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Effect of sintering atmosphere on structure and properties of austeno-ferritic stainless steels
Materials Science and Engineering A 517 (2009) 328–333
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of sintering atmosphere on structure and properties of austeno-ferritic stainless steels R. Mariappan ∗ , S. Kumaran, T. Srinivasa Rao Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli 620015, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 28 November 2008 Received in revised form 27 March 2009 Accepted 3 April 2009 Keywords: Austeno-ferritic stainless steels Sintering atmosphere Densification rate Bi-phase structure
a b s t r a c t Prealloyed 316L and 430L stainless steel powders along with Cu, Cr, Mo and Ni have been used to produce different austeno-ferritic stainless steels through powder metallurgy route. Cylindrical green compacts of 30 mm diameter and 12 mm height have been made at three different compaction pressures and sintered in nitrogen and argon atmospheres at 1350 ◦ C for 4 h, to study the effect of sintering on densification behaviour and mechanical properties. Stainless steels sintered in argon atmosphere exhibited better densification rate than the nitrogen atmosphere sintering. The microstructure of stainless steel sintered in nitrogen atmosphere revealed lamellar constituents with grain boundary Cr2 N in ferritic matrix. Composition A (50 wt% 316L + 50 wt% 430L) of austeno-ferritic stainless steel sintered in argon atmosphere showed bi-phase structure with high strength and better elongation. The XRD patterns are in line with the microstructure. The ferrite content was measured by ferrite scope and is coherent with the strength and structures. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Individually ferritic stainless steels and austenitic stainless steels exhibit low toughness and low corrosion resistance respectively, however, austeno-ferritic stainless steels having combination of both microstructures are characterized by outstanding combination of strength, toughness and corrosion resistance [1]. These steels are also known as duplex stainless steels (DSS). Combinations of such properties make the duplex stainless steel very attractive for numerous applications. Duplex stainless steels obtained by powder metallurgy technology could be used in many industrial branches due to their high mechanical properties and good corrosion resistance. Sintered duplex stainless steel seems to be very promising and many research groups across the globe are working on developing duplex stainless steels through powder metallurgy (P/M) route. Powder metallurgy processing followed by hot isostatic pressing (HIP) of heavy super duplex stainless steel components can yield consistent quality with higher alloying elements such as Cr, Mo and N. P/M + HIP duplex stainless steels have already shown their potential applications in process industry, off shore, pulp and paper industry. Typical applications for P/M duplex stainless steels are flanges, valve bodies, fittings, pumps, mixers, and manifold sections [2–5]. Powder metallurgy enables
∗ Corresponding author. Tel.: +91 9865582348; fax: +91 431 2500133. E-mail addresses: [email protected] (R. Mariappan), [email protected] (S. Kumaran), [email protected] (T.S. Rao). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.04.011
production of duplex stainless steels by several methods. The first method is based on fully prealloyed powder with a required duplex composition [6]. The approach for the next one is mixing the elemental powders such as Cr, Ni, Mo powders with austenitic and ferritic alloyed powders, in proper ratios to ensure required duplex microstructure [7–9]. Application of powder metallurgy technology for producing bi-phase duplex stainless steels enables precise control of their chemical composition and phase combination as well as elimination of number of technological difficulties that are present during the production of same kind of steels using traditional methods. However, these materials present several problems during their manufacture and service conditions. Heat treatments may provoke undesirable metallurgical transformations, generating the precipitation of secondary phases, such as intermetallics, carbides and nitrides. Appearance of these brittle phases reduces the ductility and produces chromium depleted areas, resulting in decrease in the corrosion resistance [9]. Attempts are being made to control these undesirable metallurgical transformations. Generally wrought duplex stainless steels are having higher yield strength (above 425 MPa) and better elongation than the P/M duplex stainless steels. Dakhlaui et al. reported from the elasto-plastic self consistent model that yield stress of the wrought hot rolled duplex stainless steel is affected by the initial residual stresses present within the material. Due to the superposition of initial stresses with applied stresses, the yield points are different for compressive and tensile tests [10]. In the present work different combinations of austeno-ferritic stainless steels were produced from premixes of 316L and 430L powders, elemental Cu, Cr, Mo and Ni pow-
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Fig. 1. SEM micrographs of powder mixtures of (a) composition A and (b) composition C respectively.
