2020 Volume 60 Issue 1 Pages 193-195
This paper suggests a procedure of an improved digestion method, which can be applied to a variety of steel samples, for preparing the sample solution to quantify alloyed elements and sulfur simultaneously in inductively coupled plasma atomic emission spectrometry. A conventional digestion method using a mixture of hydrochloric and nitric acid has a poor ability to decompose tool steel completely. Alternatively, a microwave digestion method, in which an acid mixture of hydrochloric acid, hydrofluoric acid, nitric acid, and phosphoric acid was prepared with 1:1:1:1 volume ratio, enabled various steel samples including tool steel and stainless steel to be fully decomposed. Due to no addition of sulfuric acid, the sulfur content in the samples could be determined. The suggested procedure was applicable to determine sulfur, vanadium, chromium, manganese, cobalt, nickel, molybdenum, and tungsten in a variety of steel alloys using the same dissolution procedure.
In the production process of steel materials, alloyed elements should be analyzed with high accuracy for quality control. Further, it is also important to control the content of sulfur strictly, because sulfur compounds exert a negative effect on the physical characteristics of the steel, such as ductility, ultimate tensile strength, and toughness. For the analysis with inductively coupled plasma emission spectrometry (ICP-AES), several solution preparation methods have been tested for high-alloyed steel samples including high-speed tool steel and stainless steel. It is difficult to digest a tool steel sample without remaining a residue and precipitating no tungsten hydrate.1) For example, an acid mixture containing hydrochloric and nitric acids,2) which easily digests a stainless steel sample, cannot completely decompose metal carbides and oxides (e.g., aluminum, calcium, and chromium3)) in a steel sample. Digestion of such carbides and oxides should be conducted by fusion procedures with potassium disulfate4) and sodium carbonate.5) Authors were previously suggested lithium tetraborate fusion procedures for a high-speed tool steel sample on ICP-AES1) and X-ray fluorescence6,7) analyses. Although these methods were successfully employed to determine alloyed elements, they cannot be applied for sulfur quantification because of the loss of sulfur during its fusion process. In addition, the emission intensity might be severely influenced by a flux reagent in a specimen solution.8) Furthermore, tungsten hydride would be precipitated in the HCl–HNO3 solution. In general, a tool steel sample containing tungsten is decomposed by fuming with another acid mixture containing sulfuric and phosphoric acids.9) Various metal carbides and nitrides can be completely digested with this acid mixture.3) Further, the H2SO4–H3PO4 solution without tungsten precipitation can be prepared. Unfortunately, this acid mixture cannot be used to analyze sulfur.
In this work, microwave digestion with a quaternary acid mixture of hydrochloric acid, hydrofluoric acid, nitric acid and phosphoric acid was tested to prepare a sample solution for the simultaneous quantification of alloyed elements together with sulfur in high-alloyed steel samples based on ICP-AES. Suggested solution preparation was successfully used to quantify sulfur, vanadium, chromium, manganese, cobalt, nickel, molybdenum, and tungsten in various steel samples including stainless steel and high-speed tool steel.
A spectrometer of Arcos MV130 (SPECTRO Analytical Instruments GmbH, Kleve, Germany) was used for ICP-AES measurement. A 27.12-MHz radio-frequency generator was operated at 1.20 kW. The following flow-rate conditions of argon gas were used: 13.0 dm3 min−1 for coolant gas, 1.00 dm3 min−1 for auxiliary gas, and 0.65 dm3 min−1 for nebulizer gas. Solution introduction was conducted with a polytetrafluoroethylene introducing system with a concentric nebulizer and a cyclone spay camber. The rate of a peristaltic pump was 15 rounds per minute. By observing the plasma radially, the emission intensities were accumulated for 10 s. The mean value of triplicate measurements was used for analytical calculations. The following emission lines were used for the present quantification: S 180.731 nm and 182.034 nm; V 292.402 and 311.071 nm; Cr 284.984 nm; Mn 403.076 nm; Co 228616 nm; Ni 231.604 nm; Mo 202.095 nm; W 224.875 nm; and Pd 340.458 nm. For the quantification calculations of sulfur and vanadium, mean value of each result for multiple emission lines was regarded as an analytical result.
