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Effect of boron nitride on microstructure of Fe-Cr-Mo-BN-C steel sintered in vacuum
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ScienceDirect Materials Today: Proceedings 5 (2018) 9409–9416
www.materialstoday.com/proceedings
The 10th Thailand International Metallurgy Conference (The 10th TIMETC)
Effect of boron nitride on microstructure of Fe-Cr-Mo-BN-C steel sintered in vacuum Patiparn Ninpetcha, Apichit Luechaisirikula, Monnapas Morakotjindac, Thanyaporn Yotkaewc, Roongthip Krataitongc, Nattaya Tosangthumc, Sithipong Mahathanabodeea,b, * and Ruangdaj Tongsric 0F
a
Department of Production Engineering, Faculty of Engineering, King Mongkut’s University of Technology North Bangkok (KMUTNB) 1518 Pracharat 1, Wongsawang, Bang-Sue, Bangkok 10800,Thailand b Tribo-Systems for Industrial Tools and Machinery Research, King Mongkut’s University of Technology North Bangkok (KMUTNB) 1518 Pracharat 1, Wongsawang, Bang-Sue, Bangkok 10800,Thailand c Particulate Materials Processing Technology Laboratory (PMPT), National Metal and Materials Technology Center 114 Paholyothin Road, Khlong Nueng, Khlong Luang, Pathum Thani 12120, Thailand
Abstract In this work, a pre-alloyed powder of Fe-3% Cr-0.5% Mo was admixed with boron nitride (BN) and graphite powder (0 to 0.4%). The tensile test specimens with the green density about 6.5 g/cm3 were compacted. The specimens were sintered in vacuum furnace at 1280°C for 45 min. After sintering process, specimens were cooled in N2 at various cooling rate. The microstructures, mechanical properties of sintered steels were studied. The results showed tensile strength and hardness increased with the increasing carbon content and cooling rate. Furthermore, the addition of BN showed the tensile strength and hardness improvement at low cooling rate. BN resulted on the coarse grain microstructure and progressive grain coarsening. The liquid phases were found at grain boundaries. The microstructures of sintered steels were mixture of ferrite, bainite and martensite. Finally, the addition of boron nitride was found to improve the transformation of bainite. Therefore, the tensile strength and hardness of sintered steel were improved © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference. Keywords: Boron nitride; cooling rate; sintering, liquid phase; vacuum furnace
* Corresponding author. Tel.: +662-555-2000 Ext. 8212, 8215, 8222 Ext 417; fax: +662-587-0029. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference.
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1. Introduction Chromium-molybdenum (Fe-Cr-Mo) pre-alloyed powder admixed with graphite is attractive in PM manufacturing due to its high performances. Sintered Fe-Cr-Mo-C steel shows good strength, hardenability and cost-effective when it is compared with the traditional PM alloy steels [1]. The mechanical properties of Fe-Cr-MoC steels correlated to the changes of their phase portion of ferrite, bainite, mixed bainite and martensite to fully martensite. Additions of elemental powders, such as Cu [2], Mn [3,4], Si [5] and SiC [6] powder in Fe-Cr-Mo-C steel improved the hardenability of sintered steels. However, strength improvement of sintered steels is preferred for the processing with slow cooling rate condition. In this work, boron nitride (BN) powder is expected that is potential to be a strength and hardenability enhancer for the Fe-Cr-Mo-C steel due to both boron and nitrogen element has ability in hardening steel [1,7]. However, the performance of boron nitride is depending on its content and the rate of cooling during the hardening process. Therefore, this work aims to study the effect of boron nitride content and postsintering cooling rate on microstructure of sintered Fe-Cr-Mo steels. 2. Experimental The specimens were prepared by powder metallurgy process. Water atomized pre-alloyed powder of Fe-3Cr0.5Mo (Astaloy CrM® supplied by Höganäs AB Sweden) with average particle size of 40-150 µm was mixed with 0.5wt% of boron nitride (in a hexagonal form (h-BN)) powder (NX1 grade supplied by Momentive Performance Inc. USA) and various content of graphite powder (0, 0.1, 0.2, 0.3 and 0.4wt%). Compositions were mixed in the tumbling mixture for an hour at rotating speed of 30 rpm. Zinc stearate of 1 wt% was added to the powder mixtures as lubricant. Subsequently, all compositions were compacted in the uniaxial die to produce tensile test specimens according to the MPIF standard 10 [8] with the green density of 6.5 g/cm3. Green specimens were de-lubricated at 600°C for 30 min and then sintered at 1280°C for 45 min in vacuum. During cooling process, specimens were cooled with N2 in various pressures of 0, 2.5 and 5 bar (Correspond to the cooling rates of 0.1, 4.0 and 5.4°C/s). The heating and cooling profile of sintering process and tensile specimens are shown in Fig. 1. The green and sintered density was determined according to the MPIF standard 42 [9]. Sintered specimens were cut, ground, polished and etched for optical microscopy and microhardness observations. Tensile strength, yield strength and hardness were determined according to MPIF standards 10 and 51, respectively [8,10].
