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Microstructure and mechanical properties of stainless steel produced by centrifugal-SHS process
Journal of Materials Processing Technology 137 (2003) 1–4
Microstructure and mechanical properties of stainless steel produced by centrifugal-SHS process Wenjun Xi*, Sheng Yin, Hoyi Lai School of Materials Science and Engineering, University of Science and Technology, Beijing 100083, PR China
Abstract The microstructure and mechanical properties of the stainless steel layer of a composite pipe produced by the centrifugal-SHS process were investigated. The possibility of crack formation is discussed and the methods for increasing ductility and avoiding cracking are given. The influence of solution treatment on the microstructure and mechanical properties are also investigated. It is shown that the stainless steel is mainly composed of austenite phase between which a thin ferrite layer is distributed. The inter-metallic compound AlNi precipitates in the ferrite phase and sigma-phase precipitates at the phase boundaries between the austenite and ferrite. Solution treatment significantly increases the tensile strength and ductility of the stainless steel. # 2002 Published by Elsevier Science B.V. Keywords: SHS-centrifugal; Stainless steel; Microstructure; Properties
1. Introduction As a simple, rapid, and economical method, the centrifugal-SHS process or centrifugal–thermit (C–T) process has been employed to make ceramic-lined composite steel pipes that have been used in industry recently [1–3]. Using the same technique, a type of stainless steel-lined composite pipe has been explored [4,5]. The major advantage is its ultra-low carbon content in the stainless steel layer. Good corrosion resistance can be achieved. A problem with this composite pipe is of poor ductility of the stainless steel layer, its elongation being almost zero. Another problem is the cracking of the lined layer resulting from thermal stress. In this paper, the microstructure of the stainless steel layer has been investigated. The possibility of crack formation is discussed and methods of increasing ductility and avoiding cracking are given. The influence of solution treatment on the microstructure and mechanical properties are also investigated.
2. Experimental procedures A thermit powder mixture was prepared by blending Al, Fe2O3, Cr2O3, CrO3 and NiO powders, which were weighted according to the following chemical reactions (Eqs. (1)–(4)). *
Corresponding author.
0924-0136/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 1 0 5 0 - 6
In order to improve the fluidity of the molten steel, a certain amount of CaF2 powder was used Fe2 O3 þ 2Al ¼ 2Fe þ Al2 O3 ; DH ¼ 836 kJ
(1)
Cr2 O3 þ 2Al ¼ 2Cr þ Al2 O3 ; DH ¼ 530 kJ
(2)
CrO3 þ 2Al ¼ Cr þ Al2 O3 ;
DH ¼ 1094 kJ
(3)
3NiO þ 2Al ¼ 3Ni þ Al2 O3 ; DH ¼ 928 kJ
(4)
A low carbon steel pipe with an outer diameter of 76 mm, a wall thickness of 4 mm and a length of 100 mm was mounted on a centrifuge machine. The low carbon steel pipe was loaded with the thermit powder mixture, which was ignited by a tungsten filament, once a given rotational velocity of the centrifuge machine had been reached. The combustion reactions shown in Eqs. (1)–(4) occurred. During the reactions, all of the oxides were reduced by molten Al, resulting in the formation of the metals Fe, Cr, Ni and alumina. The combustion temperature was so high that all the reacted products was changed to liquid state. Under the centrifugal force field, the liquid state metals of Fe, Cr, Ni moved in the inner surface of the low carbon steel pipe and formed a stainless steel-lined layer with a normal composition of 12–14%Cr, 14–16%Ni, 70–74%Fe. The slag layer, containing alumina, at the same time, covered the stainless steel layer due to its low density. A solution treatment at temperature 1000 8C was carried out on the composite stainless steel pipe. The tensile strength and the 0.2% proof stress of the resulting stainless steel-lined
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layer was examined using a material test system (MTS). The fracture morphology and microstructure of the stainless steel-lined layer were observed by means of scanning electron microscope (SEM) and transmission electron microscope (TEM).
