normal stress.
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Figure 10: Average coefficient of friction with iron-phosphide volume fraction.
Figure 10 shows the variation of the average coefficient of friction with iron-phosphide volume fraction at given loads used in the present investigation. A similar trend is observed at other stresses of 0.17, 0.26 and 0.35MPa. The average value of changing frictional force was recorded at a regular interval of time for calculating the coefficient of friction.The average coefficient of friction is found to decrease more or less linearly with increasing volume fraction of iron-phosphide from 21 (Fe-1.3wt. %P) to 34 vol. pct phosphide (Fe-3wt. %P). The average value ranged from about 0.47 to 0.41 at 3m/s. As phosphorus content increases in the alloy, the coefficient of friction decreases for a given load [20]. Further, it can be observed that coefficient of friction also decreases with increase in load for given phosphorus content in the alloy (Fig.10). This can be explained on the basis of the presence of compacted layer between the specimen and the disc. These compacted layers consist of iron oxide and iron-phosphide as the presence of oxygen and phosphorus is detected on the worn surface of the specimen by SEMEDAX (Fig.11).
Figure 11. (a) Worn surface of Fe-3wt. %P under SEM (b) Elemental analysis of worn surface using energy dispersive analysis of X-rays. It shows the compacted layer of Phosphorus at the worn surface.
The presence of iron oxide and phosphide layer suggests that the oxidation of iron and phosphorus takes place due to severe plastic deformation of wear debris in the presence of oxygen of the atmospheric air. The scanning electron micrographs of worn surfaces of Fe-1.3wt. %P, Fe-2wt. %P and Fe-3wt. %P are shown in Fig. 12. Wear tracks can be observed on the worn surface. Figures 13 clearly show that the deformed layers are along the direction of sliding
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which experienced a significant amount of plastic deformation during wear. During dry sliding wear at sliding speed 3m/s and stress 0.35MPa, strains were accumulated on sliding surfaces by repeating contact of the counter surface. It would form surface deformation layers with increased dislocation density subsequently.
Figure 12. SEM of the worn surface at 0.35MPa stress, 3m/s sliding speed and sliding distance 4000m showing surface covered with wear debris. (a) Fe-1.3wt.% P (b)Fe-2wt.% P (c) Fe-3wt.% P
The localized plastic deformation and the associated strain hardening leads to the formation of subsurface cracks which results in the delaminating of the surface layers as shown at higher magnification in Fig. 13. The delamination of thin oxide film increases the frictional coefficient at high load but in turn, reduces the mass loss. The 1.3wt. % or above phosphorus-containing alloys show little evidence of the phosphide phase deformed with pulling the layers out of the surface (Fig. 13(a, b and c)). SEM of the same samples was taken after the ultrasonic cleaning to remove the wear debris from the worn surface.
Figure 13: SEM of the worn surface at 0.35MPa stress, 3m/s sliding speed and sliding distance 4000m after ultrasonic cleaning. (a) Fe-1.3wt. %P shows oxide layer as well as plastic deformation. (b) Fe-2wt. %P shows grooves in the direction of sliding(c) Fe-3wt. %P shows the plastic deformation with adhesion mark.
4. Conclusions The effect of 1.3, 2 and 3wt. % P on the wear characteristics of a Fe based powder metallurgy alloys using the pin-on-disc technique with sliding speed 3 m/s and normal stresses of 0.17, 0.26 and 0.35MPa has been investigated. The followings are the major conclusions: 1. Increasing the phosphorus content in iron from 1.3 to 3wt. % increases the iron-phosphide which leads to improving the wear property of the alloys. 2. With the increase in phosphorus content, wear rate decreases. Lowest wear rate 0.05 x10-5 mm3 /m for Fe-3wt. %P and highest wear rate 0.28 x10-5 mm3/m for Fe-1.3wt. %P sample for a given load is observed. Fe-2wt. %P has intermediate wear rate, i.e 0.16 x10-5 mm3/m. 3. The average coefficient of friction decreases with increase in phosphorus content and normal load. It varies from 0.47 to o.41.