Fig. 1. (a) Time-temperature profile and (b) typical tensile specimens
3. Results and discussion 3.1 Sintered density and hardness Density of sintered Fe-Cr-Mo-BN-C steels from different cooling rates is shown in Fig. 2(a). Density of sintered specimens with only boron nitride addition (6.97-7.05 g/cm3) was lower than of the specimens added with boron
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nitride and graphite (7.15-7.33 g/cm3). However, the density of sintered steel was about 90% of theory density. Effect of boron nitride and carbon content on the hardness of sintered specimens under different cooling rates is shown in Fig. 2(b). Sintered specimens showed that the hardness increased corresponding to increasing of carbon content and cooling rate. In addition, the hardness was improved by addition of boron nitride.
Fig. 2. Variation of density and hardness of sintered Fe-Cr-Mo-BN-C steels in different cooling rates; (a) Density and (b) Hardness
3.2 Tensile strength, yield strength and elongation Carbon content and cooling rate also affected on the increasing of tensile strength and yield strength of sintered specimens as shown in Fig. 3. Boron nitride addition showed enhancement of tensile and yield strength of specimens with carbon content 0.2-0.4 wt% at the low cooling rate (0.1°C/s) as shown in Fig. 3(a). Tensile and yield strength of sintered steel were improved from the high cooling rate (4.0°C/s). Moreover, the addition of boron nitride indicated the tensile strength improvement as shown in Fig.3(b). In contrast, the increase of cooling rate and carbon content showed the reduction of elongation as shown in Fig. 4. The addition of boron nitride resulted in the decreasing of elongation at cooling rate of 0.1°C/s but the elongation improved at the cooling rate of 4.0 and 5.4°C/s.
Fig. 3. Tensile and yield strength of sintered Fe-Cr-Mo-BN-C steels in different cooling rates; (a) 0.1°C/s (b) 4.0°C/s
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Fig. 4. Elongation of sintered Fe-Cr-Mo-BN-C steel in different cooling rates
Fig. 5. Typical micrographs of sintered specimens at cooling rate of 0.1°C/s; Without BN: (a) 0.1 wt%C, (b) 0.2 wt%C and (c) 0.3 wt%C; With BN: (d) 0 wt%C, (e) 0.2 wt%C and (f) 0.3 wt%C
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3.3 Microstructures 3.3.1 Slow cooling rate Microstructures of sintered Fe-Cr-Mo-BN-C steels from cooling rate of 0.1°C/s are shown in Fig.5. The microstructure of sintered specimen without boron nitride and graphite addition showed ferrite (F) phases with good bonding and rounded pores in microstructures as shown in Fig 5(a). Pearlite (P) phases were found in microstructure of 0.2 wt%C (Fig. 5(b)) whereas bainite (B) microstructures obtained in the microstructure of 0.3 wt%C as shown in Fig. 5(c). In the Fe-Cr-Mo-C system, carbon diffusion was hindered by chromium and molybdenum [11]. This is attributed to formation of pearlite and bainite in the microstructure of Fe-Cr-Mo-C steels. The increasing of bainite microstructures resulted in the increasing of tensile strength and hardness. In contrast, it was resulted in decreasing of elongation of sintered steels. The addition of 0.5 wt% boron nitride showed that the liquid phases remained segregated at the grain boundary as shown in Fig.5d. The segregation phase is formed due to the reaction between boron nitride and pre-alloyed Fe-CrMo particles. The liquid phase of boron nitride addition has been reported, under addition small amount of boron elements into Fe-Cr-Mo-C system, resulting in the formation of boride phase along grain boundaries [12, 13]. In addition, the spherical pores were found at the original sites of the boron nitride powders. The ferrite-bainite microstructure appeared in the specimens added with 0.2 wt%C as shown in Fig. 5(e). In this case, the fraction of bainitic structures was found higher than specimens without boron nitride addition (Fig.5(c)). It is indicated that the addition of boron nitride influenced on the formation of bainitic structure in sintered Fe-Cr-Mo-BN-C steels. The effect of boron and alloying elements (such as Cr, Mo, Ni) on the transformation of microstructures is still controversial discussed in the previous reports [7, 13-15] In addition, the bainitic structures increased corresponding to the increasing of carbon content as shown in Fig. 5(f). 