The development and application of metals and their composite materials often require effective control and accurate determination of the carbon and sulfur content.
The carbon in metal materials mainly exists in the form of free carbon, solid solution carbon and compound carbon, as well as gaseous carbon and surface protection carburization and coated organic carbon.
The current methods for analyzing carbon content in metals mainly include the combustion method, emission spectrometry, gas volumetric method, non-aqueous titration method, infrared absorption method and chromatography, etc.
Since each measurement method has a certain scope of application, and the measurement result is affected by many factors, such as the form of carbon, whether the carbon can be released completely during oxidation, and the blank value, the same method has certain accuracy in different occasions. difference.
This article summarizes the current analysis methods, sample processing, instruments 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 carbon (and sulfur) quantitative analysis.
The principle is to burn the sample in an oxygen stream to generate CO2.
Under 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 before and after the CO2 gas flows through the infrared absorber.
Principle of combustion-infrared absorption method
In recent years, infrared gas analysis technology has developed rapidly, and various analytical instruments using the principles of high-frequency induction heating and combustion and infrared spectrum absorption have also appeared rapidly.
For the determination of carbon and sulfur by the high-frequency combustion infrared absorption method, the following factors should generally be considered: sample dryness, electromagnetic sensitivity, geometric size, sample size, type of flux, proportion, order of addition, and amount of addition, Blank value settings, etc.
The advantage of this method is 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 during 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 return to the ground state spontaneously.
In the process of returning from the excited state to the ground state, the characteristic spectrum of each element will be released, and its content can be determined according to the intensity of the characteristic spectrum.
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 first choice in this industry because they can quickly obtain stable results.
But this method has specific requirements for sample preparation.
For example, when analyzing cast iron samples by spark spectroscopy, it is required that the carbon on the surface of the analysis is in the form of carbides, without free graphite, otherwise the analysis results will be affected.
Some users take advantage of the characteristics of fast cooling and whitening of thin slices to detect the carbon content in castings by spark spectroscopy method after the samples are made into slices.
When using spark spectroscopy to analyze carbon steel wire samples, the samples must be processed strictly and the samples must be placed “upright” or “flat” on the spark table for analysis using a small sample analysis fixture to improve the precision of the analysis.
3. Wavelength dispersive X-ray method
The wavelength dispersive X-ray analyzer can quickly and simultaneously measure 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 to emit secondary X-rays (ie X-ray fluorescence).
The wavelength dispersive X-ray fluorescence spectrometer (WDXRF) uses a crystal to split light, and the detector receives the diffracted characteristic X-ray signal.
If the spectroscopic crystal and the controller move synchronously and continuously change the diffraction angle, the characteristic X-ray wavelength and the intensity of each wavelength 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 and has attracted attention due to the simultaneous determination of multiple components in complex systems.
Especially in the geology department, this kind of instrument has been deployed successively, and the analysis speed has been significantly improved, which has played an important role.
However, the light element carbon has a low fluorescence yield due to the longer wavelength of characteristic radiation.
In heavy matrix materials such as steel, the absorption and attenuation of the characteristic radiation of carbon by the matrix are very large and other reasons, which often cause certain difficulties in the XRF analysis of carbon.
In addition, when measuring carbon in steel with an X-ray fluorescence instrument, if the ground surface is measured 10 times continuously, it can be found that the carbon content value is increasing.
Therefore, the application of this method is not as extensive as the previous two.
4. Non-aqueous titration method
The non-aqueous titration method is a method of titration in a non-aqueous solvent.
This method allows certain weak acids and bases that cannot be titrated in an aqueous solution to be titrated after selecting an appropriate solvent to increase their acidity and alkalinity.
The carbonic acid generated by CO2 in water solution is weakly acidic and can be accurately titrated by selecting different organic reagents.
The following is a commonly used non-aqueous titration method:
① The sample is burned at a high temperature in an electric arc furnace equipped with a carbon and sulfur analyzer.
② The carbon dioxide gas emitted by combustion is absorbed by the ethanol-ethanolamine solution, and the carbon dioxide reacts with ethanolamine to form a relatively stable 2-hydroxyethylamine carboxylic acid.
③ Use KOH for non-aqueous titration.
The reagents used in this method are toxic, 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.
A little carelessness will cause carbon to run out and cause low results.
The reagents used in the non-aqueous titration method are mostly flammable products, and high-temperature heating operations are involved in the experiment. The operators must have sufficient safety awareness.
The flame atomization detector is combined with gas chromatography to heat the sample in hydrogen, and then use the flame atomization detector-gas chromatography to detect the emitted gases (such as CH4 and CO).
Some users use this method to test the trace amount of 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 for test results.
6. Electrochemical methods
Some users introduced the use of potentiometric analysis to determine the low carbon content 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 product to determine the carbon concentration.
This method is particularly 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.
This method has few practical applications and mostly stays in the experimental research stage.
7. Online 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.
Some scholars in the metallurgical industry have introduced examples of using waste gas information to estimate carbon concentration:
When using the oxygen consumption and concentration in the vacuum vessel in the vacuum decarburization process and the flow rate of oxygen and argon, the carbon content in molten steel is estimated.
There are also users who have developed methods and related instruments for the 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 suitable for quality management and performance control in the steelmaking production process.