Metallographic specimen preparation techniques and specific methods

Metallic materials microstructure inspection is a basic project in the physical properties of metal materials testing, the chemical composition of the material, microstructure morphology, grain size, processing technology, heat treatment process and so on are closely related to its physical and mechanical properties, which can be seen in the importance of microstructure inspection of the material. Metallographic examination is through the optical microscope on the grinding, polishing and etching treatment of the specimen to observe, can analyse the real microstructure and morphology characteristics of the specimen, is the basis of the study of the mechanical properties of materials. The microstructure inspection of metal materials is a basic project in the physical property testing of metal materials. In this paper, different types of metal materials, especially high-purity tungsten, high nitrogen stainless steel, automotive steel, coated materials specimen cutting, setting, grinding, polishing and etching process of sample making skills and specific methods are summarised, in order to prepare a better metallographic specimen.

specimen cutting and setting and notes

Specimen cutting and precautions

Specimen cutting has a variety of methods, such as wheel cutting, machining line cutting, etc., according to the need to choose, but the choice of specimen cutting method should not have an impact on the microstructure of the specimen, such as the generation of deformation, overheating organisation, the surface of the coating off and so on. Selection of wheel cutting machine to take samples, sometimes the cutting rate is too fast, insufficient cooling, resulting in changes in the specimen cut surface organisation due to overheating, the need to remove the heat-affected layer caused by the cutter to intercept the specimen in the subsequent process of sample making. Can be ground on the grinder, but in the grinding process, the specimen should be continuously cooled into the water, repeat the grinding, cooling process, until the real organisation layer; can also be rough grinding of the specimen on the grinder, the rough grinding process needs to keep the cooling water unobstructed, but in the grinder grinding speed is slower, and need to spend more time.

Specimen inlay and precautions

When the size of smaller, thinner, too soft, fragile or irregularly shaped specimens need to do edge inspection, need to be set on the specimen first. The specimen should be cleaned before inlaying, so that the specimen and the resin inlay material can be better combined to avoid gaps between the specimen and the resin after inlaying. For surface-modified specimens or composite specimens of two or more materials, choose a resin that is close to the hardness of the surface of the material for inlaying. If the structure of the surface layer of the material is loose, such as oxidised skin, or the combination of the surface layer and the base material is poor, it is necessary to be inlaid. When inlaying, choose the cold inlay resin with small shrinkage rate to ensure that the surface layer of the organisation is well protected.

Specimen grinding and polishing and precautions

As some specimens are smaller, thinner, too soft or fragile, grinding and polishing the normal specimen to be smaller, about grinding and polishing the normal specimen strength of 1/2, can not press the specimen, in order to avoid damage to the grinding surface of the specimen. For fragile specimens, if the force is too large, it will lead to specimen fragmentation, fall off; soft specimens, if the force is too large, it may lead to specimen deformation or grinding traces of heavy, which will affect the observation of microstructure.

Specimen etching and precautions

According to the type of material to choose the appropriate etching solution, especially different types of materials to form a composite material, due to the different corrosion resistance of the material, generally the first poor corrosion resistance of the etching, its observation, and then repeat the etching process, so that the corrosion-resistant material microstructure can be shown in full, and then again on its observation. Etching can be immersed in the specimen into the etching solution or with degreasing cotton dipped in the etching solution to wipe the surface of the specimen.

