Anal. Chem. 1996, 68, 3300-3303
Determination of Hydrogen in Steel by Using a Levitation Melting Method Masayuki Nishifuji,† Akihiro Ono,† and Koichi Chiba*,‡
Materials Characterization Laboratory, Nippon Steel Corporation, 1618 Ida, Nakahara, Kawasaki 211, Japan, and Department of Applied Chemistry, School of Engineering, Nagoya University, Nagoya 464-01, Japan
A new quantitative technique for determination of hydrogen in steels has been developed using levitation, which permits flotation and melting of samples by magnetic pressure under application of a high-frequency magnetic field to steel specimens. Steel samples are melted in a state of levitation in a flow of nitrogen gas. The discharged hydrogen gas is detected and measured quantitatively with a thermal conductivity detector. This method needs no crucible because the melting occurs in flotation. A cylindrical-shaped steel sample of 6 mm diameter and 6 mm length (about 1.5 g) was melted using the levitation melting technique. Radio frequency power of 1.5 kW with 200 kHz was applied to the levitation coil. Hydrogen was completely extracted within 1 min after melting. Working reference materials of stainless steel samples were analyzed in order to evaluate the present analytical method. The calibration curve showed an excellent linearity in a range of 0.4-6.7 µg/g, with a relative standard deviation of about 13% at the 1 µg/g level. Hydrogen in steel deteriorates the mechanical properties of steel products in such ways as hydrogen embrittlement, delayed cracking, and decrease in toughness. Therefore, improvements in the techniques for removing hydrogen in the steel-making process have progressed. Presently, hydrogen concentrations in steel can be decreased to as low as about 0.5 µg/g by some vacuum degassing treatments.1 Hydrogen in steel is usually determined by the inert gas carrier fusion/thermoconductometric method (IF-TC).2 The method is based on the extraction of hydrogen in steel as H2 gas by heating and melting the steel and the measurement of H2 gas. The specimen is heated and melted in a graphite crucible in a flow of inert gas, and the discharged hydrogen is detected and measured quantitatively with a thermal conductivity detector (TCD). However, the melted specimen for analysis erodes the crucible, and contaminant materials are released from the inside of the crucible even if it has been sufficiently baked. It is difficult to eliminate completely the contamination originating from crucibles. Therefore, satisfactorily reliable analysis cannot be performed in the determination of hydrogen below 0.5 µg/g for the purpose of quality evaluation and production management of highly purified steel products. There are great demands for development of a novel analytical method for determination of ultratrace levels of hydrogen in steel. †
Nippon Steel Corp. Nagoya University. (1) Sasabe, M. Refining Limits of Impurities in Steel and Progresses in Steelmaking Art; Japan Iron and Steel Institute: Tokyo, 1992; pp 3-25. (2) Jpn. Ind. Stand. 1990, JIS-Z-2614. ‡
3300 Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
The levitation melting method, which enables metals to be melted while floating, is widely used in the research field of metal refining and metallurgical reactions.3,4 The levitation melting method melts a metal specimen by 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 use of a container such as crucibles and without contact with other materials. Although levitation is effective as a means for melting of metals, it does not appear to have been applied to previous analytical techniques. In the present study, a new method has been developed using levitation melting for extraction of hydrogen in order to establish a highly accurate technique for quantitative analysis of a small content of hydrogen in steels. This paper reports on the newly designed analysis system and the results of quantitative determination of hydrogen in stainless steels. PRINCIPLE OF LEVITATION MELTING METHOD The levitation melting method is a technique for simultaneous floating and melting of conductive metals. The principle is schematically shown in Figure 1. As shown in Figure 1a, when a metallic specimen is put into an induction coil and a highfrequency current is applied to the coil, an electric current is induced in the specimen in the opposite direction of the current in the induction coil by the generated magnetic field, according to the electromagnetic induction theory. The induced highfrequency current tends to concentrate on the surface region of metal specimen. This phenomenon is well known as the skin effect. Upon interaction of the induced current on the surface with the magnetic field, Lorentz’s force acts from the surface to the center of specimen. The coil is wound in the shape of conical spiral turned upside-down, and it makes the Lorentz force weaker in the upper part and stronger in the lower part, so that the resultant force acts on the whole specimen in the upward direction. When the resultant force is balanced against the gravity, the metallic sample begins to float. In addition, the resistance to the induced current produces Joule heat, which melts the metal specimen (Figure 1b). The levitation melting method is able to melt a metal specimen floating in the air without use of any container. EXPERIMENTAL SECTION Analytical System. The construction of the analytical system is schematically shown in Figure 2. The whole system was constructed in our laboratory. The system consists of a hydrogen (3) Sassa, K.; Asai, S. Netsusyori 1990, 30 (2), 80-86. (4) Richard Weber, J. K.; Krishma, S.; Nordine, P. J. Met. 1991, 43 (7), 8-14. S0003-2700(96)00377-0 CCC: $12.00
© 1996 American Chemical Society
Table 1. Chemical Composition of Samples (Wt %)
Figure 1. Principle of levitation melting method.