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4. Elemental analysis of worn surface using energy dispersive analysis of X-rays shows that the accumulation of phosphorus at the worn surface of the phosphorus-containing alloy. 5. Increasing the phosphorus content from 1.3 to 3wt. % increases the macrohardness from 125 to 243HV because the ferrite matrix is strengthened by the solid solution of phosphorus in iron and increase in phosphide volume fraction. 6. SEM features of the worn surfaces indicate the presence of compacted layer which consists of oxides of iron and phosphorus. The delamination of thin oxide film increases the frictional coefficient at high load but in turn, reduces the mass loss. References [1] N. Carlbaum, P. Engdahl, Powder Met. 35 (1992) 137-140. [2] Gouthama and R. Balasubramaniam, Bulletin of Mater. Sci. 26 (2003) 483-491. [3] R. Ding, A. Islam, S.Wu, J. Knott, Mater. Sci. and Tech. 21 (2005)467-475. [4] W.J.Tomlinson, G.J.Vandrill, Wear (1987) 375-379. [5] W.J.Tomlinson, G. Dennison, Tribology Internat. (1989) 259-264. [6] R. Balasubramaniam, Corro. Sci. 42.12 (2000)2103-2129. [7] G. Wranglén, Corro. Sci. 10.10 (1970) 761-770. [8] K. H. Moyer, Magn. Mater. and Properties for powder met. part appls. ASM Hand Book. 7 (1998) 1006-1020. [9] J.A. Bas, J. A. Calero, M. J. Dougan, J. Magn. Magn. Mater. 254 (2003) 391-398. [10] B. Węglinski, J. Kaczmar, Powder Met. 23.4 (1980) 210-216. [11] S.K Chaurasia, U. Prakash, K.Chandra, P. S.Misra, Mater. Sci. Forum, 710 (2012) 297-302. [12] G. Straffelini, V. Fontanari, A. Molinari, B.Tesi, Powder met. 36 (1993) 135-141. [13] J.W. Stewart, J. A. Charles, E. R. Wallach, Mater. Sci. and Tech. 16.3 (2000) 275-282. [14] P. Lindskog, J.Tengzelius, S.A. Kvist, Int. J. Powder met. 10 (1977) 97-101. [15] S.K Chaurasia, U. Prakash, P.S. Misra, K. Chandra, Bull. of Mat. Sci. 35 (2012) 191-196. [16] J. Das, K. Chandra, P.S. Mishra, B. Sarma, J. Magn. Magn. Mater. 320 (2008) 906-915. [17] W.J.Tomlinson, G.Dennison, Tribology Internat. 22 (1989) 259-264. [18] D. Krecar,V. Vassileva, H. Danninger, H. Hutter, Analy. Bioanaly. Chem. 379 (2004) 610–618. [19] T. S. Eyre, P. Williams, Wear 24 (1973) 337-349. [20] K.V. Sudhakar, P. Sampathkumaran, E.S. Dwarakadasa, Wear 242 (2000) 207–212. [21] M. H. Jiao, B. W. Su, S. B. Ma, B. C. Wan, Y. G. Yin, Appl. Mech. and Mater. (2012) 105-109. [22] I. M. Fedorchenko, G. M. Derkacheva, I. I. Panaioti, Powder Met. and Metal Cera. 8 (1969) 945-947.
ScienceDirect Materials Today: Proceedings 5 (2018) 17170–17179
www.materialstoday.com/proceedings
AMPCO-2017
Dry Sliding Wear of Fe Based Powder Processed Through Hot Powder Forging Technique S. K. Chaurasiaa*, Ujjwal Prakasha, Vikram Dabhadea, S. K. Natha a
Department of Metallurgical and Materials Engineering Indian Institute of Technology Roorkee, Uttarakhand-247667, India
Abstract This research investigates the effect of normal load, alloy composition and hardness on the coefficient of friction and wear rate of Iron-Phosphorus Powder metallurgy (Fe-P P/M) alloys. The specimens were prepared from hot forged alloys (Fe-1.3wt.%P, Fe2wt.%P and Fe-3wt.%P) were tested using Pin on disc apparatus. The wear rate increases as increasing in the normal loads. The coefficient of friction was inversely proportional to hardness. Since the phosphorus addition increases from 1.3 to 3 wt.% the volume percentage of phosphide (Fe3P) increased from 21 to 34vol. % which improves the hardness. The wear mechanism of the alloys is presented and discussed. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Advances in Materials & Processing: Challenges & Opportunities (AMPCO-2017). Keywords: Contact; Removal; Adhesive wear; Wear rate; Powder metallurgy.
