Samples must be representative and suitable<\/li>\n<\/ul>\nPreparation Steps<\/h4>\n
Sampling \u2192 Mounting \u2192 Grinding \u2192 Polishing \u2192 Etching<\/p>\n
Selection of Metallographic Samples<\/h4>\n
Suitable for inspection items such as grain size, decarburization layer, microstructure, network structure, plating thickness, etc.<\/p>\n
Based on material processing characteristics:<\/strong><\/p>\n\n- Longitudinal sampling:<\/strong> Sampling along the forging\/rolling direction of steel \u2014 suitable for grain size, decarburization layer, microstructure, network structure, and plating thickness inspection<\/li>\n
- Transverse sampling:<\/strong> Sampling perpendicular to the forging\/rolling direction of steel \u2014 suitable for non-metallic inclusions, banded structure, white spots, carbide unevenness, ferrite content, etc.<\/li>\n<\/ul>\n
Based on part load and failure characteristics:<\/strong><\/p>\n\nli>Cut samples from both the failed area and intact area for comparative analysis<\/li>\n<\/ul>\nSpecial sampling:<\/strong><\/p>\n\n- Follow special regulations for special part sampling<\/li>\n
- For carburized automotive gear parts: Martensite and retained austenite sampling at the pitch circle of the tooth face; carbide sampling at the tooth top corner; surface decarburization layer sampling at the tooth root<\/li>\n<\/ul>\n
Sampling Method<\/h4>\n
Use hand saw, abrasive cutting machine, wire cutting machine, turning, planing, and grinding.<\/p>\n
Note:<\/strong> Do not cause structural changes in the sample due to deformation or overheating.<\/p>\nSample Size<\/h4>\n
Typically cylindrical samples with diameter \u03c612~15mm, height 12~15mm, or square samples with side length 12~15mm.<\/p>\n
Sample Mounting<\/h4>\n
Mounting is required for irregular shapes, wire materials, plate materials, small workpieces, surface treatment and\u6e17\u5c42, plating layers, surface decarburization materials, etc.<\/p>\n
Hot Mounting:<\/strong> Heat the mounting material and apply a certain pressure and maintain for a certain time for tight bonding (requires a mounting machine such as XQ-2B mounting machine).<\/p>\nCold Mounting:<\/strong> Stir the cold mounting material evenly, pour into the mounting mold, and remove after solidification for a certain time. Suitable for samples that cannot be heated, softer or low melting point metals.<\/p>\nSample Grinding<\/h4>\n
Grinding Steps:<\/strong><\/p>\nCoarse Grinding (Leveling):<\/strong> The\u622a\u53d6 or selected sample should first be leveled using a grinding wheel or angle grinder to prepare for the next sandpaper grinding. During the leveling process, the sample must be cooled with water so that the metal structure does not change due to heating.<\/p>\nFine Grinding (Polishing):<\/strong> The grinding of samples can be done manually or mechanically. Generally grind to 500# water sandpaper or metallographic sandpaper for macro inspection. For micro metallographic samples, polishing is still required.<\/p>\n\u0627\u0644\u062a\u0644\u0645\u064a\u0639:<\/strong> Remove the fine grinding marks and surface deformation layer left by fine grinding. Common polishing methods include mechanical polishing, electrolytic polishing, and chemical polishing.<\/p>\nAbrasive Material Selection<\/h4>\n
Water Sandpaper (by abrasive):<\/strong> Silicon carbide, aluminum oxide<\/p>\n\n- Coarse grinding:<\/strong> 120#, 140#, 180#, 200#, 240#, 280#, 320#, etc.<\/li>\n
- Fine grinding:<\/strong> 400#, 500#, 600#, 800#, 1000#, 1200#, etc.<\/li>\n
- Ultra-fine grinding:<\/strong> 1600#, 1800#, 2000#, 2500#, 3000#, etc.<\/li>\n<\/ul>\n
Metallographic Sandpaper (dry and wet use):<\/strong><\/p>\n\n- Coarse grinding:<\/strong> 120#, 140#, 180#, 200#, 240#, 280#, 320#, etc.<\/li>\n
- Fine grinding:<\/strong> 400#, 500#, 600#, 800#, 1000#, 1200#, etc.<\/li>\n
- Ultra-fine grinding:<\/strong> 1600#, 1800#, 2000#, 2500#, 3000#, etc.<\/li>\n<\/ul>\n