A microwave oven of speedwave MWS-3 Microwave Pressure Digestion Unit (Berghof Products + Instruments GmbH, Eningen, Germany) was used for the steel digestion. Digestion was conducted with a 17 cm3 of polytetrafluoroethylene digestion vessel and a polytetrafluoroethylene lid.
2.2. ReagentsAll acid reagents for electronics industry (FUJIFILM Wako Pure Chemical Corp., Osaka, Japan) were used: 35 mass% hydrochloric acid, 50 mass% hydrofluoric acid, 70 mass% nitric acid and 85 mass% phosphoric acid. Pure iron was used as a matrix component of steel materials in the calibrating solutions. Pure metals of vanadium, chromium, manganese, cobalt, nickel, molybdenum, and palladium were used to prepare each acid solution as the metal standard solution. Potassium sulfate and sodium tungstate were used to prepare each aqueous solution as the standard solution.
2.3. StandardsSeveral CRMs of steel were used to validate the present analysis: JSS 602-11 of tool steel, JSS 606-9 of high-speed tool steels and JSS 654-15 of stainless steel (Japanese Iron and Steel Federation, Tokyo, Japan), and ECRM 231-2 of stainless steel (Bundesanstalt für Materialforschung und –prüfung, Berlin, Germany).
2.4. Optimization for Microwave DigestionDigestion was optimized on the basis of five acid mixtures and three heating conditions. The following five acid mixtures were prepared: (1) 1:1:1 volume ratio of HF, HNO3 and H3PO4 (ternary acid mixture without HCl), (2) 1:1:1 volume ratio of HCl, HNO3 and H3PO4 (ternary acid mixture without HF), (3) 1:1:1 volume ratio of HCl, HF, and H3PO4 (ternary acid mixture without HNO3), (4) 1:1:1 volume ratio of HCl, HNO3 and HF (ternary acid mixture without H3PO4), and (5) 1:1:1:1 volume ratio of HCl, HF, HNO3, and H3PO4 (acid mixture with 4 acids). Samples were ECRM 231-2 of stainless steel and JSS 602-11 of tool steel. Weighed steel sample (0.20 g) was placed in the digestion vessel. One of the acid mixtures (4.0 cm3) was added in the vessel, and then it was sealed with the lip. Sealed vessels were heated in the microwave oven using the following three conditions: (A) increasing temperature to 433 K in 10 min and keeping the temperature for 20 min, (B) increasing temperature to 433 K in 10 min and keeping the temperature for 40 min, and (C) increasing temperature to 473 K in 10 min and keeping the temperature for 20 min. After cooling the vessel to room temperature, the resulting solution was placed in a 100 cm3 of polypropylene volumetric flask.
2.5. Sample PreparationValidation tests for the present quantification were conducted with three CRMs: JSS 602-11, JSS 606-9, and ECRM 231-2. Weighed steel sample (0.20 g) was placed in the digestion vessel. Mixed acid (4.0 cm3) with 1:1:1:1 volume ratio of HCl, HF, HNO3 and H3PO4 was added in the vessel, and then it was sealed with the lip. Sealed vessels were heated in the microwave oven using the following conditions: increasing temperature to 473 K in 10 min and keeping the temperature for 20 min. After cooling the vessel to room temperature, the resulting solution was placed in a 100 cm3 of polypropylene volumetric flask. The specimen solutions, which contained 10.0 μg cm−3 of palladium, were prepared to be conducted internal standard method. In the same manner, calibrating solution was prepared using pure iron. Standard solutions were added in the solution containing the pure iron.
