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Fabrication of lotus-type porous carbon steel via continuous zone melting and its mechanical properties
Materials Science and Engineering A 524 (2009) 112–118
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Fabrication of lotus-type porous carbon steel via continuous zone melting and its mechanical properties M. Kashihara a,∗ , H. Yonetani a , T. Kobi a , S.K. Hyun b,1 , S. Suzuki b , H. Nakajima b a b
Mori Seiki Co. Ltd., 201, MIDAI, Iga, Mie 519-1414, Japan The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan
a r t i c l e
i n f o
Article history: Received 2 September 2008 Received in revised form 4 June 2009 Accepted 8 June 2009
Keywords: Lotus-type porous carbon steel Nitrogen Porous metals Porosity Continuous zone melting Tensile strength Yield strength Young’s modulus Solid-solution hardening
a b s t r a c t Lotus-type porous carbon steel (lotus carbon steel) AISI1018 rods with long cylindrical pores aligned in one direction were fabricated using the continuous zone melting technique under nitrogen gas pressure of 2.5 MPa. The porosity decreased with increasing transference velocities of 40–160 m s−1 . Tensile tests of the fabricated lotus-type carbon steel rods were performed. The elongation of lotus carbon steel increased after normalizing at 1200 K. The tensile strength and the Young’s modulus decreased with increasing porosity. In contrast, the yield strength of lotus carbon steel did not decrease, even with a porosity of 20%, compared with that of non-porous carbon steel. This superior characteristic is attributed to solid-solution strengthening by solute nitrogen. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, machine tools have shown remarkable progress with regard to high-performance and high-precision machining. Lighter structures and further improvements in speed and energy conservation are now in demand. Casting materials have been used for many years for machine tools, and will be used into the future. However, further improvements in productivity depend on the development of materials for use in moving structural parts of machine tools, with characteristics such as lighter weight, good vibration damping, and improved rate of heat exchange. Recently, fabrication techniques, as well as various properties, of lotus-type porous metals have been actively investigated. This material has the potential to exhibit the characteristics [1–10] mentioned above. However, many of its characteristics remain uncharacterized. It has been reported that lotus-type porous carbon steel is a highly useful metal when considered for certain applications [10,11]. Ordinary lotus-type porous metals of a given porosity can be fabricated using hydrogen gas. However, the use of hydrogen
has drawbacks, including its combustibility and the potential for hydrogen embrittlement, which make this method less desirable for mass production. Therefore, nitrogen gas was chosen because it is inert and provides solid-solution strengthening of iron [7]. However, there are a number of problems with this technique; the thermal conductivity of carbon steel is low, and the pore morphology is not uniform in lotus carbon steel fabricated via the conventional mold casting technique (Gasar process) [3,6]. In this study, the continuous zone melting technique [1,10], which can be used to fabricate homogeneous lotus metals with a low thermal conductivity, was applied to fabricate lotus carbon steel. Lotus carbon steels were fabricated under various pressure conditions using nitrogen gas. As the solubility gap of nitrogen between liquid and solid is similar to that of hydrogen, it is expected that lotus carbon steel can be produced using nitrogen. The mechanical properties of these materials were investigated. This paper reports the results of the fabrication of lotus carbon steel using nitrogen, as well as the mechanical properties of the material. 2. Experimental procedure
∗ Corresponding author. Fax: +81 595 45 4365. E-mail address: [email protected] (M. Kashihara). 1 Present address: School of Materials Science and Engineering, Inha University, Inchon 402-751, Republic of Korea. 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.06.009
2.1. Fabrication of lotus carbon steel The schematic of the continuous zone melting apparatus used in this study [1,10,11] is shown in Fig. 1. It consists of an induction
M. Kashihara et al. / Materials Science and Engineering A 524 (2009) 112–118
113
Table 1 Fabrication conditions for lotus-type porous carbon steel and non-porous carbon steel. Transference velocity (m s−1 )
Gas pressure (MPa)
80
Ar 2.5
160 80 40
N2 2.5
330 330
H2 1.5 + He 1.0 H2 2.5
ular and parallel to the transference direction. After embedding in resin, the specimen surface was polished using a polishing machine (Model PRO-126, Refine Tec Co.). These cross sections were examined with an optical microscope (Model VH-6300, Keyence Co.). The porosity, p, was calculated from the bulk density, , of the lotus carbon steel and the density, 0 , of the non-porous carbon steel, using the following formula: p= Fig. 1. Schematic drawing of the apparatus of continuous zone melting technique.