3.3.2 High cooling rate The microstructures of sintered Fe-Cr-Mo-BN-C steels from the cooling rate of 4.0°C/s are shown in Fig. 6. For the low carbon specimen, the mixture of allotriomorphic ferrite (AF), Widmanstätten ferrite (WF), quasi-polygonal ferrite (QF) and granular ferrite (GF) were found in microstructures as shown in Fig. 6(a). These microstructures of low carbon steel at high cooling rate are described in previous works [16-18]. The transformation of microstructure due to high cooling rate resulted in the mechanical improvement as shown in Fig. 2(b) and 3(b). When carbon content in sintered specimen was increased up to 0.2 wt%, the bainitic structures appeared in microstructures as shown in Fig. 6(b). The bainite-martensite microstructure was found as carbon content increasing as shown in Fig. 6(c). At the cooling rate of 5.4°C/s, bainite-martensite fraction in microstructure increased. The microstructure of 0.3 wt%C specimen with cooling rate 5.4°C/s is shown in Fig. 8(a). The hardness and tensile strength improved due to the increasing of bainite-martensite fraction in microstructure as shown in Fig. 2(b) and 3(b). In the specimens with boron nitride addition, the liquid phases remained along the grain boundaries were found in the microstructures. It is indicated that the liquid phase exists along particle boundaries during sintering process. The solid particles rearrange and lead to densification by coarsening. Some pores vanish due to liquid phase accelerates the change in grain morphology and assists pore removal [19]. The grain coarsening microstructures due to liquid phase generated during sintering process are shown in Fig. 6(d-f). In the specimens of 0 wt%C, the mixture of AF, WF and QF were found in microstructures as shown in Fig. 6(d). The bainitic structures appeared in the microstructure of sintered steels added with carbon as shown in Fig. 6(e). These phases increased corresponding to the carbon content. In addition, martensitic microstructure was found in the microstructure of sintered specimens with carbon content up to 0.3 wt%C as shown in Fig. 6(f). There are resulted in the improvement of tensile strength and hardness of sintered Fe-Cr-Mo-BN-C steels was observed, as shown in Fig. 2(b) and 3(b). Additionally, thinner layer of liquid phase remained at grain boundary at high cooling rate, as shown in Fig. 7. This is due to phase transformation at the high cooling rate, which resulted in some boron atoms dissolved in the matrix. The amount of boron atoms to form boride liquid phase then reduced. The diffusion of boron into the matrix grain resulted in enhancement of microhardness of sintered steel [13]. The microhardness of 0.1 wt%C specimens with boron nitride
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addition at cooing rate of 0.1 and 4.0°C/s were 110-118 HV0.02 and 223-262 HV0.02, respectively. However, the effects on liquid phase sintering of cooling rate and alloying elements such as chromium, carbon and boron are stilled unclear. Then it is necessary to study in the further works. At the cooling rate of 5.4°C/s, bainite-martensite fraction in microstructures increased in the specimens with 0.4 wt%C in high cooling rate as shown in Fig. 8(b).
