Ultra Purification of Electrolytic Iron by Cold Crucible Induction Melting and Induction Heating Floating Zone Melting in Ultra High Vacuum

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(1)Materials Transactions, JIM, Vol. 41, No. 1 (2000) pp. 2 to 6 Special Issue on Ultra-High Purity Metals c °2000 The Japan Institute of Metals. Ultra-Purification of Electrolytic Iron by Cold-Crucible Induction Melting and Induction-Heating Floating-Zone Melting in Ultra-High Vacuum Seiichi Takaki and Kenji Abiko Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan The effect of melting under ultra-high vacuum of 10−7 Pa on the purification of the high-purity electrolytic iron developed was investigated using a new cold-crucible induction melting furnace which was able to melt high-purity iron of 10 kg and a new induction-heating floating-zone melting furnace, which were designed and constructed using ultra-high vacuum technology. Ultra-high vacuum melting was quite useful even for the ultra-purification of iron 10 kg in weight. The high-purity electrolytic iron of 7.5 kg with residual resistivity ratio of 2000 to 2700 was melted using the cold-crucible induction melting furnace, and the pressure during melting after melt-down was kept from 1 × 10−6 to 8 × 10−7 Pa for 9 min. The 7.5 kg ultra-pure iron ingot of more than 99.9988 mass% purity after the chemical analysis of 33 elements, most of which were lower than each detection limit, was successfully obtained, even if this ingot contains their all elements of the detection limit value. Also the ultra-pure iron 35 g in weight of more than 99.9988 mass% purity was obtained from electrolytic iron of 99.996 mass% purity by the combination of cold-crucible induction melting in ultra-high vacuum and induction-heating floating-zone melting in UHV of 4 × 10−7 Pa after the analysis of 35 elements. The concentration of C + N + O + S decreases from 14.4 mass ppm before zone-leveling to 2.4 mass ppm after four zone-leveling passes in ultra-high vacuum. For further ultra-purification of iron it is absolutely necessary to develop and establish the trace analysis techniques of measuring accurately the concentration of non-metallic impurity elements at levels below 0.1 mass ppm and metallic impurity elements at levels below 0.01 or 0.001 mass ppm. (Received January 12, 2000) Keywords: high-purity electrolytic iron, ultra-high purity iron, ultra-purification, ultra-high vacuum, cold-crucible induction-melting, induction-heating floating-zone melting, electron-beam floating-zone melting, trace analysis. 1. Introduction For the fundamental research on the intrinsic properties of iron and also to determine the inherent effects of each impurity element on the properties ultra-high purity iron is essential. There have been many studies of the purification of iron in the literature,1) but few which describe the effect of ultrahigh vacuum (UHV) on it, because it is not easy to attain to ultra-high vacuum better than 1 × 10−6 Pa during melting of iron in a large melting furnace. We previously reported2) that ultra-high purity iron 15 g in weight with the residual resistivity ratio (R R R H =64 kA/m ) of more than 5000 was prepared by electron-beam floatingzone melting (EBFZM) under an ultra-high vacuum of the order of 10−7 Pa using Johnson-Matthey pure iron as starting iron bars. The production of high-purity electrolytic iron has been developed by us3) and the purity attainable is increasing year by year. On the other hand, we have made surface analysis experiments on Fe–P alloys under an ultrahigh vacuum of 4 × 10−9 Pa.4) We also designed and made a new high-temperature optical microscope using UHV technology.5) Then, on the basis of such our experience and the background mentioned above, we started to design and build new purification apparatus to prepare ultra-high purity base metal specimens, and first in UHPM-94 we reported on the construction of a new arc melting furnace by using UHV technology,6) second in UHPM-95 on the construction of a new induction-heating floating-zone melting (IHFZM) furnace7) and then on the construction of a new cold-crucible inductionmelting (CCIM) furnace8) in UHPM-97 and a part of the results of the purification effect by CCIM.9, 10) The importance of UHV in a purification apparatus is described in detail in. Ref. 6). The purpose of the present work is to investigate the effect of melting under ultra-high vacuum of 10−7 Pa on the purification of the high-purity electrolytic iron as follows and to establish a new purification procedure to obtain ultra-pure iron: (1) two kinds of electrolytic iron about 10 kg in weight are melted under UHV of 10−7 Pa in CCIM furnace and (2) the iron bars less than 50 g in weight prepared from iron ingot obtained by CCIM in UHV of 10−6 Pa are further zone-melted in UHV of 10−7 Pa using the IHFZM furnace. The UHV attainable in both our CCIM and IHFZM furnaces is better than 1 × 10−7 Pa. 2. Experimental Procedure 2.1 Starting materials Two kinds of electrolytic iron (EFe) developed by us,3) high-purity iron and higher-purity iron were used as the starting materials. The analytical results of two batches of high-purity electrolytic iron, EFe-1 and EFe-2, are shown in Table 1. The purity was more than 99.996 mass% after the analysis of 35 elements. For the higher-purity electrolytic iron, EFe-3, not chemical analysis but residual resistivity ratio (R R R H ) measurement was performed and the R R R H value was from 2000 to 2700 for 16 batchs of higher-purity electrolytic iron. 2.2 Cold-crucible induction melting The new induction melting furnace with a cold-copper crucible is described in detail in Ref. 8). This furnace can make an iron ingot up to 10 kg. The power supply provides max. 240 kW and 10 kHz. The main chamber which has two flanges of 1.4 m in diameter and a volume of about 2.3 m3 can.

