What are the advantages of metallographic microscopy for quantitative analysis of impurities?
Metallographic microscopes and metallographic inspection technologies are widely used in steel inspection applications. Because metallographic microscopes have the advantages of simple equipment and practical methods, the use of metallographic microscopes to detect steel is still one of the more practical and simple methods today.
In most cases, the grain boundaries are diffusely reflected and cannot enter the objective lens, so the grain boundaries appear black in most cases. What is divided by the grain boundaries is the organizational structure of the steel. Qualitative analysis of the steel can be carried out based on the results of the metallographic microscope, including: the material's organizational morphology, grain size, non-metallic impurities - oxides, sulfides, etc. The content and distribution in the material; the relationship between the organizational structure of the material and its chemical composition; the microstructure of various materials after different processing techniques can be determined; the quality of the material can be judged.
Malleable cast iron is in an annealed state, and graphite is a black flocculent structure, similar to cotton wool, with a relatively regular shape. No etching is performed and the matrix appears white. The test sample is white cast iron green billet. Through annealing solid-state graphitization treatment, primary, secondary, and tertiary cementite are fully graphitized.
Under a metallographic microscope, graphite is a black flake structure. Because it has not been etched, it is basically not displayed and appears white. Graphite is mainly scattered on the matrix in the form of individual flakes, which are mainly separated and not related to each other. . Flake graphite varies in length and has different properties.
Analysis of Impurities in Steel
The analysis of impurities using a metallographic microscope is mostly quantitative analysis, using bright field to observe the color, shape, size and distribution of impurities; using dark field to observe the inherent color and transparency of impurities; using polarized light Various optical properties under orthogonal conditions can be used to observe impurities and then judge the type of impurities. In most cases, silicates alone present a solitary particle shape distribution, while oxides such as aluminum oxide, ferrous oxide, and manganese oxyhydroxide aggregate into groups and present a string-like distribution, while ferrous sulfide, ferrous sulfide·ferrous oxide It is distributed along the grain boundaries.
Phase contrast analysis with polarized light microscopy
In steel structures, sometimes the properties of reflected light are the same or similar, and there is only a small structure on the surface. The two tissues show that when the incident light wave hits them and is reflected, the amplitudes of the two types are basically the same, but their cycles are different. This kind of reflected light with the same amplitude but different phase is difficult to distinguish with the naked eye. The solution is to use an annular light diaphragm and a phase plate to use the transmitted light to reflect or lag 1/4 of the wavelength, thereby producing a positive or negative phase difference. That is to convert the light with a phase difference into a light with a difference in intensity, thereby improving the discrimination ability.
