Test method of carbon content in steel
“Carbon” is located in the fourth group of the second cycle of the periodic table of elements. It is one of the most common elements in nature. In the earth’s crust, atmosphere and biology, it mainly exists in the form of simple substances and compounds. Along with mineral processing, mineral smelting, material manufacturing and other processes, it will inevitably be introduced into metal materials. Carbon has an important influence on the mechanical properties, microstructure and technology of metal materials. Therefore, accurate determination of carbon content in metal materials and related raw materials has important guiding significance for smelting and manufacturing process.
Determination of carbon content
Table of Contents
- 1 Determination of carbon content
- 2 Seven methods for determination of carbon content in steel
- 3 Summary
According to the relationship between chemical properties and morphological transformation of carbon, the determination methods of carbon content in metal materials can be divided into chemical method, physical method and physical chemical method;
Chemical and physicochemical methods
It belongs to the special method of carbon quantitative analysis, which uses the high temperature combustion method to convert the carbon in the sample into CO2, separate it from the sample, and then determine the amount of CO2 by appropriate method, which is composed of high temperature combustion system and detection system.
This method is suitable for metal alloys, rock minerals and inorganic nonmetallic materials which can be processed into chip, granular and powder. Among them, high frequency combustion infrared absorption method has been widely and mature used in carbon content analysis of metal alloy materials such as steel, ferroalloy, common nonferrous metals, nickel based alloys, refractory metals, cemented carbides, rare earth metals, etc.
At present, the measurement principle and application range of two common high temperature combustion methods in metal material carbon content test are as follows:
High temperature combustion gas volumetric method
The determination principle is shown in Fig.1: the sample is heated in a high-temperature furnace and burned with oxygen, so that the carbon in the sample is quantitatively oxidized to CO2. The mixed gas is collected in the measuring tube after sulfur removal agent, and the volume is measured. Then, the mixed gas is allowed to pass through the absorber with potassium hydroxide solution to absorb CO2, and the remaining oxygen is returned to the gas measuring tube, Is the volume of CO2 generated and the carbon content is calculated.
The method has the advantages of rapid operation, low cost, simple procedure and high accuracy. It is suitable for the determination of more than 0.10% carbon content. The improved gas volumetric method is used to measure high carbon content. The measurement range is 5% – 21%, and the measurement accuracy is about 0.03%, which can meet the requirements of carbon measurement accuracy of cemented carbide.
High frequency combustion infrared absorption method
The measuring principle of this method is that in the presence of flux, oxygen is introduced into the high-frequency induction furnace. The high-frequency furnace makes the sample heat up and melt rapidly, and the CO2 gas generated by the furnace enters into the infrared absorption cell. After the infrared light is absorbed by the CO2 gas in the absorption cell, it is incident on the detector. The light intensity corresponding to the concentration of CO2 gas is measured on the detector, which is converted into electrical signal by photoelectric detector in the computer After normalization, the mass fraction of carbon was obtained.
The heating temperature can reach 1700-2000 ℃, which is conducive to the determination of refractory samples and low carbon content, and is suitable for the determination of 0.001-10% carbon content.
According to the intensity of the spectral line emitted by the sample under high temperature excitation, the carbon content can be directly measured, which belongs to the multi-element and multi-channel simultaneous rapid analysis method. According to the different detection principles, it can be divided into emission spectrometry and other methods.
The application of this method is mainly focused on iron and steel materials. Because of its special requirements on the shape and size of samples, or unable to achieve accurate quantitative analysis, its application field is limited.
Emission spectrometry was used
Qualitative and quantitative analysis is realized by using characteristic spectra and intensities of atoms and elements. According to the difference of excitation light source, it can be divided into spark source emission spectrometry (Spark-OES), glow discharge emission spectrometry (GD-OES) and laser-induced emission spectrometry (LIBS). Spark source emission spectrometry is suitable for rapid analysis of bulk metal alloy, and can realize automatic intelligent analysis of steel production. Glow discharge emission spectrometry (Gdes) is suitable for surface inspection and depth analysis of metal materials. It has been used to determine carbon in some iron and steel materials. Laser induced emission spectrometry is suitable for nondestructive (minimally invasive) in-situ analysis of pitting, and is suitable for the composition analysis of steel.
