As is well known, metallographic sample preparation generally includes four processes: sampling, embedding, grinding and polishing, and etching observation. But how about the specific operation? Today, I would like to talk to you about the process of preparing metallographic specimens, as follows:

     1、 Sampling


The selection of samples should be based on the purpose of the study, and representative parts should be selected. After determining the location, the sample can be cut off using a metallographic cutting machine. The size of the sample is usually a cylinder with a diameter of 12-20mm and a height of 12-15mm, or a square sample with a side length of 12-15mm. During the cutting process, it is not advisable to raise the temperature of the sample too much to avoid causing changes in the metal structure and affecting the analysis results.


2、 Inlay


If the sample size is too small or irregular, it may be difficult to grind it directly by hand. At this time, we can choose to use a metallographic inlay machine to embed the sample in bakelite powder.

    3、 Grinding and polishing


Rough grinding: The purpose of rough grinding is to obtain a flat surface. It is usually carried out on a Bench grinder, but attention should be paid to the following points during grinding: the pressure of the sample on the grinding wheel should not be too large, otherwise deep grinding marks will form on the surface of the sample, increasing the difficulty of fine grinding and polishing; Always cool the sample with water to avoid tissue changes caused by heat; If the edges and corners of the sample do not have a preserved surface, they can be rounded (chamfered) first to avoid tearing the sandpaper or polishing cloth during fine grinding and polishing, and even causing the sample to fly out of the metallographic polishing machine and injure people.


Fine grinding: Although the surface of the sample is relatively flat after rough grinding, there are still deep grinding marks. The purpose of precision grinding is to eliminate these grinding marks and obtain a flat and smooth grinding surface, preparing for the next polishing step. Fine grinding is carried out on a set of metallographic sandpaper with varying degrees of thickness, from coarse to fine. During precision grinding, place the sandpaper on a polishing machine with a rotating disc, with the sample ground facing downwards, and press it evenly down onto the sandpaper or polishing cloth using a fixed method of the polishing machine. When replacing finer sandpaper, the grinding direction of the sample must be adjusted 90 °;.


Polishing: Fine ground specimens also need to be polished. The purpose of polishing is to remove the fine grinding marks left by fine grinding and obtain a shiny mirror surface. The polishing of the sample is carried out on a dedicated polishing machine, and the grinding disc speed is generally 10-700r/min (adjustable). During polishing, velvet and other fabrics are laid on the surface of the polishing disc, and polishing agents are continuously added. During polishing, the polished surface of the sample should be evenly pressed onto the rotating polishing disc and continuously move radially from the edge to the center of the disc until a bright mirror surface is obtained.


4、 Erosion observation


Rinse the polished sample thoroughly with water or wipe off any remaining dirt on the surface with alcohol. Then immerse the polished surface of the sample in corrosive agent, or use bamboo or wood clips to hold cotton balls and wipe them with corrosive agent on the polished surface. The polished surface gradually loses its luster; After the corrosion of the sample is suitable, immediately rinse it with water and alcohol, and dry the ground surface of the sample with a hair dryer. Then, it can be observed and analyzed under a microscope. At high magnification, the corrosion should be slightly shallower, while at low magnification, the corrosion should be deeper.


The equipment required for the metallographic sample preparation process used above, including manual (automatic) metallographic cutting machine, fully automatic hot pressing inlay machine, metallographic polishing machine, Leica microscope, etc., can be purchased from Chain Energy Metallographic Experimental Equipment Nanjing Co., Ltd. In addition to metallographic equipment, various metallographic consumables required during the sample preparation process can also be matched in the chain metallography. One stop service allows you to easily prepare samples without any worries.