(2) Ultra-Purification of Electrolytic Iron by Cold-Crucible Induction Melting and Induction-Heating Floating-Zone Melting in Ultra-High Vacuum Table 1. Results of chemical analysis of iron samples (mass ppm).. Starting iron. EFe-1. EFe-2. Iron ingot IFe-1. IFe-2. IFe-3. EFe-1+EFe-2 CCIM, >4 × 10−6 Pa. EFe-1+EFe-2 CCIM, >2 × 10−6 Pa. EFe-3 CCIM, >8 × 10−7 Pa. C 3.2 3.5 N <0.1 0.1 O 24 12 S 1.3 1.1 H 1.3 1.0 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Al 3.1 <0.2 As 1.2 0.8 B 0.6 0.3 Ba <0.1 <0.1 Bi <0.1 <0.1 Ca <0.2 <0.2 Cd <0.001 <0.001 Co 0.3 <0.1 Cr <0.3 <0.3 Cu 0.4 0.2 Ga <0.2 0.3 Hf <0.06 <0.06 Mg <0.01 <0.01 Mn <0.01 <0.01 Mo 0.1 <0.1 Nb <0.05 <0.05 Ni <0.1 0.1 P 0.1 0.7 Pb <0.01 0.12 Sb <0.3 <0.3 Se <0.04 <0.04 Si 1 1 Sn <0.4 <0.4 Ta <0.6 <0.6 Te <0.03 <0.03 Ti <0.2 <0.2 V 0.03 0.03 W <1 <1 <1 Zn 1.6 14.2 Zr <0.2 <0.2 Purity. >99.996%. 3. >99.996%. 1.0 0.7 0.6 <0.1 0.8 <0.1 7.2 6.0 1.9 0.5 0.7 0.8 0.7 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 0.2 <0.4 <0.4 1.1 1.6 1.4 0.03 0.38 0.12 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.2 <0.001 <0.001 <0.001 0.7 <0.1 0.6 0.2 0.5 <0.5 0.5 0.2 0.2 0.2 <0.3 <0.3 <0.06 <0.07 <0.07 <0.01 <0.02 <0.02 <0.01 <0.01 <0.01 0.1 <0.3 <0.3 <0.05 <0.03 <0.04 0.1 <0.1 0.2 0.1 0.3 0.3 0.04 <0.03 <0.03 <0.3 <0.3 <0.3 <0.04 <0.04 <0.04 1 1 1 <0.4 <0.5 <0.5 <0.6 <0.9 <0.9 <0.03 <0.02 <0.02 <0.1 <0.2 <0.2 0.05 <0.03 <0.03 1 1 0.6 <0.1 <0.1 0.2 <0.07 <0.07 >99.9982%. >99.9983%. >99.9988%. be evacuated to a base pressure of 6.7 × 10−8 Pa by a system of an oil diffusion pump and a cold trap. The total pressure and the partial pressure of mass number between 1 and 50 in the main chamber were measured before, during and after melting as seen below. The ingot obtained was heated in high-purity Ar atmosphere at 1150 K and then forged, rolled and machined to rods 8 mm in diameter for FZM.. flow of high-purity hydrogen or in UHV. The UHV attainable in this IHFZM furnace is better than 1 × 10−7 Pa. An example of the vacuum during each zone-leveling pass in UHV is shown in Table 3. The zone travel rate was 2.5 mm/min and the distance was about 90 mm for FZM only in UHV, about 120 mm for FZM only in hydrogen and about 55 mm for FZM in hydrogen and then in UHV. This new IHFZM apparatus is described in detail in Ref. 7).. 2.3 Induction-heating floating-zone melting Iron bars 8 mm in diameter for FZ refining were prepared from the ingot IFe-1 obtained by cold-crucible induction melting of EFe-1 and EFe-2 at the pressure of 10−6 Pa. The further purification by floating-zone melting was performed under the following conditions. Four zone-leveling passes, which means two up-and-down passes, were made in the slow. 2.4 Analysis The samples for analysis were cut from the starting materials and the ingots and bars melted in the present experiment, and polished mechanically and then electrolytically. The gaseous impurities in these samples were analyzed by combustion infrared absorption method for carbon and sulfur on the LECO CS444LS and by fusion thermal conductivity.

