The development and application of metals and their composites often require effective control and accurate determination of their carbon and sulphur content.
Carbon in metallic materials is mainly in the form of free carbon, soluble carbon and synthetic carbon, as well as gaseous carbon and surface protection carburizing and coated organic carbon.
The current methods for analyzing carbon content in metals include combustion, emission spectroscopy, gas volume method, non-aqueous solution titration, infrared absorption method chromatography and etc.
Because each method has a certain scope of application, and the measurement results are influenced by many factors, such as the presence of carbon form, whether carbon can be released completely during oxidation, blank value, etc., so the accuracy of the same method in different occasions has some differences.
This article collates the current methods of analysis of carbon in metals, sample processing, instruments used and applications.
1. Infrared absorption method
Combustion infrared absorption method developed based on infrared absorption method is a special method for quantitative analysis of carbon (and sulfur).
The principle is that the specimen is burned in a stream of oxygen to produce CO2.
Under a certain pressure, the energy of CO2 absorbing infrared light is proportional to its concentration, so the energy change before and after the CO2 gas flows through the infrared absorber is measured, and the carbon content can be calculated.
Principle of combustion-infrared absorption
In recent years, infrared gas analysis technology has developed rapidly, and a variety of analytical instruments using the principle of high-frequency induction heating combustion and infrared spectral absorption have also rapidly emerged.
For the determination of carbon and sulphur by high-frequency combustion infrared absorption method, the following factors should be considered in general: the dryness of the specimen, electromagnetic inductance, geometry, specimen volume, the type of flux, proportion, order of addition and the amount of addition, the setting of the blank value, etc.
This method has the advantage of quantitative accuracy and fewer interfering items.
It’s suitable for users with high requirements for carbon accuracy and sufficient time for testing in production.
2. Emission spectroscopy
Elements leap from the base state to the excited state when excited by heat or electricity, and the excited state returns to the base state spontaneously.
In the process of returning from the excited state to the base state, the signature line of each element is released and its content can be determined based on the strength of the signature line.
Emission spectrometer principle
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, not just the carbon content, in a very short period of time.
Spark direct-read emission spectrometers have become the industry’s first choice due to their ability to get stable results quickly.
However, the method has specific requirements for sample preparation.
For example, spark spectroscopy for the analysis of cast iron specimens requires that the carbon on the analytical surface is all in the form of carbide and cannot have free graphite, otherwise it will affect the results.
Some users use the characteristics of rapid cooling of thin slices of samples, which are good for white-mouthed, to determine the carbon content in cast iron by spark spectroscopy after the samples are made into thin slices.
When analyzing carbon steel wire samples by spark spectroscopy, the samples must be processed strictly and placed “upright” or “flat” on the spark table for analysis using a small sample analysis jig to improve the precision of analysis.
3. Wavelength dispersion X-ray method
Wavelength dispersion X-ray analyzers allow fast simultaneous determination of multiple elements.
Wavelength-dispersion X-ray fluorescence spectrometer principle
Under X-ray excitation, secondary X-rays (i.e., X-fluorescence) are emitted from the inner electrons of the atoms of the element under test as a result of the energy level shift.
Wavelength-dispersion X-ray fluorescence spectroscopy (WDXRF) is the use of crystalline spectroscopy and the reception of diffracted characteristic X-ray signals by the detector.
The wavelengths of the characteristic X-rays produced by the elements in the sample and the intensity of the X-rays at each wavelength can be obtained if the spectroscopic crystals and the controller are moving in synchronous motion and the diffraction angle is constantly changed, and qualitative and quantitative analysis can be carried out.
This instrument was developed in the 1950s and attracted attention for its ability to perform simultaneous multi-component determinations of complex systems, especially in the geological sector, where its configuration has played an important role in significantly increasing the speed of analysis.
However, light elemental carbon has a low fluorescence yield due to the long wavelength of the characteristic radiation.
In heavy matrix materials such as steel, the matrix has a great influence on the absorption attenuation of the characteristic radiation of carbon, often causing certain difficulties for the XRF analysis of carbon.
In addition, when measuring the carbon in steel with X-ray fluorometer, if the ground surface is measured 10 times in a row, the carbon content value can be found to be increasing.
Therefore, the application of this method is not as widespread as the first two.
4. Non-aqueous titration
Non-aqueous titration is a method of titration in a non-aqueous solvent. This method can be used to titrate some weak acids and bases that could not be titrated in an aqueous solution.
After selecting an appropriate solvent to enhance its acidity and alkalinity, it can be titrated.
The carbonic acid produced by CO2 in aqueous solution is less acidic and can be accurately titrated by selecting different organic reagents.
The following is a common non-aqueous titration method.
① The sample is burned at high temperature in an electric arc furnace equipped with a carbon and sulfur analyzer.
The carbon dioxide gas emitted by combustion is absorbed by the ethanol-ethanolamine solution, and the carbon dioxide reacts with the ethanolamine to form a relatively stable 2-hydroxyethylamine carboxylic acid.
③ Non-aqueous solution titration using KOH.
The reagents used in this method are toxic, and long-term exposure will affect human health, and it is difficult to operate, especially when the carbon content is high, the solution must be pre-set, and a little carelessness will run off the carbon, resulting in low results.
The reagents used in the non-aqueous solution titration method are mostly flammable, and the experiment involves high-temperature heating operation, and the operator must have sufficient safety awareness.
The flame atomization detector is used in combination with gas chromatography to heat the sample in hydrogen, and then the flame atomization detector-gas chromatography is used to detect the released gases (such as CH4 and CO).
Some users use this method to test trace carbon in high-purity iron, the content is 4μg / g, and the analysis time is 50min.
This method is suitable for users with extremely low carbon content and high requirements on test results.
6. Electrochemical method
A user described the determination of low carbon content in alloys using potential analysis.
After oxidation of the iron sample in an induction furnace, gaseous products were determined by electrochemical differential cell analysis consisting of a potassium carbonate solid electrolyte to determine the concentration of carbon.
This method is particularly suitable for the determination of very low carbon concentrations and allows the precision and sensitivity of the analysis to be controlled by changing the composition of the reference gas and the oxidation rate of the sample.
The method has had less practical application and has remained mostly in the experimental research phase.
7. Online analysis method
When refining steel, it is often necessary to control the carbon content of the molten steel in the vacuum furnace in real time, and there are examples of metallurgical industry scholars who have described the use of exhaust gas information to estimate carbon concentration.
The amount of carbon in the molten steel was estimated using the consumption, concentration and flow rate of oxygen and argon in the vacuum vessel during the vacuum decarburization process.
A user-developed method for the rapid determination of trace carbon in molten steel and related equipment.
A carrier gas is drummed into the molten steel and the carbon content of the molten steel is estimated from the carbon that has been oxidized in the carrier gas.
This kind of similar online analysis methods are applicable to quality management and performance control in the steel production process.