Table 1 Chemical composition of various grades/mixtures. Powder grade
Cr
C
Ni
Si
Mn
Mo
Fe
Flow rate/50 g (s)
Apparent density (g/cm3 )
316L 430L
16.60 16.56
0.023 0.012
12.43 –
0.90 1.20
0.10 0.10
2.10 –
Balance Balance
25.4 27.4
2.77 2.88
Mixtures
Composition A (50% 316L + 50% 430L) wt% Composition B (49% 316L + 49% 430L + 2% Cu) wt% Composition C (45% 316L + 45% 430L + 4% Cr + 3% Mo + 3% Ni) wt% Composition D (44% 316L + 44% 430L + 4% Cr + 3% Mo + 3% Ni + 2% Cu) wt%
Table 2 Chemical composition of investigated powder mixtures. Composition
A B C D
Elements concentration (%wt) Cr
C
Ni
Si
Mn
Mo
16.58 16.24 17.84 16.59
0.018 0.016 0.018 0.016
6.22 6.09 7.09 7.47
1.05 1.03 0.95 0.92
0.10 0.10 0.09 0.09
1.10 1.08 2.48 2.47
ders, and subsequently sintered in two different atmospheres to study the effect of sintering atmosphere on densification behaviour, microstructure and mechanical properties. 2. Experimental procedure Water atomized 316L austenitic and 430L ferritic stainless steel powders supplied by M/s Hoganas India Ltd. were used in the present work along with elemental powders such as Cu, Cr, Mo and Ni to produce four different compositions of austeno-ferritic stainless steels. All these powders exhibit irregular shape which results better compactability and sinterability (Fig. 1). The chemical compositions of 316L and 430L powders and combination of four powder mixtures, i.e. compositions A, B, C and D are given in Table 1. Schaffler’s diagram was taken into consideration while preparing the powder mixtures. Although its proper application is in welding, it is possible to extend its use in the field of powder metallurgy. Thus, CrE (Cr equivalent) and NiE (Ni equivalent) (CrE = % Cr + % Mo + 1.5% Si + 0.5% Nb; NiE = % Ni + 30% C + 0.5% Mn) are obtained introducing the wt% quantity of the corresponding element into the formula, which locate all the products in a well-defined area, at least in a theoretical point of view. Table 2 presents the chemical compositions of four different austeno-ferritic stainless steels along with chromium and nickel equivalent (CrE and NiE ) values derived from green composition and Fig. 2 indicates their location on Schaffler’s diagram. Cylindrical green compacts of 30 mm diameter and 12 mm height were prepared at three different pressure levels such as 420, 490 and 560 MPa. The green compacts were sintered at 1350 ◦ C for 4 h in two different controlled atmospheres,
Fe Bal Bal Bal Bal
Cu – 2.00 – 2.00
NiE
CrE
6.81 6.62 9.13 8.92
19.70 19.30 25.94 25.52
such as argon and nitrogen. After sintering, the stainless steel compacts were cooled at the rate of 20 ◦ C per min. Density of both green compacts and sintered compacts was measured by mass and physical dimensions to understand the effect of compaction pressure and sintering atmosphere on densification. Microstructural analysis (structural changes) was made with the help of image analyzer. Structural evolution and phase transformation of the sintered steels was studied by X-ray Diffractometer (Make: Rigaku, Japan) with Cu K␣ target. Mechanical properties such as tensile strength and
Fig. 2. Schaffler’s diagram indicating CrE and NiE for A, B, C and D compositions [2].
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Fig. 3. Microstructures of austeno-ferritic stainless steels sintered in nitrogen atmosphere (a) composition A, (b) composition B, (c) composition C and (d) composition D.
Fig. 4. Microstructures of austeno-ferritic stainless steels sintered in argon atmosphere (a) composition A, (b) composition B, (c) composition C and (d) composition D.
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Table 3 Percent theoretical density of austeno-ferritic stainless steels sintered in nitrogen atmosphere at 1350 ◦ C for 4 h. Composition
A B C D
Compaction pressure 420 MPa
Compaction pressure 490 MPa
Compaction pressure 560 MPa
Green density
Sintered density
Green density
Sintered density
Green density
Sintered density
72.77 72.69 71.83 72.45
77.43 79.14 76.82 77.58
75.08 75.35 72.93 75.91
80.05 80.15 6.273 79.95
77.09 77.18 75.45 77.11
82.17 82.52 80.20 81.82
Table 4 Percent theoretical density of austeno-ferritic stainless steels sintered in argon atmosphere at 1350 ◦ C for 4 h. Composition
A B C D
Compaction pressure 420 MPa
Compaction pressure 490 MPa
Compaction pressure 560 MPa
Green density
Sintered density
Green density
Sintered density
Green density
Sintered density
72.52 72.57 70.58 71.08
83.72 86.02 84.61 84.84
75.11 74.22 74.45 73.72
85.83 86.77 85.13 85.66
78.56 80.52 78.43 78.88
87.00 88.32 89.59 88.75
hardness of sintered austeno-ferritic stainless steels were evaluated by using Hounsfield Tensometer and Rockwell hardness tester respectively. The percent elongation of different stainless steels was measured by the displacement of the grips. The ferrite content was measured by a magnetic method using Fischer ferrite scope.