(3) 4. S. Takaki and K. Abiko. Fig. 1 Changes in the total pressure and the partial pressure of gases of mass numbers 2, 18, 28, 32 and 44 during induction heating of higher-purity electrolytic iron, EFe-3.. method for oxygen and nitrogen on the LECO TC43611) in the accuracy of 0.1 mass ppm in Japan Analyst Corporation. The metallic impurities in these samples were analyzed by several methods in our institute.12, 13) 3. Results and Discussion 3.1 Purification by cold-crucible induction melting A high-purity iron ingot of 8 kg, IFe-1, was made from high-purity electrolytic iron 7 kg EFe-1 and 1 kg EFe-2,9, 10) 10 kg IFe-2 from 8 kg EFe-1 and 2 kg EFe-2, and 7.5 kg IFe-3 from 7.5 kg EFe-3. Figure 1 shows the changes in the total pressure and the partial pressure of gases of mass numbers 2, 18, 28, 32 and 44, which seem to be H2 , H2 O, CO, O2 and CO2 , during induction heating and melting of higherpurity electrolytic iron, EFe-3. Roughly speaking, the total pressure and the partial pressure are two orders of magnitude lower than those in the case of IFe-1 melting (see Fig. 2 of Ref. 9)) during heating before melt-down and one order of magnitude lower during melting after melt-down. It is the point worthy of mention that the pressure during melting after melt-down for IFe-3 of weight 7.5 kg was kept from 1 × 10−6 to 8 × 10−7 Pa for 9 min, because the lowest pressure during melting was 2 × 10−6 Pa for IFe-2 and 4 × 10−6 Pa for IFe-1. The picture of the high-purity iron ingot, IFe-3, is shown in Fig. 2. The analytical results of these high-purity iron ingots made by cold-crucible induction melting in UHV are shown in Table 1. Almost every impurity but oxygen is less than 1 mass ppm and most of impurities analyzed are lower than each detection limit of analysis. The total amount of impurities was reduced from about 40 mass ppm in electrolytic iron (99.996 mass% purity) to less than 18 mass ppm in ingot IFe-1 (more than 99.9982 mass% purity) after the analysis of 35 elements, to less than 17 mass ppm in IFe-2 (more than 99.9983% purity) and to less than 12 mass ppm in IFe-3 (more than 99.9988% purity) after the analysis of 33 elements, even. Fig. 2 The high-purity iron ingot, IFe-3.. if these ingots contain all impurities analyzed of each detection limit value. 3.2 Purification by floating-zone melting Table 2 shows the changes in concentration of gaseous impurities in high-purity iron before and after floating-zone leveling in hydrogen and/or in UHV. An iron bar was melted only in UHV by FZL. The changes in pressure during the FZL passes are shown in Table 3, comparing with the results of EBFZM in UHV,2) by which we successfully prepared ultrahigh purity iron with R R R of more than 50002) and oxygen content of less than 3 mass ppm,14) which was the detection limit in those days. At the first pass the vacuum in the present FZL is already two orders of magnitude better than that in EBFZM and after four passes the pressure in the present FZL.