Manual Grinding<\/h4>\n
After the leveled sample is cleaned and dried, lay the sandpaper flat on smooth glass, metal, or other boards. Make the sample fully contact the sandpaper surface. Grind sequentially from coarse to fine on each grade of sandpaper by hand. Each time the sandpaper is changed, the sample or grinding direction must be rotated 90\u00b0 perpendicular to the old grinding marks.<\/p>\n
Notes for manual grinding:<\/strong><\/p>\n\n- Apply uniform pressure when pushing forward, lift the sample on the return stroke. Do not grind back and forth on the sandpaper<\/li>\n
- Each time the sandpaper is changed, grind until the old grinding marks completely disappear and the new grinding marks are uniform<\/li>\n
- Each time the sandpaper is changed, clean the sample with water to prevent coarse sand particles from being brought to the fine sandpaper<\/li>\n
- Do not apply too much force when grinding; each grinding time should not be too long to avoid overheating the sample<\/li>\n<\/ul>\n
Mechanical grinding<\/strong> uses a mechanical grinding machine. Water cooling is used during grinding, and other steps are the same.<\/p>\nMechanical Polishing<\/h4>\n
Required equipment and materials:<\/strong> Polishing machine, polishing agent, polishing cloth<\/p>\n\n- Polishing agents:<\/strong> Diamond, chromium oxide, aluminum oxide; available in 0.25, 0.5, 1, 3, 7, 9 microns, etc.<\/li>\n
- Polishing cloth:<\/strong> Canvas, velvet, silk, etc.<\/li>\n
- Rough polishing:<\/strong> Use a polishing machine equipped with nylon, velvet, or fine canvas cloth, together with aluminum oxide, magnesium oxide, chromium oxide abrasives. Polishing time: 2~5 minutes. Wash and dry with water after polishing.<\/li>\n
- Fine polishing:<\/strong> Use a polishing machine with nylon silk, velvet, or other fine uniform silk velvet, together with fine polishing powder of different grain sizes or fine diamond abrasive paste for final polishing.<\/li>\n
- Notes:<\/strong> Apply uniform force, heavy first then light, perpendicular to the sample initially, rotate the sample later.<\/li>\n<\/ul>\n
Precautions During Mechanical Polishing<\/h4>\n\n- Use light force; polish back and forth from the center to the edge of the disc, and occasionally add a small amount of polishing powder suspension<\/li>\n
- The humidity of the cloth should be such that the water film on the surface can completely evaporate within 2~3 seconds when the sample is taken off the disc for observation<\/li>\n
- Polish until all grinding marks are completely removed and the surface is mirror-like<\/li>\n
- Wash and dry the polished sample with water to prevent water marks or dirt residue<\/li>\n
- If coarse grinding marks are difficult to remove during polishing, or if pits are found under the microscope after polishing that affect the test results, the sample should be re-ground<\/li>\n<\/ul>\n
Electrolytic Polishing<\/h4>\n
Electrolytic polishing:<\/strong> Based on the principle of anodic dissolution during electrochemical reaction, the sample is used as the anode, and a carbon rod or other material is used as the cathode. Select the appropriate polishing solution, voltage, current, temperature, and time according to the sample material, and obtain a bright surface through electrochemical reaction.<\/p>\nApplicable to:<\/strong> Metals or single-phase alloys with lower hardness, easily deformed alloys such as austenitic stainless steel, high manganese steel, etc.<\/p>\nNot applicable to:<\/strong> Cast iron, inclusion inspection, heavily segregated metal materials.<\/p>\nCommon Electrolytic Polishing Solutions (Table 3-1)<\/h4>\n
\n\n| No.<\/th>\n | Composition<\/th>\n | Polishing Specifications<\/th>\n | \u0627\u0644\u062a\u0637\u0628\u064a\u0642<\/th>\n | Notes<\/th>\n<\/tr>\n |
\n| 1<\/td>\n | Perchloric acid 15%~20%, Alcohol 80%~85%<\/td>\n | Current density (0.1~0.3) A\/cm\u00b2, below 50\u00b0C, time 15~90 min<\/td>\n | Steel<\/td>\n | When preparing, slowly add perchloric acid to alcohol<\/td>\n<\/tr>\n |
