Friends who have done metallographic experiments and material testing know one sentence: metallographic observation, seven-point sampling, three-point observation.
Qualified metallographic specimens are the basis for clear observation of metal grains, phase change structure, inclusions, defects, and weld structure.
Many people have metallographic images with many scratches, blurred organization, poor contrast, and false positives. In addition to equipment problems, focus on the microscope and parameter settings, and then pay attention to whether the preliminary sample preparation is standardized.
Years of experience in microscopy and related sample preparation equipment based on Instrument Card Ink Optical Instruments
Today, we summarize and analyze the whole process of universal, standardized, and zero redundancy metallographic sampling, which novice can do directly, and veteran can be used for error correction.
Full standard process: sampling (cutting) → inlay (optional) → coarse grinding/fine grinding → polishing → corrosion → cleaning drying → microscopic observation
01 Sampling: the first step in sampling, eliminating the illusion of tissue
The core principle of sampling: retain the true metallographic structure, pay attention to high temperature, mechanical deformation damage. This step is wrong, and subsequent processes cannot be remedied.
1. Select the sampling location: According to the testing requirements, select representative areas, such as heat treatment area, weld area, fracture failure area, material surface, core, etc.
• Wire Cutting (EDM): First Choice! Cryogenic cutting, extremely thin deformation layer, almost no damage to the original tissue
• Water-cooled grinding wheel cutting: suitable for large specimens and must be cooled by water throughout
• Manual sawing: only for soft small metal specimens
3. Standard sample size: side length/diameter 10–25mm, height 3–10mm, convenient for grinding and inlay observation.
Uncooled high-speed cutting, cutting speed too fast, will produce 0.1–0.3mm thermal damage to the deformation layer, resulting in the subsequent observation of "false tissue, false grains", the test results are completely distorted.
Large and regular samples can be omitted from the inlay, but thin sheets, filaments, small pieces, hollow, fragile, and coated samples must be inlaid, otherwise they cannot be flattened, polished, and easily chamfered and chipped.
It adopts phenolic resin and transparent resin, and is pressurized and cured by heating, so it has fast forming, high hardness and strong adaptability.
Process: The sample is placed flat with the grinding surface facing → down. The mold → filling resin is → heated to 130–180 °C, and the mold is kept under pressure for → cooling and demolding.
Contraindications: Not suitable for quenching parts, thermal coatings, phase change materials, high temperatures will change the original metallographic structure.
2. Cold inlay (Precision/sensitive specimens preferred)
Epoxy resin + curing agent curing at room temperature, no high temperature, no pressure, no tissue damage, is the best choice for thin walls, coatings, composites, porous materials.
Process: The specimen is flattened and fixed. The mixture → of resin and curing agent is → slowly poured to prevent air bubbles from standing and curing at → room temperature (12–24h), and the constant temperature accelerated curing can be shortened to 2–4h.
03 Grinding: eliminating the deformation layer and leveling the surface of the specimen
The grinding is divided into rough grinding + fine grinding, and the core purpose is to completely remove the remaining deformation layer, knife marks, and burrs for polishing.
Serial number of ✅ standard sandpaper items (skipping is not recommended)
180 # → 400 # → 600 # → 800 # → 1200 # → 2000 # → 3000 # (depending on the hardness of the sample)
1. Flowing water and wet grinding throughout the whole process: cooling and cooling, washing away abrasive chips to avoid secondary scratches and surface thermal deformation.
2. Each time the sandpaper is changed, the specimen is rotated 90°, and all scratches of the previous process must be completely removed before the fine sandpaper can be upgraded.
3. Lightly grind at a uniform speed, and do not grind under heavy pressure for a long time to prevent deep plastic deformation.
4. Finish grinding to 3000 #, the surface of the specimen presents a uniform, delicate and uniform slight grain, without large scratches can enter the polishing process.
Jumping sandpaper, not changing the polishing direction, excessive pressure, will leave deep scratches, subsequent polishing can not be completely removed, and finally imaging is full of miscellaneous marks.
04 Polishing: Mirror molding, determines the imaging texture
Polishing is a key step in removing subtle scuff marks, eliminating surface deformation, and obtaining a mirror-shiny surface, which directly determines the clarity of the metallographic picture. It is divided into rough throwing and fine throwing.
Rough polishing: canvas polishing cloth + 3μm diamond polish
Finishing: Velvet/wool polishing cloth + 0.25-0.5μm diamond polish
Soft metals (aluminum, copper, lead) can be finished with a silica suspension for a more uniform and fine surface.
1. The polishing disc runs at a low speed, a small amount of polishing agent is added several times, and dry throwing is strictly prohibited.
2. Lightly adhere the specimen to the disk surface, intermittently lift up and observe, avoid fixed-point heavy pressure and continuous polishing for a long time.
3. The total polishing time is recommended to be ≤ 5 minutes, and the overtime is prone to problems such as tail dragging, relief, orange peel, and surface graying.
4. Polishing endpoint judgment: the macroscopic observation is a bright mirror, and there are no scratches or numb spots under the low-power microscope.
05 Corrosion: a key step in making the metallographic structure “visible”
After polishing, the specimen has no bright-dark contrast in grain and phase structure, and cannot be directly observed. The grain boundaries, pearlite, ferrite, martensite and other microstructures can be clearly displayed by using the difference in corrosion resistance of different microstructures and forming microscopic unevenness and convexity through chemical corrosion.
1. After polishing, rinse the sample with water, and wipe with alcohol to remove surface stains;
2. Clamp the specimen into the corrosive agent, slightly shaking at a uniform speed to ensure uniform corrosion;
3. Control the corrosion time (seconds to tens of seconds) according to the material and room temperature;
4. Immediately after the tissue lining reaches the standard, rinse with plenty of water to stop corrosion;
5. Anhydrous ethanol is dehydrated, cold air is quickly blown to dry, and hot air (prone to water stains and oxidation spots) is prohibited.
Insufficient corrosion: the grain boundary is blurred and the organization is not layered; excessive corrosion: overcorrosion of the organization, coarse grains, and the appearance of numbness.
After the sample preparation is completed, confirm that there are no water stains, fingerprints, dust, and oxidation points on the surface of the sample, place it flat on the metallographic microscope stage, and press and fix it. Observations follow the principle of low to high magnification, first overall and then local, step-by-step focusing observation and photographic recording.
One last SOP process (Instrument Card Optical Instrument Nanny Level Tutorial)
Water-cooled precision sampling → On-demand inlay → progressive wet grinding (no sand jumping, commutation grinding) → Coarse polishing + fine polishing Mirror treatment → Fresh water Alcohol cleaning → Precision corrosion → Cold wind blow drying → Microscopic observation
Tell us the material, the feature you need to see and the magnification range. We will recommend a configuration and quote it.