3. Results and discussion 3.1. Microstructure The SEM and TEM micrographs of the stainless steel layer are shown in Figs. 1 and 2. The stainless steel is composed mainly of austenite phase, which grew dendritically in the radial direction, with a thin ferrite layer being distributed at the austenite grain boundary region. Fig. 3 shows the ferrite region in which some dislocation loops appear: within the loops, particles of precipitating phase of NiAl exist. The orientation relationship between the precipitate and ferrite matrix have been determined to be: ð1 1 0ÞAlNi ==ð1 1 0Þa-Fe ;
Fig. 1. An SEM back-scattered electron image of the stainless steel layer.
½0 0 1AlNi ==½0 0 2a-Fe
The sigma-phase (FeCr) precipitates at the phase boundary between the austenite and the ferrite (Fig. 4). The sigmaphase has a simple tetragonal lattice structure with the lattice parameters of a ¼ 0:8799 nm, c ¼ 0:4556 nm. The orientation relationship between the sigma-phase and the ferrite phase is ð1 1 0Þs ==ð1 1 0Þa-Fe ;
½0 0 2s ==½0 0 2a-Fe
3.2. Inclusions and cracking The thermal expansion coefficient of the stainless steel is higher than that of the carbon steel. The stainless steel layer contracts much more than the ceramic layer and the carbon steel during cooling, which leads to tensile stress remaining in the stainless steel layer. The calculated residual tensile stress is 513 MPa, which is above the critical tensile strength of the stainless steel layer. Therefore the thermal stress is high enough to cause the stainless steel layer to crack, if it is not relaxed by plastic
Fig. 2. A TEM micrograph of the ferrite layer between the austenite grains.
deformation [4]. On the other hand, if the separation of the ceramic and metal had not been completed, the ceramic (Al2O3) would remain in the stainless steel as brittle inclusions and decrease the plasticity of the steel, causing stress concentration and brittle fracture. In the C–T process, the molten products lasted for only 10–60 s. There is not enough time to allow the oxide
Fig. 3. Showing (a) the morphology, (b) the electron diffraction pattern of the NiAl phase in the ferrite region.
W. Xi et al. / Journal of Materials Processing Technology 137 (2003) 1–4
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Fig. 4. Showing (a) the morphology, (b) the electron diffraction pattern of the s phase at the phase boundary.
inclusions to be removed from the molten steel. It is necessary to prolong the duration of the molten products and to increase the fluidity that facilitates the separation of metal and ceramic. The present experiments show that the time for the retention of the liquid state of the molten products can be increased by preheating the carbon steel pipe before the C–T process. In addition, improving the fluidity of the molten products by introducing CaF2 into the thermit is helpful in reducing the inclusions. The CaF2 also acted as a desulphurzer and dephosphorizer. After the improvement of process, the inclusions in the stainless steel were significantly reduced. The elongation of the stainless steel layer was increased from 0 to 5–8% and cracks were not observed in the plastic stainless steel. Fig. 5. A TEM micrograph of the stainless steel layer after solution treatment at 1000 8C for 20 min.
Table 1 The mechanical properties of the stainless steel layer before and after solution treatment at 1000 8C for 20 min Materials
Tensile strength (MPa)
Proof stress, 0.2% (MPa)
Elongation (%)
Before solution treatment After solution treatment
316 473
265 259
8 21
3.3. Solution treatment The TEM micrograph of the stainless steel layer after solution treatment at 1000 8C for 20 min is shown in Fig. 5. The stainless steel still consists of austenite and ferrite, but the precipitating phases in the ferrite region cannot be found. High density dislocations exist in the austenite region. The mechanical properties of the stainless steel layer after solution treatment are shown in Table 1. It can be seen that the solution treatment slightly lowers 0.2% proof stress, but significantly increases the elongation
Fig. 6. The fracture micrographs of the stainless steel layer (a) before and (b) after the solution treatment.