\n| 2<\/td>\n | Perchloric acid 18%~20%, Acetic acid 80%~85%<\/td>\n | Voltage 20~40V, temperature 70~80\u00b0C, current density (0.7~1.0) A\/cm\u00b2<\/td>\n | Steel<\/td>\n | When preparing, slowly add perchloric acid to acetic acid<\/td>\n<\/tr>\n |
\n| 3<\/td>\n | Phosphoric acid 48% (by weight), Glycerin 50% (by weight), Water 2% (by weight)<\/td>\n | Current density 1~5 A\/cm\u00b2, temperature 70~80\u00b0C, time 1~3 min<\/td>\n | \u0627\u0644\u0641\u0648\u0644\u0627\u0630 \u0627\u0644\u0645\u0642\u0627\u0648\u0645 \u0644\u0644\u0635\u062f\u0623<\/td>\n | Lead as cathode<\/td>\n<\/tr>\n<\/table>\nChemical Polishing<\/h4>\nUse chemical reagents to dissolve the sample surface unevenly to obtain a bright surface. This method can only make the sample surface smooth, not achieve the surface flatness requirement.<\/p>\n Common Chemical Polishing Solutions<\/h4>\n\n\n| No.<\/th>\n | Composition<\/th>\n | Polishing Specifications<\/th>\n | \u0627\u0644\u062a\u0637\u0628\u064a\u0642<\/th>\n<\/tr>\n | \n| 1<\/td>\n | Nitric acid (specific gravity 1.33) 30ml, Hydrofluoric acid (specific gravity 1.12) 70ml, Water 300ml<\/td>\n | Temperature 60\u00b0C<\/td>\n | Low carbon steel<\/td>\n<\/tr>\n | \n| 2<\/td>\n | Hydrogen peroxide 100ml, Oxalic acid 2.5~3g, Hydrofluoric acid 5~10ml<\/td>\n | \u2014<\/td>\n | Low alloy steel<\/td>\n<\/tr>\n | \n| 3<\/td>\n | Hydrochloric acid 30% (by weight), Sulfuric acid 40% (by weight), Titanium chloride 5.5% (by weight), Water 24.5% (by weight)<\/td>\n | Temperature 70~80\u00b0C<\/td>\n | \u0627\u0644\u0641\u0648\u0644\u0627\u0630 \u0627\u0644\u0645\u0642\u0627\u0648\u0645 \u0644\u0644\u0635\u062f\u0623<\/td>\n<\/tr>\n<\/table>\nSample Etching<\/h4>\nIf the polished sample is observed directly under the metallographic microscope, only a bright light can be seen. Except for certain non-metallic inclusions (MnS and graphite, etc.), it is impossible to distinguish various constituents and their morphological characteristics.<\/p>\n An etchant must be used to etch the sample surface to clearly show the true situation of the microstructure.<\/p>\n The most commonly used etchant for steel materials is 1~5% nitric acid alcohol solution.<\/p>\n The most commonly used metallographic structure display method is chemical etching.<\/p>\n Chemical Etching Method<\/h4>\n\n- Etching time:<\/strong> Depends on the nature of the metal material, the concentration of the etching solution, the test temperature, etc. It should be appropriate to clearly display the metal structure under the microscope.<\/li>\n
- After etching, quickly rinse with water, then clean with absolute alcohol and dry with hot air. If the etching is insufficient, continue etching or re-polish and then etch; if over-etched, generally re-polish and then etch; if severely over-etched, re-grind, polish, and then etch.<\/li>\n
- The etched sample should be observed and photographed immediately.<\/li>\n<\/ul>\n
Common Metallographic Chemical Etchants (Table 3-2)<\/h4>\n\n\n| Etchant Name<\/th>\n | Composition<\/th>\n | Applicable Range<\/th>\n<\/tr>\n | \n| Nitric acid alcohol solution<\/td>\n | Nitric acid 1-5mL, Alcohol 100mL<\/td>\n | Quenched martensite, pearlite, cast iron, etc.<\/td>\n<\/tr>\n | \n| Picric acid alcohol solution<\/td>\n | Picric acid 4g, Alcohol 100mL<\/td>\n | Pearlite, martensite, bainite, cementite, etc.