In addition to spectral methods, there are X-ray fluorescence spectrometry (XRF), X-ray photoelectron spectroscopy (XPS), glow discharge mass spectrometry (GD-MS) and so on. XRF is suitable for on-site inspection and laboratory quantitative analysis of metal alloys, geological samples and non-metallic materials; XPS is suitable for semi quantitative analysis of surface composition and element valence state analysis of powder samples; GD-MS is suitable for micro trace and ultra trace element analysis of high-purity materials and metal alloys, which is slightly mentioned in the determination of carbon content in low-alloy steel and high-temperature alloy.
Seven methods for determination of carbon content in steel
At present, the main methods to analyze carbon content in metals include combustion method, emission spectrometry method, gas volumetric method, non-aqueous solution titration method, infrared absorption method and chromatography method. Because each method has a certain scope of application, and the results are affected by many factors, such as the form of carbon, whether the carbon can be completely released during oxidation, blank value, etc., the accuracy of the same method is different in different occasions. In this paper, the analytical methods of carbon in metals, sample treatment, instruments and application fields are summarized.
Infrared absorption method
Combustion infrared absorption method based on infrared absorption method is a special method for quantitative analysis of carbon (and sulfur).
The principle is that the sample is burned in oxygen flow to generate CO2. Under a certain pressure, the energy absorbed by CO2 is directly proportional to its concentration. Therefore, the carbon content can be calculated by measuring the energy change before and after CO2 gas flowing through the infrared absorber.
In recent years, infrared gas analysis technology has developed rapidly, and various analytical instruments based on the principle of high-frequency induction heating combustion and infrared spectrum absorption are also emerging rapidly. For the determination of carbon and sulfur by high frequency combustion infrared absorption method, the following factors should be considered: dryness, electromagnetic induction, geometric size, sample quantity, flux type, ratio, adding sequence and amount, and blank value setting.
This method has the advantages of accurate quantification and less interference. It is suitable for users who have high requirements for carbon content accuracy and have enough time for detection in production.
When an element is thermally or electrically excited, it will transition from the ground state to the excited state, and the excited state will return to the ground state spontaneously. In the process of returning from the excited state to the ground state, the characteristic spectral line of each element will be released, and its content can be determined according to the intensity of the characteristic spectral line.
In the metallurgical industry, due to the urgency of production, it is necessary to analyze the content of all the main elements in the furnace water in a short time, not just the carbon content. Spark direct reading emission spectrometer has become the first choice in the industry because of its fast and stable results. However, this method has specific requirements for sample preparation.
For example, in the analysis of cast iron samples by spark spectrometry, the carbon on the analysis surface must be in the form of carbides and free graphite is not allowed, otherwise the analysis results will be affected. The carbon content in cast iron was determined by spark spectrometry after the sample was made into flake by using the characteristics of quick cooling and good white cast.
When analyzing linear carbon steel samples by spark spectrometry, it is necessary to process the samples strictly and put the samples “upright” or “lying down” on the spark table by using small sample analysis fixture, so as to improve the analysis precision.
Wavelength dispersive X-ray method
Wavelength dispersive X-ray analyzer can be used for the rapid simultaneous determination of multiple elements.
Under the excitation of X-ray, the inner layer electrons of the tested element atom undergo energy level transition and emit secondary x-ray (i.e. X-ray fluorescence). Wavelength dispersive X-ray fluorescence spectrometer (WDXRF) uses crystal to divide the light, and then the detector receives the diffraction characteristic X-ray signal. If the crystal and the detector move synchronously and the diffraction angle is changed constantly, the wavelength and intensity of characteristic X-ray produced by various elements in the sample can be obtained, which can be used for qualitative and quantitative analysis. This kind of instrument was produced in the 1950s. It has attracted much attention because it can simultaneously determine the multi-component of complex system. Especially in the geological department, this instrument has been configured successively, which has significantly improved the analysis speed and played an important role.
