- Email: [email protected]
Nano-ferrite formation and strain-induced-ferrite transformation in an SUS316L austenitic stainless steel
Scripta mater. 44 (2001) 2039 –2042 www.elsevier.com/locate/scriptamat
NANO-FERRITE FORMATION AND STRAIN-INDUCEDFERRITE TRANSFORMATION IN AN SUS316L AUSTENITIC STAINLESS STEEL H. Fujiwara, H. Inomoto*, R. Sanada* and K. Ameyama** Department of Environmental Systems Engineering, Kochi University of Technology, Tosayamadacho, Kochi 782-8502, Japan *Graduate School, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan **Department of Mechanical Engineering, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan (Received August 21, 2000) (Accepted November 28, 2000) Keywords: Powder processing; TEM; Steels; Phase transformations
Introduction Grain refinement is an effective method for an improvement of the mechanical property. Recently the grain refinement by the powder metallurgy process has received much attention. The high strain powder metallurgy (HS-PM) process is one of the new processes combining mechanical milling, mechanical alloying, heat treatment and sintering processes [1–3]. The process has three interesting features. Firstly a heavy cold deformation by the milling treatment results in a non-equilibrium phase. Secondary a nano grain structure can be obtained from non-equilibrium phases by controlling the heat treatment condition, which influences the phase transformation, recovery, recrystallization and grain growth. Lastly it has a good workability because of the fine grain structure. Therefore, the HS-PM process is the most efficient and useful non-equilibrium powder metallurgy process because both an improvement of mechanical property and workability by the control of microstructure are simultaneously possible. In this study, the HS-PM process was applied to an SUS316L austenitic stainless steel powder, and the nano ferrite grain structure formation is discussed.
Experimental Procedure An SUS316L austenitic stainless steel powder (C: 0.013, Si: 0.25, Mn: 1.69, P: 0.035, S: 0.011, Ni: 12.09, Cr: 16.26, Mo: 2.01, Fe: bal. (mass%)) was introduced an extremely heavy deformation with the mechanical milling equipment. This SUS316L powder was produced by the plasma rotating electrode process (PREP) and has an average particle size of 1.05 mm. PREP has the advantage that the product powder is hardly contaminated by impurities such as oxygen or nitrogen gases during the process [4]. A planetary ball mill with SUS316L stainless steel vials and SUS304 stainless steel balls was used for mechanical milling under an Ar atmosphere. A milling time was 360 ks and 720 ks. The milling temperature was estimated as less than 323 K because of cooling fins attached to the vials and a strong ventilating system [1,2]. The milled powders were characterized by means of X-ray diffraction (XRD), SEM and TEM. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00858-2
2040
AUSTENITIC STAINLESS STEEL
Vol. 44, Nos. 8/9
Figure 1. SEM micrographs of (a) cross section and (b) surface appearance of the PREP powder milled for 360 ks.
Results and Discussion Figure 1 shows SEM micrographs of (a) cross section and (b) surface appearances of the PREP powder milled for 360 ks. In Figure 1 (a) it is observed that the interior structure consists of dendrite and it is obviously different from the vicinity of the surface. The rough surface of the PREP particle shown in Figure 1 (b) indicates that the HS-PM process introduced an extremely heavy deformation to the PREP powder. Figure 2 shows XRD patterns of (a) cross section and (b) surface of the PREP powder milled for 360 ks. XRD peak profiles of FCC are observed in Figure 2 (a), while XRD peak profiles of BCC in Figure 2 (b). Figures 1 and 2 indicate that an interior of the PREP powder particle consists of an austenite phase and the microstructure in the vicinity of the surface is composed of a ferrite phase. The sections that follow provide a more detailed description of a microstructure in the vicinity of the surface. Figure 3 shows TEM micrograph of the region that has a distance of approximately 15 m from the surface of the PREP powder milled for 360 ks. This microstructure can be classified into two areas. These areas are named as (A) and (B) in order of increasing distance from the surface. The fine grain structure is formed in area (A). The microstructure with elongated grains is observed in area (B). The microstructure in the vicinity of the surface of the PREP powder shown in Figure 1 consists of a nano BCC structure that is formed by an extremely heavy deformation of the HS-PM process. Figure 4 demonstrates TEM micrograph of area (A) shown in Figure 3. Ultra fine grains of approximately 15 nm in diameter are observed in this area. A diffraction pattern indicates that this nano structure consists of BCC phase, that is, ferrite phase and has high angle boundaries. Figure 5 shows TEM micrograph of the interface area between (A) and (B) shown in Figure 3. Narrow grains with the size of approximately 20 nm in width are observed and they seem to have a
Figure 2. XRD results of (a) cross section and (b) surface of the PREP powder milled for 360 ks.