(1)
The bulk density, , is given by the mass, W, and the volume, V: =
heating coil, a specimen rod and holders, which are installed in a stainless-steel pressure chamber capable of withstanding pressures up to 2.8 MPa. The specimen rod was clamped with the specimen holders, and was pulled down at a given transference velocity. The central part of the rod was heated via induction heating until it melted. The temperature around the melting zone was measured using a non-contact radiation thermometer (Tokyo Seiko Co.). In this work, nitrogen and hydrogen were dissolved into the melt, while argon and helium were used as external pressure gases. After the specimen rod was melted in the center, nitrogen or hydrogen dissolved into the melt to the equilibrium concentration. When the melt was solidified along one direction, insoluble gas evolved into the pores, creating lotus carbon steels using fabrication conditions shown in Table 1. The chemical composition of the as-received carbon steel AISI1018 was analyzed by the optical emission spectrometer (Table 2). For comparison, non-porous carbon steel, which does not have pores, was also fabricated under an argon pressure of 2.5 MPa. Some of lotus carbon steel specimens were fabricated under a hydrogen pressure of 2.5 MPa and a transference velocity of 330 m s−1 . Other lotus carbon steel specimens were fabricated in a gas mixture of hydrogen (1.5 MPa) and helium (1.0 MPa) at a transference velocity 330 m s−1 . The fabricated specimens were cut using a spark-erosion wire cutting machine (Model A320D, Sodick Co.) in directions perpendic-
0 − × 100. 0 W . V
(2)
The specimens were shredded after the tensile tests to open the pores and release the nitrogen gas. The shredded specimen was analyzed using a nitrogen analyzer (Model TC-300, LECO Co.) to determine the concentration of nitrogen dissolved in the metallic matrix. 2.2. Heat treatment The fabricated lotus carbon steel specimens were etched with 3% nitric acid alcohol, and the microstructure was observed with an optical microscope (Model BX60M, Olympus Co.). A Widmanstätten structure, with ferrite in a spicular configuration, was observed in the specimen. Such structures are generally soft and brittle, and Table 2 Chemical composition of carbon steel AISI1018 rod. Element
Mass (%)
C Si Mn P S
0.17 0.29 0.72 0.022 0.020
Fig. 2. The matrix structures before and after normalizing of lotus-type porous carbon steel fabricated under nitrogen gas of 2.5 MPa at a transference velocity of 80 m s−1 . The normalizing was carried out at 1200 K for 7.2 ks.
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strain of up to 0.1, and a speed of 0.5 mm min−1 at strains higher than 0.1. The ultimate tensile strength, UTS , is calculated by UTS =
Fmax , A0
(3)
where Fmax is the maximum load at which the sample is ruptured and A0 is the original cross-sectional area of the test specimen. The tensile strain, ε, is calculated as ε= Fig. 3. Sample shape used for tensile tests (in mm).
not suitable as a structural material without further heat treatment [11]. To improve the microstructure, the specimens were normalized in a vacuum of 1 × 10−5 Pa. First the specimen was heated for 2.4 ks, then it was kept at a constant temperature of 1200 K for 7.2 ks, and finally cooled down. After heat treatment, the Widmanstätten structure, with the spicular ferrite and pearlite composed of rough crystal grains, was changed into the standard ferrite and pearlite structures composed of refined crystal grains, as shown in Fig. 2. 2.3. Tensile tests Using a spark-erosion wire cutting machine, the tensile-test specimens were cut into the shapes illustrated in Fig. 3. The tensile direction is parallel to the transference direction, which was approximately along the pore-growth direction. For comparison, tensile-test specimens without normalizing were also prepared. Three to nine tensile tests were carried out using specimens cut from the same ingot. The tensile tests were carried out using a tensile testing machine (Model 5582, Instron Co.) at room temperature. The speed of the cross-head was set up to be 0.2 mm min−1 with
l1 − l0 , l0
(4)
where l1 is the gauge length after loading and l0 is the original gauge length before the tensile test. The yield strength, y , is the stress at the residual strain of 0.2%. The Young’s moduli of the lotus-type porous and non-porous carbon steel were calculated from the slope of the proportional region in the stress–strain curve. 3. Results and discussion 3.1. Effect of the transference velocity on the pore morphology Fig. 4 shows the cross-sectional views of the fabricated lotus carbon steel specimens perpendicular and parallel to the transference direction. It was confirmed that the pore-growth direction is parallel to the transference direction. Fig. 5 shows the dependence of the porosity and the average pore diameter of the transference velocity. The porosity and the average pore diameter decrease with increasing transference velocity. The decrease in porosity with increasing transference velocity was a much different result compared to previous results from lotus stainless steel fabricated by the continuous zone melting technique under hydrogen pressure. When hydrogen is used, the porosity is
Fig. 4. Cross sections, porosity, pore diameter and nitrogen concentration of lotus-type porous carbon steel fabricated in nitrogen gas at 2.5 MPa. Transference velocities were 40, 80 and 160 m s−1 .
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Fig. 5. Dependence of porosity and average pore diameter on transference velocity.
independent of the transference velocity [1]. This difference may be attributed to a difference in the diffusion rates for hydrogen and nitrogen. When the transference velocity increases, supercooling is enhanced so that the number of nucleation sites for pores increases. Since hydrogen diffusion is fast, insoluble hydrogen can diffuse into the pores, growing them directionally. However, since nitrogen diffusion is one order of magnitude slower than that of hydrogen, a large amount of insoluble nitrogen cannot diffuse into the pores, disabling them from growing and causing a reduction in the porosity. To discuss these results, we consider a system where NN L mol of nitrogen atoms are dissolved in liquid steel containing 1 mol of iron at just above the liquidus temperature. If the metal is solidified, some of the nitrogen atoms will remain soluble. Other nitrogen atoms are excluded from the solid due to the solubility gap of nitrogen between liquid and solid steel. Some of the excluded nitrogen atoms form pores, others escape to the atmosphere. Therefore, the number of nitrogen atoms in the solid steel, in the pores and in the atmosphere should be equal to NN L . NN L = NN S + NN pore + NN out ,
(5)
where NN S , NN pore and NN out are the number of nitrogen atoms in solid, in pores and in atmosphere, respectively. NN S and NN pore can be estimated from the experimental results. The concentration of nitrogen, N S (at%), in the solid matrix of the carbon steel can be converted from the value expressed in CN (wt%). This is shown in Fig. 4 under the assumption that the matrix is a ternary system of Fe, C and N, and that the concentration of carbon is 0.17 wt%, as shown in Table 2. As the metal contains 1 mol of iron atoms, the number of nitrogen atoms NN S can be estimated as NN S = N S
MFe CN /MN MFe = × , CFe CFe CN /MN + CC /MC + CFe /MFe
Fig. 6. Number of nitrogen atoms in pores, NN pore , in the matrix, NN S and the sum of the two, NN S + NN pore , for lotus-type porous carbon steels fabricated under nitrogen gas of 2.5 MPa. Transference velocities were 40, 80 and 160 m s−1 .