<\/td>\n<\/tr>\n | \n| Hydrochloric acid, picric acid alcohol solution<\/td>\n | Hydrochloric acid 5mL, Picric acid 1g, Alcohol 100mL<\/td>\n | Tempered martensite and austenite grain<\/td>\n<\/tr>\n | \n| Hydrochloric acid nitric acid solution<\/td>\n | Hydrochloric acid 10mL, Nitric acid 3mL, Alcohol 100mL<\/td>\n | High-speed steel after tempering, nitrided layer, carbonitrided layer<\/td>\n<\/tr>\n | \n| Ferric chloride, hydrochloric acid aqueous solution<\/td>\n | Ferric chloride 5g, Hydrochloric acid 50mL, Water 100mL<\/td>\n | Austenitic stainless steel, austenite-ferrite austenite<\/td>\n<\/tr>\n | \n| Ferric chloride hydrochloric acid aqueous solution<\/td>\n | Ferric chloride 5g, Hydrochloric acid 15mL, Water 100mL<\/td>\n | Pure copper, brass and other copper alloys<\/td>\n<\/tr>\n | \n| Sodium hydroxide aqueous solution<\/td>\n | Sodium hydroxide 1g, Water 100mL<\/td>\n | Aluminum and aluminum alloys<\/td>\n<\/tr>\n<\/table>\nElectrolytic Etching Method<\/h4>\nSuitable for alloys with extremely high chemical stability, such as stainless steel, heat-resistant steel, etc.<\/p>\n Common Metallographic Electrolytic Etchants (Table 3-3)<\/h4>\n\n\n| Electrolyte Composition<\/th>\n | Current Density (A\/cm\u00b2)<\/th>\n | Time (s)<\/th>\n | Cathode<\/th>\n | \u0627\u0644\u062a\u0637\u0628\u064a\u0642<\/th>\n<\/tr>\n | \n| Potassium ferricyanide 10g, Water 90mL<\/td>\n | 0.2-0.3<\/td>\n | 40-80<\/td>\n | \u2014<\/td>\n | Stainless steel, High-speed steel<\/td>\n<\/tr>\n | \n| Oxalic acid 10g, Water 100mL<\/td>\n | 0.1-0.3<\/td>\n | 40-60<\/td>\n | Platinum<\/td>\n | Heat-resistant steel, stainless steel<\/td>\n<\/tr>\n | \n| CrO\u2083 10g, Water 90mL<\/td>\n | 0.2-0.3<\/td>\n | 30-70<\/td>\n | \u2014<\/td>\n | Stainless steel, high alloy steel, high-speed steel<\/td>\n<\/tr>\n<\/table>\nOn-site Metallographic Inspection<\/h4>\n\n- Directly perform inspection on the workpiece, including grinding \u2192 polishing \u2192 etching \u2192 observation \u2192 photographing<\/li>\n
- When the position is not convenient for direct inspection, prepare the sample and then make a replica (AC paper or organic glass sheet), and bring it to the laboratory for observation<\/li>\n
- Materials needed for on-site sample preparation: angle grinder, electric hand grinder, small grinding wheels of different grain sizes, sandpaper patches, polishing cloth patches, grinding paste, reagents, etc.<\/li>\n<\/ul>\n
\nSection 2: Metallographic Microscope<\/h3>\nMicroscope Overview<\/h4>\nMicroscopes used for reflected light illumination of opaque objects are generally referred to as metallographic microscopes.<\/p>\n The evolution of microscopes includes: mid-19th century microscopes, early 20th century microscopes, microscopes with image analysis, Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM).<\/p>\n Amplification Principle of Metallographic Microscope<\/h4>\n\n- The microscope consists of two lenses (objective and eyepiece). The multi-group convex lenses of the objective and eyepiece progressively amplify the object image and reflect it on the retina.<\/li>\n
- The total magnification of the microscope is the product of the magnification of the objective and the magnification of the eyepiece.<\/li>\n
- Mo = f1 \/ fo, where Mo represents the objective magnification, f1 represents the tube lens focal length, and fo represents the objective focal length.<\/li>\n<\/ul>\n
Main Indicators<\/h4>\nInclude resolution capability, total magnification, depth of field, field width, image brightness, working distance, etc.<\/p>\n Composition and Main Components<\/h4>\nThe microscope consists of optical and mechanical systems.<\/p>\n \n- Optical system:<\/strong> Light source, aperture diaphragm, field diaphragm, condenser, objective, eyepiece, etc.<\/li>\n
- Mechanical system:<\/strong> Focusing mechanism, stage.<\/li>\n<\/ul>\n
Optical System<\/h4>\nThe objective is the core optical component of the microscope. It is composed of different shaped lenses made of various types of glass. The plano-convex lens at the front of the objective is called the front lens, used for magnification. The other lenses below it are correction lenses, used to correct various optical defects caused by the front lens.<\/p>\n Example objective marking (NIKON PLAN APO 60X 0.95 DIC M \u221e\/0.11-0.23 WD 0.15):<\/strong><\/p>\n\n- NIKON \u2014 Manufacturer: Nikon Corporation<\/li>\n