3. Results and discussion 3.1. Densification behaviour The green and sintered density values of austeno-ferritic stainless steels are reported in Tables 3 and 4. Table 3 corresponds to nitrogen atmosphere sintering and Table 4 corresponds to argon atmosphere sintering. It is understood from these tables that the density increases with increasing compaction pressure, but the effect of composition is negligible on density. The stainless steels sintered in nitrogen atmosphere show 4–6% density increments irrespective of compaction pressure. In other words, the densification rate achieved varies from 5–8%. However, the steels sintered in argon atmosphere exhibit higher density increments, in the range of 8–14%, i.e. the densification rate is in the order of 10–21%. The main reason for differential densification rate in two sintering atmospheres is due to formation of Cr2 N, which is absorbed from the nitrogen atmosphere which in turn reduces the diffusion rate resulting in lower sintered density. In argon atmosphere sintering, the densification rate is better for the steels compacted at low pressure and the same decreases with increasing compaction pressure. The steels containing 2% Cu do not show any significant increment in sintered density.
3.2. Microstructure and phase evolution Figs. 3 and 4 show optical micrographs of four different austenoferritic stainless steels sintered in nitrogen and argon atmospheres respectively. Fig. 5 shows the optical micrographs with higher magnification for composition A sintered in nitrogen and argon atmospheres respectively. From Fig. 3 the microstructures of four austeno-ferritic stainless steels sintered in nitrogen atmosphere contain lamellar constituents with ferritic and austenitic phases. The lamellar constituent is a mixture of Cr2 N and ferrite, commonly seen in Fe–Cr–Mn–Ni alloys, melted under high nitrogen gas pressure and was reported as ‘false pearlite’ by Garcia and his co-workers. These constituents are also reported in heat resistant alloys of higher nickel and chromium content, and the same was increased with increasing nitrogen gas pressure [11]. From the microstructures, apart from lamellar constituents, Cr2 N is also observed along the grain boundaries. The grain boundary nitrides and the mixture of Cr2 N with ferrite are clearly observed from the optical micrograph of Fig. 5a. Optical microstructures of compositions B and D exhibit more amount of austenite compared to the compositions A and C. This could be due to the addition of copper in the compositions B and D which would have acted as austenitic stabilizer. The microstructures of austeno-ferritic stainless steels sintered in argon atmosphere (Fig. 4) are entirely different from the steels sintered in nitrogen atmosphere. There is no indication of grain boundary nitrides and lamellar constituents, instead, the sintered products reveal bi-phase structure, namely austenite and ferrite with varying volume fraction. Composition A shows more ferrite than composition B, this is due to the addition of copper which would have stabilized the austenite. Microstructure of
Fig. 5. Microstructures of composition A of austeno-ferritic stainless steel (a) sintered in nitrogen and (b) sintered in argon atmospheres respectively (magnification 800×).
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Fig. 6. XRD patterns of austeno-ferritic stainless steels sintered in nitrogen atmosphere.
composition B exhibits more austenite with acicular ferrite, whereas compositions C and D exhibit balanced amounts of ferrite and austenite. This is mainly attributed to balance between CrE and NiE as per Schaffler’s diagram. From Fig. 5b it is clear that there is no grain boundary nitrides and lamellar constituents. Fig. 6 shows the XRD patterns of austeno-ferritic stainless steel sintered in nitrogen atmosphere. From the above XRD patterns, apart from austenite and ferrite peak traces of Cr2 N and hard peaks such as are also observed. Formation of chromium nitride is due to sintering of stainless steels in nitrogen atmosphere. Austenite phase peak is dominant in Cu containing systems indicating more amount of austenite as it is evident from optical microstructures. XRD patterns of the stainless steels sintered in argon atmosphere reveal only austenitic and ferritic peaks (Fig. 7). Compositions A and C exhibit ferritic peaks with higher intensity compared to austenitic peaks indicating more amount of ferrite. Whereas compositions B and D reveal predominant peaks of austenite.
Fig. 8. Stress–strain curves for different austeno-ferritic stainless steels sintered in nitrogen atmosphere.
Fig. 9. Stress–strain curves for different austeno-ferritic stainless steels sintered in argon atmospheres.
3.3. Evaluation of mechanical properties Figs. 8 and 9 show the stress–strain curves for different compositions of austeno-ferritic stainless steels sintered at 1350 ◦ C for 4 h in nitrogen and argon atmospheres respectively. Mechanical properties such as yield strength, ultimate tensile strength, elongation and hardness of these steels are given in Tables 5 and 6. Since composition A has more amount of Cr2 N compared to composition B due to sintering in nitrogen atmosphere, there is an improvement in tensile strength as shown in Fig. 8. Similarly composition C exhibits more tensile strength than composition D for the same reason. The effect of Cu addition in compositions B and D
does not show any improvement on density. However, Cu addition has facilitated the formation of austenite, which is a softer phase, hence reduction in tensile strength. Due to the presence of additional elements, such as Cr, Ni and Mo based on Schaffler’s diagram in composition C, microstructure exhibits more amount of Cr2 N resulting in improved strength (694 MPa) and hardness (66 HRA) Table 5 Mechanical properties of different austeno-ferritic stainless steels sintered at 1350 ◦ C for 4 h in nitrogen atmosphere. Composition
A B C D
Compaction pressure 560 MPa Yield strength (MPa)
Ultimate tensile strength (MPa)
% Elongation
Hardness HRA
426 366 447 386
666 530 694 583
4.91 5.54 2.44 4.52
66 56 61 57
Table 6 Mechanical properties of different austeno-ferritic stainless steels sintered at 1350 ◦ C for 4 h in argon atmosphere. Composition
Fig. 7. XRD patterns of austeno-ferritic stainless steels sintered in argon atmosphere.