Fig. 7 shows the temperature distribution measured in the rod specimen during continuous zone melting at transference velocities of 40, 80 and 160 m s−1 . The temperature gradients in the liquid at the solid–liquid interface are similar for the three curves, and are measured to be about 32 K mm−1 . The cooling rate, dT/dt, can be estimated as the multiplication of the temperature gradient, G, and the transference velocity, v: dT = Gv. dt
(8)
The cooling rate was estimated to be 1.3, 2.6 and 5.2 K s−1 for transference velocities of 40, 80 and 160 m s−1 , respectively. Because the diffusion coefficient of nitrogen in ␦-Fe, DN (3.3 × 10−9 m2 s−1 [12]), at a temperature just below the solidus line is much lower than that of hydrogen, DH (5 × 10−8 m2 s−1 [13]), the diffusion distance of nitrogen is much shorter than that of hydrogen. The number of pore nucleation sites would increase with a faster transference velocity resulting from supercooling. However, the supply of nitrogen is not sufficient to grow the pores because of the slow diffusion of nitrogen. Insoluble nitrogen may form the clustering in the solid. This contrasts with what is seen with hydrogen.
(6)
where M and C represent the atomic weight and the concentration (wt%), respectively. The subscripts represent the elements. The number of nitrogen atoms, NN pore , which formed pores can be calculated from NN pore = 2NN2 pore =
2PN2 RTS
×
p MFe , × 1−p Fe
(7)
under the assumption that the volume change during cooling is negligible. Fig. 6 shows NN S , NN pore and the sum of the two as a function of the porosity. The sum of NN S and NN pore remains nearly constant; the increment of NN S with decreasing porosity is roughly equal to the decrease in NN pore . Thus, the decrease in porosity with increasing transference velocity is attributed to an increase in the number of nitrogen atoms in the solid.
Fig. 7. Temperature distribution during fabrication of lotus-type porous carbon steels in nitrogen gas of 2.5 MPa by the continuous zone melting technique. Transference velocities were 40, 80 and 160 m s−1 . The dashed line shows the liquidus temperature of the carbon steel.
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Fig. 8. Stress–strain curves obtained from tensile tests of lotus-type porous and non-porous carbon steels. Transference velocity is 80 m s−1 . Fig. 10. Porosity dependence of the yield strength of lotus-type porous carbon steels.
3.2. Mechanical properties of fabricated lotus carbon steels Fig. 8 shows the stress–strain curves obtained from the tensile tests for lotus-type porous and non-porous carbon steels. These results show that the maximum stress (i.e. tensile strength) decreases and the elongation increases after normalizing. This may be attributed to the refinement of crystal grains, since soft, brittle Widmanstätten with spicular ferrite is changed to a standard ferrite–pearlite structure via normalizing. It was reported in a previous work that the micro-Vickers hardness of the matrix decreases from 210–233 to 175–186 HV by normalizing [11]. It is reasonable that normalizing reduces both the tensile strength and the hardness. Fig. 9 shows the porosity dependence of the tensile strength of as-fabricated and normalized lotus carbon steels. The tensile strength decreases with increasing porosity. The dotted straight lines are the interpolation of the strength of non-porous steel and zero strength at a porosity of 100%, which are exhibited for both as-fabricated and normalized specimens. While the data obtained for as-fabricated specimens deviated from the dotted straight line, the data from normalized specimens were not as scattered. This is because the microstructures of lotus carbon steels are homogenized by the normalizing treatment.
Fig. 9. Porosity dependence of the tensile strength of lotus-type porous carbon steels. Non-porous carbon steel, lotus carbon steels with porosity up to 30% and with porosity from 40% to 60% were fabricated in argon, nitrogen, and a mixed gas atmosphere of hydrogen and helium, respectively.
The tensile strengths of specimens fabricated with nitrogen were in the vicinity of the dotted straight line. The tensile strength deviates downward when the porosity reaches 40–60%. The specimens fabricated with hydrogen have large pores with diameters of about 1 mm. The cracking of the tensile-test specimens tended to progress along the thin cell walls of pores. Thus, the stress concentration takes place in the thin cell walls, causing a decrease in the tensile strength. Fig. 10 shows the relationship between the yield strength, y , and the porosity of as-fabricated and normalized specimens. The dotted straight lines are drawn in the same manner as for the case of the tensile strength. The yield strength of lotus carbon steels, up to the porosity of 20%, is higher than that of non-porous carbon steel. The yield strength also decreases after normalizing. These results show that the yield strength of lotus carbon steel fabricated under pressurized nitrogen gas, with a porosity in the range of 10–30%, is higher than the dotted straight line. Figs. 9 and 10 show the apparent strength (stress) calculated using a 6.8 × 1.4 mm2 cross-sectional area of a specimen, including pores. Therefore, the strength of the metallic part can be calculated as: apparent strength/(1 − p/100). Figs. 11 and 12 show the calculated results for the tensile strength and the yield strength, respectively. Although the tensile strength of the metal-
Fig. 11. Porosity dependence of the tensile strength of the metal in lotus-type porous carbon steels.