Vol. 44, Nos. 8/9
AUSTENITIC STAINLESS STEEL
2041
Figure 3. TEM micrograph of SUS316L milled for 360ks. (⬃15m from the surface).
bamboo like structure; that is, the narrow grains have been sectioned to several nano grains by boundaries. Grains X and Y are the one of those grains constructing the narrow grains. Micro-microdiffraction patterns (MMDP) taken from the grains X and Y are indicated right. These MMDP were obtained under the different incident beam conditions. From the MMDP, grain X has a FCC structure while grain Y has a BCC structure. Those BCC and FCC grains are very similar in grain size and are neighboring very close, so that we can say that the BCC phase presumably formed from the FCC phase by some transformation mechanism through the grain refinement. The mechanism of such a FCC to BCC transformation can be considered as follows. The total interface energy of the austenite phase can be estimated as approximately 1200 J/mol when the austenite phase consists of the grains with average grain size of 15 nm and their interfacial energy is assumed as 0.85 J/m2 [5], respectively. In the austenite to ferrite or martensite phase transformation 200⬃1000 J/mol is required for the driving force [6], and this amount is less than the total interface energy described above. Therefore, the nano ferrite grain structure formed by the milling process can be referred to as a strain-induced-ferrite transformation.
Figure 4. TEM micrograph of area (A) shown in Figure 3.
2042
AUSTENITIC STAINLESS STEEL
Vol. 44, Nos. 8/9
Figure 5. TEM micrograph of the interface area between (A) and (B) shown in Figure 3.
Conclusion The high strain powder metallurgy (HS-PM) process was applied to an SUS316L powder. The conclusions are as follows: (1) MM process produced a new layer on the powder surface. (2) The surface layer was composed of a nano ferrite grain structure. (3) The nano ferrite grain structure with average grain size of 15 nm was considered to form by a strain-induced-ferrite transformation since the estimated total interface energy in the austenite matrix was larger than the driving force of an austenite to ferrite or martensite transformation. References 1. 2. 3. 4. 5. 6.
K. Ameyama, Scripta Metall. Mater. 38, 517 (1998). K. Ameyama, M. Hiromitsu, and N. Imai, Tetsu-to-Hagane. 84, 357 (1998). H. Fujiwara and K. Ameyama, Mater. Sci. Forum. 304 –306, 47 (1999). K. Isonishi, K. Ameyama, M. Tokizane, and R. Kumagaya, in Proceedings of the 1993 Powder Metallurgy World Congress, Kyoto, 31 (1993). L. H. Van Vlack, Trans. AIME. 191, 251 (1951). T. Maki, Nishiyama Memorial Lecture, No. 161–162, 3 (1996).
NANO-FERRITE FORMATION AND STRAIN-INDUCEDFERRITE TRANSFORMATION IN AN SUS316L AUSTENITIC STAINLESS STEEL H. Fujiwara, H. Inomoto*, R. Sanada* and K. Ameyama** Department of Environmental Systems Engineering, Kochi University of Technology, Tosayamadacho, Kochi 782-8502, Japan *Graduate School, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan **Department of Mechanical Engineering, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan (Received August 21, 2000) (Accepted November 28, 2000) Keywords: Powder processing; TEM; Steels; Phase transformations
Introduction Grain refinement is an effective method for an improvement of the mechanical property. Recently the grain refinement by the powder metallurgy process has received much attention. The high strain powder metallurgy (HS-PM) process is one of the new processes combining mechanical milling, mechanical alloying, heat treatment and sintering processes [1–3]. The process has three interesting features. Firstly a heavy cold deformation by the milling treatment results in a non-equilibrium phase. Secondary a nano grain structure can be obtained from non-equilibrium phases by controlling the heat treatment condition, which influences the phase transformation, recovery, recrystallization and grain growth. Lastly it has a good workability because of the fine grain structure. Therefore, the HS-PM process is the most efficient and useful non-equilibrium powder metallurgy process because both an improvement of mechanical property and workability by the control of microstructure are simultaneously possible. In this study, the HS-PM process was applied to an SUS316L austenitic stainless steel powder, and the nano ferrite grain structure formation is discussed.
Experimental Procedure An SUS316L austenitic stainless steel powder (C: 0.013, Si: 0.25, Mn: 1.69, P: 0.035, S: 0.011, Ni: 12.09, Cr: 16.26, Mo: 2.01, Fe: bal. (mass%)) was introduced an extremely heavy deformation with the mechanical milling equipment. This SUS316L powder was produced by the plasma rotating electrode process (PREP) and has an average particle size of 1.05 mm. PREP has the advantage that the product powder is hardly contaminated by impurities such as oxygen or nitrogen gases during the process [4]. A planetary ball mill with SUS316L stainless steel vials and SUS304 stainless steel balls was used for mechanical milling under an Ar atmosphere. A milling time was 360 ks and 720 ks. The milling temperature was estimated as less than 323 K because of cooling fins attached to the vials and a strong ventilating system [1,2]. The milled powders were characterized by means of X-ray diffraction (XRD), SEM and TEM. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00858-2
2040
AUSTENITIC STAINLESS STEEL
Vol. 44, Nos. 8/9
Figure 1. SEM micrographs of (a) cross section and (b) surface appearance of the PREP powder milled for 360 ks.