A B C D
Compaction pressure 560 MPa Yield strength (MPa)
Ultimate tensile strength (MPa)
% Elongation
Hardness HRA
404 336 376 332
644 512 556 532
9.02 5.08 4.5 7.12
64 56 59 57
R. Mariappan et al. / Materials Science and Engineering A 517 (2009) 328–333 Table 7 Ferrite content of austeno-ferritic stainless steels sintered in different atmospheres. Composition
Sintered in nitrogen atmosphere (% ferrite)
Sintered in argon atmosphere (% ferrite)
A B C D
21 18 27 21
38 24 23 17
with concomitant loss of ductility (2.44% elongation) as is evident from XRD patterns (Fig. 6). From Fig. 9, composition A exhibits more tensile strength (644 MPa), better ductility (9.03% elongation) and hardness (64 HRA) compared to other compositions. In general the steels sintered in nitrogen atmosphere exhibited an improvement in UTS by 5–8% for compositions A and B, 25–32% for compositions C and D. This shows mechanical properties of the austenoferritic stainless steels depend on composition and sintering atmospheres. 3.4. Ferrite content The determination of ferrite percentage in P/M austeno-ferritic stainless steels is carried out by magnetic measurements by using Fischer’s ferrite scope and the details are given in Table 7. The amount of ferrite measured in the composition C for the stainless steels sintered in nitrogen atmosphere with an average of 27%. Similarly more quantity of ferrite (average of 38%) is observed in the composition A for the stainless steels sintered in argon atmosphere. In both sintering atmospheres, compositions B and D exhibit lower amount of ferrite compared to other compositions. This is due to the addition of copper, which leads to the formation of more austenite. 4. Conclusions In the present work, effect of sintering atmosphere on densification behaviour, microstructure and mechanical properties was studied. The salient features of the present study are summarized as follows:
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• Densification rate of austeno-ferritic stainless steels sintered in argon atmosphere is better (10–21%) than the steels sintered in nitrogen atmosphere (8–14%). • Sintering of stainless steels in nitrogen atmosphere reveals lamellar constituents with grain boundary Cr2 N in the ferritic matrix, whereas sintering in argon atmosphere shows only bi-phase structure. • Austeno-ferritic stainless steels sintered in nitrogen atmosphere exhibit higher strength and lower elongation than the steels sintered in argon atmosphere. • XRD patterns for compositions B and D for both sintering atmospheres show better intensity for austenitic peaks due to the addition of copper. Acknowledgements The authors would like to thank Dr. M. Chidambaram, Director, NIT, Tiruchirappalli 620015 for providing financial support and permission to publish this research paper. Authors also would like to acknowledge the assistance rendered by Mr. T. Gnana Prakasam, Mechanic-A and Mr. S. Arumugasamy, Technical Assistant, Dept. of MME, NIT, Tiruchirappalli for their help during experimental work. References [1] P. Datta, G.S. Upadhyaya, Materials Chemistry and Physics 67 (2001) 234–242. ´ [2] L.A. Dobrzanski, Z. Brytan, M. Actis Grande, M. Rosso, E.J. Pallavicini, Journal of Materials Processing Technology 157–158 (2004) 312–316. [3] P.K. Samal, J.B. Terrell, Metal Powder Report (2001) 28–34. [4] A.J. Rawlings, H.M. Kopech, H.G. Rutz, Proceedings of International Conference on Powder Metallurgy & Particulate Materials PM2TEC’97, Chicago, USA, 1997. [5] D. Schaefer, C.J. Trombino, State of North American P/M industry 2005, PM2TEC’ Montreal, Canada, 2005. [6] C.J. Munez, M.V. Utrilla, A. Urena, Journal of Alloys and Compounds (2007) 1–23. [7] J. Kaziora, M. Nykiela, T. Pieczonkab, T. Marcu Puscasc, A. Molinaric, Journal of Materials Processing Technology 157–158 (2004) 712–717. [8] Olena Smuk, Microstructure and properties of modern P/M super duplex stainless steel, Doctoral thesis, Dept. of Mat. Sci.and Eng., Royal Institute of Technology, Sweden, 2004, pp. 1–5. [9] F. Iacoviello, M. Boniardi, G.M. La Vecchia, International Journal of Fatigue 21 (1999) 957–963. [10] R. Dakhlaui, A. Baczman’ski, C. Braham, S. Wron’ski, K. Wierzbanowski, E.C. oliver, Acta Meteriallia 54 (2006) 5027–5039. [11] C. García, F. Martín, P. de Tiedra, L. García Cambronero, Corrosion Science 49 (2007) 1718–1736.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of sintering atmosphere on structure and properties of austeno-ferritic stainless steels R. Mariappan ∗ , S. Kumaran, T. Srinivasa Rao Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli 620015, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 28 November 2008 Received in revised form 27 March 2009 Accepted 3 April 2009 Keywords: Austeno-ferritic stainless steels Sintering atmosphere Densification rate Bi-phase structure