(4) Ultra-Purification of Electrolytic Iron by Cold-Crucible Induction Melting and Induction-Heating Floating-Zone Melting in Ultra-High Vacuum. 5. Table 2 Changes in concentration of gaseous impurities in high-purity iron before and after floating-zone leveling in hydrogen and/or in UHV (mass ppm). After FZL. Before FZL 4 passes in UHV. 4 passes in H2. 4 passes in H2 +4 passes in UHV. C N O S. 6.1 0.7 7.0 0.6. 0.2 0.0 2.0 0.2. 0.7 0.6 1.0 0.6. 0.5 0.3 0.8 0.6. C+N+O+S. 14.4. 2.4. 2.9. 2.2. Table 3. Changes in vacuum with zone-melting passes (Pa).. Pass number. The present IHFZL. EBFZM [2]. 1 2 3 4 8 10 11 12. 1 × 10−6 6 × 10−7 5 × 10−7 4 × 10−7. 1 × 10−4 1 × 10−5 7 × 10−6 3 × 10−6 1 × 10−6 4 × 10−7 4 × 10−7 4 × 10−7. Fig. 3 Carbon and nitrogen plot of ultra-pure iron after four zone-leveling passes in UHV from LECO CS444LS and TC436, respectively.. became 4×10−7 Pa, which corresponds to that after 10 passes in EBFZM. This means that the purity of the present starting iron bar is better than that of JM-iron used in the EBFZM experiment. After four zone-leveling passes in UHV, carbon and nitrogen decreased to 0.2 and 0.0 mass ppm, respectively. Figure 3 shows the signal in the carbon and nitrogen analysis,. which is almost on the level of the detection limit. Oxygen content decreased from 7.0 to 2.0 mass ppm after four passes in UHV. To decrease oxygen further, another iron bar was melted by FZL in hydrogen and the half length of the bar cut was further melted by FZL in UHV; as a result, oxygen decreases in concentration from 7.0 mass ppm to 1.0 mass ppm after four passes in hydrogen and to 0.8 mass ppm after further four passes in UHV. The melting in hydrogen is known to be more effective in reducing oxygen in high-purity iron. In the present investigation ultra-high purity iron with the concentration of C + N + O + S of 2.4, 2.9 and 2.2 mass ppm were obtained after four zone-leveling passes in UHV, in hydrogen and further four zone-leveling passes in UHV, respectively. Thus it follows that ultra-high purity iron less than 50 g in weight of more than 99.9988 mass% purity were obtained from electrolytic iron of 99.996 mass% purity by the present purification process, even if the total amount of metallic impurities of 9.1 mass ppm was not reduced during zone-melting process. 3.3 Effects of ultra-high vacuum on purification Ultra-high vacuum attained in the chamber of a melting furnace is thought to have three effects on the purification of iron; first, a direct purification effect by evaporation of impurities of which vapor pressure is high at the melting point, for example, in CCIM and EBFZM in UHV, and second, an indirect effect by less contamination of gas atmosphere by outgassing from the inner wall and parts of the chamber, for example, on arc melting, CCIM and IHFZM in gas atmosphere such as Ar, He or H2 . In addition to these effects, third, the ultra-high vacuum acts as a reducing atmosphere, because the residual gas in ultra-high vacuum is not water vapor but mostly hydrogen, if the chamber is clean and well outgassed by baking; thus, the oxygen concentration is reduced by the ultrahigh vacuum melting. These agree with the results obtained in the present experiments as seen in Tables 1 and 2. We conclude that ultra-high vacuum is very important and effective for the ultra-purification of iron, in particular for the reduction of gaseous impurities. 4. Conclusion The effect of melting under ultra-high vacuum of 10−7 Pa on the purification of the high-purity electrolytic iron was investigated using a new cold-crucible induction melting fur-.

