The preparation process and precautions of metallographic sample are briefly described

Date：2022-09-07 16:46:21Views：753

Metallographic analysis is to observe the sample after grinding, polishing and etching through an optical microscope, which can analyze the true microstructure and morphological characteristics of the sample, and is the basis of the study of material mechanical properties. To a certain extent, the metallographic inspection of ordinary steel is not difficult, but with the application of new materials and technologies, the inspection range is constantly increasing, and the types of samples are also increasing. More countermeasures and technical methods are needed for practitioners.

1、 Sampling

Selecting appropriate and representative samples is an extremely important step for metallographic and microscopic analysis, including selecting sampling positions, inspection surfaces, determining cutting methods and sample sizes.

1. Selection of sampling position and inspection surface

The sampling position and inspection surface shall be selected according to the inspection purpose. For example, when analyzing the causes of metal defects and damages, samples shall be taken at the parts where defects and damages occur, and at the same time, samples shall be taken at the intact parts for comparison;

Transverse cross section shall be taken for testing decarburization layer, chemical heat treated infiltration layer, quenching layer, grain size, etc; The longitudinal section should be taken when studying the deformation of banded structure and cold plastically deformed workpiece and inclusions.

2. Sample cutting method

The sample cutting method may vary according to the properties of metal materials. For soft materials, sawing, turning, planing and other methods can be used; For hard materials, such methods as grinding wheel slicer cutting or electric discharge cutting can be used; For hard and brittle materials, such as white cast iron, hammering can be used; Oxygen cutting and other methods can be used for sampling on large workpieces.

When cutting with grinding wheel or electric spark, cooling measures shall be taken to reduce the change of sample structure caused by heating. The deformation layer or burning layer on the sample caused by cutting must be removed in the subsequent process.

3. Sample size and shape

The size and shape of metallographic specimen shall be easy to hold and grind, and the diameter is usually used ф 15 to 20 mm high, or 15 to 20 mm long cube.

2、 Grind

It is divided into two steps: rough grinding and fine grinding. After the sample is taken down, first carry out rough grinding. If it is steel material, the sample can be roughly ground with a grinding wheel; if it is very soft material (such as aluminum, copper and other non-ferrous metals), it can be filed with a file. When grinding on the grinding wheel, hold the sample tightly to make the stress on the sample uniform, the pressure should not be too large, and water should be used to cool it at any time to prevent the change of metal structure caused by heat.

In addition, in general, the periphery of the sample should be rounded with a grinding wheel or file to avoid scratching the sandpaper and polishing fabric during polishing and polishing. However, for the sample that needs to observe the surface structure (such as carburized layer and decarburized layer), the edge cannot be rounded, and this kind of sample is best inlaid.

3、 Polishing

Objective to remove the grinding marks left by fine grinding on the metallographic grinding surface and make it a smooth and traceless mirror surface.

Generally, the preparation of metallographic samples goes through the following steps: sampling, inlaying (sometimes omitted), polishing (rough grinding and fine grinding), polishing and corrosion.

Every operation must be carried out carefully and strictly according to the operation requirements, because any operation error may affect the subsequent steps, and in extreme cases, it may also cause false organization, so as to draw wrong conclusions. Metallographic sample preparation is a technology closely related to the sample preparation experience of preparers. The level of preparers determines the quality of sample preparation.

matters needing attention:

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, grinding surface direction, etc. of the sample taken shall be in strict accordance with the corresponding standards.

Sampling principle of metallographic samples: selecting representative metallographic samples is the first step of metallographic research. Failure to pay attention to the importance of sampling will often affect the success or failure of test results.

1. The part where the sample is taken must be able to characterize the characteristics of materials or components and the purpose of inspection.

① When metallographic analysis is carried out on the cause of the fracture of the mechanical part, the sample shall be taken at the fracture position of the part. In order to obtain more data, it is also necessary to intercept the reference sample at a position far from the fracture source for comparative study.

② For materials or components with different process or heat treatment, the cut-off position of the sample should also be changed accordingly.

③ To study and analyze the metallographic structure of the casting, it is necessary to observe it from the surface layer to the center at the same time. According to the difference of the structure of each part, the degree of segregation of the casting can be understood. For small parts, the cross section perpendicular to the mold wall can be directly cut, while for large parts, several samples shall be cut from the surface to the center on the cross section perpendicular to the mold wall.

④ The sampling of rolled profiles or forgings should consider whether there are decarburization, folding and other defects on the surface layer, as well as the identification of non-metallic inclusions, so the samples should be taken in the transverse and longitudinal directions.

Transverse samples are mainly used to study the distribution of surface defects and non-metallic inclusions. For long profiles, samples should be cut at both ends to compare the segregation of inclusions; The longitudinal specimen is mainly used to study the shape of inclusions, identify the types of inclusions, observe the grain length, and estimate the degree of cold deformation in the process of reverse deformation.

⑤ The microstructure of the parts subjected to various heat treatments is relatively uniform, so it is only necessary to cut the samples on any section, and the surface conditions, such as decarburization, 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 shall indicate the sampling position and the direction of the grinding surface.

① Main research contents of cross section: A. change of metallographic microstructure from the outer edge to the center of the sample. b. Inspection of surface defects, such as oxidation, decarbonization, overburning, folding, etc. c. Observation of surface treatment results, such as surface coating, surface quenching, chemical heat treatment, etc. d. Distribution of non-metallic inclusions on the cross section. e. Determination of grain size.

② Main research contents of longitudinal section: a. the quantity, shape and size of non-metallic inclusions, and the situation of inclusions have a great relationship with the sampling position, so it must be noted that the sampling position can represent the whole material. b. Measure the degree of grain elongation to understand the degree of cold deformation of the material. c. Identify the banded structure of steel and the effect of heat treatment on eliminating banded structure.

3. Section cutting method of metallographic sample: appropriate method must be adopted for sample cutting to avoid the change of microstructure caused by improper cutting and processing.

There are two possibilities of causing organizational changes that must be noted:

① Reverse deformation changes the metallographic structure. For example, the grains in low-carbon steel and non-ferrous metals are compressed, elongated or distorted, the appearance of deformation contractures in polycrystalline zinc grains, and the increase of slip lines in austenite steel grains are all prone to occur.

② The metallographic structure changes of materials caused by heat, such as quenching martensite, are often tempered by grinding heat, resulting in tempered martensite. ③ According to the hardness of the materials, different methods are used to cut the samples.

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