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W. Xi et al. / Journal of Materials Processing Technology 137 (2003) 1–4
(from 8 to 21%). This correlates with the dissolving of precipitating phases which inhibit the movement of dislocations. The solution treatment also increases the tensile strength from 316 to 473 MPa. Before the solution treatment, the stainless steel layer consists of nearly paralleled columnar austenite grains between which a thin layer ferrite exists, as shown in Fig. 2. The grain boundary strength was reduced by the sigma-phases which is distributed at the phase boundaries between the austenite and the ferrite. The tensile stress is also perpendicular to the axis of the columnar grain, thus cracking occurs at the grain boundaries first. A fracture micrograph of the stainless steel layer before the solution treatment is shown in Fig. 6(a). Parallel columnar material can be found at the fracture surface. The figure clearly displays the characteristics of crystalline fracture. After the solution treatment, the grain boundary strength was increased due to the dissolving of the sigma-phase and the elimination of columnar grains. This leads to the increase not only in tensile strength but also in ductility. Fig. 6(b) shows the fracture micrograph of a tensile test specimen after solution treatment. There are numerous dimples on the fracture surface, which indicates the characteristics of toughness fracture.
4. Conclusions 1. The stainless steel produced by C–T process is composed mainly of austenite phase which grows in columnar grains. A thin ferrite layer is distributed at the boundary region of the austenite grains. 2. The inter-metallic compound AlNi precipitates in the ferrite phase and sigma-phase precipitates at the phase boundaries between the austenite and the ferrite. 3. Cracking in the stainless steel layer can be avoided by reducing inclusions in the stainless steel layer and increasing its plasticity. 4. Solution treatment significantly increases the tensile strength and ductility of the stainless steel. References [1] A.G. Merzhanov, in: Z.A. Munir, J.B. Holt (Eds.), Proceedings of the Combustion and Plasma Synthesis of High-temperature Materials, VCH, New York, 1990, p. 1. [2] O. Odawara, J. Am. Ceram. Soc. 73 (1990) 629. [3] Y. Sheng, M. Liu, Z. Guo, Int. J. SHS 1 (1993) 69. [4] H.-P. Duan, Doctoral Dissertation, University of Science and Technology, Beijing, 1996. [5] H.-P. Duan, Y. Sheng, M. Liu, L. Hoyi, J. Mater. Sci. Lett. 15 (1996) 1060.
Microstructure and mechanical properties of stainless steel produced by centrifugal-SHS process Wenjun Xi*, Sheng Yin, Hoyi Lai School of Materials Science and Engineering, University of Science and Technology, Beijing 100083, PR China
Abstract The microstructure and mechanical properties of the stainless steel layer of a composite pipe produced by the centrifugal-SHS process were investigated. The possibility of crack formation is discussed and the methods for increasing ductility and avoiding cracking are given. The influence of solution treatment on the microstructure and mechanical properties are also investigated. It is shown that the stainless steel is mainly composed of austenite phase between which a thin ferrite layer is distributed. The inter-metallic compound AlNi precipitates in the ferrite phase and sigma-phase precipitates at the phase boundaries between the austenite and ferrite. Solution treatment significantly increases the tensile strength and ductility of the stainless steel. # 2002 Published by Elsevier Science B.V. Keywords: SHS-centrifugal; Stainless steel; Microstructure; Properties
1. Introduction As a simple, rapid, and economical method, the centrifugal-SHS process or centrifugal–thermit (C–T) process has been employed to make ceramic-lined composite steel pipes that have been used in industry recently [1–3]. Using the same technique, a type of stainless steel-lined composite pipe has been explored [4,5]. The major advantage is its ultra-low carbon content in the stainless steel layer. Good corrosion resistance can be achieved. A problem with this composite pipe is of poor ductility of the stainless steel layer, its elongation being almost zero. Another problem is the cracking of the lined layer resulting from thermal stress. In this paper, the microstructure of the stainless steel layer has been investigated. The possibility of crack formation is discussed and methods of increasing ductility and avoiding cracking are given. The influence of solution treatment on the microstructure and mechanical properties are also investigated.
2. Experimental procedures A thermit powder mixture was prepared by blending Al, Fe2O3, Cr2O3, CrO3 and NiO powders, which were weighted according to the following chemical reactions (Eqs. (1)–(4)). *
Corresponding author.