(5) 6. S. Takaki and K. Abiko. nace and a new induction-heating floating-zone melting furnace. It is concluded that ultra-high vacuum melting is very important and effective even for the ultra-purification of iron 10 kg in weight. (1) The high-purity electrolytic iron of 7.5 kg with the residual resistivity ratio of 2000 to 2700 was melted using the cold-crucible induction melting furnace and the pressure during melting after melt-down was kept from 1 × 10−6 to 8×10−7 Pa for 9 min. The 7.5 kg ultra-pure iron ingot of more than 99.9988 mass% purity after the chemical analysis of 33 elements, most of which were lower than detection limit, was successfully obtained, even if this ingot contains their all elements of detection limit value. (2) Ultra-pure iron 35 g in weight of more than 99.9988 mass% purity were obtained from electrolytic iron of 99.996 mass% purity by the combination of cold-crucible induction melting in UHV and induction-heating floating-zone melting in UHV of 4 × 10−7 Pa after the analysis of 35 elements. The concentration of C + N + O + S decreases from 14.4 mass ppm before zone-leveling to 2.4 mass ppm after four zone-leveling passes in ultra-high vacuum. (3) For further ultra-purification of iron it is absolutely necessary to develop and establish the analysis techniques of measuring accurately the concentration of non-metallic impurity element at levels below 0.1 mass ppm and metallic impurity element at levels below 0.01 or 0.001 mass ppm. Acknowledgements We would like to acknowledge the support from the Ministry of Education, Culture and Science, and from the Program of Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Co. We wish. to thank Toho Zinc Co. Ltd. for supplying high-purity electrolytic iron. We are grateful to Mr. Y. Morimoto, Japan Analyst Co., Dr. K. Takada and the members of the analytical group in our institute for analysis of trace amounts of elements in high-purity iron and also to Mr. N. Harima and Mr. T. Nakajima for help in this experiments. REFERENCES 1) Ultra High Purity Base Metals, Proc. 1st. Int. Conf. on Ultra High Purity Base Metals (UHPM-94), ed. by K. Abiko, K. Hirokawa and S. Takaki, The Japan Institute of Metals, Sendai, (1995). 2) S. Takaki and H. Kimura: Scripta Metall., 10 (1976), 1095–1100. 3) T. Daicho, N. Arisawa and K. Abiko: Ultra High Purity Base Metals, eds. K. Abiko, K. Hirokawa and S. Takaki, The Japan Institute of Metals, (1995), pp. 397–402. 4) K. Abiko: Ultra High Purity Base Metals, eds. K. Abiko, K. Hirokawa and S. Takaki, The Japan Institute of Metals, (1995), pp. 1–26. 5) K. Abiko: Proc. JIMIS-4 Conf. on Grain Boundary Structure and Related Phenomena, Supplement to Trans. JIM, Vol. 27 (1986), pp. 633– 640. 6) R. Oiwa, Y. Ashino, S. Takaki and K. Abiko: Ultra High Purity Base Metals, eds. K. Abiko, K. Hirokawa and S. Takaki, The Japan Institute of Metals, (1995), pp. 516–519. 7) S. Takaki, Y. Ashino, Y. Morimoto, M. Tanino and K. Abiko: J. Physique (IV) C7, 5 (1995), 159–164. 8) K. Abiko, T. Nakajima, N. Harima and S. Takaki: Phys. Stat. Sol. (a), 167 (1998), 347–355. 9) K. Abiko and S. Takaki: Vacuum, 53 (1999), 93–96. 10) S. Takaki and K. Abiko: Vacuum, 53 (1999), 97–100. 11) D. Lawrenz: Ultra High Purity Base Metals, eds. K. Abiko, K. Hirokawa and S. Takaki, The Japan Institute of Metals, (1995), pp. 183– 190. 12) K. Hirokawa and K. Takada: Ultra High Purity Base Metals, eds. K. Abiko, K. Hirokawa and S. Takaki, The Japan Institute of Metals, (1995), pp. 161–170. 13) K. Takada: Phys. Stat. Sol. (a), 160 (1997), 561–565. 14) R. Onodera, S. Takaki, K. Abiko and H. Kimura: Mater. Sci. Eng., A141 (1991), 261–267..

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Figure

Table 1Results of chemical analysis of iron samples (mass ppm).

Table 1.

Results of chemical analysis of iron samples mass ppm . View in document p.2
Fig. 1Changes in the total pressure and the partial pressure of gases of mass numbers 2, 18, 28, 32 and 44 during induction heating ofhigher-purity electrolytic iron, EFe-3.

Fig 1.

Changes in the total pressure and the partial pressure of gases of mass numbers 2 18 28 32 and 44 during induction heating ofhigher purity electrolytic iron EFe 3 . View in document p.3
Fig. 2The high-purity iron ingot, IFe-3.

Fig 2.

The high purity iron ingot IFe 3 . View in document p.3
Table 2Changes in concentration of gaseous impurities in high-purity iron before and after floating-zone leveling in hydrogen and/or inUHV (mass ppm).

Table 2.

Changes in concentration of gaseous impurities in high purity iron before and after oating zone leveling in hydrogen and or inUHV mass ppm . View in document p.4
Table 3Changes in vacuum with zone-melting passes (Pa).

Table 3.

Changes in vacuum with zone melting passes Pa . View in document p.4
Fig. 3Carbon and nitrogen plot of ultra-pure iron after four zone-levelingpasses in UHV from LECO CS444LS and TC436, respectively.

Fig 3.

Carbon and nitrogen plot of ultra pure iron after four zone levelingpasses in UHV from LECO CS444LS and TC436 respectively . View in document p.4

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