Seven methods for determining carbon content in steel

Currently, the main methods for analyzing carbon content in metals include combustion method, emission spectroscopy, gas volumetric method, non-aqueous solution titration method, infrared absorption method, and chromatography method, etc. Since each determination method has its own applicable range, and the determination results are affected by many factors, such as the form of carbon present, whether carbon can be completely released during oxidation, blank value, etc., the accuracy of the same method varies in different situations. This article summarizes the current analysis methods for carbon in metals, sample processing, the instruments used, and application fields, etc. 

1. Infrared Absorption Method

The combustion infrared absorption method developed based on the infrared absorption method is a specialized method for quantitative analysis of carbon (and sulfur).

The principle is that the sample is burned in an oxygen stream, generating CO2. Under a certain pressure, CO2 absorbs the energy of infrared rays and its concentration is proportional. Therefore, by measuring the energy change of the CO2 gas before and after passing through the infrared absorption device, the carbon content can be calculated. 

In recent years, infrared gas analysis technology has developed rapidly, and various analytical instruments based on the principles of high-frequency induction heating of combustion and infrared spectral absorption have emerged rapidly. For the determination of carbon and sulfur by the high-frequency combustion infrared absorption method, the following factors should generally be considered: the dryness of the sample, electromagnetic susceptibility, geometric size, sample quantity, types and ratios of fluxing agents, addition sequence and amount, and the setting of blank values, etc. 

The advantage of this method is its high degree of accuracy and fewer interfering factors. It is suitable for users who have high requirements for the accuracy of carbon content and have sufficient time for testing during production. 

2. Emission Spectroscopy

When an element is subjected to thermal or electrical excitation, it will transition from the ground state to the excited state, and the excited state will spontaneously return to the ground state. During the process of returning from the excited state to the ground state, characteristic spectral lines of each element will be released. The content of the element can be determined based on the intensity of these characteristic spectral lines. 

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 within a very short period of time, not just the carbon content. The spark direct-read emission spectrometer, because it can obtain stable results quickly, has become the preferred choice in this industry. However, this method has specific requirements for sample preparation. 

For example, when using spark spectroscopy to analyze cast iron samples, it is required that all the carbon on the surface exists in the form of carbides and there should be no free graphite. Otherwise, it will affect the analysis results. Some users take advantage of the characteristic that thin-pressed samples cool rapidly and become white-ironized well, and after making the samples into thin sheets, they use spark spectroscopy to determine the carbon content in the cast iron. 

When using spark spectroscopy to analyze linear samples of carbon steel, it is necessary to process the samples carefully and use a small sample analysis fixture to place the samples "upright" or "flat" on the spark table for analysis, in order to improve the precision of the analysis. 

3. Wavelength Dispersive X-ray Method

The wavelength dispersive X-ray analyzer can conduct rapid and simultaneous determination of multiple elements. 

Under X-ray excitation, the inner electrons of the tested element atoms undergo energy level transitions and emit secondary X-rays (i.e., X fluorescence). The wavelength-dispersive X-ray fluorescence spectrometer (WDXRF) uses a crystal for dispersion and then receives the diffracted characteristic X-ray signals through a detector. If the dispersion crystal and the detector move synchronously and continuously change the diffraction angle, the wavelengths and intensities of characteristic X-rays produced by various elements within the sample can be obtained. Based on this, qualitative and quantitative analyses can be conducted. This type of instrument was developed in the 1950s. Due to its ability to simultaneously measure multiple components in complex systems, it has gained attention. Especially in the geological department, such instruments have been configured, and the analysis speed has significantly increased, playing an important role. 

However, due to the longer wavelength of the characteristic radiation of light elements such as carbon, the fluorescence yield is low. Moreover, in heavy base material materials like steel, the absorption and attenuation of the characteristic radiation of carbon by the matrix is also significant. These reasons often cause certain difficulties in the XRF analysis of carbon. Additionally, when measuring carbon in steel using an X-ray fluorescence instrument, if the polished sample surface is measured continuously for 10 times, it can be observed that the carbon content value keeps increasing. Therefore, the application scope of this method is not as wide as the previous two. 

4. Non-aqueous Solution Titration Method

The non-aqueous solution titration method is a technique that conducts titration in non-aqueous solvents. This method enables certain weak acids and weak bases that cannot be titrated in aqueous solutions to be titrated after selecting appropriate solvents to enhance their acidity and basicity. The carbonic acid formed by the dissolution of CO2 in water has a relatively weak acidity. By choosing different organic reagents, accurate titration can be achieved. 

The following is a commonly used non-aqueous titration method:

① The sample is subjected to high-temperature combustion in an arc combustion furnace provided with a carbon-sulfur analyzer.

② The carbon dioxide gas released during combustion is absorbed by an ethanol-ethanolamine solution. The carbon dioxide reacts with ethanolamine to form a relatively stable 2-hydroxyethylamine carboxylic acid.

③ Non-aqueous titration is carried out using KOH. 

The reagents used in this method are toxic and prolonged exposure can affect human health. Moreover, they are difficult to handle. Especially when the carbon content is high, a pre-prepared solution must be set up. If not careful, carbon may escape and result in an inaccurate test result. The reagents used in the non-aqueous solution titration method are mostly flammable substances. In the experiment, high-temperature heating operations are also involved, so the operators must have sufficient safety awareness. 

5. Chromatography Method

The flame atomicization detector is combined with gas chromatography. The sample is heated in hydrogen, and then the released gases (such as CH4 and CO) are detected using the flame atomicization detector - gas chromatography method. Some users have used this method to test trace amounts of carbon in high-purity iron, with a content of 4 μg/g, and the analysis time was 50 minutes. 

This method is applicable to users whose carbon content is extremely low and who have very high requirements for the test results. 

6. Electrochemical Method

One user has introduced the use of potentiometric analysis to determine the content of low carbon in alloys: After the iron sample is oxidized in an induction furnace, an electrochemical concentration cell composed of potassium carbonate solid electrolyte is used to analyze and determine the gaseous products, thereby determining the carbon concentration. This method is particularly suitable for the determination of very low concentrations of carbon and can control the precision and sensitivity of the analysis by changing the composition of the reference gas and the oxidation rate of the sample. 

This method is rarely put into practical use and mostly remains at the stage of experimental research. 

7. Online Analysis Method

When refining steel, it is often necessary to control the carbon content in the molten steel within the vacuum furnace in real time. A scholar from the metallurgical industry has presented an example of estimating the carbon concentration using the information from exhaust gas: The carbon content in the molten steel was estimated by using the consumption, concentration of oxygen in the vacuum container during vacuum decarburization, as well as the flow rates of oxygen and argon. 

Some users have also developed methods and related instruments for quickly determining trace amounts of carbon in molten steel: By blowing a carrier gas into the molten steel, the carbon that has been oxidized in the carrier gas can be used to estimate the carbon content in the molten steel. 

Similar online analysis methods are applicable to quality management and performance control in the steel production process.