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Fig. 12. Porosity dependence of the yield strength of the metal in lotus-type porous carbon steels.
lic part decreases, the yield strength increases with increasing porosity, in the range of 10–20%, for as-fabricated lotus carbon steels. On the other hand, the strength decreases when the porosity is in the 40–60% range for lotus carbon steels fabricated with hydrogen. One of the causes of the difference between the two porosity ranges is the nitrogen concentration in the matrix. The specimens with a porosity of 10–20% were fabricated by nitrogen and have 0.13 wt% nitrogen in the matrix. On the other hand, the specimens with the porosity of 40–60% were fabricated by hydrogen and have only 0–0.005 wt% nitrogen. Based on these results, the increase in yield strength of the lotus carbon steels fabricated in a nitrogen atmosphere is caused by the presence of a solid-solution of nitrogen in carbon steel [14]. Fig. 13 shows the porosity dependence of the Young’s moduli. Before and after normalizing, the Young’s moduli do not change; the Young’s modulus is not affected by heat treatment [15]. The Young’s moduli are lower than the reference values obtained from measurement of the lotus iron using resonant ultrasound spectroscopy [16]. The data scattering was caused by the tensile test, which has several error sources such as clamping and estimation of Young’s moduli
Fig. 13. Porosity dependence of the Young’s moduli of lotus-type porous carbon steels.
Fig. 14. SEM micrographs of the fracture surface after tensile tests of lotus-type porous and non-porous carbon steels. (a) Fracture surface in Widmanstätten structures, (b) pearlite colonies, (c) internal surface of pores, (d) dimple fracture surface, (e) cleavage plane, (f) deep dimple fracture surface and (g) deep dimple fracture surface.
from the stress–strain curves. For more precise measurements of the Young’s moduli, a more suitable method is necessary, such as resonant ultrasound spectroscopy [16]. The effect of normalizing on tensile strength and yield strength is apparent. Fracture surfaces in the tensile-test specimens were observed via SEM, and Fig. 14 shows secondary electron images of fracture surfaces in each tensile-test specimen. In lotus-type porous and non-porous carbon steels before normalizing, reticulate, shallow dimples and cleavage planes are shown in Fig. 14a, d and e, respectively. These types of steels have a fracture mode that is typical for brittle materials with low ductility. This is attributed to fracture formation in materials with Widmanstätten structures hardened by a nitrogen solid-solution. However, as shown in Fig. 14f and g, fracture surfaces formed after normalizing have deeper dimples, typical of fracture formation in materials of high ductility. The fracture surfaces shown in Fig. 14b are observed before and after normalizing. These are pearlite colonies, which are formed when perpendicular fracturing occurs in a pearlite structure. The type of structure shown in Fig. 14c can be seen before and after normalizing. This structure represents the internal surface of the pores, which existed before fracturing, and is unrelated to the formation of fractures. It is clear that the normalizing affects the tensile strength and the yield strength because brittle structure of the matrix is changed into a ductile one. Lotus metals have long cylindrical pores that are aligned in one direction, causing the material to have anisotropic strength [2]. The strength in the direction parallel to the pore growth is higher than the strength in the perpendicular direction. For practical use, the
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anisotropic strength and the direction of the load should be taken into account in the design of the structure. 4. Conclusion Lotus carbon steel was fabricated using the continuous zone melting technique under nitrogen gas pressure of 2.5 MPa. The nitrogen concentration in the metal of fabricated lotus carbon steel increased with decreasing porosity. The yield strength did not decrease significantly up to a porosity of 20%. The yield strength for material with a porosity up to 20% was higher than that of the non-porous carbon steel both in as-fabricated and normalized specimens. Such superior strength is attributed to the solid-solution hardening of nitrogen. Lotus carbon steels were normalized to improve the microstructure. This was necessary because the specimens had Widmanstätten structures before normalizing. These pre-normalizing microstructures were characterized by the presence of brittle modes and demonstrated significant variation in strength. However, lotus carbon steels can be used without normalizing for certain applications. Acknowledgements This study was supported by the New Energy and Industrial Technology Development Organization (NEDO) as a part of
a research program on new manufacturing technologies for the development of lightweight, high stiffness structural materials and evaluation technologies, and for the development of new structural materials for machining systems. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