1. Introduction In recent years there has been a considerable amount of work directed towards the development of high performance (improved mechanical properties) steels for various refineries, structural and ships etc. As automotive manufacturing is an area under high-cost pressure, less expensive and more easily processed materials are needed [1]. There are various technological methods to develop the steel alloys for the critical applications. One of the technological process that is being used to replace traditional metal-forming operations because of its low relative energy consumption, high material utilization and the low capital cost is powder metallurgy (P/M). Therefore, P/M
* Corresponding author. Tel.: +91-9412489182. E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Advances in Materials & Processing: Challenges & Opportunities (AMPCO-2017).
S.K.Chaurasia et.al / Materials Today: Proceedings 5 (2018) 17170–17179
17171
steels have been found wide application in the automotive industry, in engine and transmission systems. However, in the present research work, a process has been developed to achieve high performance (nearly full density) alloys for the tribological study. In this process, phosphorus (in the form of ortho-phosphoric acid) was mixed with an appropriate amount of water-atomized iron powder to prepare a master alloy of the required amount of phosphorus content for developing further Fe-P alloys. Phosphorus is normally considered an undesirable impurity in steels. It causes cold and hot shortness in ferrous alloys. It may also lead to grain boundary failure due to segregation of phosphorus at grain boundaries [2-3]. For these reasons, the amount of phosphorus is limited to 0.04% in the majority of steels. Ironphosphorus (Fe-P) P/M alloys show better mechanical properties in terms of high hardness, high strength. They show low friction coefficient and excellent wear resistance [4-5], enhanced corrosion resistance [6-7] and improved magnetic properties [8-11]. Phosphorus provides solid solution strengthening to the ferrite matrix [12-14]. Phosphorus addition may lead to increase in the density of P/M iron-based alloys [8, 15-16]. In Fe-P alloys prepared by melting and casting, the last liquid to solidify is rich in phosphorus and this can lead to embrittlement. In Fe-P P/M alloys developed by conventional pressing and sintering route, heavy volume shrinkage is experienced. Also, it takes a long time to complete the process. This problem has been overcome by using powder forging. The excess amount of phosphorus forms an iron phosphide which increases the tribological behavior of the alloy [17]. Phosphoric iron shows high wear resistance. The hard parts of phosphide favor an increase in the hardness and wear resistance of the alloy [18]. The role of phosphide in the wear behavior of iron has generally been suggested, to be due to one or more of the following. (i) The phosphide may stand proud of the surface and carry the applied load, (ii) The phosphide increases the strength of the iron under compressive conditions and this increases the resistance to plastic flow, (iii) Phosphide may spread over the surface and form a continuous film, (iv) The phosphide may melt and cause a redistribution of stress [19]. However, a few works have been reported on hot forging route of Fe-P alloys (less amount of P) in the liquid phase (above 1100̊C) region [16], But there is also need to go for the high P content alloys. In the present investigation Fe-P alloys were developed by forging of hot solid coated powder which may also allow us to go to the high phosphorus level without the problem of the segregation of phosphorus and shrinkage as observed in convention powder metallurgy process and melting and casting route. Hence, the wear and friction behavior research has been relatively scarce compared to steels developed by another route. Most of the mechanical components are made by powder metallurgy technique which exhibits good wear resistance [20]. Phosphorus is used as an alloying element in the iron-based alloy for special applications. Due to its high wear resistance and lack of sparking, high content of phosphorus in the iron would best material suitable for the brake. The role Fe-P alloys also exhibit improved corrosion resistance and magnetic permeability. 2. Experimental Procedure Fe-P with 1.3%, 2% and 3% phosphorus (all compositions in weight %) P/M alloys were developed by hot solid powder forging technique in the present investigation. In this process, water-atomized Fe powder (~8 microns) supplied by M/S HOGANAS AB, Sweden was used in the present study. An Fe-P master alloy was prepared by mixing the iron powder with the required amount of laboratory grade of ortho-phosphoric acid, H3PO4 with purity 98%. The ortho-phosphoric acid was diluted by slowly mixing it into an adequate amount of distilled water so that the volume of the diluted ortho-phosphoric acid was sufficient to completely cover the iron powder, which was laid in a tray. For the preparation of 1 kg master alloy with 5wt. % P, 950 gm atomized iron powder was mixed with 100 ml ortho-phosphoric acid and left for two hours. The mixture was dried in air for 24 hours. The ortho-phosphoric acid would react with iron particle and form a surface coated iron powder as Fe3 (PO4)2 after the chemical reaction such as:
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3Fe + 2H3PO4 Fe3 (PO4) + 3H2 (1) The coated (Fe3 (PO4)2) powder so produced is poured out and dried in open atmosphere. By using atomized Fe powder and master alloy (Fe3 (PO4)2), the powder blends were used for making the different composition of Fe-P alloys. Mixing carried out mechanically in a laboratory ball mill for 2-3 hours. The total weight of the mixture was taken 1 Kg. The distribution of P in a typical Fe-P powder mixture is shown in Fig. 1.