2.6. Calibration MethodsThe calibrations were conducted by using the following sets of calibrating solutions: (1-a) six solutions containing 0–0.63 μg cm−3 of sulfur, (1-b) ten calibrating solutions containing 0–10.0 μg cm−3 of sulfur; (2) seven solutions containing 0–19.8 μg cm−3 of vanadium, 0–8.3 mg cm−3 of chromium, 0–25.6 μg cm−3 of manganese, 0–2.1 μg cm−3 of cobalt, 0–5.4 μg cm−3 of nickel, 0–7.8 μg cm−3 of molybdenum, and 0–114 μg cm−3 of tungsten; and (3) six solutions containing 0–543 μg cm−3 of chromium, 0–401 μg cm−3 of nickel, and 0–458 μg cm−3 of tungsten.
The results of the optimization for microwave digestion were listed in Table 1. Heating condition (C), where the temperature increased to 473 K in 10 min and it was kept for 20 min, was superior to the other conditions of (A) and (B). Hydrochloric acid should be added because of decomposing a steel sample containing high content of chromium. Hydrofluoric acid must be added to prevent a precipitation of tungsten. On the digestion of JSS 602-11 with the ternary acid mixture without HF, precipitation of tungsten, which was filtered from the resulting solution, was detected with wavelength dispersive X-ray fluorescence (WDXRF) measurement as shown in Fig. 1. Nitric acid must be added to decompose a tool steel sample. Black-colored residue was remained on the digestion of JSS 602-11 with the ternary acid mixture without HNO3. This residue contained vanadium, chromium and tungsten (Fig. 1). An oxidizing agent of HNO3 might be essential for such decomposition. Phosphoric acid should be added to obtain a clear solution without a residue. Additionally, phosphoric acid had an effect to prevent bumping on the addition of an acid mixture to the digestion vessel containing a tool steel sample. In conclusion, the quaternary acid mixture of HCl, HF, HNO3, and H3PO4 was the most effective digesting agent for the present analysis.
| Acid mixtures with the same volume ratio | |||||
|---|---|---|---|---|---|
| (1) Without HCl | (2) Without HF | (3) Without HNO3 | (4) Without H3PO4 | (5) With 4 acids | |
| (A) Increasing temperature to 433 K in 10 min and keeping for 20 min | |||||
| ECRM 231-2 of stainless steel (18.071 mass% of Cr) | n1 | y | y | y | y |
| JSS 602-11 of tool steel (3.57 mass% of W) | y | n2 | n3 | n2 | n2 |
| (B) Increasing temperature to 433 K in 10 min and keeping for 40 min | |||||
| ECRM 231-2 of stainless steel | n1 | y | y | y | y |
| JSS 602-11 of tool steel | y | n2 | n3 | n2 | n2 |
| (C) Increasing temperature to 473 K in 10 min and keeping for 20 min | |||||
| ECRM 231-2 of stainless steel | y1 | y | y | y | y |
| JSS 602-11 of tool steel | y | n4 | n3 | n2 | y |
y: Digestion was successfully conducted.
y1: Digestion was conducted; however, another steel sample of JSS 654-15 containing higher Cr (24.92 mass%) was not digested completely.
n1: Steel sample was not decomposed completely.
n2: Brown-colored residue was left in the resulting solution.
n3: Black-colored residue was left in the resulting solution.
n4: Digestion was apparently conducted, but precipitation of tungsten was separated from the solution by filtration. Tungsten was detected on the filtered paper by X-ray fluorescence measurement.
Wavelength dispersive X-ray fluorescence spectra of residues left on the filtered paper after decomposition of tool steel (JSS 602-11). ●: blank filter paper, ○: a quaternary acid mixture of HCl, HF, HNO3 and H3PO4, ■: the ternary acid mixture without HF, and □: the ternary acid mixture without HNO3.