T. Ikeda, T. Aoki, H. Nakajima, Metall. Mater. Trans. A 36A (2005) 77–86. H. Nakajima, Prog. Mater. Sci. 52 (2007) 1091–1173. V. Shapovalov, Proc. MRS Symp. 52 (1998) 281–290. H. Nakajima, S.K. Hyun, K. Murakami, Mater. Sci. Eng. A299 (2001) 241–248. S.K. Hyun, H. Nakajima, Mater. Trans. 43 (2002) 526–531. T. Ikeda, M. Tsukamoto, H. Nakajima, Mater. Trans. 43 (2002) 2678–2684. S.K. Hyun, T. Ikeda, H. Nakajima, Sci. Tech. Adv. Mater. 5 (2004) 201–205. Z.K. Xie, M. Tane, S.K. Hyun, Y. Okuda, H. Nakajima, Mater. Sci. Eng. A417 (2006) 129–133. H. Chiba, T. Ogushi, H. Nakajima, T. Ikeda, JSME Int. J. Ser. B 47 (2004) 516–517. T. Kujime, S.K. Hyun, H. Nakajima, Metall. Mater. Trans. 37A (2006) 393–398. M. Kashihara, S.K. Hyun, H. Yonetani, T. Kobi, H. Nakajima, Scr. Mater. 54 (2006) 509–512. P. Grieveson, E.T. Turkdogan, Trans. Metall. Soc. AIME 230 (1964) 1604–1609. H. Schenk, K.W. Lange, Arch. Eisenhütt. 37 (1966) 809–812. J.C. Rawers, Mater. Sci. Eng. A187 (1994) 167–176. R. Le Roux, Métaux 340 (1953) 489. M. Tane, T. Ichitsubo, H. Nakajima, S.K. Hyun, M. Hirao, Acta Mater. 52 (2004) 5195–5201.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Fabrication of lotus-type porous carbon steel via continuous zone melting and its mechanical properties M. Kashihara a,∗ , H. Yonetani a , T. Kobi a , S.K. Hyun b,1 , S. Suzuki b , H. Nakajima b a b
Mori Seiki Co. Ltd., 201, MIDAI, Iga, Mie 519-1414, Japan The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan
a r t i c l e
i n f o
Article history: Received 2 September 2008 Received in revised form 4 June 2009 Accepted 8 June 2009
Keywords: Lotus-type porous carbon steel Nitrogen Porous metals Porosity Continuous zone melting Tensile strength Yield strength Young’s modulus Solid-solution hardening
a b s t r a c t Lotus-type porous carbon steel (lotus carbon steel) AISI1018 rods with long cylindrical pores aligned in one direction were fabricated using the continuous zone melting technique under nitrogen gas pressure of 2.5 MPa. The porosity decreased with increasing transference velocities of 40–160 m s−1 . Tensile tests of the fabricated lotus-type carbon steel rods were performed. The elongation of lotus carbon steel increased after normalizing at 1200 K. The tensile strength and the Young’s modulus decreased with increasing porosity. In contrast, the yield strength of lotus carbon steel did not decrease, even with a porosity of 20%, compared with that of non-porous carbon steel. This superior characteristic is attributed to solid-solution strengthening by solute nitrogen. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, machine tools have shown remarkable progress with regard to high-performance and high-precision machining. Lighter structures and further improvements in speed and energy conservation are now in demand. Casting materials have been used for many years for machine tools, and will be used into the future. However, further improvements in productivity depend on the development of materials for use in moving structural parts of machine tools, with characteristics such as lighter weight, good vibration damping, and improved rate of heat exchange. Recently, fabrication techniques, as well as various properties, of lotus-type porous metals have been actively investigated. This material has the potential to exhibit the characteristics [1–10] mentioned above. However, many of its characteristics remain uncharacterized. It has been reported that lotus-type porous carbon steel is a highly useful metal when considered for certain applications [10,11]. Ordinary lotus-type porous metals of a given porosity can be fabricated using hydrogen gas. However, the use of hydrogen
has drawbacks, including its combustibility and the potential for hydrogen embrittlement, which make this method less desirable for mass production. Therefore, nitrogen gas was chosen because it is inert and provides solid-solution strengthening of iron [7]. However, there are a number of problems with this technique; the thermal conductivity of carbon steel is low, and the pore morphology is not uniform in lotus carbon steel fabricated via the conventional mold casting technique (Gasar process) [3,6]. In this study, the continuous zone melting technique [1,10], which can be used to fabricate homogeneous lotus metals with a low thermal conductivity, was applied to fabricate lotus carbon steel. Lotus carbon steels were fabricated under various pressure conditions using nitrogen gas. As the solubility gap of nitrogen between liquid and solid is similar to that of hydrogen, it is expected that lotus carbon steel can be produced using nitrogen. The mechanical properties of these materials were investigated. This paper reports the results of the fabrication of lotus carbon steel using nitrogen, as well as the mechanical properties of the material. 2. Experimental procedure
∗ Corresponding author. Fax: +81 595 45 4365. E-mail address: [email protected] (M. Kashihara). 1 Present address: School of Materials Science and Engineering, Inha University, Inchon 402-751, Republic of Korea. 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.06.009
2.1. Fabrication of lotus carbon steel The schematic of the continuous zone melting apparatus used in this study [1,10,11] is shown in Fig. 1. It consists of an induction
M. Kashihara et al. / Materials Science and Engineering A 524 (2009) 112–118
113
Table 1 Fabrication conditions for lotus-type porous carbon steel and non-porous carbon steel. Transference velocity (m s−1 )
Gas pressure (MPa)
80
Ar 2.5
160 80 40
N2 2.5
330 330
H2 1.5 + He 1.0 H2 2.5
ular and parallel to the transference direction. After embedding in resin, the specimen surface was polished using a polishing machine (Model PRO-126, Refine Tec Co.). These cross sections were examined with an optical microscope (Model VH-6300, Keyence Co.). The porosity, p, was calculated from the bulk density, , of the lotus carbon steel and the density, 0 , of the non-porous carbon steel, using the following formula: p= Fig. 1. Schematic drawing of the apparatus of continuous zone melting technique.
