Anal. Chem. 1998, 70, 2866-2869
Levitation Melting/Thermoconductometric Method for Determination of Nitrogen in Steel Masayuki Nishifuji,† Masanori Fujinami,† Akihiro Ono,† and Koichi Chiba*,‡
Materials Characterization Laboratory, Nippon Steel Corporation, 3-35-1 Ida, Nakahara, Kawasaki 211, Japan, and Department of Applied Chemistry, School of Engineering, Nagoya University, Nagoya 464-01, Japan
A levitation melting/thermoconductometric method for determination of nitrogen in steel has been developed, in which metal samples are melted in a state of floatation by magnetic pressure. The present method needs no crucible, so it is free from contamination from crucibles. Nitrogen extracted from a steel sample in a state of floatation was measured quantitatively with a thermal conductivity detector. About 5 min was required for nitrogen to be sufficiently extracted, versus only about 2 min for hydrogen. This difference was explained in terms of their diffusion velocities in molten iron. The calibration curve for nitrogen was linear to 156 µg/g, with a relative standard deviation of 6.5% at the 20 µg/g level. The present method was applied to determination of nitrogen in certified reference material JSS GS-5a, and the analytical result (17.5 ( 1.1 µg/g) was in good agreement with the certified value (17.0 µg/g). Vacuum degassing technology in steel-making processes has been remarkably improved in the last two decades, so that concentration levels of nitrogen, hydrogen, oxygen, and carbon are reduced to about 20, 1, 20, and 20 µg/g, respectively, in highly purified iron such as interstitial free iron.1 Some mechanical properties of steel are strongly dependent on even very low concentrations of nitrogen.2,3 For example, the tensile strength and yield stress of steel increase with increases in nitrogen concentration, whereas the elongation and drawing properties of steel decrease. And nitrogen even in trace levels also causes the age hardening of steels. It is very important to reduce nitrogen content in steel when producing some special steels that are formed into complicated shapes, such as car bodies. It is expected that nitrogen concentration in steel will be as low as 5 µg/g at the beginning of the 21st century.4 Nitrogen in steel is conventionally determined by the inert gas carrier fusion/thermoconductometric method (IF-TC).5 In this method, a specimen is heated and melted in a graphite crucible in an inert gas atmosphere, and the discharged nitrogen is †
Nippon Steel Corp. Nagoya University. (1) Akisue, O.; Hada, T. Nippon Steel Tech. Rep. 1994, 354, 1-5. (2) A Handbook of Iron and Steel Materials; Iron and Steel Institute of Japan: Tokyo, Japan, 1967; p 24. (3) Enzian. G. H. J. Met. 1950, 188, 346-351. (4) Sasabe, M. Refining Limits of Impurities in Steel and Progresses in Steelmaking Art; Iron and Steel Institute of Japan: Tokyo, Japan 1992; pp 3-25. (5) Jpn. Ind. Stand. 1990, JIS-Z-2614. ‡
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detected quantitatively with a thermal conductivity detector (TCD). The determination limit of this method is 3-5 µg/g.6 However, satisfactory determination of low nitrogen levels is difficult by this method. The specimen melted during analysis erodes and reacts with the crucible materials, releasing contaminant materials from the inside of the crucible even if it has been sufficiently preconditioned. It is quite difficult to eliminate crucible contamination, so development of a new analytical method for ultratrace levels of nitrogen in steel is strongly desired. The levitation melting method, in which metals are melted in a state of floating, is used in the research fields of metal refining and metallurgical reactions.7,8 The levitation melting method floats and melts a metal specimen by a magnetic pressure generated by application of a high-frequency magnetic field to the specimen placed inside an induction coil. Thus, the metallic specimen can be melted without contact with other materials, such as crucibles. In a previous study,9 the present authors proposed a method for the determination of ultratrace levels of hydrogen in steel with levitation melting and a TCD. In this paper, the levitation melting/ thermoconducitomeric method is applied to the quantitative determination of very low levels of nitrogen in steels, and the extraction behavior of nitrogen from molten steel has been investigated in comparison to the hydrogen extraction. EXPERIMENTAL SECTION Analytical System. The schematic diagram of the analytical system is shown in Figure 1. The system was constructed in our laboratory, and its details and the principle of levitation melting were described in a previous paper.9 The system consists of a nitrogen extraction part using the levitation melting method, a carrier gas supply line, columns for removing components interfering with nitrogen detection, and a detector. In the nitrogen extraction part, an extraction cell was installed in the center of an induction coil with a 0.2-MHz radio frequency power supply (Fuji-Denpa-Koki Co., Saitama, Japan). The cell was transparent quartz with i.d. and length of 16 and 200 mm, respectively. The quartz sample holder holds the sample before flotation and after analysis and can be elevated as much as 30 mm to set the specimen at the proper cell position. The specimen temperature during levitation melting was measured with a twocolor pyrometer (Chino Co., Japan). The extraction cell and the (6) Inamoto, I. Bunseki 1990, 328-336. (7) Sassa, K.; Asai, S. Netsusyori 1990, 30 (2), 80-86. (8) Richard Weber, J. K.; Krishma, S.; Nordine, P. J. Met. 1991, 43 (7), 8-14. (9) Nishifuji, M.; Ono, A.; Chiba, K. Anal. Chem. 1996, 68, 3300-3303. S0003-2700(97)01326-7 CCC: $15.00
© 1998 American Chemical Society Published on Web 06/04/1998
Figure 1. Schematic diagram of levitation melting/thermoconductometric system.