Figure 2. Schematic diagram of analytical system.
extraction part, a carrier gas supply line, columns for removing the interfering components with hydrogen detection, and a detector. Hydrogen Extraction Part. The hydrogen extraction part is composed of an extraction cell, an induction coil, and a radio frequency power supply. The cell was made from transparent quartz. Its inside diameter is 16 mm and its length 250 mm, so the working volume is 40 mL. The sample holder was made from quartz to hold the sample before flotation and after analysis. It was designed to be capable of elevation as much as 30 mm. The induction coil and radio frequency power supply system used were purchased from Fuji-Denpa-Koki Co. (Saitama, Japan). The
sample
H
C
Si
Mn
P
A B C D
0.00004 0.00033 0.00067 0.00012
0.03 0.07 0.02 0.02
0.48 0.44 0.53 0.26
2.05 1.32 1.21 1.18
0.015 0.032 0.030 0.030
S
Ni
Cr
Mo
0.03 20.88 25.53 2.33 0.011 8.37 18.16 0.14 0.005 10.13 18.71 0.12 0.001 19.60 24.66 0.08
frequency of the current was 0.2 MHz. The main coils for levitation melting were five turns, and two extra turns with larger diameters were prepared for stabilization of sample floating in the upper part. Water flowed inside the induction coil to cool it. The extraction cell and the induction coil were placed in an insulated box for safety from electric shock and heat of the sample. A sample was observed through a cobalt blue glass window during levitation. Carrier Gas Supply Line. Gas supply lines were set up by stainless steel pipes and Teflon joints. Gas lines connected a gas cylinder, an extraction cell, removal columns, and a detector in series. There was an injection port to inject a hydrogen standard gas just before the extraction cell. High-purity nitrogen gas (Nippon Sanso) was used as the carrier gas after the water vapor was removed by passing the gas through a column filled with 5 Å molecular sieves. Gas flow rate was controlled by a needle valve and a flow meter before the extraction cell. Columns for Removal of Interfering Components with Hydrogen Detection. When a steel sample was heated and melted to extract hydrogen in an inert gas atmosphere, other gases such as CO, CO2, N2, and water vapor were also expected to be extracted. These gases must be removed, because they interfere with the detection of hydrogen by TCD. Three columns were provided in the exit side of the extraction cell: the first column for oxidation of CO, filled with iodine pentaoxide; the second for removal of CO2, filled with molecular sieves; and the third for removal of water vapor, filled with magnesium perchlorate. Hydrogen Detection. TCD was used for the detection of hydrogen. A stainless steel column of 2 mm i.d. and 3 m length filled with Gaskuropac 54 was installed just before the detector to suppress the fluctuation of the carrier gas pressure due to rapid changes in temperature and pressure on heating of samples. It was named a buffer column. It had another purpose: to separate H2 from CO, CO2, N2, and water if they were not sufficiently removed through the separation columns. Analytical Procedure. Initially, a steel specimen was set on the sample holder in the hydrogen extraction cell. The atmosphere in the cell was thoroughly replaced with N2 gas. Radio frequency current was applied to the levitation coil under continuous flow of nitrogen gas in the cell. While the current was gradually raised to 20 A, the sample began floating and started melting. The sample holder was withdrawn downward to the bottom about 20 mm below the coil as soon as the sample lifted from the holder. Hydrogen extracted from the melted sample was led to TCD for detection and quantitative determination. Chemical Composition of Samples. The chemical compositions of stainless steel samples are listed in Table 1. Those samples are the working reference materials. They are used for the calibration of the conventional hydrogen determination systems used in Nippon Steel Corp. Their hydrogen contents were determined by the IF-TC method, and other elements were determined by a conventional spark emission spectrometry. Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
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Figure 4. Detection peak measuring H2 standard gas.
Figure 3. Levitation melting. Top, solid sample; bottom, melted sample.
The stainless steel samples were cut into a cylindrical shape of 6 mm diameter and 6 mm length, ground to remove oxide films on the surface, and rinsed with ethanol in an ultrasonic bath for 3 min. RESULTS AND DISCUSSION Floating Conditions. Gaseous elements analysis of steel usually requires more than 1 g of sample in order to avoid the effects of segregation of the element. Thus, the floating conditions for a sample of about 1-2 g were examined. Although spherical samples may be favorable for keeping balance at the start of floating, their use takes a lot of handling in preparation. Thus, samples were shaped into cylinders of about 6 mm in diameter and 6 mm in length, or cubes of about 6 mm along an edge. The sample shape changed into a spherical body due to its surface tension upon melting (as can be seen in Figure 3). The optimum high-frequency power applied to the levitation coil was found to be a voltage of 7 kV and a current of 2 A. If the power applied was much higher, the sample jumped out from the levitation coil zone, and if it was much lower, the sample could not be melted. Temperature of Samples. The temperature rising pattern of a stainless sample during levitation melting was measured with a two-color pyrometer. The temperature rose up to the melting point (1723 K) in about 10 s after starting the current application and to the maximum temperature (about 1970 K) in a further 10 s. On the temperature rising pattern, a small shoulder, which corresponded to the melting of a specimen, was observed. Detection of Hydrogen. The optimum conditions for hydrogen detection were examined by using hydrogen standard gas. The optimum carrier gas flow was found to be at 1.5 kPa pressure and 40 mL/min flow rate. Figure 4 shows an example of the 3302 Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
Figure 5. Extraction profiles obtained from stainless steel sample.
chromatogram obtained under the above conditions, when 100 µL of hydrogen standard gas was injected. The chromatogram still shows broadening of peaks, tailing in particular. This may be a consequence of diffusion of hydrogen in the extraction cell, removal columns, and buffer column. Therefore, hydrogen was determined by peak area. The standard deviation of 10 µL standard gas measurement was