- PLAN APO \u2014 Objective name: Plan Apochromat objective<\/li>\n
- 60X \u2014 Magnification: 60x<\/li>\n
- 0.95 \u2014 Numerical aperture: 0.95<\/li>\n
- DIC M \u2014 Function: Can perform DIC<\/li>\n
- \u221e\/0.11-0.23 \u2014 Indicates infinity tube length \/ cover glass thickness<\/li>\n
- WD 0.15 \u2014 Indicates working distance<\/li>\n<\/ul>\n
Objective Classification<\/h4>\n\n- By medium:<\/strong> Dry (medium is air), Wet\/oil immersion (medium is high refractive index liquid)<\/li>\n
- By working distance:<\/strong> Normal objective, Long working distance objective<\/li>\n
- By function:<\/strong> Phase contrast objective, DIC objective, HMC objective, polarizing objective, multi-function objective<\/li>\n
- By chromatic aberration correction:<\/strong> Achromatic, Plan achromatic, Semi-apochromatic, Apochromatic<\/li>\n
- For special purposes:<\/strong> Water immersion objective, TIRF dedicated objective, super fluorescence objective, other special objectives<\/li>\n<\/ul>\n
Eyepiece<\/h4>\nThe eyepiece is mainly used to further magnify the image already magnified by the objective. Divided into normal eyepiece, correction eyepiece, and projection eyepiece.<\/p>\n \n- Why can’t the eyepiece improve resolution?<\/strong> It only magnifies the image formed by the objective on the intermediate focal plane of the eyepiece, and does not directly image with the object.<\/li>\n
- Why are standard eyepieces all 10x?<\/strong> The field of view is appropriate, and the magnification matches well.<\/li>\n<\/ul>\n
Illumination System<\/h4>\nThere are two methods of observing objects: 45\u00b0 plane glass reflection and prism total reflection. Both methods are to make the light vertically turn and project onto the object. The illumination methods in metallographic work are divided into bright field and dark field illumination.<\/p>\n How to judge the quality of the illumination system?<\/strong> High lens transmittance, uniform light, high utilization rate.<\/p>\nLight Sources<\/h4>\n\n- Halogen lamp:<\/strong> Thermal excitation luminescence, continuous spectrum, visible + UV + IR, brighter peak than LED, economical, good overall effect, but short lifespan of 2000h.<\/li>\n
- Mercury lamp:<\/strong> Atomic excitation luminescence, line spectrum, bands at 365nm, 420nm, 455nm, high heat and temperature, selecting the filter is key, lifespan 200h.<\/li>\n
- Xenon lamp:<\/strong> White light monochromaticity, higher color temperature, intensity and brightness than halogen lamp, transmitted through light guide, lifespan 2000h.<\/li>\n
- LED:<\/strong> White light, lifespan 10000h, not suitable for research-grade microscopes.<\/li>\n
- Laser:<\/strong> Monochromaticity, high brightness, good convergence, good polarization, but expensive.<\/li>\n<\/ul>\n
Diaphragms<\/h4>\n\n- Aperture diaphragm:<\/strong> Located before the condenser lens of the light source. Controls the thickness of the incident beam to change the numerical aperture of the objective. When the aperture diaphragm is enlarged, the resolution improves, and the image contrast decreases. Generally, it should be adjusted so that the light beam fills the rear lens of the objective.<\/li>\n
- Field diaphragm:<\/strong> Located after the aperture diaphragm. After passing through the optical system, it is imaged on the metallographic grinding surface. Adjusting the field diaphragm can change the size of the observation field. The smaller the field diaphragm, the better the image contrast.<\/li>\n<\/ul>\n
Filters<\/h4>\nAn important auxiliary tool for metallographic microscope photography. Its function is to absorb unwanted wavelengths of light from the white light emitted by the light source and only allow the desired wavelengths to pass, to obtain light of a certain color, thereby obtaining metallographic pictures that can clearly express various constituents.<\/p>\n \n- Improve the resolution of colored structures during photography<\/li>\n
