Selecting suitable and representative samples is an extremely important step in metallographic microscopic analysis, including selecting sampling site, inspection surface, determining interception method and sample size.
1, Selection of sampling position and inspection surface
The selection of sampling location and inspection surface should be based on the purpose of inspection. For example, when analyzing the causes of defects and breakage of metals, samples should be taken at the parts where defects and breakage occur, and samples should be taken at the intact parts for comparison;
Detection of decarburization layer, chemical heat treatment layer, quenching layer and grain size. , should take the cross section; When studying the structure of banded structure and cold plastic deformation workpiece and the deformation of inclusions, the longitudinal section should be cut.
2, the sample interception method
The method of sample interception can be different according to the properties of metal materials. For soft materials, methods such as sawing, turning and planing can be used; For hard materials, grinding wheel slicer or EDM can be used for cutting. For hard and brittle materials, such as white cast iron, hammering can be used; Sampling of large workpieces can be accomplished by oxygen cutting.
When cutting with grinding wheel or EDM, cooling measures should be taken to reduce the structural changes of the sample caused by heating. The deformed layer or combustion layer produced by interception on the sample must be removed in the subsequent process.
3. Sample size and shape
The size and shape of metallographic specimen should be convenient for clamping and grinding. Generally, a cylinder with a diameter of Ф 15 ~ 20mm, a height of 15 ~ 20mm or a cube with a side length of15 ~ 20mm is used.
Second, grinding
It is divided into two steps: rough grinding and fine grinding. After the sample is taken out, it is roughly ground first. For iron and steel materials, the sample can be roughly ground with a grinding wheel first, and for very soft materials (such as aluminum, copper and other non-ferrous metals), it can be leveled with a file. When grinding on the grinding wheel, it is necessary to hold the sample tightly, so that the stress on the sample is uniform and the pressure cannot be too high, and it should be cooled with water at any time to prevent the metal structure from changing due to heating.
In addition, in general, the periphery of the sample should be rounded with a grinding wheel or a file to avoid cutting the sandpaper and polishing cloth when polishing. However, for the samples whose surface structures (such as carburized layer and decarburized layer) need to be observed, the edges should not be rounded, so it is better to embed such samples.
Third, polishing
Objective to remove the grinding marks left by fine grinding on the metallographic grinding surface and make it a smooth and seamless mirror.
Generally speaking, the preparation of metallographic samples goes through the following steps: sampling, inlay (sometimes omitted), polishing (rough grinding and fine grinding), polishing and corrosion.
Every operation must be carried out in strict accordance with the operation requirements, because any operation error may affect the subsequent steps, and in extreme cases, it may also lead to wrong organization, thus drawing wrong conclusions. Metallographic sample preparation is a technology closely related to the experience of sample preparation personnel, and the level of sample preparation personnel determines the quality of sample preparation.
Extended data:
Sampling is the first process of metallographic sample preparation. If the sampling is improper, the inspection purpose will not be achieved. Therefore, the size, position and grinding direction of the samples taken should be strictly in accordance with the corresponding standards. Sampling principle of metallographic samples: selecting representative metallographic samples is the first step of metallographic research, and ignoring the importance of sampling will often affect the success or failure of test results.
1. The location of the intercepted sample must be able to characterize the characteristics of the material or part and the purpose of inspection.
(1) When metallographic analysis is carried out on the fracture causes of parts, samples should be taken at the fracture parts. In order to obtain more information, it is necessary to intercept reference samples far away from the fracture source for comparative study.
(2) For materials or parts with different technological processes or heat treatments, the interception part of samples shall be changed accordingly.
③ To study and analyze the metallographic structure of castings, we must observe them from the surface to the center. According to the difference of microstructure in different parts, the segregation degree of castings can be understood. Small parts can directly intercept the cross section perpendicular to the die wall, and large parts should intercept several samples from the surface to the center on the cross section perpendicular to the die wall.
(4) The sampling of rolled profiles or forgings should consider whether there are decarburization, folding and other defects on the surface and the identification of non-metallic inclusions, so the samples should be intercepted horizontally and vertically.
Transverse samples mainly study the distribution of surface defects and non-metallic inclusions. For very long profiles, samples should be cut at both ends to compare the segregation of inclusions. Longitudinal specimens mainly study the shape of inclusions, identify the types of inclusions, observe the degree of grain growth and estimate the degree of cold deformation during reverse deformation.
⑤ The microstructure of parts after various heat treatments is relatively uniform, so it is only necessary to take samples at any cross section, and at the same time, the surface conditions, such as decarbonization, carburization, surface coating and oxidation, should be considered.
2. Determine the metallographic grinding surface of the sample: the metallographic photos on the research results or test reports should indicate the sampling position and the direction of the grinding surface.
①? The main research contents of the cross section are as follows: a. The change of metallographic structure from the outer edge to the center of the sample. B. Check surface defects, such as oxidation, decarbonization, overburning, folding, etc. Observe the results of surface treatment, such as surface coating, surface quenching, chemical heat treatment, etc. Distribution of nonmetallic inclusions in cross section. E. determination of particle size.
② Main research contents of longitudinal section: A. The number, shape and size of nonmetallic inclusions have a great relationship with the sampling position, so it must be noted that the sampling position can represent the whole material. B. Determine the degree of grain elongation to know the degree of cold deformation of the material. Identify the banded structure of steel and the influence of heat treatment on eliminating banded structure.
3. Interception method of metallographic sample: The sample must be intercepted by appropriate methods to avoid the change of microstructure caused by improper cutting processing.
There are two aspects that must be paid attention to in the possibility of causing organizational change:
① Reverse deformation changes the metallographic structure. For example, the grains in low carbon steel and nonferrous metals are stretched or twisted under compression, deformation twins appear in polycrystalline zinc grains, and slip lines increase in austenitic steel grains. Are easy to happen.
(2) The metallographic structure changes of materials caused by heating, such as quenched martensite structure, are often tempered due to the influence of grinding heat, resulting in tempered martensite. (3) According to the hardness of materials, different methods are used to intercept samples.
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