a b s t r a c t Prealloyed 316L and 430L stainless steel powders along with Cu, Cr, Mo and Ni have been used to produce different austeno-ferritic stainless steels through powder metallurgy route. Cylindrical green compacts of 30 mm diameter and 12 mm height have been made at three different compaction pressures and sintered in nitrogen and argon atmospheres at 1350 ◦ C for 4 h, to study the effect of sintering on densification behaviour and mechanical properties. Stainless steels sintered in argon atmosphere exhibited better densification rate than the nitrogen atmosphere sintering. The microstructure of stainless steel sintered in nitrogen atmosphere revealed lamellar constituents with grain boundary Cr2 N in ferritic matrix. Composition A (50 wt% 316L + 50 wt% 430L) of austeno-ferritic stainless steel sintered in argon atmosphere showed bi-phase structure with high strength and better elongation. The XRD patterns are in line with the microstructure. The ferrite content was measured by ferrite scope and is coherent with the strength and structures. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Individually ferritic stainless steels and austenitic stainless steels exhibit low toughness and low corrosion resistance respectively, however, austeno-ferritic stainless steels having combination of both microstructures are characterized by outstanding combination of strength, toughness and corrosion resistance [1]. These steels are also known as duplex stainless steels (DSS). Combinations of such properties make the duplex stainless steel very attractive for numerous applications. Duplex stainless steels obtained by powder metallurgy technology could be used in many industrial branches due to their high mechanical properties and good corrosion resistance. Sintered duplex stainless steel seems to be very promising and many research groups across the globe are working on developing duplex stainless steels through powder metallurgy (P/M) route. Powder metallurgy processing followed by hot isostatic pressing (HIP) of heavy super duplex stainless steel components can yield consistent quality with higher alloying elements such as Cr, Mo and N. P/M + HIP duplex stainless steels have already shown their potential applications in process industry, off shore, pulp and paper industry. Typical applications for P/M duplex stainless steels are flanges, valve bodies, fittings, pumps, mixers, and manifold sections [2–5]. Powder metallurgy enables
∗ Corresponding author. Tel.: +91 9865582348; fax: +91 431 2500133. E-mail addresses: [email protected] (R. Mariappan), [email protected] (S. Kumaran), [email protected] (T.S. Rao). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.04.011
production of duplex stainless steels by several methods. The first method is based on fully prealloyed powder with a required duplex composition [6]. The approach for the next one is mixing the elemental powders such as Cr, Ni, Mo powders with austenitic and ferritic alloyed powders, in proper ratios to ensure required duplex microstructure [7–9]. Application of powder metallurgy technology for producing bi-phase duplex stainless steels enables precise control of their chemical composition and phase combination as well as elimination of number of technological difficulties that are present during the production of same kind of steels using traditional methods. However, these materials present several problems during their manufacture and service conditions. Heat treatments may provoke undesirable metallurgical transformations, generating the precipitation of secondary phases, such as intermetallics, carbides and nitrides. Appearance of these brittle phases reduces the ductility and produces chromium depleted areas, resulting in decrease in the corrosion resistance [9]. Attempts are being made to control these undesirable metallurgical transformations. Generally wrought duplex stainless steels are having higher yield strength (above 425 MPa) and better elongation than the P/M duplex stainless steels. Dakhlaui et al. reported from the elasto-plastic self consistent model that yield stress of the wrought hot rolled duplex stainless steel is affected by the initial residual stresses present within the material. Due to the superposition of initial stresses with applied stresses, the yield points are different for compressive and tensile tests [10]. In the present work different combinations of austeno-ferritic stainless steels were produced from premixes of 316L and 430L powders, elemental Cu, Cr, Mo and Ni pow-
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Fig. 1. SEM micrographs of powder mixtures of (a) composition A and (b) composition C respectively.