Fig. 6 Typical micrographs of sintered specimens at cooling rate of 4.0°C/s; Without BN: (a) 0.1 wt%C, (b) 0.2 wt%C and (c) 0.3 wt%C; With BN: (d) 0 wt%C, (e) 0.2 wt%C and (f) 0.3 wt%C
Fig. 7 Typical SEM micrographs of sintered specimens at different cooling rates; (a) 0.1°C/s and (b) 5.4°C/s
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Fig. 8 Typical micrographs of sintered specimens at cooling rates of 5.4°C/s; (a) 0.3%C without BN and (b) 0.4%C with BN
4. Conclusion Fe-Cr-Mo steel with boron nitride and graphite additions were sintered at 1280°C, cooled in various cooling rate in vacuum furnace, microstructurally investigated and tested for their mechanical. The following results were obtained: 1. Tensile strength and hardness of sintered specimens increased with the increasing carbon content and cooling rate. Boron nitride added resulted in the yield strength and hardness were higher than the specimen without BN, especially at 0.1°C/s. 2. Boron nitride addition resulted in the coarse grain microstructure and progressive grain coarsening in specimens. The liquid phases formed surrounding grain boundaries induced densification improvement. However, liquid phases remained segregated along grain boundaries reduced at high cooling rate. 3. The microstructures of sintered Fe-Cr-Mo-BN-C steels were mixture of ferrite, bainite and martensite. Bainite-martensite fraction increased corresponding to the increasing of carbon content and cooling rate. In addition, boron nitride was found to improve the transformation of bainite in Fe-Cr-Mo-BN-C steels. Acknowledgements The authors would like to thank King Mongkut’s University of Technology North Bangkok for financial support under Contact No. KMUTNB-GEN-59-03. Our gratitude is also extended to the National Metal and Materials Technology Center, Pathum Thani of Thailand for laboratory equipment, characterization facilities and personal help. The courtesy of Acme International (Thailand) Ltd. and Höganäs AB (Sweden) for AstaloyCrM® powder. We also would like to thank Momentive Performance Materials, Inc., USA, for supplying h-BN powder. References [1] J. Kazior, C. Janczur, T. Pieczonka, J. Ploszczak, Surface and Coatings Tech. 151-152 (2002) 333-337. [2] N. Hirose, T. Itahara, Y. Takeda. Hardening Effect of Cr Containing Material with Cu and Fe Addition without Rapid Cooling. in World Congress PM2014, Orlando USA (2014) 1-10. [3] T. Pieczonka, M. Sułowski, A. Ciaś., Arch. Metall. Mater. 57(4) (2012) 1001-1009. [4] M. Sułowski, Arch. Metall. Mater. 59(4) (2014) 1499-1505. [5] M. Azadbeh, N.P. Ahmadi Curr. Appl. Phys. 9(4) (2009) 777-782. [6] M. Hebda, S. Gądek, K. Miernik, J. Kazior, Adv. Powder Tech. 25-2 (2014) 543-550. [7] H.K. Zeytin, H. Yildirim, B. Berme, S. Duduoĝlu, G. Kazdal, A. Deniz, J. Iron Steel Res. Int. 18(11) (2011) 31-39. [8] MPIF Standard 10: Method for Determination of Tensile Properties of Powder Metallurgy Materials, in Standard Test Methods for Metal Powders and Powder Metallurgy Products. 2002: Princeton, (New Jersey, USA).