(1)
The bulk density, , is given by the mass, W, and the volume, V: =
heating coil, a specimen rod and holders, which are installed in a stainless-steel pressure chamber capable of withstanding pressures up to 2.8 MPa. The specimen rod was clamped with the specimen holders, and was pulled down at a given transference velocity. The central part of the rod was heated via induction heating until it melted. The temperature around the melting zone was measured using a non-contact radiation thermometer (Tokyo Seiko Co.). In this work, nitrogen and hydrogen were dissolved into the melt, while argon and helium were used as external pressure gases. After the specimen rod was melted in the center, nitrogen or hydrogen dissolved into the melt to the equilibrium concentration. When the melt was solidified along one direction, insoluble gas evolved into the pores, creating lotus carbon steels using fabrication conditions shown in Table 1. The chemical composition of the as-received carbon steel AISI1018 was analyzed by the optical emission spectrometer (Table 2). For comparison, non-porous carbon steel, which does not have pores, was also fabricated under an argon pressure of 2.5 MPa. Some of lotus carbon steel specimens were fabricated under a hydrogen pressure of 2.5 MPa and a transference velocity of 330 m s−1 . Other lotus carbon steel specimens were fabricated in a gas mixture of hydrogen (1.5 MPa) and helium (1.0 MPa) at a transference velocity 330 m s−1 . The fabricated specimens were cut using a spark-erosion wire cutting machine (Model A320D, Sodick Co.) in directions perpendic-
0 − × 100. 0 W . V
(2)
The specimens were shredded after the tensile tests to open the pores and release the nitrogen gas. The shredded specimen was analyzed using a nitrogen analyzer (Model TC-300, LECO Co.) to determine the concentration of nitrogen dissolved in the metallic matrix. 2.2. Heat treatment The fabricated lotus carbon steel specimens were etched with 3% nitric acid alcohol, and the microstructure was observed with an optical microscope (Model BX60M, Olympus Co.). A Widmanstätten structure, with ferrite in a spicular configuration, was observed in the specimen. Such structures are generally soft and brittle, and Table 2 Chemical composition of carbon steel AISI1018 rod. Element
Mass (%)
C Si Mn P S
0.17 0.29 0.72 0.022 0.020
Fig. 2. The matrix structures before and after normalizing of lotus-type porous carbon steel fabricated under nitrogen gas of 2.5 MPa at a transference velocity of 80 m s−1 . The normalizing was carried out at 1200 K for 7.2 ks.
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strain of up to 0.1, and a speed of 0.5 mm min−1 at strains higher than 0.1. The ultimate tensile strength, UTS , is calculated by UTS =
Fmax , A0
(3)
where Fmax is the maximum load at which the sample is ruptured and A0 is the original cross-sectional area of the test specimen. The tensile strain, ε, is calculated as ε= Fig. 3. Sample shape used for tensile tests (in mm).
not suitable as a structural material without further heat treatment [11]. To improve the microstructure, the specimens were normalized in a vacuum of 1 × 10−5 Pa. First the specimen was heated for 2.4 ks, then it was kept at a constant temperature of 1200 K for 7.2 ks, and finally cooled down. After heat treatment, the Widmanstätten structure, with the spicular ferrite and pearlite composed of rough crystal grains, was changed into the standard ferrite and pearlite structures composed of refined crystal grains, as shown in Fig. 2. 2.3. Tensile tests Using a spark-erosion wire cutting machine, the tensile-test specimens were cut into the shapes illustrated in Fig. 3. The tensile direction is parallel to the transference direction, which was approximately along the pore-growth direction. For comparison, tensile-test specimens without normalizing were also prepared. Three to nine tensile tests were carried out using specimens cut from the same ingot. The tensile tests were carried out using a tensile testing machine (Model 5582, Instron Co.) at room temperature. The speed of the cross-head was set up to be 0.2 mm min−1 with
l1 − l0 , l0
(4)
where l1 is the gauge length after loading and l0 is the original gauge length before the tensile test. The yield strength, y , is the stress at the residual strain of 0.2%. The Young’s moduli of the lotus-type porous and non-porous carbon steel were calculated from the slope of the proportional region in the stress–strain curve. 3. Results and discussion 3.1. Effect of the transference velocity on the pore morphology Fig. 4 shows the cross-sectional views of the fabricated lotus carbon steel specimens perpendicular and parallel to the transference direction. It was confirmed that the pore-growth direction is parallel to the transference direction. Fig. 5 shows the dependence of the porosity and the average pore diameter of the transference velocity. The porosity and the average pore diameter decrease with increasing transference velocity. The decrease in porosity with increasing transference velocity was a much different result compared to previous results from lotus stainless steel fabricated by the continuous zone melting technique under hydrogen pressure. When hydrogen is used, the porosity is
Fig. 4. Cross sections, porosity, pore diameter and nitrogen concentration of lotus-type porous carbon steel fabricated in nitrogen gas at 2.5 MPa. Transference velocities were 40, 80 and 160 m s−1 .