Sample making techniques and reasonable methods for several metal materials

High-purity tungsten

Tungsten is currently the highest melting point of the metal, the hardness is also large, so it is widely used in the defence and military industry and machinery industry, has become a very important basic material. Due to the high hardness and good wear resistance of tungsten, in the process of metallographic inspection, coarse grinding, fine grinding and polishing take a lot of energy and time. In grinding, a slight uneven force of the hand will result in a tilted grinding surface, so the hand should hold the sample steadily and the force should be even to avoid a tilted grinding surface or multiple grinding surfaces. The sandpaper should be from coarse to fine, each grinding time is not less than 6min, when changing the next sandpaper, rotate the specimen by 90°, perpendicular to the previous grinding marks, until this sandpaper grinding marks evenly cover the entire grinding surface. Rinse the specimen with tap water before polishing, and the polishing time is not less than 10 min. when polishing, the specimen is first polished on the silk polishing cloth sprayed with diamond polishing agent for 8 min, and then turn on the water flow of the polishing machine. The size of the water flow is measured as the specimen polishing process water flow is not splash. Water flow is too large or too small need to be adjusted. Polishing time under the condition of water flow is not less than 2 min. after closing the water flow of the polishing machine, in the polishing disc after shaking off the dry alcohol instead of water as a lubricant to continue to polish for 2 min. the polishing process should be hard, more than the ordinary specimen of the polishing force, and pay attention to the polishing cloth has no damage, otherwise the specimen polishing surface will be penetrating scratches. Once found that the polishing cloth is broken, it should be replaced immediately. Statistics, a high-purity tungsten average sample preparation time of 1.5 ~ 2 hour. High-purity tungsten etching using alkaline reagents, can not be separated from sodium hydroxide, potassium hydroxide, ammonia, potassium ferricyanide and other alkaline substances, etching time and reagent concentration related to the need for 6 ~ 10 s, the etching surface becomes grey and then immediately rinse the specimen, otherwise there will be a phenomenon of over-corrosion of the grain boundaries. It should be noted that the etching agent is alkaline and can not be mixed with acid, because potassium ferricyanide reacts with acid to produce toxic substances, which will cause some harm to the human body. In addition, the etching process must be carried out in the test room with a fume hood, and the operator needs to wear gloves and other protective equipment to ensure safety.

High nitrogen stainless steel

High nitrogen stainless steel is a new type of stainless steel material with nitrogen instead of nickel, the addition of nitrogen improves the strength, plasticity and corrosion resistance of the material, and reduces the cost of the material. Due to its excellent performance, it is widely used in defence industry, power plant, medical equipment, oil and other related fields. Compared with high-purity tungsten, high-nitrogen stainless steel sampling is simpler and less time-consuming, but high-nitrogen stainless steel is very sensitive to etching agents. With the traditional aqua regia etching is prone to the phenomenon of uneven etching, some positions are seriously overcorroded, and some positions of the microstructure has not yet been shown completely, the effect is poor, especially in the preparation of metallographic specimens of high-nitrogen stainless steel welded overlay materials with different nitrogen element content, and its microstructure morphology after etching is shown in Figure 1.

Figure 1: microstructure morphology of high-nitrogen stainless steel weld overlay specimens after aqua regia etching

After exploration, it was found that the better etching reagent is ferric chloride hydrochloric acid glycerol mixed solution, which can effectively solve the problem of uneven etching of high nitrogen stainless steel. The above different proportions of the etching solution, placed at least 12h, so that the solution is fully mixed and reacted. Etching using the dip test method, the high nitrogen stainless steel to be etched surface into the etchant, followed by rapid removal, the hand constantly shaking the specimen, so that the surface dipped in the etching solution in the surface to be etched back and forth, and carefully observe the changes in the surface of the etching surface until the surface of the etching surface of the mirror surface disappears and becomes greyish-white. If the dipping of the etching solution is not enough, you can repeat the dipping, the hand continues to shake the specimen. High nitrogen stainless steel specimen is best to a successful etching, if rinsed off the etchant and blow-drying, the use of optical microscope observation found that the microstructure morphology of the lighter, and then again etching the specimen will appear specimens dipped in the etchant directly after the over-erosion of the phenomenon, the need to repeat the grinding and polishing of the specimen, increasing the workload. Glycerol in many nickel-based alloy specimen etching agent can also play a good role in reconciling the concentration of reagents to improve the corrosion strength of the etching agent, can be made good use of. The microstructure morphology after the use of iron trifluoride hydrochloric acid glycerol etching as shown in Figure 2, can be seen on the surface of the specimen etching uniformity, clear organisation, even in the micro-organisation of different welds, can also clearly show the organisation, the effect is better.