0924-0136/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 1 0 5 0 - 6
In order to improve the fluidity of the molten steel, a certain amount of CaF2 powder was used Fe2 O3 þ 2Al ¼ 2Fe þ Al2 O3 ; DH ¼ 836 kJ
(1)
Cr2 O3 þ 2Al ¼ 2Cr þ Al2 O3 ; DH ¼ 530 kJ
(2)
CrO3 þ 2Al ¼ Cr þ Al2 O3 ;
DH ¼ 1094 kJ
(3)
3NiO þ 2Al ¼ 3Ni þ Al2 O3 ; DH ¼ 928 kJ
(4)
A low carbon steel pipe with an outer diameter of 76 mm, a wall thickness of 4 mm and a length of 100 mm was mounted on a centrifuge machine. The low carbon steel pipe was loaded with the thermit powder mixture, which was ignited by a tungsten filament, once a given rotational velocity of the centrifuge machine had been reached. The combustion reactions shown in Eqs. (1)–(4) occurred. During the reactions, all of the oxides were reduced by molten Al, resulting in the formation of the metals Fe, Cr, Ni and alumina. The combustion temperature was so high that all the reacted products was changed to liquid state. Under the centrifugal force field, the liquid state metals of Fe, Cr, Ni moved in the inner surface of the low carbon steel pipe and formed a stainless steel-lined layer with a normal composition of 12–14%Cr, 14–16%Ni, 70–74%Fe. The slag layer, containing alumina, at the same time, covered the stainless steel layer due to its low density. A solution treatment at temperature 1000 8C was carried out on the composite stainless steel pipe. The tensile strength and the 0.2% proof stress of the resulting stainless steel-lined
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W. Xi et al. / Journal of Materials Processing Technology 137 (2003) 1–4
layer was examined using a material test system (MTS). The fracture morphology and microstructure of the stainless steel-lined layer were observed by means of scanning electron microscope (SEM) and transmission electron microscope (TEM).
3. Results and discussion 3.1. Microstructure The SEM and TEM micrographs of the stainless steel layer are shown in Figs. 1 and 2. The stainless steel is composed mainly of austenite phase, which grew dendritically in the radial direction, with a thin ferrite layer being distributed at the austenite grain boundary region. Fig. 3 shows the ferrite region in which some dislocation loops appear: within the loops, particles of precipitating phase of NiAl exist. The orientation relationship between the precipitate and ferrite matrix have been determined to be: ð1 1 0ÞAlNi ==ð1 1 0Þa-Fe ;
Fig. 1. An SEM back-scattered electron image of the stainless steel layer.
½0 0 1AlNi ==½0 0 2a-Fe
The sigma-phase (FeCr) precipitates at the phase boundary between the austenite and the ferrite (Fig. 4). The sigmaphase has a simple tetragonal lattice structure with the lattice parameters of a ¼ 0:8799 nm, c ¼ 0:4556 nm. The orientation relationship between the sigma-phase and the ferrite phase is ð1 1 0Þs ==ð1 1 0Þa-Fe ;
½0 0 2s ==½0 0 2a-Fe
3.2. Inclusions and cracking The thermal expansion coefficient of the stainless steel is higher than that of the carbon steel. The stainless steel layer contracts much more than the ceramic layer and the carbon steel during cooling, which leads to tensile stress remaining in the stainless steel layer. The calculated residual tensile stress is 513 MPa, which is above the critical tensile strength of the stainless steel layer. Therefore the thermal stress is high enough to cause the stainless steel layer to crack, if it is not relaxed by plastic
Fig. 2. A TEM micrograph of the ferrite layer between the austenite grains.
deformation [4]. On the other hand, if the separation of the ceramic and metal had not been completed, the ceramic (Al2O3) would remain in the stainless steel as brittle inclusions and decrease the plasticity of the steel, causing stress concentration and brittle fracture. In the C–T process, the molten products lasted for only 10–60 s. There is not enough time to allow the oxide
Fig. 3. Showing (a) the morphology, (b) the electron diffraction pattern of the NiAl phase in the ferrite region.