Results and Discussion Figure 1 shows SEM micrographs of (a) cross section and (b) surface appearances of the PREP powder milled for 360 ks. In Figure 1 (a) it is observed that the interior structure consists of dendrite and it is obviously different from the vicinity of the surface. The rough surface of the PREP particle shown in Figure 1 (b) indicates that the HS-PM process introduced an extremely heavy deformation to the PREP powder. Figure 2 shows XRD patterns of (a) cross section and (b) surface of the PREP powder milled for 360 ks. XRD peak profiles of FCC are observed in Figure 2 (a), while XRD peak profiles of BCC in Figure 2 (b). Figures 1 and 2 indicate that an interior of the PREP powder particle consists of an austenite phase and the microstructure in the vicinity of the surface is composed of a ferrite phase. The sections that follow provide a more detailed description of a microstructure in the vicinity of the surface. Figure 3 shows TEM micrograph of the region that has a distance of approximately 15 m from the surface of the PREP powder milled for 360 ks. This microstructure can be classified into two areas. These areas are named as (A) and (B) in order of increasing distance from the surface. The fine grain structure is formed in area (A). The microstructure with elongated grains is observed in area (B). The microstructure in the vicinity of the surface of the PREP powder shown in Figure 1 consists of a nano BCC structure that is formed by an extremely heavy deformation of the HS-PM process. Figure 4 demonstrates TEM micrograph of area (A) shown in Figure 3. Ultra fine grains of approximately 15 nm in diameter are observed in this area. A diffraction pattern indicates that this nano structure consists of BCC phase, that is, ferrite phase and has high angle boundaries. Figure 5 shows TEM micrograph of the interface area between (A) and (B) shown in Figure 3. Narrow grains with the size of approximately 20 nm in width are observed and they seem to have a
Figure 2. XRD results of (a) cross section and (b) surface of the PREP powder milled for 360 ks.
Vol. 44, Nos. 8/9
AUSTENITIC STAINLESS STEEL
2041
Figure 3. TEM micrograph of SUS316L milled for 360ks. (⬃15m from the surface).
bamboo like structure; that is, the narrow grains have been sectioned to several nano grains by boundaries. Grains X and Y are the one of those grains constructing the narrow grains. Micro-microdiffraction patterns (MMDP) taken from the grains X and Y are indicated right. These MMDP were obtained under the different incident beam conditions. From the MMDP, grain X has a FCC structure while grain Y has a BCC structure. Those BCC and FCC grains are very similar in grain size and are neighboring very close, so that we can say that the BCC phase presumably formed from the FCC phase by some transformation mechanism through the grain refinement. The mechanism of such a FCC to BCC transformation can be considered as follows. The total interface energy of the austenite phase can be estimated as approximately 1200 J/mol when the austenite phase consists of the grains with average grain size of 15 nm and their interfacial energy is assumed as 0.85 J/m2 [5], respectively. In the austenite to ferrite or martensite phase transformation 200⬃1000 J/mol is required for the driving force [6], and this amount is less than the total interface energy described above. Therefore, the nano ferrite grain structure formed by the milling process can be referred to as a strain-induced-ferrite transformation.
Figure 4. TEM micrograph of area (A) shown in Figure 3.
2042
AUSTENITIC STAINLESS STEEL
Vol. 44, Nos. 8/9
Figure 5. TEM micrograph of the interface area between (A) and (B) shown in Figure 3.
Conclusion The high strain powder metallurgy (HS-PM) process was applied to an SUS316L powder. The conclusions are as follows: (1) MM process produced a new layer on the powder surface. (2) The surface layer was composed of a nano ferrite grain structure. (3) The nano ferrite grain structure with average grain size of 15 nm was considered to form by a strain-induced-ferrite transformation since the estimated total interface energy in the austenite matrix was larger than the driving force of an austenite to ferrite or martensite transformation. References 1. 2. 3. 4. 5. 6.
K. Ameyama, Scripta Metall. Mater. 38, 517 (1998). K. Ameyama, M. Hiromitsu, and N. Imai, Tetsu-to-Hagane. 84, 357 (1998). H. Fujiwara and K. Ameyama, Mater. Sci. Forum. 304 –306, 47 (1999). K. Isonishi, K. Ameyama, M. Tokizane, and R. Kumagaya, in Proceedings of the 1993 Powder Metallurgy World Congress, Kyoto, 31 (1993). L. H. Van Vlack, Trans. AIME. 191, 251 (1951). T. Maki, Nishiyama Memorial Lecture, No. 161–162, 3 (1996).