How to grind the metallographic specimen? Detailed step process

Date：2022-11-04 14:08:00Views：1056

Metallographic section, also known as section, is a sample preparation method that uses a special liquid resin to wrap and seal the sample, and then carries out grinding and polishing. The detection process includes sampling, sealing, grinding, polishing, and finally providing morphological photos, crack delamination size judgment, or size and other data. It is the most commonly used sample preparation method to observe the cross-sectional structure of samples. Next, we will briefly introduce the steps of metallographic sample preparation and grinding. Let's have a look!

Metallographic specimen preparation is a very important part of metallographic research, which includes specimen interception, specimen inlay, specimen polishing, specimen polishing, and metallographic microstructure display. The following are discussed respectively:

To correctly inspect and analyze the microstructure of metals, excellent metallographic samples must be available. The prepared sample shall be able to observe the real structure, without abrasion marks, pitting and water marks, and keep the inclusions, graphite, etc. in the metal structure from falling off. Otherwise, the correctness of microscopic analysis will be seriously affected. The preparation of metallographic samples includes several steps, such as sampling, grinding, polishing, and microstructure display (etching).

1. Grinding

There are two steps: coarse grinding and fine grinding. After the sample is removed, rough grinding shall be carried out first. If it is steel material sample, rough grinding wheel can be used first. If it is very soft material (such as aluminum, copper and other non-ferrous metals), file can be used to smooth. When grinding on the grinding wheel, hold the sample tightly to make the sample bear uniform force and not too much pressure, and cool it with water at any time to prevent the change of metal structure caused by heating. In addition, in general, the circumference of the sample shall be rounded with grinding wheel or file to avoid scratching the sandpaper and polished fabric during polishing and polishing. However, for the samples that need to observe the surface structure (such as carburized layer and decarburized layer), the edges cannot be rounded, and such samples are easy to be inlaid.

Fine grinding is to eliminate the wear 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 blown dry, the ground surface shall be polished on the metallographic abrasive paper from coarse to fine. The number of commonly used sandpaper is 01, 02, 03 and 04. The smaller the sandpaper is, the coarser the abrasive particles are, and the larger the sandpaper is, the finer the abrasive particles are. During grinding, the sandpaper shall be laid flat 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 downward and in contact with the sandpaper. The sample shall be pushed forward under slight pressure with uniform force to ensure stability, 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. This shall be repeated "one-way and one-way" until the old wear marks on the grinding surface are removed and the new wear marks are uniform. When replacing the next finer sandpaper, remove the wear debris and sand particles from the sample, and turn it 90 ° to make the new and old wear marks vertical.

In addition to making the surface smooth and flat, it is more important to reduce the surface damage as much as possible. For each polishing process, the deformation layer caused by the previous process must be removed (at least the deformation layer produced by the previous process should be reduced to the depth of the deformation layer produced by this process), rather than only removing the wear marks of the previous process; At the same time, the damage of this process itself should be minimized to facilitate the next process. The depth of the deformation layer produced in the last polishing process should be very shallow, which should be removed in the next polishing process.

When grinding cast iron samples, a thin layer of graphite or soap can be coated on the sandpaper as a lubricant to prevent graphite from falling off or tail dragging. When grinding soft non-ferrous metal samples, in order to prevent the abrasive particles from embedding into the soft metal and reduce the scratch of the grinding surface, a layer of engine oil, gasoline, soap solution or glycerin solution can be coated on the sandpaper as a lubricant.

2. Mechanical polishing

Mechanical polishing is carried out on a special polishing machine, which is mainly composed of a motor and a polishing disc（ Ф 200~300mm), and the speed of the polishing disc is more than 200~600r/min. Fine canvas, woolen cloth, silk, etc. are laid on the polishing disc. During polishing, continuously drip polishing liquid on the polishing disc. Al2O3, MgO or Cr2O3 and other fine powders (particle size is about 0.3-1) are usually used as polishing solution μ m) A suspension in water. Mechanical polishing is to eliminate wear marks by relative grinding and hydraulic action between extremely fine polishing powder and grinding surface. During the operation, the grinding surface of the sample shall be uniformly pressed on the rotating polishing disc, and the disc shall continuously move radially and reciprocally from the edge to the center. The polishing time is generally 3~5min. After polishing, the polished surface of the sample shall be bright without trace, and the graphite or inclusions shall not be thrown away or have tail. At this time, wash the specimen with clean water, then clean the grinding surface with anhydrous alcohol, and finally dry it with a hair dryer.

3. Organization display

The etching method is to immerse the ground surface of the sample into the etchant, or use cotton to dip the etchant to control the test surface. Etching time shall be appropriate. Generally, it can be stopped when the grinding surface of the sample becomes dark. If the etching is insufficient, the etching can be repeated. After etching, rinse the tube with clean water, then with alcohol, and finally dry it with a hair dryer. The metallographic sample thus prepared can be observed and analyzed under a microscope. If the etching is excessive, the sample needs to be polished again, even polished on No. 04 sandpaper, and then etched.

The above is related to the preparation and grinding of metallographic samples for wound core testing. I hope it will be helpful to you. Our company has a team of professional engineers and industry elites, and has built three standardized laboratories with an area of more than 1800 square meters, which can undertake a variety of test projects such as electronic component test verification, IC authenticity identification, product design material selection, failure analysis, functional testing, factory incoming material inspection and tape weaving.