induction coil were placed in an insulated box for safety from electric shock and sample heat. The sample could be observed through a cobalt blue glass window of the box during levitation. A thermal conductivity detector (TCD) was used for the detection of N2 gas. The buffer column filled with Gaskuropac 54 (GL Science Co., Japan) was installed just before the detector to suppress fluctuation of the carrier gas pressure due to rapid changes in temperature and pressure on heating of samples, and to separate any CO, CO2, H2, and water vapor what was not removed by the separation columns mentioned below. Gas supply lines connected a gas cylinder, an extraction cell, an electric furnace, removal columns, and a detector in series. There was an injection port to inject N2 standard gas just before the extraction cell. High-purity helium gas (Nippon Sanso Co., Japan) was used as carrier gas after removing water vapor by passing through a column filled with 5-Å molecular sieves (GL Science Co.). Gas flow rate was controlled by a needle valve and flow meter before the extraction cell. When a steel sample was heated and melted to extract nitrogen in an inert gas atmosphere, other gases, such as CO, CO2, H2, and water vapor, are also extracted and must be removed because they would produce a TCD response. As can be seen in Figure 1, an electric furnace and two columns were provided in the exit side of the extraction cell to remove those gases. The electric furnace was filled with CuO heated at 550 °C and oxidized CO and H2 to CO2 and H2O; the first column, filled with molecular sieves, removes CO2, and the second, filled with magnesium perchlorate, removes water vapor, leaving only the extracted nitrogen to be carried to the TCD by helium gas flow. Analytical Procedure. A steel specimen was placed in the nitrogen extraction cell, which was then thoroughly purged with He gas. Radio frequency current was applied to the levitation coil under a continuous flow of He gas in the cell. As the current was gradually raised to 2 A with a voltage of 7 kV for 10 s, the sample began floating according to the levitation principle and started melting by the Joule heat produced from resistance of iron to the induced current.9 The sample holder was withdrawn
downward to the bottom about 20 mm below the coil, as soon as the sample raised from the holder. Nitrogen extracted from the melted sample was led to TCD for detection and quantitative determination. Samples. The chemical compositions of the steel samples used are listed in Table 1. JSS GS-6b, JSS GS-5b, and JSS GS-2b are certified reference materials issued from the Iron and Steel Institute of Japan (ISIJ). Their nitrogen contents were determined by the alkalimetric method after distillation.10 The other samples are working reference materials from Nippon Steel Corp., which are used for the calibration of conventional nitrogen determination systems routinely. Their nitrogen contents were determined by the IF-TC method. The concentrations of other elements of all samples were determined by conventional spark emission spectrometry. Gaseous element analysis of steel conventionally needs more than 1 g of sample in order to avoid the effects of segregation of the element. The certified reference materials from ISIJ are spherical-shaped drops, so 1-1.5 g of a drop was used as a sample specimen. The working reference materials, which were rods in shape, were cut into cylinders of 6 mm diameter and 6 mm length and weighed about 1.5 g. Specimens were polished with a grinder to remove oxide films on the surface and rinsed with ethanol in an ultrasonic bath for 3 min before measurement. RESULTS AND DISCUSSION Detection of Nitrogen with Levitation Melting System. The optimum conditions for nitrogen detection were examined with N2 standard gas. The tentative optimum carrier gas flow rate was found to be 40 mL/min, which was to be a maximum value for the present system. A higher gas flow rate should give a better shaped plug flow, but our system could not stand up to higher pressure. Figure 2 shows the plug flow obtained under the above conditions, when 100 µL of N2 standard gas was injected. This amount corresponds to about 110 µg/g of nitrogen in steel for a 1-g specimen. The plug flow shows broadening of peaks, tailing in particular. This may be due to diffusion of nitrogen gas in the extraction cell, the electric furnace, the removal columns, and the buffer column. Therefore, nitrogen concentration was determined by measuring peak area intensity. Replicate measurements of 10 µL of N2 standard gas (corresponding to 11 µg/g of nitrogen in 1 g of steel) had a standard reference deviation of about 1% (n ) 10). It is, thus, found that the proposed system has the ability to be applied to the determination of nitrogen extracted from steel. Figure 2 also shows a typical plug flow of nitrogen extracted from steel with levitation melting for 5 min for certified reference material JSS GS-2b with 156 µg/g nitrogen concentration. The specimen temperature rose to a maximum of 2000 K in 20 s after starting the current application and was maintained at 2000 K during the levitation melting. Only a peak corresponding to N2 appeared in the plug flow, and no signals originating from H2, CO, CO2, and H2O were observed. It was verified that those gases were completely removed during passage through the electric furnace and removal columns. Extraction Behavior of Nitrogen from Steel with Levitation Melting System. As can be seen in Figures 2, the peak shape of nitrogen extracted was much broader than that of N2 standard (10) Jpn. Ind. Stand. 1980, JIS-G-1228.
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Table 1. Chemical Composition of Samples (wt %) sample
N
C
Si
Mn
P
S
JSS GS6b JSS GS-2b JSS GS-5a A B
0.005 0.0156 0.0017 0.0095 0.0005
0.99 0.30 0.13 0.45 0.005
0.21 0.18 0.06 0.20
