Metallographic sample preparation

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Metallographic sample preparation

The preparation of metallographic specimen is a very important part of metallographic research, including sample cutting, inlay, polishing, polishing, metallographic microstructure display. Metallographic structure reflects the specific morphology of metal metallography, such as martensite, austenite, ferrite, pearlite and so on.

1. Principle of metallographic sample interception: selecting representative metallographic samples is the first step of metallographic research, and neglecting the importance of sampling will often affect the success or failure of test results. The part of the sample must be able to represent the characteristics of the material or component and the purpose of the inspection. Determine the metallographic grinding surface of the sample: the metallographic photos on the research results or test reports shall indicate the sampling position and grinding surface direction. The size and shape of the metallographic sample shall be subject to the convenience of holding and grinding. Generally, the cylinder with diameter of ф 15-20 mm, height of 15-20 mm or cube with side length of 15-20 mm shall be used.

2. Metallographic specimen inlay is applicable to the compression of thermosetting plastic for the micro metallographic specimen which is not shaped or easy to take, such as wire rod, small pipe, thin plate, hammering fragment, etc. It is not easy to hold during polishing. Inlay into standard size test block by inlay method, then cut and polish. The commonly used inlay methods are low melting point alloy inlay and plastic inlay.

3. The grinding of metallographic sample is divided into two steps: coarse grinding and fine grinding. After the sample is removed, rough grinding is carried out first. Fine grinding is to eliminate the grinding marks produced during rough grinding and prepare for the polishing of the grinding surface of the sample. After the rough ground sample is washed with clean water and dried, the grinding surface is then polished on each metallographic sandpaper from coarse to fine. During grinding, the sandpaper shall be laid on the thick glass plate, the left hand shall hold the sandpaper, and the right hand shall hold the sample, so that the grinding surface is facing down and in contact with the sandpaper. Under the action of slight pressure, the sample shall be pushed forward and grinded with even force, so as to be stable, otherwise the grinding mark will be too deep, and the grinding surface of the sample will be deformed. When the sample is returned, it shall not contact with the sandpaper. In this way, it shall be repeated "one-way and one-way" until the old wear mark on the grinding surface is removed and the new wear mark is uniform. When changing the next finer sandpaper, remove the debris and sand on the sample, and turn the angle of 90 ° to make the new and old wear marks vertical. During grinding, the specimen shall be held tightly so that the stress on the specimen is even and the pressure is not too large, and the specimen shall be cooled with water at any time to prevent the change of metal structure caused by heat.

4. The purpose of polishing the metallographic specimen is to remove the grinding marks left by fine grinding on the metallographic grinding surface and make it a smooth and traceless mirror surface. The polishing of metallographic samples can be divided into three categories: mechanical polishing, electrolytic polishing and chemical polishing. Mechanical polishing is easy and widely used. During polishing, the polishing liquid is continuously dripped on the polishing disc, and the grinding surface of the sample is evenly pressed on the rotating polishing disc during operation. After polishing, the grinding surface of the sample shall be bright without any trace, and the graphite or inclusions shall not be thrown away or have the phenomenon of tail dragging. At this time, the sample is washed with clear water, then washed with anhydrous alcohol, and finally dried with a blower.

5. Structure display: due to the difference of alloy composition and structure in the metal, the corrosion ability is different. After corrosion, there is a certain degree of contrast between various structures, grain boundary and crystal, and the metal structure can be displayed. The commonly used methods of metallographic structure display are: (1) chemical etching; (2) electrolytic etching; (3) special metallographic structure display, of which chemical etching is the most commonly used. If the polished sample is observed directly under the microscope, only one bright light can be seen. Except for some non-metallic inclusions (such as MNS and graphite, etc.), it is impossible to distinguish various components and their morphological characteristics. It is necessary to "etch" the sample surface with etchant to clearly see the real situation of the microstructure. The most commonly used etchant for steel materials is 3% ~ 4% nitric acid alcohol solution or 4% picric acid alcohol solution. The etching method is to immerse the grinding surface of the sample into the etchant or to control the test surface with cotton. The etching time should be appropriate. Generally, it can be stopped when the grinding surface of the sample is dark. If the etching is insufficient, the etching can be repeated. After etching, wash the meter with clear water immediately, then with alcohol, and finally dry it with a blower. The metallographic samples can be observed and analyzed under the microscope. If the etching is excessive, the sample needs to be polished again, or even polished on No. 04 sandpaper before etching.