Figure 1: Elemental mapping of powder mixture alloy.
This M.S (Fig.2) capsule was a hollow tube of 63.5 mm outer diameter (OD), 2 mm wall thickness and 80 mm length fabricated with hemispherical end caps at both sides. A hole of 10mm was provided at the center of end caps. The end caps were joined by electric arc welding with the hollow can as well as with 12mm diameter hollow pipe of length 1000 mm for the passing of hydrogen gas during heating. The mixed powder is encapsulated in an M.S capsule (Fig.4a).
Figure 2: Hollow mild steel capsule.
The encapsulated powders were thus heated below the melting point (liquid phase) of the alloy (upto1048̊C) in the presence of hydrogen gas (H2 gas) at the rate of 18̊C/min and held for 30 minutes in the tubular furnace prior to forging. The following reaction [16] takes place during the heating of encapsulated powder: 3Fe + Fe3(PO4)2 + 8H2 → 2Fe3P + 8H2O
(2)
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The Fe-P phase diagram is shown in the Fig.3 [14]. The cleaning of the powder during heating provides the good bonding between the particles which helps to get the nearly full density.
Figure 3: Fe-P phase diagram [14]. During heating coated iron powder with iron-phosphate converted into iron-phosphide prior to the forging. The powders lead to a solid solution of phosphorus in the iron-based matrix. The hot capsule was taken out from the furnace and placed in the channel die (220 × 75 × 25 mm3) for hot forging in a 100 metric ton friction screw driven forge press (Fig. 4b). The schematic forging process is shown in Figure 4.
Figure 4: Schematic diagram of a process for development of Fe-P alloys.
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After cutting the end gas flow pipes, re-forging of the slab was carried out in a 100 metric ton friction screw driven forge press to improve the density level nearly full density. The forged slabs were then homogenized at 1100̊C for 2 h in a muffle furnace followed by air cooling to ensure uniform distribution of alloying elements. After homogenization, the end hemispherical caps of mild steel capsule were cut off and the mild steel encapsulation was also removed by machining. A forged plate measuring 120 × 73 × 10 mm was thus obtained (Fig.4d). Samples were cut off from the plates for further characterization. Samples measuring 10x10x10 mm3 were cut for SEM, microstructure studies, hardness measurement and density measurement. All the samples were annealed by keeping them in muffle furnace at 600̊C for 1hour to relieve the residual stresses and then cooling in the furnace. The annealed samples were mechanically polished to 1 micron grade diamond powder finish and etched with 2% nital solution for metallographic examination. The metallographic structures were analyzed to determine the volume fraction of iron-phosphide in iron-phosphorus alloys by Radical Meta Check 5.0 phase analyzer software from polished samples in the optical microscope. Hardness of the annealed alloys was measured with Vickers’s Hardness tester using 5kg load. The hardness of matrix and precipitates was measured in a UHL VHMT micro-hardness testing machine on polished samples using a 50g load. The samples were prepared for SEM examination on Zeiss EVO18 Special Edition and micro-structural studies on optical microscope (Leica). The density of the alloys was determined by Archimedes principle and it was in the range of 7.84-7.87 g/cc (Table 1). Cylindrical pin samples (40mm x 6mmø) of the developed Fe-P alloy were used for the current investigation. All the specimens were kept in a muffle furnace at 600̊C for an one hour. The wear tests were conducted using Pin samples having flat surfaces in the contact regions and rounded