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[9] MPIF Standard 42: Method for Determination of Density of Compacted or Sintered Powder Metallurgy Products, in Standard Test Methods for Metal Powders and Powder Metallurgy Products. 2002: Princeton, (New Jersey, USA). [10] MPIF Standard 43: Method for Determination of the Apparent Hardness of Powder Metallurgy Products, in Standard Test Methods for Metal Powders and Powder Metallurgy Products. 2002: Princeton, (New Jersey, USA). [11] Y. Yu, Thermodynamic and Kinetic Behaviours of Astaloy CrM. in Proceedings of 2000 Powder Metallurgy World Congress. 2000. Kyoto, Japan: Höganäs AB, Sweden. [12] M.-W. Wu, Y.-C. Fan, H.-Y. Huang, W.-Z. Cai, Metall. and Mater. Trans. A, 46(11) (2015) 5285-5295. [13] M. Selecká, A. Šalak, H. Danninger, J. Mater. Proc. Tech. 143-144 (2003) 910-915. [14] S. Khare, K. Lee, H.K.D.H. Bhadeshia, Int. J. Mater. Res. 100(11) (2009) 1513-1520. [15] T.E. Perez, G.R. Gomez, Bainitic steels with boron. (2010) Google Patents. [16] W. Srichumphan et al., Effect of carbon content and cooling rate on microstructure and mechanical property of sintered low-carbon steels, in Burapha University International Conference 2015, Chonburi, Thailand, “Moving Forward to a Prosperous and Sustainable Community”. (2015) 463-470. [17] W. Kiatdherarat et al., Key Eng. Mater. 658 (2015) 69-75. [18] P. Cizek, B.P. Wynne, C.H.J. Davies, B.C. Muddle, P.D. Hodgson, Metall. and Mater. Trans. A, 33(5) (2002) 1331-1349. [19] R.M. German, Chapter Nine - Sintering With a Liquid Phase, in Sintering: from Empirical Observations to Scientific Principles. 2014, Butterworth-Heinemann: Boston. 247-303.
ScienceDirect Materials Today: Proceedings 5 (2018) 9409–9416
www.materialstoday.com/proceedings
The 10th Thailand International Metallurgy Conference (The 10th TIMETC)
Effect of boron nitride on microstructure of Fe-Cr-Mo-BN-C steel sintered in vacuum Patiparn Ninpetcha, Apichit Luechaisirikula, Monnapas Morakotjindac, Thanyaporn Yotkaewc, Roongthip Krataitongc, Nattaya Tosangthumc, Sithipong Mahathanabodeea,b, * and Ruangdaj Tongsric 0F
a
Department of Production Engineering, Faculty of Engineering, King Mongkut’s University of Technology North Bangkok (KMUTNB) 1518 Pracharat 1, Wongsawang, Bang-Sue, Bangkok 10800,Thailand b Tribo-Systems for Industrial Tools and Machinery Research, King Mongkut’s University of Technology North Bangkok (KMUTNB) 1518 Pracharat 1, Wongsawang, Bang-Sue, Bangkok 10800,Thailand c Particulate Materials Processing Technology Laboratory (PMPT), National Metal and Materials Technology Center 114 Paholyothin Road, Khlong Nueng, Khlong Luang, Pathum Thani 12120, Thailand
Abstract In this work, a pre-alloyed powder of Fe-3% Cr-0.5% Mo was admixed with boron nitride (BN) and graphite powder (0 to 0.4%). The tensile test specimens with the green density about 6.5 g/cm3 were compacted. The specimens were sintered in vacuum furnace at 1280°C for 45 min. After sintering process, specimens were cooled in N2 at various cooling rate. The microstructures, mechanical properties of sintered steels were studied. The results showed tensile strength and hardness increased with the increasing carbon content and cooling rate. Furthermore, the addition of BN showed the tensile strength and hardness improvement at low cooling rate. BN resulted on the coarse grain microstructure and progressive grain coarsening. The liquid phases were found at grain boundaries. The microstructures of sintered steels were mixture of ferrite, bainite and martensite. Finally, the addition of boron nitride was found to improve the transformation of bainite. Therefore, the tensile strength and hardness of sintered steel were improved © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference. Keywords: Boron nitride; cooling rate; sintering, liquid phase; vacuum furnace
* Corresponding author. Tel.: +662-555-2000 Ext. 8212, 8215, 8222 Ext 417; fax: +662-587-0029. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference.