Linear calibration curves for the present quantification were successfully obtained. Each calibration curve were evaluated by lower use-limit of calibration curve (LUC), which was previously demonstrated in published work,6) for the present steel quantification. The LUC (in mass%) was calculated by using the following equation: LUC = (3/b) [{Σ(Ii – Icalc., i)2}/(m – 2)]1/2 × 100 × 10−6/0.20 × 100, where b is the slope of a calibration curve (in cps μg−1 cm3), Ii is the measured emission intensity of ith calibrating standard solution (in cps), Icalc., i is the calculated emission intensity of ith calibrating standard solution estimated from the calibration curve (in cps), and m is the number of calibrating standard solutions. The following quantification ranges were used for each calibration curve: (1-a) 0.0005 (LUC)–0.03 mass% of sulfur without Pd internal standard; (1-b) 0.03 (LUC: 0.005)–0.50 mass% of sulfur without Pd internal standard; (2-a) 0.02 (LUC)–0.99 mass% of vanadium with Pd internal standard; (2-b) 0.009 (LUC)–0.42 mass% of chromium with Pd internal standard; (2-c) 0.02 (LUC)–1.3 mass% of manganese with Pd internal standard; (2-d) 0.004 (LUC)–0.10 mass% of cobalt with Pd internal standard; (2-e) 0.008 (LUC)–0.27 mass% of nickel with Pd internal standard; (2-f) 0.001 (LUC)–0.39 mass% of molybdenum with Pd internal standard; (2-g) 0.2 (LUC)–5.7 mass% of tungsten without Pd internal standard; (3-a) 0.42–27.2 mass% of chromium with Pd internal standard; (3-b) 0.27–20.0 mass% of nickel without Pd internal standard; and (3-c) 5.7–22.9 mass% of tungsten without Pd internal standard. For the sulfur emission line of 180.731 nm, spectral interferences were collected by using the emission intensities of manganese, nickel, and tungsten. Another spectral interference on the sulfur emission line of 182.034 nm was also collected by using the emission intensity of manganese.
3.3. Validation AnalysisSeveral CRMs were analyzed by using the present sample preparation and ICP-AES measurements. Analytical results were listed in Table 2. The difference between the analytical result and the corresponding certified value could be regarded very slightly. In conclusion, the suggested method might be applicable to quantify alloyed elements and sulfur in various steel samples including high-alloyed steels of stainless steel and high-speed tool steel.
| ECRM231-2 (Stainless steel) | JSS 602-11 (Tool steel) | JSS 606-9 (High-speed steel) | ||||
|---|---|---|---|---|---|---|
| Present method | Certified value | Present method | Certified value | Present method | Certified value | |
| S | 0.025 ± 0.001 | 0.0250 | 0.101 ± 0.004 | 0.0102 | 0.004 ± 0.000 | 0.00432 |
| V | 0.071 ± 0.000 | 0.0708 | 0.223 ± 0.001 | 0.218 | 0.834 ± 0.004 | 0.837 |
| Cr | 18.1 ± 0.2 | 18.071 | 0.362 ± 0.001 | 0.366 | 4.01 ± 0.02 | 4.00 |
| Mn | 1.25 ± 0.00 | 1.263 | 0.300 ± 0.000 | 0.301 | 0.302 ± 0.000 | 0.304 |
| Co | 0.041 ± 0.000 | 0.0402 | < 0.004* | ― | 0.083 ± 0.000 | 0.0820 |
| Ni | 10.0 ± 0.1 | 10.105 | 0.155 ± 0.000 | 0.1547 | 0.038 ± 0.000 | 0.0400 |
| Mo | 0.301 ± 0.001 | 0.301 | 0.047 ± 0.000 | 0.0455 | 0.157 ± 0.001 | 0.163 |
| W | < 0.2* | ― | 3.62 ± 0.06 | 3.57 | 16.9 ± 0.2 | 17.17 |
―: no certified value; *: less than the lower use-limit of calibration curve.
The data in this paper demonstrates a digestion method for the determination of alloyed elements and sulfur in various steel samples by ICP-AES, using an acid solution of hydrochloric acid, hydrofluoric acid, nitric acid, and phosphoric acid with 1:1:1:1 volume ratio. The best combination of decomposition acids was determined by our systematic experiments in which different chemical properties of alloyed elements in steel were fully considered. This acid mixture for microwave digestion had a practical advantage beyond the conventional ones, because various kinds of steels, including stainless steel and high-speed tool steel, could be digested in the same manner.
