The development and application of metals and their composite materials often require effective control and accurate determination of carbon and sulfur content. Carbon in metal materials mainly exists in the forms of free carbon, solid solution carbon, and combined carbon, as well as gaseous carbon and surface protected carburization and coated organic carbon.

At present, the main methods for analyzing carbon content in metals include combustion method, emission spectroscopy, gas volumetric method, non aqueous solution titration, infrared absorption method, and chromatography. Due to the applicability of each measurement method and the influence of many factors on the measurement results, such as the presence of carbon, whether carbon can be completely released during oxidation, blank values, etc., the accuracy of the same method varies in different situations. This article summarizes the current analysis methods, sample processing, instruments used, and application fields of carbon in metals.

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

The principle is to burn the sample in an oxygen stream to generate CO2. Under a certain pressure, the energy absorbed by CO2 in infrared radiation is directly proportional to its concentration. Therefore, by measuring the energy changes before and after the CO2 gas flows through the infrared absorber, the carbon content can be calculated.

Principle of combustion infrared absorption method

In recent years, infrared gas analysis technology has developed rapidly, and various analytical instruments utilizing high-frequency induction heating combustion and infrared spectral absorption principles have also rapidly emerged. For the determination of carbon and sulfur using high-frequency combustion infrared absorption method, the following factors should generally be considered: dryness of the sample, electromagnetic susceptibility, geometric size, sample size, type, ratio, addition order and amount of flux, blank value setting, etc.

The advantage of this method is accurate quantification and fewer interference terms. Suitable for users who have high requirements for carbon content accuracy and have sufficient time for testing during production.

element is thermally or electrically excited, it transitions from the ground state to the excited state, and the excited state spontaneously returns to the ground state. During the process of returning from the excited state to the ground state, the characteristic spectral lines of each element will be released, and their content can be determined based on the strength of the characteristic spectral lines.

Principles 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 carbon content. Spark direct reading emission spectrometer has become the preferred choice in the industry due to its ability to quickly obtain stable results. However, this method has specific requirements for sample preparation.

For example, when analyzing cast iron samples using spark spectroscopy, it is required to analyze the surface carbon in the form of carbides, without free graphite, otherwise it will affect the analysis results. Some users take advantage of the characteristics of rapid cooling and good whitening of thin samples, and after making the samples into thin slices, the carbon content in cast iron is determined by spark spectroscopy analysis.

When analyzing carbon steel linear samples using spark spectroscopy, it is necessary to strictly process the samples and use a small sample analysis fixture to place them "upright" or "flat" on a spark stage for analysis, in order to improve the accuracy of the analysis.

The wavelength dispersive X-ray analyzer can quickly and simultaneously determine multiple elements.

Principle of wavelength dispersive X-ray fluorescence spectrometer

Under X-ray excitation, the inner electrons of the measured element atoms undergo energy level transitions and emit secondary X-rays (i.e. X-ray fluorescence). Wavelength dispersive X-ray fluorescence spectrometer (WDXRF) is a device that uses crystals to separate light and then receives diffracted characteristic X-ray signals from the detector. If the spectroscopic crystal and the controller move synchronously and continuously change the diffraction angle, the wavelength and intensity of characteristic X-rays generated by various elements in the sample can be obtained, which can be used for qualitative and quantitative analysis. This type of instrument was developed in the 1950s and has gained attention due to its ability to simultaneously determine multiple components in complex systems. Especially in the geological department, this instrument has been configured successively, significantly improving the analysis speed and playing an important role.

However, light element carbon often poses certain difficulties in XRF analysis of carbon due to its long wavelength of characteristic radiation, low fluorescence yield, and significant absorption and attenuation of carbon characteristic radiation by the matrix in heavy matrix materials such as steel. In addition, when measuring carbon in steel using an X-ray fluorescence instrument, if the ground sample surface is continuously measured 10 times, it can be observed that the carbon content value is continuously increasing. Therefore, the application scope of this method is not as extensive as the first two.

Non aqueous solution titration is a method of titration in non aqueous solvents. This method can titrate certain weak acids and bases that cannot be titrated in aqueous solutions by selecting appropriate solvents to enhance their acidity and alkalinity. The carbonic acid generated by CO2 in aqueous solution 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 subjected to high-temperature combustion in an electric arc furnace equipped with a carbon sulfur analyzer.

② The carbon dioxide gas released from combustion is absorbed by the ethanol ethanolamine solution, and the carbon dioxide reacts with ethanolamine to generate a relatively stable 2-hydroxyethylamine carboxylic acid.

③ Use KOH for non aqueous solution titration.

The reagents used in this method are toxic, long-term exposure can affect human health, and are difficult to operate. Especially when the carbon content is high, it is necessary to preset the solution, and slight carelessness may cause carbon leakage and lower results. The reagents used in non aqueous solution titration are mostly flammable, and the experiment involves high-temperature heating operations. Operators should have sufficient safety awareness.

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

This method is suitable for users with extremely low carbon content and high requirements for detection results.

A user introduced the use of potential analysis method to determine the low carbon content in alloys: after oxidation of iron samples in an induction furnace, gas products were analyzed using an electrochemical concentration cell composed of potassium carbonate solid electrolyte to determine the concentration of carbon. This method is particularly suitable for the determination of very low concentration carbon, and the precision and sensitivity of the analysis can be controlled by changing the reference gas composition and the oxidation rate of the sample.

This method has few practical applications and mostly remains in the experimental research stage.

When refining steel, it is often necessary to control the carbon content in the molten steel in a vacuum furnace in real time. Scholars in the metallurgical industry have introduced an example of using information from exhaust gas to estimate carbon concentration: using the consumption and concentration of oxygen in the vacuum container during the vacuum decarbonization process, as well as the flow rate of oxygen and argon, to estimate the carbon content in the molten steel.

There are also users who have developed methods and related instruments for rapid 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.

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