Sharing of sample selection methods for metallographic analysis

Date：2022-11-07 16:28:00 Views：910

The sample selection is the first step of metallographic analysis. The quality of sample selection directly determines the quality of the experimental results, and its importance is beyond doubt. Metallographic analysis is one of the important means of metal material test and research. Based on the principle of quantitative metallography, the three-dimensional spatial morphology of the alloy structure is determined by the measurement and calculation of the metallographic microstructure of the two-dimensional metallographic sample grinding surface or film, so as to establish the quantitative relationship between the alloy composition, structure and properties.

Generally speaking, the selection of samples includes the selection of sampling position and inspection surface, sample cutting process, and sample size. For the samples with special shape or small size that are difficult to hold, they should also be inlaid or mechanically clamped.

1. Longitudinal sampling:

Longitudinal sampling refers to sampling along the forging and rolling direction of steel. The main inspection contents are: deformation degree of non-metallic inclusions, grain distortion degree, plastic deformation degree, various microstructures after deformation, and overall situation of heat treatment.

2. Horizontal sampling:

The transverse sampling is only perpendicular to the steel forging direction. The main inspection contents are: microstructure, microstructure state, grain size grade, carbide network, surface defect depth, oxide layer depth, decarburization layer depth, corrosion layer depth, surface chemical heat treatment and coating thickness of metal material carbon sulfur analyzer from surface to center.

3. Sampling for defect or failure analysis:

The sample for defect analysis shall include the defective part of the part. For example, it includes the fracture surface when the part is broken, or the cross section of the crack is taken to observe the depth of the crack and the change of the surrounding organization. During sampling, the defects shall not be damaged or even disappear during grinding.

The size of the sample should be less than 400mm for the ground surface and 15-20mm for the height. The sample can be cut by hand saw, grinding wheel cutter, micro slicer, chemical cutting device, electric spark cutter, shear, saw, planer, turning, milling, etc. The carbon sulfur analyzer can also be cut by gas cutting when necessary. Hard and brittle metals can be sampled by hammering. No matter which method is used for cutting, care shall be taken not to change the structure of the specimen due to deformation or overheating. For specimens cut at high temperature, the heat affected part must be removed.

Selection of sampling position and inspection surface of the sample:

The selection of sampling position and inspection surface shall be determined according to the characteristics of the analyzed material, processing technology and heat treatment process. The sampling position, shape and size of samples used for routine inspection in production are clearly specified.

1. In the study of failure analysis materials, samples should be taken from the failure parts and intact parts of the materials according to the causes of failure, so as to facilitate comparative analysis.

2. The research sample is a casting, and its structure difference must be observed from the surface to the center and from the top to the bottom to understand the segregation, and the influence of shrinkage porosity and cooling rate on the structure.

3. For the sampling of rolled profiles or forgings, it shall be considered whether there are decarburization, folding and other defects on the surface layer, as well as the identification of non-metallic inclusions. Therefore, it is necessary to cut samples horizontally and longitudinally. The transverse samples mainly study the distribution of surface defects and non-metallic inclusions. For very long profiles, samples should be taken at both ends to compare the segregation of inclusions. The longitudinal samples mainly study the shape of inclusions; Identify the type of inclusions, observe the extent of grain size, and estimate the extent of cold deformation during reverse deformation.

4. As the metallographic structure of the parts after heat treatment is uniform, samples of any section can be taken, but the sampling of surface chemical heat treatment and coating parts shall be perpendicular to the surface, so as to observe its structure and test its thickness.

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