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Fig. 5. Dependence of porosity and average pore diameter on transference velocity.
independent of the transference velocity [1]. This difference may be attributed to a difference in the diffusion rates for hydrogen and nitrogen. When the transference velocity increases, supercooling is enhanced so that the number of nucleation sites for pores increases. Since hydrogen diffusion is fast, insoluble hydrogen can diffuse into the pores, growing them directionally. However, since nitrogen diffusion is one order of magnitude slower than that of hydrogen, a large amount of insoluble nitrogen cannot diffuse into the pores, disabling them from growing and causing a reduction in the porosity. To discuss these results, we consider a system where NN L mol of nitrogen atoms are dissolved in liquid steel containing 1 mol of iron at just above the liquidus temperature. If the metal is solidified, some of the nitrogen atoms will remain soluble. Other nitrogen atoms are excluded from the solid due to the solubility gap of nitrogen between liquid and solid steel. Some of the excluded nitrogen atoms form pores, others escape to the atmosphere. Therefore, the number of nitrogen atoms in the solid steel, in the pores and in the atmosphere should be equal to NN L . NN L = NN S + NN pore + NN out ,
(5)
where NN S , NN pore and NN out are the number of nitrogen atoms in solid, in pores and in atmosphere, respectively. NN S and NN pore can be estimated from the experimental results. The concentration of nitrogen, N S (at%), in the solid matrix of the carbon steel can be converted from the value expressed in CN (wt%). This is shown in Fig. 4 under the assumption that the matrix is a ternary system of Fe, C and N, and that the concentration of carbon is 0.17 wt%, as shown in Table 2. As the metal contains 1 mol of iron atoms, the number of nitrogen atoms NN S can be estimated as NN S = N S
MFe CN /MN MFe = × , CFe CFe CN /MN + CC /MC + CFe /MFe
Fig. 6. Number of nitrogen atoms in pores, NN pore , in the matrix, NN S and the sum of the two, NN S + NN pore , for lotus-type porous carbon steels fabricated under nitrogen gas of 2.5 MPa. Transference velocities were 40, 80 and 160 m s−1 .
Fig. 7 shows the temperature distribution measured in the rod specimen during continuous zone melting at transference velocities of 40, 80 and 160 m s−1 . The temperature gradients in the liquid at the solid–liquid interface are similar for the three curves, and are measured to be about 32 K mm−1 . The cooling rate, dT/dt, can be estimated as the multiplication of the temperature gradient, G, and the transference velocity, v: dT = Gv. dt
(8)
The cooling rate was estimated to be 1.3, 2.6 and 5.2 K s−1 for transference velocities of 40, 80 and 160 m s−1 , respectively. Because the diffusion coefficient of nitrogen in ␦-Fe, DN (3.3 × 10−9 m2 s−1 [12]), at a temperature just below the solidus line is much lower than that of hydrogen, DH (5 × 10−8 m2 s−1 [13]), the diffusion distance of nitrogen is much shorter than that of hydrogen. The number of pore nucleation sites would increase with a faster transference velocity resulting from supercooling. However, the supply of nitrogen is not sufficient to grow the pores because of the slow diffusion of nitrogen. Insoluble nitrogen may form the clustering in the solid. This contrasts with what is seen with hydrogen.
(6)
where M and C represent the atomic weight and the concentration (wt%), respectively. The subscripts represent the elements. The number of nitrogen atoms, NN pore , which formed pores can be calculated from NN pore = 2NN2 pore =
2PN2 RTS
×
p MFe , × 1−p Fe
(7)
under the assumption that the volume change during cooling is negligible. Fig. 6 shows NN S , NN pore and the sum of the two as a function of the porosity. The sum of NN S and NN pore remains nearly constant; the increment of NN S with decreasing porosity is roughly equal to the decrease in NN pore . Thus, the decrease in porosity with increasing transference velocity is attributed to an increase in the number of nitrogen atoms in the solid.
Fig. 7. Temperature distribution during fabrication of lotus-type porous carbon steels in nitrogen gas of 2.5 MPa by the continuous zone melting technique. Transference velocities were 40, 80 and 160 m s−1 . The dashed line shows the liquidus temperature of the carbon steel.
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Fig. 8. Stress–strain curves obtained from tensile tests of lotus-type porous and non-porous carbon steels. Transference velocity is 80 m s−1 . Fig. 10. Porosity dependence of the yield strength of lotus-type porous carbon steels.
3.2. Mechanical properties of fabricated lotus carbon steels Fig. 8 shows the stress–strain curves obtained from the tensile tests for lotus-type porous and non-porous carbon steels. These results show that the maximum stress (i.e. tensile strength) decreases and the elongation increases after normalizing. This may be attributed to the refinement of crystal grains, since soft, brittle Widmanstätten with spicular ferrite is changed to a standard ferrite–pearlite structure via normalizing. It was reported in a previous work that the micro-Vickers hardness of the matrix decreases from 210–233 to 175–186 HV by normalizing [11]. It is reasonable that normalizing reduces both the tensile strength and the hardness. Fig. 9 shows the porosity dependence of the tensile strength of as-fabricated and normalized lotus carbon steels. The tensile strength decreases with increasing porosity. The dotted straight lines are the interpolation of the strength of non-porous steel and zero strength at a porosity of 100%, which are exhibited for both as-fabricated and normalized specimens. While the data obtained for as-fabricated specimens deviated from the dotted straight line, the data from normalized specimens were not as scattered. This is because the microstructures of lotus carbon steels are homogenized by the normalizing treatment.
Fig. 9. Porosity dependence of the tensile strength of lotus-type porous carbon steels. Non-porous carbon steel, lotus carbon steels with porosity up to 30% and with porosity from 40% to 60% were fabricated in argon, nitrogen, and a mixed gas atmosphere of hydrogen and helium, respectively.
