During vacuum induction melting, laser-induced breakdown spectroscopy is used to perform real-time quantitative analysis of the main-ingredient elements of a molten alloy.
Vacuum induction melting is a popular method for refining high purity metal and alloys. Traditionally, standard process control in metallurgy involves several steps, include drawing samples, cooling, cutting, transport to the laboratory, and analysis. The whole analysis process requires more than 30 minutes, which hinders on-line process control. Laser-induced breakdown spectroscopy is an excellent on-line analysis method that can satisfy the requirements of vacuum induction melting because it is fast and noncontact and does not require sample preparation. The experimental facility uses a lamp-pumped Q-switched laser to ablate melted liquid steel with an output energy of 80 mJ, a frequency of 5 Hz, a FWHM pulse width of 20 ns, and a working wavelength of 1,064 nm. A multi-channel linear charge coupled device (CCD) spectrometer is used to measure the emission spectrum in real time, with a spectral range from 190 to 600 nm and a resolution of 0.06 nm at a wavelength of 200 nm. The protocol includes several steps: standard alloy sample preparation and an ingredient test, smelting of standard samples and determination of the laser breakdown spectrum, and construction of the elements concentration quantitative analysis curve of each element. To realize the concentration analysis of unknown samples, the spectrum of a sample also needs to be measured and disposed with the same process. The composition of all main elements in the melted alloy can be quantitatively analyzed with an internal standard method. The calibration curve shows that the limit of detection of most metal elements ranges from 20-250 ppm. The concentration of elements, such as Ti, Mo, Nb, V, and Cu, can be lower than 100 ppm, and the concentrations of Cr, Al, Co, Fe, Mn, C, and Si range from 100-200 ppm. The R2 of some calibration curves can exceed 0.94.
Due to its unique features, such as remote sensing, fast analysis, and no need for sample preparation, laser-induced breakdown spectroscopy (LIBS) offers unique capabilities for on-line concentration determination1,2,3. Although the use of the LIBS technique in different fields has been investigated4,5,6, a considerable attempt to develop its capabilities in industrial applications is ongoing.
Analysis of molten material contents during the course of industrial processes can effectively improve the product quality, which is a promising development direction of LIBS. Experimental findings have been reported about the application of LIBS in the industrial field, such as findings about argon oxygen liquid steel7,8,9,10,11, molten aluminium alloy12, molten salt13, and molten silicon14. The majority of these materials exist in the environment of air or an assistant gas. However, vacuum induction melting (VIM) is another good application field of LIBS to realize processing control. A VIM furnace can realize smelting at temperatures higher than 1,700 °C for alloy refining; it is the most popular method for refining high-purity metal and alloys such as iron-base or nickel-base alloys, high purity alloys, and clean magnetic alloys. During the course of melting, the pressure in a furnace is always in the region of 1-10 Pa, and the composition of air in the furnace mainly includes the air absorbed on the sample or the inner wall of the furnace and some vaporous oxide or nitride metal. These working situations induce quite different LIBS measurement situations for smelting in air. Here, we report an experimental investigation of the analysis of molten alloy during the course of VIM by LIBS.
An optical window is added to a furnace for laser ablation and radiant light detection. A silica glass with a diameter of 80 mm serves as the window. An emitting laser and gathering of radiant light employ the same window; it is a co-axial optical structure that focuses on the same point. The working focal length is approximately 1.8 m, and the focusing length of the experimental setup can be adjusted from 1.5 to 2.5 m.
Based on the practicality of industrial online analysis, precision, repeatability and stability is more important than the low limit of detection (LOD) during molten alloy ingredient analysis. The technical route of a four-channel linear CCD spectrometer is chosen, the spectral range of the spectrometer ranges from 190 to 600 nm, the resolution is 0.06 nm, and the wavelength is 200 nm. A laser diode pumped Q-switched laser (constructed in house) is used to ablate molten alloy, with an output energy of 100 mJ, a frequency of 5 Hz, an FWHM pulse width of 20 ns, and a working wavelength of 1064 nm. The remaining part will present the VIM LIBS-analysing process and live measurement, followed by an introduction of the data processing results.
1. Preparation of Standard Samples
NOTE: This step is not essential.
2. Test Ingredient of Standard Alloy Samples
3. Smelt Samples
4. Determine Laser Breakdown Spectra of Standard Samples
5. Construct Calibration Curve of Quantitative Analysis
6. Elemental Composition Analysis of Molten Alloy
NOTE: The experimental setup has been divided to two parts, namely, the detector head and the control cabinet, as shown in Figure 1. The same laser and spectrometer parameters, moulting, and spectrum gathering process employed in the previous process are utilized to ensure accurate quantitative analysis results.
Ten nickel-based alloy samples (#1-#10) are used to construct internal-standard calibration curves. The compositions of all samples are listed in Table 1. The elemental concentrations of these samples are orthogonally designed to avoid signal interference. The concentration of each element in all samples is measured with chemical analysis methods.
