第 3 章：金相检测技术与设备

第 3 章：金相检测技术与设备

Section 1: Preparation of Metallographic Specimens

1.1 Specimen Selection

According to GB/T 135298-2015 “Metal Microstructure Testing Method”, specimen selection should be based on:

• Manufacturing method, testing purpose, technical conditions, or mutual agreements
• Material processing characteristics
• Inspection items: grain size, decarburization layer, non-metallic inclusions, etc.

Sampling directions:

• Longitudinal sampling: Along the steel forging/rolling direction
• Transverse sampling: Perpendicular to the forging/rolling direction
• For gear components: carbide detection at the tooth top corner, surface decarburization detection at the root

Sampling precautions:

• Avoid specimen deformation or overheating during sampling that may cause microstructural changes
• For irregular shapes, wires, plates, small workpieces, surface treatments, carburized layers, plated layers, and surface decarburization materials, special handling is required

1.2 Specimen Mounting (Embedding)

For specimens that are irregular in shape or difficult to hold, mounting is necessary:

• Cold mounting: Using epoxy resin or other cold-mounting materials, suitable for specimens that cannot be heated, soft metals, or low melting point metals
• Hot mounting: Using thermosetting plastic (bakelite) with heat and pressure, suitable for most metal specimens

1.3 Specimen Grinding

After selecting the specimen, first flatten it with a grinding wheel or angle grinder to prepare for the next sandpaper grinding step. During flattening, water cooling is required to prevent microstructural changes due to heat.

Grinding can be performed manually or with a mechanical grinding machine. Generally, water sandpaper or metallographic sandpaper is used for macro inspection, while micro metallographic specimens require additional polishing.

Sandpaper grit classification:

• Coarse grinding: 120#, 140#, 180#, 200#, 240#, 280#, 320#
• Fine grinding: 400#, 500#, 600#, 800#, 1000#, 1200#
• Ultra-fine grinding: 1600#, 1800#, 2000#, 2500#, 3000#

Manual grinding procedures:

• Lay sandpaper flat on smooth glass, metal, or other boards
• Rotate specimen 90° when changing sandpaper to grind perpendicular to previous marks
• Apply uniform pressure forward, lift specimen on return stroke — never grind back and forth
• Each sandpaper change should continue until old scratches completely disappear and new scratches are uniform
• Wash specimen with water between each sandpaper change to avoid transferring coarse particles to finer paper
• Do not apply excessive force or grind too long to avoid specimen overheating

1.4 Polishing

Polishing removes fine scratches and surface deformation layers left by fine grinding. Common polishing methods include mechanical polishing, electrolytic polishing, and chemical polishing.

Mechanical Polishing:

• Equipment and materials: Polishing machine, polishing agent, polishing cloth
• Abrasives: Diamond, chromium oxide, aluminum oxide, etc. (0.25, 0.5, 1, 3, 7, 9 microns)
• Polishing cloths: Canvas, velvet, silk, etc.
• Rough polishing: 2-5 minutes, then wash with water and dry
• Fine polishing: Apply uniform pressure, start heavy then light, initially perpendicular then rotate specimen

Precautions during mechanical polishing:

• Use light pressure, polish from center to edge of disc
• Periodically add polishing suspension
• Cloth humidity: water film should evaporate within 2-3 seconds when specimen is removed
• After polishing, wash with water and dry to avoid water marks or residue

Electrolytic Polishing:

• Uses anodic dissolution principle — specimen as anode, carbon rod or other material as cathode
• Suitable for: low hardness metals, single-phase alloys, metals prone to work hardening (e.g., austenitic manganese steel)
• Not suitable for: cast iron, inclusion inspection, severe segregation materials

组成	规格	Notes
Perchloric acid 15-20%, Alcohol 80-85%	Current density 0.1-0.3 A/cm², <50°C, 15-90 min	Add perchloric acid slowly to alcohol
Perchloric acid 18-20%, Acetic acid 80-85%	Voltage 20-40V, 70-80°C, current density 0.7-1.0 A/cm²	Add perchloric acid slowly to acetic acid
Phosphoric acid 48%, Glycerin 20%, Water 32%	Current density 1-5 A/cm², 70-80°C, 1-3 min	—

Chemical Polishing:

• Uses chemical reagents for uneven dissolution of specimen surface to achieve a bright surface
• Can make surface smooth but cannot achieve the same quality as electrolytic polishing

1.5 Specimen Etching

After polishing, specimens under the metallographic microscope only show a bright field. Unless certain non-metallic inclusions (MnS, graphite, etc.) are present, various constituents and their morphological characteristics cannot be distinguished.

Etchants must be used to etch the specimen surface to clearly reveal the true microstructure.

The most commonly used etchant for steel materials is 1-5% nitric acid alcohol solution.

The most commonly used method for revealing metallographic structure is chemical etching.