The tensile strengths of specimens fabricated with nitrogen were in the vicinity of the dotted straight line. The tensile strength deviates downward when the porosity reaches 40–60%. The specimens fabricated with hydrogen have large pores with diameters of about 1 mm. The cracking of the tensile-test specimens tended to progress along the thin cell walls of pores. Thus, the stress concentration takes place in the thin cell walls, causing a decrease in the tensile strength. Fig. 10 shows the relationship between the yield strength, y , and the porosity of as-fabricated and normalized specimens. The dotted straight lines are drawn in the same manner as for the case of the tensile strength. The yield strength of lotus carbon steels, up to the porosity of 20%, is higher than that of non-porous carbon steel. The yield strength also decreases after normalizing. These results show that the yield strength of lotus carbon steel fabricated under pressurized nitrogen gas, with a porosity in the range of 10–30%, is higher than the dotted straight line. Figs. 9 and 10 show the apparent strength (stress) calculated using a 6.8 × 1.4 mm2 cross-sectional area of a specimen, including pores. Therefore, the strength of the metallic part can be calculated as: apparent strength/(1 − p/100). Figs. 11 and 12 show the calculated results for the tensile strength and the yield strength, respectively. Although the tensile strength of the metal-
Fig. 11. Porosity dependence of the tensile strength of the metal in lotus-type porous carbon steels.
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Fig. 12. Porosity dependence of the yield strength of the metal in lotus-type porous carbon steels.
lic part decreases, the yield strength increases with increasing porosity, in the range of 10–20%, for as-fabricated lotus carbon steels. On the other hand, the strength decreases when the porosity is in the 40–60% range for lotus carbon steels fabricated with hydrogen. One of the causes of the difference between the two porosity ranges is the nitrogen concentration in the matrix. The specimens with a porosity of 10–20% were fabricated by nitrogen and have 0.13 wt% nitrogen in the matrix. On the other hand, the specimens with the porosity of 40–60% were fabricated by hydrogen and have only 0–0.005 wt% nitrogen. Based on these results, the increase in yield strength of the lotus carbon steels fabricated in a nitrogen atmosphere is caused by the presence of a solid-solution of nitrogen in carbon steel [14]. Fig. 13 shows the porosity dependence of the Young’s moduli. Before and after normalizing, the Young’s moduli do not change; the Young’s modulus is not affected by heat treatment [15]. The Young’s moduli are lower than the reference values obtained from measurement of the lotus iron using resonant ultrasound spectroscopy [16]. The data scattering was caused by the tensile test, which has several error sources such as clamping and estimation of Young’s moduli
Fig. 13. Porosity dependence of the Young’s moduli of lotus-type porous carbon steels.
Fig. 14. SEM micrographs of the fracture surface after tensile tests of lotus-type porous and non-porous carbon steels. (a) Fracture surface in Widmanstätten structures, (b) pearlite colonies, (c) internal surface of pores, (d) dimple fracture surface, (e) cleavage plane, (f) deep dimple fracture surface and (g) deep dimple fracture surface.
from the stress–strain curves. For more precise measurements of the Young’s moduli, a more suitable method is necessary, such as resonant ultrasound spectroscopy [16]. The effect of normalizing on tensile strength and yield strength is apparent. Fracture surfaces in the tensile-test specimens were observed via SEM, and Fig. 14 shows secondary electron images of fracture surfaces in each tensile-test specimen. In lotus-type porous and non-porous carbon steels before normalizing, reticulate, shallow dimples and cleavage planes are shown in Fig. 14a, d and e, respectively. These types of steels have a fracture mode that is typical for brittle materials with low ductility. This is attributed to fracture formation in materials with Widmanstätten structures hardened by a nitrogen solid-solution. However, as shown in Fig. 14f and g, fracture surfaces formed after normalizing have deeper dimples, typical of fracture formation in materials of high ductility. The fracture surfaces shown in Fig. 14b are observed before and after normalizing. These are pearlite colonies, which are formed when perpendicular fracturing occurs in a pearlite structure. The type of structure shown in Fig. 14c can be seen before and after normalizing. This structure represents the internal surface of the pores, which existed before fracturing, and is unrelated to the formation of fractures. It is clear that the normalizing affects the tensile strength and the yield strength because brittle structure of the matrix is changed into a ductile one. Lotus metals have long cylindrical pores that are aligned in one direction, causing the material to have anisotropic strength [2]. The strength in the direction parallel to the pore growth is higher than the strength in the perpendicular direction. For practical use, the
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anisotropic strength and the direction of the load should be taken into account in the design of the structure. 4. Conclusion Lotus carbon steel was fabricated using the continuous zone melting technique under nitrogen gas pressure of 2.5 MPa. The nitrogen concentration in the metal of fabricated lotus carbon steel increased with decreasing porosity. The yield strength did not decrease significantly up to a porosity of 20%. The yield strength for material with a porosity up to 20% was higher than that of the non-porous carbon steel both in as-fabricated and normalized specimens. Such superior strength is attributed to the solid-solution hardening of nitrogen. Lotus carbon steels were normalized to improve the microstructure. This was necessary because the specimens had Widmanstätten structures before normalizing. These pre-normalizing microstructures were characterized by the presence of brittle modes and demonstrated significant variation in strength. However, lotus carbon steels can be used without normalizing for certain applications. Acknowledgements This study was supported by the New Energy and Industrial Technology Development Organization (NEDO) as a part of
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