Nickel is the internal standard element. The calibration curves of Cu, Ti, Mo, Al, and Cr are constructed. Figure 2 auf Figure 6 show the calibration results. In these figures, the X-axis represents the concentration of the calibrated elements, and the Y-axis represents the relative signal intensity ratio of the calibrated element after the disposal process of background correction and peak fitting. The error bar of each point in these figures shows the fluctuation range of the signal strength with twenty frame measurements. The calibration parameters of these elements are listed in Table 3 auf Table 7. The linear curve fitting results, including the residual sum of squares, Pearson's r, and the linear fitting coefficient R2, are shown from Figure 2 auf Figure 6. The intercept and slope of the coefficient of determination are also shown in these figures. The calibration curves show a near-linear relationship between the concentration of the element and the peak intensity. The spectral lines used for each element were introduced in the legend of these figures. These lines are searched by a method of filtration. All signal peaks are filtered by the signal intensity, the central of wavelength, and the Lorenz fitting effect. These selected peaks are chosen by a permutation-combination analysis of the fitting factor R2.
According to the standard of International Union of Pure and Applied Chemistry (IUPAC), 3σ limit of detection (LOD) of Cu, Ti, Mo, Al and Cr are calculated and listed in Table 8. Other elements, such as Si, C, and Nb, are analyzed. The RSD ranges from 4-6%, and the R2 exceeds 0.93. The precision can be improved if a better relative standard is employed.
Figure 1: Experimental setup of quantitative analysis in the process of vacuum induction melting by laser-induced breakdown spectroscopy. Please click here to view a larger version of this figure.
Figure 2: Calibration curves of Cu. Internal standard lines include Cu: 224.70 nm, Ni: 241.61 nm and 233.75 nm. Please click here to view a larger version of this figure.
Figure 3: Calibration curves of Ti. Internal standard lines include Ti: 444.38 nm and 337.22 nm, Ni: 445.90 nm and 313.41 nm. Please click here to view a larger version of this figure.
Figure 4: Calibration curves of Mo. Internal standard lines include Mo: 342.23 nm, 346.02 nm, and 277.44 nm, Ni: 440.16 nm and 336.68 nm. Please click here to view a larger version of this figure.
Figure 5: Calibration curves of Al. Internal standard lines include Al: 272.31 nm, 231.22 nm, and 334.85 nm, Ni: 221.65 nm, 332.23 nm, and 440.16 nm. Please click here to view a larger version of this figure.
Figure 6: Calibration curves of Cr. Internal standard lines include Cr: 286.51 nm, 302.67 nm and 342.12 nm, Ni: 224.27 nm, 233.75 nm, and 350.08 nm. Please click here to view a larger version of this figure.
Table 1: Raw material ingredients in the experiment.
Table 2: Standard nickel-based alloy samples ingredient measured results.
Table 3: Calibration data of Cu.
Table 4: Calibration data of Ti.
Table 5: Calibration data of Mo.
Table 6: Calibration data of Al.
Table 7: Calibration data of Cr.
Table 8: Limit of detection of Cu, Ti, Mo, Al, and Cr.
For elemental analysis, popular methods are X-ray fluorescence (XRF), spark discharge optical emission spectrometry (SD-OES), atomic absorption spectroscopy (AAS), and inductive couple plasma (ICP). These methods are mainly suited for a laboratory and industrial online application for molten alloys, which is determined by the characters of these technologies, is difficult. XRF uses X-rays to shock samples, and SD-OES makes sparks on the samples. The working distance of these two methods are always in the range of several centimetres. AAS and ICP yields liquid or powder samples, which requires several tens of minutes for preparation. These methods are not suitable for high-temperature samples or measurements from a distance of several metres. Compared with these analysis methods, LIBS has the advantages of long-distance analysis, fast analysis, and the need to prepare samples. LIBS is the only good method for realizing melting alloys ingredient online analysis.
The protocol includes three critical steps: using a laser to burn the molten alloy, using a spectrometer to determine the spectrum of the plasma, and quantitatively analyzing the elemental composition with the calibration curve. Preparation of the samples with gradient components and construction of the calibration curve to demonstrate the relation between the laser breakdown spectrum intensity and the elemental content are preparative steps.
Use of the LIBS to analyze the elemental composition of molten alloy has some limitations. The precision of the quantitative analysis is the most important problem. The precision of LIBS is expected to improve by an order of magnitude. The gas pressure, surface status of the samples, and focusing precision have a distinct influence on the precision; however, compensation of these errors is difficult1,2,6.
Use of the LIBS system for on-line analysis of elemental composition during vacuum melting is proven by experiments. The experimental results have shown that the plasma spectrum can be determined in a typical industrial vacuum melting furnace situation. The calibration results show that the major components of molten alloys can be quantitatively analysed.
The authors have nothing to disclose.
This study was financially supported by the National Key Scientific Instrument and Equipment Development Projects (Grant No. 2014YQ120351), the Youth Innovation Promotion Association of CAS (Grant No. 2014136), and the China Innovative Talent Promotion Plans for Innovation Team in Priority Fields (Grant No. 2014RA4051).
Laser source | Gklaser Co.,Ltd. | ||
Molten alloy to be measured | |||
Smelting furnace | Tianyu Co.,Ltd. | ||
Spectrometer | Avantes | ||
standard samples | Well known of its composition |