W. Xi et al. / Journal of Materials Processing Technology 137 (2003) 1–4
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Fig. 4. Showing (a) the morphology, (b) the electron diffraction pattern of the s phase at the phase boundary.
inclusions to be removed from the molten steel. It is necessary to prolong the duration of the molten products and to increase the fluidity that facilitates the separation of metal and ceramic. The present experiments show that the time for the retention of the liquid state of the molten products can be increased by preheating the carbon steel pipe before the C–T process. In addition, improving the fluidity of the molten products by introducing CaF2 into the thermit is helpful in reducing the inclusions. The CaF2 also acted as a desulphurzer and dephosphorizer. After the improvement of process, the inclusions in the stainless steel were significantly reduced. The elongation of the stainless steel layer was increased from 0 to 5–8% and cracks were not observed in the plastic stainless steel. Fig. 5. A TEM micrograph of the stainless steel layer after solution treatment at 1000 8C for 20 min.
Table 1 The mechanical properties of the stainless steel layer before and after solution treatment at 1000 8C for 20 min Materials
Tensile strength (MPa)
Proof stress, 0.2% (MPa)
Elongation (%)
Before solution treatment After solution treatment
316 473
265 259
8 21
3.3. Solution treatment The TEM micrograph of the stainless steel layer after solution treatment at 1000 8C for 20 min is shown in Fig. 5. The stainless steel still consists of austenite and ferrite, but the precipitating phases in the ferrite region cannot be found. High density dislocations exist in the austenite region. The mechanical properties of the stainless steel layer after solution treatment are shown in Table 1. It can be seen that the solution treatment slightly lowers 0.2% proof stress, but significantly increases the elongation
Fig. 6. The fracture micrographs of the stainless steel layer (a) before and (b) after the solution treatment.
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(from 8 to 21%). This correlates with the dissolving of precipitating phases which inhibit the movement of dislocations. The solution treatment also increases the tensile strength from 316 to 473 MPa. Before the solution treatment, the stainless steel layer consists of nearly paralleled columnar austenite grains between which a thin layer ferrite exists, as shown in Fig. 2. The grain boundary strength was reduced by the sigma-phases which is distributed at the phase boundaries between the austenite and the ferrite. The tensile stress is also perpendicular to the axis of the columnar grain, thus cracking occurs at the grain boundaries first. A fracture micrograph of the stainless steel layer before the solution treatment is shown in Fig. 6(a). Parallel columnar material can be found at the fracture surface. The figure clearly displays the characteristics of crystalline fracture. After the solution treatment, the grain boundary strength was increased due to the dissolving of the sigma-phase and the elimination of columnar grains. This leads to the increase not only in tensile strength but also in ductility. Fig. 6(b) shows the fracture micrograph of a tensile test specimen after solution treatment. There are numerous dimples on the fracture surface, which indicates the characteristics of toughness fracture.
4. Conclusions 1. The stainless steel produced by C–T process is composed mainly of austenite phase which grows in columnar grains. A thin ferrite layer is distributed at the boundary region of the austenite grains. 2. The inter-metallic compound AlNi precipitates in the ferrite phase and sigma-phase precipitates at the phase boundaries between the austenite and the ferrite. 3. Cracking in the stainless steel layer can be avoided by reducing inclusions in the stainless steel layer and increasing its plasticity. 4. Solution treatment significantly increases the tensile strength and ductility of the stainless steel. References [1] A.G. Merzhanov, in: Z.A. Munir, J.B. Holt (Eds.), Proceedings of the Combustion and Plasma Synthesis of High-temperature Materials, VCH, New York, 1990, p. 1. [2] O. Odawara, J. Am. Ceram. Soc. 73 (1990) 629. [3] Y. Sheng, M. Liu, Z. Guo, Int. J. SHS 1 (1993) 69. [4] H.-P. Duan, Doctoral Dissertation, University of Science and Technology, Beijing, 1996. [5] H.-P. Duan, Y. Sheng, M. Liu, L. Hoyi, J. Mater. Sci. Lett. 15 (1996) 1060.