Table 1 Chemical composition of various grades/mixtures. Powder grade
Cr
C
Ni
Si
Mn
Mo
Fe
Flow rate/50 g (s)
Apparent density (g/cm3 )
316L 430L
16.60 16.56
0.023 0.012
12.43 –
0.90 1.20
0.10 0.10
2.10 –
Balance Balance
25.4 27.4
2.77 2.88
Mixtures
Composition A (50% 316L + 50% 430L) wt% Composition B (49% 316L + 49% 430L + 2% Cu) wt% Composition C (45% 316L + 45% 430L + 4% Cr + 3% Mo + 3% Ni) wt% Composition D (44% 316L + 44% 430L + 4% Cr + 3% Mo + 3% Ni + 2% Cu) wt%
Table 2 Chemical composition of investigated powder mixtures. Composition
A B C D
Elements concentration (%wt) Cr
C
Ni
Si
Mn
Mo
16.58 16.24 17.84 16.59
0.018 0.016 0.018 0.016
6.22 6.09 7.09 7.47
1.05 1.03 0.95 0.92
0.10 0.10 0.09 0.09
1.10 1.08 2.48 2.47
ders, and subsequently sintered in two different atmospheres to study the effect of sintering atmosphere on densification behaviour, microstructure and mechanical properties. 2. Experimental procedure Water atomized 316L austenitic and 430L ferritic stainless steel powders supplied by M/s Hoganas India Ltd. were used in the present work along with elemental powders such as Cu, Cr, Mo and Ni to produce four different compositions of austeno-ferritic stainless steels. All these powders exhibit irregular shape which results better compactability and sinterability (Fig. 1). The chemical compositions of 316L and 430L powders and combination of four powder mixtures, i.e. compositions A, B, C and D are given in Table 1. Schaffler’s diagram was taken into consideration while preparing the powder mixtures. Although its proper application is in welding, it is possible to extend its use in the field of powder metallurgy. Thus, CrE (Cr equivalent) and NiE (Ni equivalent) (CrE = % Cr + % Mo + 1.5% Si + 0.5% Nb; NiE = % Ni + 30% C + 0.5% Mn) are obtained introducing the wt% quantity of the corresponding element into the formula, which locate all the products in a well-defined area, at least in a theoretical point of view. Table 2 presents the chemical compositions of four different austeno-ferritic stainless steels along with chromium and nickel equivalent (CrE and NiE ) values derived from green composition and Fig. 2 indicates their location on Schaffler’s diagram. Cylindrical green compacts of 30 mm diameter and 12 mm height were prepared at three different pressure levels such as 420, 490 and 560 MPa. The green compacts were sintered at 1350 ◦ C for 4 h in two different controlled atmospheres,
Fe Bal Bal Bal Bal
Cu – 2.00 – 2.00
NiE
CrE
6.81 6.62 9.13 8.92
19.70 19.30 25.94 25.52
such as argon and nitrogen. After sintering, the stainless steel compacts were cooled at the rate of 20 ◦ C per min. Density of both green compacts and sintered compacts was measured by mass and physical dimensions to understand the effect of compaction pressure and sintering atmosphere on densification. Microstructural analysis (structural changes) was made with the help of image analyzer. Structural evolution and phase transformation of the sintered steels was studied by X-ray Diffractometer (Make: Rigaku, Japan) with Cu K␣ target. Mechanical properties such as tensile strength and
Fig. 2. Schaffler’s diagram indicating CrE and NiE for A, B, C and D compositions [2].
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Fig. 3. Microstructures of austeno-ferritic stainless steels sintered in nitrogen atmosphere (a) composition A, (b) composition B, (c) composition C and (d) composition D.
Fig. 4. Microstructures of austeno-ferritic stainless steels sintered in argon atmosphere (a) composition A, (b) composition B, (c) composition C and (d) composition D.
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Table 3 Percent theoretical density of austeno-ferritic stainless steels sintered in nitrogen atmosphere at 1350 ◦ C for 4 h. Composition
A B C D
Compaction pressure 420 MPa
Compaction pressure 490 MPa
Compaction pressure 560 MPa
Green density
Sintered density
Green density
Sintered density
Green density
Sintered density
72.77 72.69 71.83 72.45
77.43 79.14 76.82 77.58
75.08 75.35 72.93 75.91
80.05 80.15 6.273 79.95
77.09 77.18 75.45 77.11
82.17 82.52 80.20 81.82
Table 4 Percent theoretical density of austeno-ferritic stainless steels sintered in argon atmosphere at 1350 ◦ C for 4 h. Composition
A B C D
Compaction pressure 420 MPa
Compaction pressure 490 MPa
Compaction pressure 560 MPa
Green density
Sintered density
Green density
Sintered density
Green density
Sintered density
72.52 72.57 70.58 71.08
83.72 86.02 84.61 84.84
75.11 74.22 74.45 73.72
85.83 86.77 85.13 85.66
78.56 80.52 78.43 78.88
87.00 88.32 89.59 88.75
hardness of sintered austeno-ferritic stainless steels were evaluated by using Hounsfield Tensometer and Rockwell hardness tester respectively. The percent elongation of different stainless steels was measured by the displacement of the grips. The ferrite content was measured by a magnetic method using Fischer ferrite scope.