- Correct residual chromatic aberration<\/li>\n
- Improve resolution<\/li>\n<\/ul>\n
Main Types of Metallographic Microscopes<\/h4>\n\n- Stereo microscope:<\/strong> Large field of view diameter, large depth of field, long working distance. Mainly used for macroscopic inspection of products, can inspect cracks, porosity, and other macroscopic defects in fractures.<\/li>\n
- Upright microscope:<\/strong> Sample observation surface placed upward; the sample observation surface must be parallel to the bottom surface to ensure alignment with the objective optical axis; the upward observation surface is not easily damaged; the sample is limited by height and shape; convenient operation, suitable for rapid inspection, photography, and image capture with high anti-vibration requirements.<\/li>\n
- Inverted microscope:<\/strong> Sample observation surface placed downward; the sample observation surface is always perpendicular to the objective optical axis; the downward observation surface is perpendicular to the stage surface and easily damaged; the sample is not limited by height and shape; not suitable for rapid inspection; good anti-vibration performance for photography and image capture.<\/li>\n<\/ul>\n
Operation and Maintenance of Metallographic Microscope<\/h4>\nOperation:<\/strong><\/p>\n\n- Fully understand the structure and principle of the instrument and equipment, use method, and strictly follow the operating procedures.<\/li>\n
- Hands must be clean during operation; the observation surface of the sample should be rinsed with alcohol and dried.<\/li>\n
- Select the objective and eyepiece according to the required magnification. Handle the lens gently. Unused lenses should be placed in the box immediately. Do not touch the lens with hands.<\/li>\n
- When observing under the microscope, generally start with low magnification 50x~100x, then use high magnification for detailed observation of certain details.<\/li>\n
- For special needs, special illumination methods such as dark field, interference, etc. can be used.<\/li>\n
- When adjusting the focus, first gently turn the coarse adjustment to bring the objective and observation surface as close as possible, then focus from the eyepiece, and then gently turn the fine adjustment until the image is clear. During adjustment, avoid collision between the objective and the sample grinding surface to prevent damage to the lens.<\/li>\n
- After use, promptly remove the objective and eyepiece, place them in the lens box, and finally disconnect the power.<\/li>\n<\/ul>\n
Maintenance:<\/strong><\/p>\n\n- The working location of the metallographic microscope must be dry, low-dust, low-vibration. Do not place in dark, damp places, and do not expose to sunlight.<\/li>\n
- Do not place near volatile, corrosive chemicals to avoid corrosive environment.<\/li>\n
- Residual liquid and oil on the sample must be cleaned during operation. If the lens is accidentally contaminated, immediately wipe it clean with cotton. Oil lenses should be carefully wiped clean with xylene after use.<\/li>\n
- The illumination bulb must be connected to a 6~8V transformer. Do not plug directly into 220V power to avoid burning out the bulb.<\/li>\n
- When rotating the coarse and fine adjustment wheels, move slowly. If a fault is encountered, report it immediately. Do not force it to avoid damaging the mechanism.<\/li>\n
- Objectives and eyepieces should generally be stored in a desiccator. If there is dust, use a dust blower to clean, then wipe with lens paper.<\/li>\n
- Dark, damp air is very harmful to the microscope, causing parts to rust and mold, leading to\u62a5\u5e9f. During the rainy season, all parts of the microscope should be protected against mold.<\/li>\n
- Do not disassemble the mechanical parts at will. Regularly add lubricating grease to ensure normal operation.<\/li>\n<\/ul>\n
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