Metallographic sample preparation is the prelude to microstructure identification. It is a process to make materials meet the requirements of metallographic observation through sampling, polishing, polishing, erosion and other steps. The prepared sample must have a clear field of view and a true tissue morphology, laying a good foundation for microstructure identification.

    1. The preparation process of metallographic specimens: This process is not static and can be increased or decreased according to different inspection purposes.

    1) Grinding of specimens

    Attention: During manual fine grinding, regardless of the thickness of the metallographic sandpaper, the entire mark left on the sandpaper after grinding the sample should have the same color, width, and size as the grinding surface of the sample, and there should be no arc marks. Only in this way can it be possible to ensure the flatness of the entire grinding surface, reduce the phenomenon of sample chamfering, and lay a good foundation for microstructure evaluation, especially for surface treatment surface microstructure evaluation

    2) Polishing cloth and polishing fluid

    The selection of polishing fabrics and polishing fluids can be determined based on the actual conditions of each laboratory and the habits of the operator.

    Attention:

    When polishing the sample, the surface often adheres to dirt, which is adhered to the surface of the sample during high-speed rotation during polishing, so it cannot be cleaned with water and alcohol. When observed under a microscope, its characteristics are:

    Regular and dense black dots or bright circles;

    Regular and relatively sparse black rice grains or ellipsoids.

    To eliminate this defect, in the later stage of polishing, pour a small amount of clean water into the center of the polishing disc. Feel the sample and the polishing fabric lightly until the surface of the sample is clean. At this time, the sample should continuously rotate.

    3. Selection of erosion time: The duration of sample erosion is related to the material, treatment status, and the degree of freshness of the etchant.

    1) The erosion time is too short, and the microstructure cannot be effectively displayed;

    2) If the erosion time is too long and the microstructure is unclear, it has a significant impact on the correct identification and accurate evaluation of microstructure;

    3) A suitable sample should be etched, and its microstructure should be clear at a glance, giving a fresh and comfortable feeling when observed.

   4) For the equilibrium structure of iron-carbon alloys, the carbon content ranges from low to high, and the erosion time ranges from long to short (industrial pure iron takes about 20s, while carbon steel above eutectoid steel takes about 10S-15s). The color change on the surface of the sample ranges from silver gray to flower color (the microstructure of other heat treated carbon steel samples takes about 10s, and the color is dark gray)