3. Results and discussion 3.1. Densification behaviour The green and sintered density values of austeno-ferritic stainless steels are reported in Tables 3 and 4. Table 3 corresponds to nitrogen atmosphere sintering and Table 4 corresponds to argon atmosphere sintering. It is understood from these tables that the density increases with increasing compaction pressure, but the effect of composition is negligible on density. The stainless steels sintered in nitrogen atmosphere show 4–6% density increments irrespective of compaction pressure. In other words, the densification rate achieved varies from 5–8%. However, the steels sintered in argon atmosphere exhibit higher density increments, in the range of 8–14%, i.e. the densification rate is in the order of 10–21%. The main reason for differential densification rate in two sintering atmospheres is due to formation of Cr2 N, which is absorbed from the nitrogen atmosphere which in turn reduces the diffusion rate resulting in lower sintered density. In argon atmosphere sintering, the densification rate is better for the steels compacted at low pressure and the same decreases with increasing compaction pressure. The steels containing 2% Cu do not show any significant increment in sintered density.
3.2. Microstructure and phase evolution Figs. 3 and 4 show optical micrographs of four different austenoferritic stainless steels sintered in nitrogen and argon atmospheres respectively. Fig. 5 shows the optical micrographs with higher magnification for composition A sintered in nitrogen and argon atmospheres respectively. From Fig. 3 the microstructures of four austeno-ferritic stainless steels sintered in nitrogen atmosphere contain lamellar constituents with ferritic and austenitic phases. The lamellar constituent is a mixture of Cr2 N and ferrite, commonly seen in Fe–Cr–Mn–Ni alloys, melted under high nitrogen gas pressure and was reported as ‘false pearlite’ by Garcia and his co-workers. These constituents are also reported in heat resistant alloys of higher nickel and chromium content, and the same was increased with increasing nitrogen gas pressure [11]. From the microstructures, apart from lamellar constituents, Cr2 N is also observed along the grain boundaries. The grain boundary nitrides and the mixture of Cr2 N with ferrite are clearly observed from the optical micrograph of Fig. 5a. Optical microstructures of compositions B and D exhibit more amount of austenite compared to the compositions A and C. This could be due to the addition of copper in the compositions B and D which would have acted as austenitic stabilizer. The microstructures of austeno-ferritic stainless steels sintered in argon atmosphere (Fig. 4) are entirely different from the steels sintered in nitrogen atmosphere. There is no indication of grain boundary nitrides and lamellar constituents, instead, the sintered products reveal bi-phase structure, namely austenite and ferrite with varying volume fraction. Composition A shows more ferrite than composition B, this is due to the addition of copper which would have stabilized the austenite. Microstructure of
Fig. 5. Microstructures of composition A of austeno-ferritic stainless steel (a) sintered in nitrogen and (b) sintered in argon atmospheres respectively (magnification 800×).
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Fig. 6. XRD patterns of austeno-ferritic stainless steels sintered in nitrogen atmosphere.
composition B exhibits more austenite with acicular ferrite, whereas compositions C and D exhibit balanced amounts of ferrite and austenite. This is mainly attributed to balance between CrE and NiE as per Schaffler’s diagram. From Fig. 5b it is clear that there is no grain boundary nitrides and lamellar constituents. Fig. 6 shows the XRD patterns of austeno-ferritic stainless steel sintered in nitrogen atmosphere. From the above XRD patterns, apart from austenite and ferrite peak traces of Cr2 N and hard peaks such as are also observed. Formation of chromium nitride is due to sintering of stainless steels in nitrogen atmosphere. Austenite phase peak is dominant in Cu containing systems indicating more amount of austenite as it is evident from optical microstructures. XRD patterns of the stainless steels sintered in argon atmosphere reveal only austenitic and ferritic peaks (Fig. 7). Compositions A and C exhibit ferritic peaks with higher intensity compared to austenitic peaks indicating more amount of ferrite. Whereas compositions B and D reveal predominant peaks of austenite.
Fig. 8. Stress–strain curves for different austeno-ferritic stainless steels sintered in nitrogen atmosphere.
Fig. 9. Stress–strain curves for different austeno-ferritic stainless steels sintered in argon atmospheres.
3.3. Evaluation of mechanical properties Figs. 8 and 9 show the stress–strain curves for different compositions of austeno-ferritic stainless steels sintered at 1350 ◦ C for 4 h in nitrogen and argon atmospheres respectively. Mechanical properties such as yield strength, ultimate tensile strength, elongation and hardness of these steels are given in Tables 5 and 6. Since composition A has more amount of Cr2 N compared to composition B due to sintering in nitrogen atmosphere, there is an improvement in tensile strength as shown in Fig. 8. Similarly composition C exhibits more tensile strength than composition D for the same reason. The effect of Cu addition in compositions B and D
does not show any improvement on density. However, Cu addition has facilitated the formation of austenite, which is a softer phase, hence reduction in tensile strength. Due to the presence of additional elements, such as Cr, Ni and Mo based on Schaffler’s diagram in composition C, microstructure exhibits more amount of Cr2 N resulting in improved strength (694 MPa) and hardness (66 HRA) Table 5 Mechanical properties of different austeno-ferritic stainless steels sintered at 1350 ◦ C for 4 h in nitrogen atmosphere. Composition
A B C D
Compaction pressure 560 MPa Yield strength (MPa)
Ultimate tensile strength (MPa)
% Elongation
Hardness HRA
426 366 447 386
666 530 694 583
4.91 5.54 2.44 4.52
66 56 61 57
Table 6 Mechanical properties of different austeno-ferritic stainless steels sintered at 1350 ◦ C for 4 h in argon atmosphere. Composition
Fig. 7. XRD patterns of austeno-ferritic stainless steels sintered in argon atmosphere.