However, the characteristic radiation wavelength of light element carbon is long, the fluorescence yield is low, and the absorption and attenuation of carbon characteristic radiation by matrix in heavy matrix materials, such as steel, are often difficult to analyze by XRF. In addition, when carbon in steel is measured by X-ray fluorescence spectrometer, if the ground sample surface is continuously measured for 10 times, it can be found that the carbon content is increasing. Therefore, the application of this method is not as wide as the first two.
Non aqueous solution titration
Nonaqueous solution titration is a method of titration in nonaqueous solvent. This method can be used to titrate some weak acids and weak bases that could not be titrated in aqueous solution by selecting appropriate solvents and enhancing their acidity and basicity. The carbonic acid formed by CO2 solution in water is weak in acidity, which can be accurately titrated by selecting different organic reagents.
The following is a commonly used non-aqueous titration method:
- ① The samples were burned at high temperature in an arc furnace equipped with a carbon sulfur analyzer.
- ② The carbon dioxide gas released from combustion is absorbed by ethanol ethanolamine solution, and carbon dioxide reacts with ethanolamine to form stable 2-hydroxyethylamine carboxylic acid.
- ③ Koh was used for non-aqueous solution titration.
The reagent used in this method is toxic, long-term contact will affect human health, and it is difficult to operate, especially when the carbon content is high, the solution must be preset, and if the carbon content is not paid attention to, the result will be low. The reagents used in non-aqueous solution titration are mostly flammable, and the high-temperature heating operation is involved in the experiment, so the operators should have enough safety awareness.
The flame atomization detector is combined with gas chromatography to heat the sample in hydrogen, and then the emitted gases (such as CH4 and CO) are detected by flame atomization detector gas chromatography. Some users have used this method to determine trace carbon in high purity iron, the content is 4 μ g / g, and the analysis time is 50 min.
The method is suitable for users with very low carbon content and high requirements for detection results.
Some users introduced the use of potentiometric analysis to determine the content of low carbon in the alloy: after the iron sample is oxidized in the induction furnace, the gaseous products are analyzed and determined by the electrochemical concentration cell composed of potassium carbonate solid electrolyte, so as to determine the carbon concentration. This method is especially suitable for the determination of very low concentration carbon. The precision of the analysis can be controlled by changing the composition of the reference gas and the oxidation rate of the sample And sensitivity.
The practical application of this method is less, most of them stay in the experimental research stage.
On line analysis
In refining steel, it is often necessary to control the carbon content in molten steel in vacuum furnace in real time. Some scholars in metallurgical industry have introduced an example of estimating carbon concentration by using the information of waste gas: the carbon content in molten steel is estimated by using the consumption and concentration of oxygen in vacuum vessel and the flow rate of oxygen and argon during vacuum decarburization..
In addition, users have developed a rapid method and related instruments for the determination of trace carbon in molten steel: the carrier gas is blown into the molten steel, and the carbon content in the molten steel is estimated from the oxidized carbon in the carrier gas.
The similar on-line analysis method is suitable for quality management and performance control in steel-making process.
After decades of development, the analytical method of carbon in metal materials has gradually formed the analytical method based on high-frequency infrared absorption method. The standards established by traditional analysis methods such as gas volumetric method are gradually replaced by the standards of high-frequency infrared absorption method. However, due to the special requirements of material and spark emission spectrometry, the application of this method is limited.
At present, the development trend of carbon detection methods is to continuously expand the application field and measurement range of high-frequency induction combustion infrared absorption method, so as to standardize the detection methods of many materials; continuously improve the accuracy and precision of multi-element solid analysis methods represented by spectral analysis; at the same time, it is necessary to develop and produce more standard samples with different material types and different carbon content gradients In order to better serve the metallurgy, mineral processing, materials and other research fields.
Source: China Stainless Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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