corners, polished up to 4/0 grade (~38µm) emery paper and cleaned with acetone to remove the dust particles from the surface of the pin. Dry sliding wear tests were carried out against the counter face of a hardened and polished disc made up of EN-31 steel having HRC 63 hardness at atmospheric temperature. A pin-on-disc wear testing apparatus, made by Magnum Engineers, Bangalore, was used to carry out the wear tests. Pin weight losses were measured at different intervals of time. Weight loss data were converted to volume loss using alloy density of 7760 kg/m3. Each test at a given load and sliding velocity was repeated three times with identical new samples on a fresh disk surface, and the average data for volume loss after each interval of time was used for the analysis of wear rate. Samples of all alloys were tested at stresses of 0.17, 0.26 and 0.35MPa and at a fixed sliding speed of 3 m/s. The variation in frictional force was continuously displayed by a digital electronic sensor installed in the equipment to calculate the average coefficient of friction (COF). The wear rate was calculated by dividing the volume of worn-out material by sliding distance traveled. The pin wear surfaces were cleaned by ultrasonic cleaning of wear debris and virtually examined. The wear surfaces, as well as the sub-surfaces of the pin specimens, were examined to identify the various mechanisms of material loss using scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDS) (ULTRA plus, Carl Zeiss, Germany). 3. Results and discussion 3.1. Characterization of forged Fe-P Samples The microstructure of the Fe-P alloys revealed dark and bright areas when etched with 2 pct Nital, as shown in Fig. 5(a) and (b). The structure of alloys has dark etching iron-phosphide and bright regions of ferrite as shown in Figure 5. The microstructure of Fe-2wt. %P indicates the phosphide in rounded shape and size. The specimen Fe3wt. %P shows the network of the phosphide of irregular shape.
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Figure 5.Optical micrograph of the etched microstructures of P/M alloy steels. (a) Fe-2wt.%P (b) Fe-3wt.%P.
Microhardness of the dark and bright regions in Fe-P alloys are given in Table 1. The volume fractions of phosphide in Fe-1.3wt. %P, Fe-2wt. %P and Fe-3wt. %P alloys, as determined by Radical Meta Check 5.0 phase analyzer software from polished samples in the optical microscope, are approximately 21, 25 and 34 vol. % respectively. Table 1. Microhardness of alloy phases. Alloy
Density (g/cc)
Regions
Phases identified
Av. Microhardness (HV) (Standard Deviation)
Fe-1.3wt.%P
7.84
Bright Dark
Ferrite Iron-phosphide
218 ( 7 480 ( 10
Fe-2wt.%P
7.87
Bright Dark
Ferrite Iron-phosphide
254 ( 8) 487 ( 9)
Fe-3wt.%P
7.76
Bright Dark
Ferrite Iron-phosphide
263 ( 7) 475 ( 10
The increase in hardness may be attributed to increasing volume fraction of iron-phosphide. 3.2. Friction and Wear characteristics
Figure 6: Variation of cumulative wear volume loss.
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The variation of cumulative wear volume loss with a sliding distance under three normal stresses (0.17, 0.26 and 0.35MPa) and at a sliding speed of 3 m/s for Fe-1.3wt. %P, Fe-2wt. %P and Fe-3wt. % P alloys are shown in Figs. 6(a-c) respectively. The variation of the wear rate with a normal load is shown in Fig. 7 for Fe-1.3wt. %P, Fe-2wt. %P and Fe-3wt. %P alloys. The wear rate, i.e., volume loss per unit sliding distance at a given load was determined from the slope of the lines by linear least-squares fit at different loads given in Figs. 6 (a, b and c).
Figure 7: Variation of the wear rate with normal stress at 3m/s.
Figure 8: Variation of wear rate with phosphorus content for three stresses.
