1、 Determination of sampling location:
The sample for metallographic observation should be taken from the parts of the sample that are prone to various defects, such as steel: according to the chemical composition of the steel, the pouring method and conditions of the ingot, as well as the deformation method, the shape and size of the finished product, there are different types, sizes, and distribution of macroscopic defects in the steel. The selection of samples should be based on the purpose of the inspection. Traditional steel selection generally follows these principles: 1) When observing surface defects, such as grinding cracks, heat treatment cracks, etc., select the outer surface of steel or steel products for metallographic preparation; 2) During sampling inspection, samples should be cut from both ends of the steel separately; 3) When analyzing and inspecting ingots and billets, one longitudinal plane and two or three (both ends or upper, middle, and lower) cross sections should be selected. For example, defects such as white spots, segregation, subcutaneous bubbles, porosity, residual shrinkage pores, and intergranular cracks along the axis in steel can be observed on the cross-section; The strip structure in steel can be observed on longitudinal specimens. 4) The samples for failure analysis and defect detection should not only be sampled at the defect location, but also in the normal area for comparison. During this rough sampling process, it is important to be careful not to cause changes in the metallographic structure of the area to be tested, such as extrusion deformation, grain growth, and cracks, due to overheating during the cutting process. The traditional reference distance between the test surface and the cutting surface during sample cutting is: not less than 20mm during thermal cutting; Not less than 10mm during cold cutting; When burning and cutting, it should not be less than 40mm. When sampling horizontally, the sample thickness is generally 20mm, and the test surface should be perpendicular to the longitudinal axis of the steel (billet) (the plane for metallographic examination should be perpendicular to the deformation direction).
In summary, the selected samples must be representative of metallographic observation, and specific operations can refer to the technical requirements of specific materials as sampling guidance.
2、 Grinding wheel cutting:
The cutting methods for the sample include cutting, sawing, cutting, etc. The tools available include metallographic cutting machines, band saws, electric drills, hand saws, or wire cutting, among other similar methods to cut from larger material blocks and then send them to the laboratory for a series of sample preparation processes. During grinding wheel cutting, there should be a circulating flow of cooling and lubricating fluid through the middle range, which not only can obtain a relatively smooth cutting surface, but also can avoid thermal deformation of the material surface.

The grinding wheel cutting blade of the metallographic cutting machine shall be selected according to the recommendation of the manufacturer of the grinding wheel cutting machine, such as ordinary Tool steel, carburized steel with a hardness of more than 60HRC, steel with a temperature of about 50HRC, steel with a temperature of 35~50HRC, steel with a temperature of 15~35HRC, or mild steel or annealed steel with a temperature of 50~90HRC, and Tool steel and hard steel with a hardness of less than 45HRC, as well as high-temperature alloy, etc; For non ferrous alloys such as titanium, aluminum, magnesium, and other moderately hard metal materials, SiC grinding wheel cutting blades should be chosen as much as possible.
The cutting medium particles of the grinding wheel cutting blade are mainly aluminum oxide or silicon carbide, and the bonding agents used include resin, rubber or a mixture of resin and rubber. Aluminum oxide (Al2O3) grinding wheel cutting blades are more suitable for cutting iron based metal materials, while silicon carbide grinding wheel cutting blades are more suitable for cutting non iron based metal and ore materials. In addition, the grinding wheel cutting blade distinguishes different bonding strengths, and the recommended grinding wheel cutting blade by the manufacturer is determined based on the appropriate bonding strength and the type of material being cut.
In addition, selecting the appropriate diameter and thickness of the grinding wheel cutting blade based on the size and sampling requirements of the cut sample is also a key factor in obtaining high-quality samples.
4、 Cutting method and cutting path:
In order to better cut the sample, the workpiece must come into contact with the cutting blade with a smaller contact surface. For smaller components, it is relatively easy to cut, while for larger components, it may not be easy to cut in the better direction and position. When a circular cutting blade cuts a circular bar in a straight cutting manner, the initial contact surface is very small, but as the cutting continues, the contact surface of the cutting becomes larger and larger, reaching the diameter of the circular sample. Then, the cutting contact surface gradually decreases until the cutting is completed, maintaining a constant load to maintain the pressure that the cutting blade bears during the cutting process does not change with the decrease or increase of the cutting contact surface. If the pressure is not appropriate, the heat generated by friction may not be easily carried away, resulting in deformation damage, phase changes, and may lead to burns or melting.
Cutting method and cutting path:
(1) Direct cutting: Traditional form of cutting, where the curvature of the contact trajectory of the cutting blade is determined by the size of the specimen, is not suitable for cutting larger workpieces and materials with higher hardness.
(2) Pulse cutting: The contact trajectory of the cutting blade is still determined by the size of the workpiece. The vibration generated by the pulse motion load peels off the cutting particles, and the cutting blade is usually prone to wear.
(3) Horizontal feed cutting: The curvature of the contact trajectory of the cutting blade can be accurately controlled through depth increment, and the transverse direction must exceed the length of the component to avoid the curvature of the contact area of the cutting blade. The equipment must set cutting parameters every time, and the cutting process must be carried out slowly.
(4) Rail cutting: Similar to lateral incremental cutting, but the contact trajectory is curved. The operation is simple and fast, and due to the minimum curvature of the contact area, the size of the part appears insignificant.