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1. Introduction Chromium-molybdenum (Fe-Cr-Mo) pre-alloyed powder admixed with graphite is attractive in PM manufacturing due to its high performances. Sintered Fe-Cr-Mo-C steel shows good strength, hardenability and cost-effective when it is compared with the traditional PM alloy steels [1]. The mechanical properties of Fe-Cr-MoC steels correlated to the changes of their phase portion of ferrite, bainite, mixed bainite and martensite to fully martensite. Additions of elemental powders, such as Cu [2], Mn [3,4], Si [5] and SiC [6] powder in Fe-Cr-Mo-C steel improved the hardenability of sintered steels. However, strength improvement of sintered steels is preferred for the processing with slow cooling rate condition. In this work, boron nitride (BN) powder is expected that is potential to be a strength and hardenability enhancer for the Fe-Cr-Mo-C steel due to both boron and nitrogen element has ability in hardening steel [1,7]. However, the performance of boron nitride is depending on its content and the rate of cooling during the hardening process. Therefore, this work aims to study the effect of boron nitride content and postsintering cooling rate on microstructure of sintered Fe-Cr-Mo steels. 2. Experimental The specimens were prepared by powder metallurgy process. Water atomized pre-alloyed powder of Fe-3Cr0.5Mo (Astaloy CrM® supplied by Höganäs AB Sweden) with average particle size of 40-150 µm was mixed with 0.5wt% of boron nitride (in a hexagonal form (h-BN)) powder (NX1 grade supplied by Momentive Performance Inc. USA) and various content of graphite powder (0, 0.1, 0.2, 0.3 and 0.4wt%). Compositions were mixed in the tumbling mixture for an hour at rotating speed of 30 rpm. Zinc stearate of 1 wt% was added to the powder mixtures as lubricant. Subsequently, all compositions were compacted in the uniaxial die to produce tensile test specimens according to the MPIF standard 10 [8] with the green density of 6.5 g/cm3. Green specimens were de-lubricated at 600°C for 30 min and then sintered at 1280°C for 45 min in vacuum. During cooling process, specimens were cooled with N2 in various pressures of 0, 2.5 and 5 bar (Correspond to the cooling rates of 0.1, 4.0 and 5.4°C/s). The heating and cooling profile of sintering process and tensile specimens are shown in Fig. 1. The green and sintered density was determined according to the MPIF standard 42 [9]. Sintered specimens were cut, ground, polished and etched for optical microscopy and microhardness observations. Tensile strength, yield strength and hardness were determined according to MPIF standards 10 and 51, respectively [8,10].
Fig. 1. (a) Time-temperature profile and (b) typical tensile specimens
3. Results and discussion 3.1 Sintered density and hardness Density of sintered Fe-Cr-Mo-BN-C steels from different cooling rates is shown in Fig. 2(a). Density of sintered specimens with only boron nitride addition (6.97-7.05 g/cm3) was lower than of the specimens added with boron
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nitride and graphite (7.15-7.33 g/cm3). However, the density of sintered steel was about 90% of theory density. Effect of boron nitride and carbon content on the hardness of sintered specimens under different cooling rates is shown in Fig. 2(b). Sintered specimens showed that the hardness increased corresponding to increasing of carbon content and cooling rate. In addition, the hardness was improved by addition of boron nitride.
Fig. 2. Variation of density and hardness of sintered Fe-Cr-Mo-BN-C steels in different cooling rates; (a) Density and (b) Hardness
3.2 Tensile strength, yield strength and elongation Carbon content and cooling rate also affected on the increasing of tensile strength and yield strength of sintered specimens as shown in Fig. 3. Boron nitride addition showed enhancement of tensile and yield strength of specimens with carbon content 0.2-0.4 wt% at the low cooling rate (0.1°C/s) as shown in Fig. 3(a). Tensile and yield strength of sintered steel were improved from the high cooling rate (4.0°C/s). Moreover, the addition of boron nitride indicated the tensile strength improvement as shown in Fig.3(b). In contrast, the increase of cooling rate and carbon content showed the reduction of elongation as shown in Fig. 4. The addition of boron nitride resulted in the decreasing of elongation at cooling rate of 0.1°C/s but the elongation improved at the cooling rate of 4.0 and 5.4°C/s.