The wear rate is observed to increase linearly with increasing stress, i.e. 0.17, 0.26 and 0.35MPa. For any given load wear rate is maximum for Fe-1.3wt. %P as compared to Fe-2wt. %P and Fe-3wt. %P alloys. It is further observed that wear rate for Fe-2wt. %P alloy is higher for all loads as compared to Fe-3wt. %P alloy. Wear rate is found to be constant for the alloys for all the three loads. The variation of wear rate with phosphorus content for three loads is shown in Figure 8. It shows that as phosphorus content increases in the ferrite, wear rate decreases. Other researchers have also found same trend [21-22]. Wear rate is maximum for 0.35MPa and minimum for 0.17MPa for Fe-1.3wt. %P. This higher wear rate is attributed to comparatively less formation of iron-phosphide. Iron phosphide is the load bearing phase in the alloy. Phosphorus addition to iron results in the formation of phosphide phase producing a dense structure with high hardness. Apart from that phosphorus also goes into iron and causes solid solution strengthening of ferrite matrix. In the present investigation, the average micro-hardness of iron-phosphide is 480VHN. Hardness affects significantly wear characteristic which is well known from Archard wear equation. The following is the Archard wear equation: Wear rate [
=
(3)
Where V is the cumulative volume loss, W a normal load, sliding distance S, and H is the hardness of softer material (Pin). K is Archard wear coefficient which is dimensionless. This equation states that as the hardness of the sample increases wear rate decreases for a given load in atmospheric conditions (at temperature: 22 ± 3̊ C, relative humidity: 50 ± 10%). The wear coefficient was calculated from the slope of linear variation of wear rate with load, by multiply with the hardness of the corresponding pin sample. For the Fe-1.3wt. %P, the wear coefficient is 1.3 x 10-7. For Fe-2wt. %P and Fe-3wt. %P, the wear coefficients are 1.1 x10-7and 0.9 x10-7 respectively. Figure 9 represents the variation of coefficient of friction (COF) with normal load. It is observed that as the load increases, the average coefficient of friction decreases linearly for all the alloys.
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Figure 9: Variation of coefficient of friction (COF) with normal stress.
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Figure 10: Average coefficient of friction with iron-phosphide volume fraction.
Figure 10 shows the variation of the average coefficient of friction with iron-phosphide volume fraction at given loads used in the present investigation. A similar trend is observed at other stresses of 0.17, 0.26 and 0.35MPa. The average value of changing frictional force was recorded at a regular interval of time for calculating the coefficient of friction.The average coefficient of friction is found to decrease more or less linearly with increasing volume fraction of iron-phosphide from 21 (Fe-1.3wt. %P) to 34 vol. pct phosphide (Fe-3wt. %P). The average value ranged from about 0.47 to 0.41 at 3m/s. As phosphorus content increases in the alloy, the coefficient of friction decreases for a given load [20]. Further, it can be observed that coefficient of friction also decreases with increase in load for given phosphorus content in the alloy (Fig.10). This can be explained on the basis of the presence of compacted layer between the specimen and the disc. These compacted layers consist of iron oxide and iron-phosphide as the presence of oxygen and phosphorus is detected on the worn surface of the specimen by SEMEDAX (Fig.11).
Figure 11. (a) Worn surface of Fe-3wt. %P under SEM (b) Elemental analysis of worn surface using energy dispersive analysis of X-rays. It shows the compacted layer of Phosphorus at the worn surface.
The presence of iron oxide and phosphide layer suggests that the oxidation of iron and phosphorus takes place due to severe plastic deformation of wear debris in the presence of oxygen of the atmospheric air. The scanning electron micrographs of worn surfaces of Fe-1.3wt. %P, Fe-2wt. %P and Fe-3wt. %P are shown in Fig. 12. Wear tracks can be observed on the worn surface. Figures 13 clearly show that the deformed layers are along the direction of sliding
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which experienced a significant amount of plastic deformation during wear. During dry sliding wear at sliding speed 3m/s and stress 0.35MPa, strains were accumulated on sliding surfaces by repeating contact of the counter surface. It would form surface deformation layers with increased dislocation density subsequently.
Figure 12. SEM of the worn surface at 0.35MPa stress, 3m/s sliding speed and sliding distance 4000m showing surface covered with wear debris. (a) Fe-1.3wt.% P (b)Fe-2wt.% P (c) Fe-3wt.% P
The localized plastic deformation and the associated strain hardening leads to the formation of subsurface cracks which results in the delaminating of the surface layers as shown at higher magnification in Fig. 13. The delamination of thin oxide film increases the frictional coefficient at high load but in turn, reduces the mass loss. The 1.3wt. % or above phosphorus-containing alloys show little evidence of the phosphide phase deformed with pulling the layers out of the surface (Fig. 13(a, b and c)). SEM of the same samples was taken after the ultrasonic cleaning to remove the wear debris from the worn surface.
Figure 13: SEM of the worn surface at 0.35MPa stress, 3m/s sliding speed and sliding distance 4000m after ultrasonic cleaning. (a) Fe-1.3wt. %P shows oxide layer as well as plastic deformation. (b) Fe-2wt. %P shows grooves in the direction of sliding(c) Fe-3wt. %P shows the plastic deformation with adhesion mark.