Figure 2: Microstructure morphology of high nitrogen stainless steel weld specimens etched with ferric chloride hydrochloric acid glycerol solution

Automotive steel plate

Automotive steel plate material is thin, easy to tilt the sample, affecting the authenticity of the microstructure, especially ferrite steel plate to test the grain size. Inlay is recommended to use the method of adhesive, that is, in the inlay machine inlay cavity at the bottom end of the top pillar of the adhesive double-sided tape, remove the double-sided tape on the side of the adhesive paper, the automotive steel plate on the double-sided adhesive, finger pressure on the specimen, and then quickly add the inlay material, which can effectively prevent the specimen from tilting. The automotive steel plate specimen with ferrite organisation is very susceptible to scratches during the polishing process. Because the polishing agent abrasive grains in the specimen surface will be accompanied by rolling and extrusion of abrasive grains, polishing cloth surface fibres will be friction on the surface of the specimen, although the polishing process with water lubrication, but the water lubrication is not enough, but also because of the relative hardness of ferrite organisation is relatively low, so the surface of the specimen is easy to produce scratches. Alcohol instead of water as a lubricant, in the polishing process, constantly adding alcohol to the polishing cloth, can effectively avoid the polishing process to produce scratches, improve the efficiency of making samples, to obtain a good effect of the microstructure morphology. For the organisation of pure ferrite automotive steel cold rolled plate specimens, polishing is easy to stain, polishing can be slightly more force, prolong the polishing time. In the speed of 650r-min-1 at least 3min polishing, and then open the polishing machine water flow to continue polishing for 1min, rinsing and blowing dry after etching, can be observed in the shape of a clear ferrite organisation.

Coating materials

In order to improve the mechanical properties of the material surface, improve the hardness of the material and wear-resistant, heat-resistant, corrosion-resistant properties, to meet the requirements of its use in aerospace, automotive, home appliances and other fields, more and more coating, plating, surface deposition and other processes are applied. When cutting coated specimens, it is important to ensure that they are not deformed or broken. If the specimen is to be clamped, the surface coating must be well protected so that the brittle coating is not cracked and the soft coating is not compressed and deformed. When cutting, in order to protect the coating, the cutting direction should be from the surface to the inside, i.e. along the order of resin, coating and base material, otherwise it will cause the coating layer to be broken and fall off. Especially when the structure of the surface layer is loose, or the combination of the surface layer and the base material is poor, it needs to be set first and cut later. Cutting should also be done in the order from the surface to the inside. With a coated, plated specimens need to be set, set according to the hardness of the surface layer to select the inlay material, such as selecting the shrinkage rate of the smaller cold inlay resin, in order to ensure that the surface layer of the organisation is better protected. Zinc - aluminium - magnesium coating specimens with a good combination of parent material, can also be used hot inlay method. Grinding should be carried out in such a way as to ensure that no chamfers are made. In the case of the same amount of grinding, try to grind more and polish less. The principle of the two is different, grinding is the abrasive grain on the surface of the specimen, resulting in plough furrows, the difference in the soft and hard materials is small. Polishing is the abrasive grain in the specimen surface and accompanied by the rolling extrusion of the abrasive grain, and polishing cloth surface fibres on the surface of the specimen friction, the material surface tends to flatten, so the polishing in the material produced by the larger differences in the polishing time is too long, prone to produce steps, affecting the observation of the microstructure of the coating. Polishing agent can be selected according to the softness and hardness of the material, with different particles of diamond or oxide. For multi-layer materials and composite layer materials, it is necessary to ensure that the layers are flat and stepless, and the direction of grinding and polishing should be parallel to the direction of the grinding disc.

Conclusion

Metallographic sample making needs to be adjusted according to the type of material, specimen size, material properties and inspection purpose. High purity tungsten hardness, grinding and polishing more time-consuming, the average sample making time of 1.5 ~ 2 hour; high nitrogen stainless steel using ferric chloride hydrochloric acid glycerol mixture of etching, can be obtained by the etching of uniform microstructure; automotive steel plate sampling to ensure that the specimen is upright, not tilted, and to avoid scratches produced by the polishing process; coating material sampling of cutting, mosaicing and grinding and polishing process should be done to ensure that the coating of the integrity of the protection work.