A B C D
Compaction pressure 560 MPa Yield strength (MPa)
Ultimate tensile strength (MPa)
% Elongation
Hardness HRA
404 336 376 332
644 512 556 532
9.02 5.08 4.5 7.12
64 56 59 57
R. Mariappan et al. / Materials Science and Engineering A 517 (2009) 328–333 Table 7 Ferrite content of austeno-ferritic stainless steels sintered in different atmospheres. Composition
Sintered in nitrogen atmosphere (% ferrite)
Sintered in argon atmosphere (% ferrite)
A B C D
21 18 27 21
38 24 23 17
with concomitant loss of ductility (2.44% elongation) as is evident from XRD patterns (Fig. 6). From Fig. 9, composition A exhibits more tensile strength (644 MPa), better ductility (9.03% elongation) and hardness (64 HRA) compared to other compositions. In general the steels sintered in nitrogen atmosphere exhibited an improvement in UTS by 5–8% for compositions A and B, 25–32% for compositions C and D. This shows mechanical properties of the austenoferritic stainless steels depend on composition and sintering atmospheres. 3.4. Ferrite content The determination of ferrite percentage in P/M austeno-ferritic stainless steels is carried out by magnetic measurements by using Fischer’s ferrite scope and the details are given in Table 7. The amount of ferrite measured in the composition C for the stainless steels sintered in nitrogen atmosphere with an average of 27%. Similarly more quantity of ferrite (average of 38%) is observed in the composition A for the stainless steels sintered in argon atmosphere. In both sintering atmospheres, compositions B and D exhibit lower amount of ferrite compared to other compositions. This is due to the addition of copper, which leads to the formation of more austenite. 4. Conclusions In the present work, effect of sintering atmosphere on densification behaviour, microstructure and mechanical properties was studied. The salient features of the present study are summarized as follows:
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• Densification rate of austeno-ferritic stainless steels sintered in argon atmosphere is better (10–21%) than the steels sintered in nitrogen atmosphere (8–14%). • Sintering of stainless steels in nitrogen atmosphere reveals lamellar constituents with grain boundary Cr2 N in the ferritic matrix, whereas sintering in argon atmosphere shows only bi-phase structure. • Austeno-ferritic stainless steels sintered in nitrogen atmosphere exhibit higher strength and lower elongation than the steels sintered in argon atmosphere. • XRD patterns for compositions B and D for both sintering atmospheres show better intensity for austenitic peaks due to the addition of copper. Acknowledgements The authors would like to thank Dr. M. Chidambaram, Director, NIT, Tiruchirappalli 620015 for providing financial support and permission to publish this research paper. Authors also would like to acknowledge the assistance rendered by Mr. T. Gnana Prakasam, Mechanic-A and Mr. S. Arumugasamy, Technical Assistant, Dept. of MME, NIT, Tiruchirappalli for their help during experimental work. References [1] P. Datta, G.S. Upadhyaya, Materials Chemistry and Physics 67 (2001) 234–242. ´ [2] L.A. Dobrzanski, Z. Brytan, M. Actis Grande, M. Rosso, E.J. Pallavicini, Journal of Materials Processing Technology 157–158 (2004) 312–316. [3] P.K. Samal, J.B. Terrell, Metal Powder Report (2001) 28–34. [4] A.J. Rawlings, H.M. Kopech, H.G. Rutz, Proceedings of International Conference on Powder Metallurgy & Particulate Materials PM2TEC’97, Chicago, USA, 1997. [5] D. Schaefer, C.J. Trombino, State of North American P/M industry 2005, PM2TEC’ Montreal, Canada, 2005. [6] C.J. Munez, M.V. Utrilla, A. Urena, Journal of Alloys and Compounds (2007) 1–23. [7] J. Kaziora, M. Nykiela, T. Pieczonkab, T. Marcu Puscasc, A. Molinaric, Journal of Materials Processing Technology 157–158 (2004) 712–717. [8] Olena Smuk, Microstructure and properties of modern P/M super duplex stainless steel, Doctoral thesis, Dept. of Mat. Sci.and Eng., Royal Institute of Technology, Sweden, 2004, pp. 1–5. [9] F. Iacoviello, M. Boniardi, G.M. La Vecchia, International Journal of Fatigue 21 (1999) 957–963. [10] R. Dakhlaui, A. Baczman’ski, C. Braham, S. Wron’ski, K. Wierzbanowski, E.C. oliver, Acta Meteriallia 54 (2006) 5027–5039. [11] C. García, F. Martín, P. de Tiedra, L. García Cambronero, Corrosion Science 49 (2007) 1718–1736.