4. Conclusions The effect of 1.3, 2 and 3wt. % P on the wear characteristics of a Fe based powder metallurgy alloys using the pin-on-disc technique with sliding speed 3 m/s and normal stresses of 0.17, 0.26 and 0.35MPa has been investigated. The followings are the major conclusions: 1. Increasing the phosphorus content in iron from 1.3 to 3wt. % increases the iron-phosphide which leads to improving the wear property of the alloys. 2. With the increase in phosphorus content, wear rate decreases. Lowest wear rate 0.05 x10-5 mm3 /m for Fe-3wt. %P and highest wear rate 0.28 x10-5 mm3/m for Fe-1.3wt. %P sample for a given load is observed. Fe-2wt. %P has intermediate wear rate, i.e 0.16 x10-5 mm3/m. 3. The average coefficient of friction decreases with increase in phosphorus content and normal load. It varies from 0.47 to o.41.
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4. Elemental analysis of worn surface using energy dispersive analysis of X-rays shows that the accumulation of phosphorus at the worn surface of the phosphorus-containing alloy. 5. Increasing the phosphorus content from 1.3 to 3wt. % increases the macrohardness from 125 to 243HV because the ferrite matrix is strengthened by the solid solution of phosphorus in iron and increase in phosphide volume fraction. 6. SEM features of the worn surfaces indicate the presence of compacted layer which consists of oxides of iron and phosphorus. The delamination of thin oxide film increases the frictional coefficient at high load but in turn, reduces the mass loss. References [1] N. Carlbaum, P. Engdahl, Powder Met. 35 (1992) 137-140. [2] Gouthama and R. Balasubramaniam, Bulletin of Mater. Sci. 26 (2003) 483-491. [3] R. Ding, A. Islam, S.Wu, J. Knott, Mater. Sci. and Tech. 21 (2005)467-475. [4] W.J.Tomlinson, G.J.Vandrill, Wear (1987) 375-379. [5] W.J.Tomlinson, G. Dennison, Tribology Internat. (1989) 259-264. [6] R. Balasubramaniam, Corro. Sci. 42.12 (2000)2103-2129. [7] G. Wranglén, Corro. Sci. 10.10 (1970) 761-770. [8] K. H. Moyer, Magn. Mater. and Properties for powder met. part appls. ASM Hand Book. 7 (1998) 1006-1020. [9] J.A. Bas, J. A. Calero, M. J. Dougan, J. Magn. Magn. Mater. 254 (2003) 391-398. [10] B. Węglinski, J. Kaczmar, Powder Met. 23.4 (1980) 210-216. [11] S.K Chaurasia, U. Prakash, K.Chandra, P. S.Misra, Mater. Sci. Forum, 710 (2012) 297-302. [12] G. Straffelini, V. Fontanari, A. Molinari, B.Tesi, Powder met. 36 (1993) 135-141. [13] J.W. Stewart, J. A. Charles, E. R. Wallach, Mater. Sci. and Tech. 16.3 (2000) 275-282. [14] P. Lindskog, J.Tengzelius, S.A. Kvist, Int. J. Powder met. 10 (1977) 97-101. [15] S.K Chaurasia, U. Prakash, P.S. Misra, K. Chandra, Bull. of Mat. Sci. 35 (2012) 191-196. [16] J. Das, K. Chandra, P.S. Mishra, B. Sarma, J. Magn. Magn. Mater. 320 (2008) 906-915. [17] W.J.Tomlinson, G.Dennison, Tribology Internat. 22 (1989) 259-264. [18] D. Krecar,V. Vassileva, H. Danninger, H. Hutter, Analy. Bioanaly. Chem. 379 (2004) 610–618. [19] T. S. Eyre, P. Williams, Wear 24 (1973) 337-349. [20] K.V. Sudhakar, P. Sampathkumaran, E.S. Dwarakadasa, Wear 242 (2000) 207–212. [21] M. H. Jiao, B. W. Su, S. B. Ma, B. C. Wan, Y. G. Yin, Appl. Mech. and Mater. (2012) 105-109. [22] I. M. Fedorchenko, G. M. Derkacheva, I. I. Panaioti, Powder Met. and Metal Cera. 8 (1969) 945-947.