Fig. 3. Tensile and yield strength of sintered Fe-Cr-Mo-BN-C steels in different cooling rates; (a) 0.1°C/s (b) 4.0°C/s
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Fig. 4. Elongation of sintered Fe-Cr-Mo-BN-C steel in different cooling rates
Fig. 5. Typical micrographs of sintered specimens at cooling rate of 0.1°C/s; Without BN: (a) 0.1 wt%C, (b) 0.2 wt%C and (c) 0.3 wt%C; With BN: (d) 0 wt%C, (e) 0.2 wt%C and (f) 0.3 wt%C
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3.3 Microstructures 3.3.1 Slow cooling rate Microstructures of sintered Fe-Cr-Mo-BN-C steels from cooling rate of 0.1°C/s are shown in Fig.5. The microstructure of sintered specimen without boron nitride and graphite addition showed ferrite (F) phases with good bonding and rounded pores in microstructures as shown in Fig 5(a). Pearlite (P) phases were found in microstructure of 0.2 wt%C (Fig. 5(b)) whereas bainite (B) microstructures obtained in the microstructure of 0.3 wt%C as shown in Fig. 5(c). In the Fe-Cr-Mo-C system, carbon diffusion was hindered by chromium and molybdenum [11]. This is attributed to formation of pearlite and bainite in the microstructure of Fe-Cr-Mo-C steels. The increasing of bainite microstructures resulted in the increasing of tensile strength and hardness. In contrast, it was resulted in decreasing of elongation of sintered steels. The addition of 0.5 wt% boron nitride showed that the liquid phases remained segregated at the grain boundary as shown in Fig.5d. The segregation phase is formed due to the reaction between boron nitride and pre-alloyed Fe-CrMo particles. The liquid phase of boron nitride addition has been reported, under addition small amount of boron elements into Fe-Cr-Mo-C system, resulting in the formation of boride phase along grain boundaries [12, 13]. In addition, the spherical pores were found at the original sites of the boron nitride powders. The ferrite-bainite microstructure appeared in the specimens added with 0.2 wt%C as shown in Fig. 5(e). In this case, the fraction of bainitic structures was found higher than specimens without boron nitride addition (Fig.5(c)). It is indicated that the addition of boron nitride influenced on the formation of bainitic structure in sintered Fe-Cr-Mo-BN-C steels. The effect of boron and alloying elements (such as Cr, Mo, Ni) on the transformation of microstructures is still controversial discussed in the previous reports [7, 13-15] In addition, the bainitic structures increased corresponding to the increasing of carbon content as shown in Fig. 5(f). 3.3.2 High cooling rate The microstructures of sintered Fe-Cr-Mo-BN-C steels from the cooling rate of 4.0°C/s are shown in Fig. 6. For the low carbon specimen, the mixture of allotriomorphic ferrite (AF), Widmanstätten ferrite (WF), quasi-polygonal ferrite (QF) and granular ferrite (GF) were found in microstructures as shown in Fig. 6(a). These microstructures of low carbon steel at high cooling rate are described in previous works [16-18]. The transformation of microstructure due to high cooling rate resulted in the mechanical improvement as shown in Fig. 2(b) and 3(b). When carbon content in sintered specimen was increased up to 0.2 wt%, the bainitic structures appeared in microstructures as shown in Fig. 6(b). The bainite-martensite microstructure was found as carbon content increasing as shown in Fig. 6(c). At the cooling rate of 5.4°C/s, bainite-martensite fraction in microstructure increased. The microstructure of 0.3 wt%C specimen with cooling rate 5.4°C/s is shown in Fig. 8(a). The hardness and tensile strength improved due to the increasing of bainite-martensite fraction in microstructure as shown in Fig. 2(b) and 3(b). In the specimens with boron nitride addition, the liquid phases remained along the grain boundaries were found in the microstructures. It is indicated that the liquid phase exists along particle boundaries during sintering process. The solid particles rearrange and lead to densification by coarsening. Some pores vanish due to liquid phase accelerates the change in grain morphology and assists pore removal [19]. The grain coarsening microstructures due to liquid phase generated during sintering process are shown in Fig. 6(d-f). In the specimens of 0 wt%C, the mixture of AF, WF and QF were found in microstructures as shown in Fig. 6(d). The bainitic structures appeared in the microstructure of sintered steels added with carbon as shown in Fig. 6(e). These phases increased corresponding to the carbon content. In addition, martensitic microstructure was found in the microstructure of sintered specimens with carbon content up to 0.3 wt%C as shown in Fig. 6(f). There are resulted in the improvement of tensile strength and hardness of sintered Fe-Cr-Mo-BN-C steels was observed, as shown in Fig. 2(b) and 3(b). Additionally, thinner layer of liquid phase remained at grain boundary at high cooling rate, as shown in Fig. 7. This is due to phase transformation at the high cooling rate, which resulted in some boron atoms dissolved in the matrix. The amount of boron atoms to form boride liquid phase then reduced. The diffusion of boron into the matrix grain resulted in enhancement of microhardness of sintered steel [13]. The microhardness of 0.1 wt%C specimens with boron nitride
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addition at cooing rate of 0.1 and 4.0°C/s were 110-118 HV0.02 and 223-262 HV0.02, respectively. However, the effects on liquid phase sintering of cooling rate and alloying elements such as chromium, carbon and boron are stilled unclear. Then it is necessary to study in the further works. At the cooling rate of 5.4°C/s, bainite-martensite fraction in microstructures increased in the specimens with 0.4 wt%C in high cooling rate as shown in Fig. 8(b).
