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Metallography as a science

Metallography is a part of metal science that studies the effect of chemical composition and processing (thermal treatment, chemical-thermal treatment, forming, etc.) on the structure of metals and alloys.

Up-to-date metallography is a complex of qualitative and quantitative methods for analyzing the structure of metallic materials. It uses modern metallographic equipment, computer technology and mathematical processing of experimental data.
The first step of metallographic research is sample preparation. The main task of this step is to create a sample that adequately represents the state of the structure of the material (or product). Since up-to-date metallography has many faces in terms of a variety of research objects, sample preparation may include the following:
• Traditional preparation of a metallographic section. The main task in this case is to create a perfect mirror surface without traces of previous processing (cutting or grinding). This is the most common metallographic preparation method.
• Obtaining a replica. It is an impression of the sample surface. It is done if for one reason or another cutting the sample is impossible. Receiving a replica is preceded by the creation of a metallographic section in the investigated area. Replicas are used in field metallography.
• Obtaining a fracture, which can be studied in various types of microscopes (optical, raster).
• Creation of samples for which special surface preparation is not required, or preparation is carried out in a minimal volume (wiping, degreasing).
Metallographic etching. It is sometimes referred to as sample preparation. However, it is more useful to consider etching as a separate operation to reveal the structure of the sample. As a rule, the research begins with the study of non-etched samples in order to determine the presence of non-metallic inclusions (sulfides, nitrides, oxides), cracks and discontinuities of various origins. The study of objects such as gray cast iron is carried out initially in a non-etched state to determine the morphology and size of the graphite inclusions. Etching is performed only for polished sections. The etching operation begins with the selection of etching method (solution, ion etching, electrolytic etching). The most common is solution etching. It begins with the selection of the appropriate etching reagent and its preparation. Etching in solutions is carried out by immersing a sample in an etching solution or by rubbing in a reagent using a swab. The etching operation ends with the washing and drying of the sample.
Dissection and etching are the preparatory stages of metallographic research.
The metallographic research itself begins with the analysis of the structure of the sample.
Metallographic analysis is subdivided into micro- and macrostructural methods. The macrostructural method is used to study the structure with the naked eye or with optical instruments at low magnifications. Basically, the macrostructure is the location of various macroscopically visible zones in the sample (weld zones, large porosity, surface damage, the nature of metal flow during plastic deformation, etc.). Microstructural analysis uses a microscope to study the structure at high magnifications. This allows us to gain sight of the grain (phase) structure of the metal and the intragranular structure.
Metallography can be both qualitative and quantitative.
Quantitative metallography includes observation and visual analysis, photographing and description of the metal structure, i.e. what we see in the image of the structure. The task of this stage is to determine all the features of the image, to separate the informative part from the artifacts and to obtain high-quality images of the microstructure that would reflect the real structure of the material.
The structure of metallic materials is not always discernible in a simple reflected light. Therefore, a metallographic microscope provides a number of options to make the image of the structure as informative as possible. These options are called optical contrast enhancers and include:
-darkfield microscopy;
-polarizing microscopy (using polarized light);
-phase contrast microscopy;
-luminescence microscopy;
-interference contrast;
- diaphragm;
-color filters.
The application of these methods allows visualization of the details of the image of the structure of materials. Dark-field illumination and polarized light are most commonly used in traditional metallography.
The final stage of metallographic research is quantitative metallography.
Quantitative metallography is an analysis of the image of the metal structure, in which quantitative characteristics are determined: grain size, number of phases or components, porosity, thickness of hardened layers, etc.
Quantitative metallography is supported by modern quantitative image processing software. These programs make it possible to select specific objects to be analyzed - grains, phases, various inclusions, layers, etc. After that the quantitative parameters of the selected objects are determined: area, length and width, perimeter, shape factor etc. The set of parameters is determined by a specific program.
After determining the quantitative parameters, it is possible to analyze the statistical characteristics of objects (grains, phases, etc.). The results are presented in the form of a graph or table, which displays the distribution of detected objects by the selected parameter (area, length, width, etc.).
The methods of quantitative metallography are based on the principles of stereometric metallography. In this part of metallography the main methods for determining the size of structural components, their amount, as well as calculating the composition of alloys based on the data of quantitative analysis are theoretically substantiated.