The development and application of metals and their composite materials often require effective control and accurate determination of the carbon and sulfur content. Carbon in metal materials mainly exists in the form of free carbon, solid solution carbon and combined carbon, as well as gaseous carbon, carburizing and coated organic carbon for surface protection.
At present, the methods for analyzing the carbon content in metals mainly include combustion method, emission spectrometry, gas volumetric method, non-aqueous solution titration method, infrared absorption method and chromatography. Since each measurement method has a certain scope of application, and the measurement results are affected by many factors, such as the form of carbon, whether carbon can be completely released during oxidation, blank value, etc., the same method has certain differences in accuracy in different occasions. This paper sorts out the current analysis methods, sample treatment, instruments used and application fields of carbon in metals.
1. Infrared absorption method
The combustion infrared absorption method developed based on the infrared absorption method is a special method for the quantitative analysis of carbon (and sulfur).
The principle is to burn the sample in an oxygen flow to generate CO2. Under a certain pressure, the energy of CO2 absorbing infrared rays is proportional to its concentration. Therefore, the carbon content can be calculated by measuring the energy change of CO2 gas before and after passing through the infrared absorber.
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Combustion-infrared absorption method principle
In recent years, infrared gas analysis technology has developed rapidly, and various analytical instruments using high-frequency induction heating combustion and infrared spectrum absorption principles have also appeared rapidly. For the determination of carbon and sulfur by high-frequency combustion infrared absorption method, the following factors should generally be considered: sample dryness, electromagnetic inductance, geometric size, sample size, type of flux, proportion, addition sequence and amount, setting of blank value, etc.
The method has the advantages of accurate quantification and less interference items. It is suitable for users who have high requirements on the accuracy of carbon content and have enough time for testing in production.
2. Emission Spectroscopy
When an element is excited by heat or electricity, it will transition from the ground state to the excited state, and the excited state will spontaneously return to the ground state. In the process of returning from the excited state to the ground state, the characteristic spectral lines of each element will be released, and the content can be determined according to the intensity of the characteristic spectral lines.
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Principle of emission spectrometer
In the metallurgical industry, due to the urgency of production, it is necessary to analyze the content of all major elements in the furnace water in a short period of time, not just the carbon content. Spark direct reading emission spectrometers have become the industry's first choice due to their ability to quickly obtain stable results. However, this method has specific requirements for sample preparation.
For example, when analyzing cast iron samples by spark spectrometry, it is required that the carbon on the analysis surface exists in the form of carbides, and there must be no free graphite, otherwise the analysis results will be affected. Some users take advantage of the characteristics of rapid cooling and whitening of thin slice samples, and after the samples are made into thin slices, the carbon content in cast iron is determined by spark spectroscopic analysis.
When analyzing carbon steel linear samples by spark spectrometry, the samples must be processed strictly and the samples should be placed on the spark stand "upright" or "flat" with small sample analysis fixtures for analysis to improve the precision of the analysis.
3. Wavelength dispersive X-ray method
Wavelength dispersive X-ray analyzers can quickly and simultaneously determine multiple elements.
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Principle of wavelength dispersive X-ray fluorescence spectrometer
Under the excitation of X-rays, the electrons in the inner layers of the atoms of the measured elements undergo energy level transitions and emit secondary X-rays (ie, X-ray fluorescence). The wavelength dispersive X-ray fluorescence spectrometer (WDXRF) uses a crystal to split the light and then the detector receives the diffracted characteristic X-ray signal. If the spectroscopic crystal and the detector move synchronously and constantly change the diffraction angle, the wavelength of the characteristic X-rays produced by various elements in the sample and the intensity of X-rays of each wavelength can be obtained, and qualitative and quantitative analysis can be carried out accordingly. This instrument was produced in the 1950s, and it has attracted attention because it can simultaneously measure multiple components in complex systems. Especially in the geological department, this instrument has been equipped successively, and the analysis speed has been significantly improved, which has played an important role.
However, due to the longer wavelength of the characteristic radiation of light element carbon, the low fluorescence yield, and the large absorption and attenuation of the characteristic radiation of carbon by the matrix in heavy matrix materials such as steel, it often causes certain difficulties for the XRF analysis of carbon. In addition, when measuring carbon in steel with an X-ray fluorescence instrument, if the ground sample surface is continuously measured 10 times, it can be found that the carbon content value is constantly increasing. Therefore, the application of this method is not as extensive as the first two.
4. Non-aqueous solution titration method
Non-aqueous titration is a method of performing titration in a non-aqueous solvent. This method can make certain weak acids and weak bases that cannot be titrated in aqueous solution can be titrated after selecting an appropriate solvent to enhance their acidity and alkalinity. The carbonic acid produced by CO2 solution in water has weak acidity and can be accurately titrated by selecting different organic reagents.
The following is a commonly used non-aqueous titration method:
① The sample is combusted at high temperature by the electric arc combustion furnace matched with the carbon and sulfur analyzer.
② The carbon dioxide gas released by combustion is absorbed by the ethanol-ethanolamine solution, and the carbon dioxide reacts with ethanolamine to generate relatively stable 2-hydroxyethylamine carboxylic acid.
③ Non-aqueous titration using KOH.
The reagents used in this method are poisonous, long-term exposure will affect human health, and it is difficult to operate, especially when the carbon content is high, the solution must be preset, and if you are not careful, the carbon will run away and the result will be low. The reagents used in the non-aqueous titration method are mostly flammable, and the experiment involves high-temperature heating operation, so the operator must have sufficient safety awareness.
5. Chromatography
Flame atomization detector coupled with gas chromatography, the sample is heated in hydrogen, and then the released gases (such as CH4 and CO) are detected using flame atomization detector-gas chromatography. Some users use this method to test trace amounts of carbon in high-purity iron, the content is 4 μg/g, and the analysis time is 50 minutes.
This method is suitable for users with extremely low carbon content and high requirements for test results.
6. Electrochemical method
A user introduced the use of potentiometric analysis to determine the low carbon content in alloys: after the iron sample was oxidized in an induction furnace, an electrochemical concentration cell composed of potassium carbonate solid electrolyte was used to analyze and measure gaseous products, thereby determining the concentration of carbon. This method is especially suitable for the determination of very low concentrations of carbon. The precision and sensitivity of the analysis can be controlled by changing the composition of the reference gas and the oxidation rate of the sample.
The practical application of this method is seldom, and most of them remain in the experimental research stage.
7. On-line analysis method
When refining steel, it is often necessary to control the carbon content in the molten steel in the vacuum furnace in real time. Scholars in the metallurgical industry have introduced an example of estimating the carbon concentration using exhaust gas information: the carbon content in the molten steel is estimated by using the consumption and concentration of oxygen in the vacuum vessel during the vacuum decarburization process, and the flow rate of oxygen and argon. .
There are also users who have developed a method for quickly measuring trace carbon in molten steel and related instruments and devices: 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.
Similar online analysis methods are suitable for quality management and performance control in steelmaking production process.