Fig. 6 Typical micrographs of sintered specimens at cooling rate of 4.0°C/s; Without BN: (a) 0.1 wt%C, (b) 0.2 wt%C and (c) 0.3 wt%C; With BN: (d) 0 wt%C, (e) 0.2 wt%C and (f) 0.3 wt%C
Fig. 7 Typical SEM micrographs of sintered specimens at different cooling rates; (a) 0.1°C/s and (b) 5.4°C/s
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Fig. 8 Typical micrographs of sintered specimens at cooling rates of 5.4°C/s; (a) 0.3%C without BN and (b) 0.4%C with BN
4. Conclusion Fe-Cr-Mo steel with boron nitride and graphite additions were sintered at 1280°C, cooled in various cooling rate in vacuum furnace, microstructurally investigated and tested for their mechanical. The following results were obtained: 1. Tensile strength and hardness of sintered specimens increased with the increasing carbon content and cooling rate. Boron nitride added resulted in the yield strength and hardness were higher than the specimen without BN, especially at 0.1°C/s. 2. Boron nitride addition resulted in the coarse grain microstructure and progressive grain coarsening in specimens. The liquid phases formed surrounding grain boundaries induced densification improvement. However, liquid phases remained segregated along grain boundaries reduced at high cooling rate. 3. The microstructures of sintered Fe-Cr-Mo-BN-C steels were mixture of ferrite, bainite and martensite. Bainite-martensite fraction increased corresponding to the increasing of carbon content and cooling rate. In addition, boron nitride was found to improve the transformation of bainite in Fe-Cr-Mo-BN-C steels. Acknowledgements The authors would like to thank King Mongkut’s University of Technology North Bangkok for financial support under Contact No. KMUTNB-GEN-59-03. Our gratitude is also extended to the National Metal and Materials Technology Center, Pathum Thani of Thailand for laboratory equipment, characterization facilities and personal help. The courtesy of Acme International (Thailand) Ltd. and Höganäs AB (Sweden) for AstaloyCrM® powder. We also would like to thank Momentive Performance Materials, Inc., USA, for supplying h-BN powder. References [1] J. Kazior, C. Janczur, T. Pieczonka, J. Ploszczak, Surface and Coatings Tech. 151-152 (2002) 333-337. [2] N. Hirose, T. Itahara, Y. Takeda. Hardening Effect of Cr Containing Material with Cu and Fe Addition without Rapid Cooling. in World Congress PM2014, Orlando USA (2014) 1-10. [3] T. Pieczonka, M. Sułowski, A. Ciaś., Arch. Metall. Mater. 57(4) (2012) 1001-1009. [4] M. Sułowski, Arch. Metall. Mater. 59(4) (2014) 1499-1505. [5] M. Azadbeh, N.P. Ahmadi Curr. Appl. Phys. 9(4) (2009) 777-782. [6] M. Hebda, S. Gądek, K. Miernik, J. Kazior, Adv. Powder Tech. 25-2 (2014) 543-550. [7] H.K. Zeytin, H. Yildirim, B. Berme, S. Duduoĝlu, G. Kazdal, A. Deniz, J. Iron Steel Res. Int. 18(11) (2011) 31-39. [8] MPIF Standard 10: Method for Determination of Tensile Properties of Powder Metallurgy Materials, in Standard Test Methods for Metal Powders and Powder Metallurgy Products. 2002: Princeton, (New Jersey, USA).
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