Etching procedures:

• Etching time depends on metal material properties, etchant concentration, test temperature, etc. — until the microstructure is clearly visible under the microscope
• After etching, rinse quickly with water, then clean with absolute alcohol and dry with hot air
• If under-etched, continue etching or re-polish and re-etch
• If over-etched, generally re-polish and re-etch; for severe over-etching, re-grind, polish, then etch
• Etched specimens should be observed and photographed immediately

Etchant Name	组成	Application Scope
Nitric acid alcohol solution	Nitric acid 1-5mL, Alcohol 100mL	Quenched martensite, pearlite, cast iron, etc.
Picric acid alcohol solution	Picric acid 4g, Alcohol 100mL	Pearlite, martensite, bainite, cementite, etc.
Hydrochloric acid, picric acid alcohol solution	HCl 5mL, Picric acid 1g, Alcohol 100mL	Tempered martensite and austenite grains
Hydrochloric acid nitric acid solution	HCl 10mL, Nitric acid 3mL, Water 100mL	High-speed steel after tempering, nitrided layer, carburized layer
Ferric chloride hydrochloric acid solution	Ferric chloride 5g, HCl 50mL, Water 100mL	Austenitic stainless steel, austenitic-ferritic duplex steel
Ferric chloride hydrochloric acid water solution	Ferric chloride 5g, HCl 15mL, Water 100mL	Pure copper, brass and other copper alloys
Sodium hydroxide water solution	Sodium hydroxide 1g, Water 100mL	Aluminum and aluminum alloys

Electrolytic Etching Method:

Applicable to alloys with extremely high chemical stability

Electrolyte Composition	Current Density (A/cm²)	Time (s)	Cathode	应用
Potassium ferricyanide 10g, Water 90mL	0.2-0.3	40-80	不锈钢	Stainless steel, high-speed steel
Oxalic acid 10g, Water 100mL	0.1-0.3	40-60	Copper	Heat-resistant steel, stainless steel
CrO₃ 10g, Water 90mL	0.2-0.3	30-70	不锈钢	High-alloy steel, high-speed steel

1.6 On-site Metallographic Testing

• Direct inspection on workpieces: grinding → polishing → etching → observation → photography
• When position is inconvenient for direct inspection, prepare replica (AC paper or organic glass sheet) and bring to laboratory for observation
• Required materials: angle grinder, electric hand grinder, different grit small grinding wheels, sandpaper patches, polishing cloth patches, grinding paste, etching solutions, etc.

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Section 2: Metallographic Microscope

2.1 Magnification Principle

The metallographic microscope consists of an objective lens and an eyepiece. It uses multiple convex lenses in the objective and eyepiece to progressively magnify the image and reflect it onto the retina.

Total magnification = Objective magnification × Eyepiece magnification

Key indicators include: resolution, total magnification, depth of field, field of view width, image brightness, working distance, etc.

2.2 Optical System

The microscope consists of optical and mechanical systems:

• Optical system: Light source, condenser, objective lens, eyepiece, etc.
• Mechanical system: Focusing mechanism, specimen stage, etc.

Objective Lens: The core optical component of the microscope, composed of various glass lenses of different shapes. The front plano-convex lens at the very front of the objective is called the front lens, used for magnification. Other lenses below correct for various optical defects caused by the front lens.

Objective Lens Classification:

• By chromatic aberration correction: Achromatic, Plan achromatic, Semi-apochromatic, Apochromatic
• By medium: Dry system (air medium), Wet/oil immersion system (oil medium)
• By working distance: Normal objective, Long working distance objective
• By function: TIRF dedicated, Phase contrast, DIC, HC, Polarizing, Multi-function, Super fluorescence, Other special objectives

Eyepiece: Used to further magnify the image already magnified by the objective. Types include normal eyepiece, compensating eyepiece, projection eyepiece.

• Why can’t eyepieces improve resolution? — They only magnify the image formed by the objective at the intermediate focal plane, not directly imaging the object.
• Why are standard eyepieces mostly 10×? — Appropriate field of view size and good matching with magnification.

2.3 Light Sources

• Tungsten lamp: Visible light + UV + IR, bright but short lifespan (~2000h)
• Halogen lamp: 320nm, 455nm, high heat and temperature
• Xenon lamp: Good monochromaticity, high brightness, transmitted through light pipe, lifespan ~2000h
• LED: Daylight white, lifespan 10000h, not suitable for research-grade microscopes
• Laser: Good monochromaticity, high brightness, but expensive

2.4 Aperture and Field Diaphragms

Two adjustable diaphragms are installed in the metallographic microscope to improve image quality:

• Aperture diaphragm: Located before the condenser lens, controls the thickness of incident light beam, changes the numerical aperture of the objective. Enlarging the aperture improves resolution but reduces image contrast. Generally adjusted until the light beam fills the rear lens of the objective.
• Field diaphragm: Located after the aperture diaphragm, forms an image on the metallographic surface through the optical system. Adjusting the field diaphragm changes the observation field size. Smaller field diaphragm = better image contrast.

2.5 Main Types of Metallographic Microscopes

• Stereo microscope: Large field diameter, large depth of field, long working distance. Mainly used for macro inspection of products, can inspect macro defects such as fractures.
• Upright microscope: Specimen observation surface placed upward; specimen surface must be parallel to the bottom to ensure alignment with objective optical axis; observation surface upward prevents damage; specimen limited by height and shape; convenient operation, suitable for rapid inspection, photography, high anti-vibration requirements during image capture.
• Inverted microscope: Specimen observation surface placed downward; observation surface always perpendicular to objective optical axis; observation surface downward contacts stage surface, easily damaged, specimen not limited by height/shape; not suitable for rapid inspection; good anti-vibration performance for photography and image capture.

2.6 Operation and Maintenance

Operation procedures:

• Fully understand the instrument structure, principles, and usage methods; strictly follow operating procedures
• Keep hands clean during operation; clean specimen observation surface with alcohol and dry
• Select objective and eyepiece based on required magnification; handle lenses gently; store unused lenses in case; never touch lenses with hands
• Generally start with low magnification (20×-100×), then use high magnification for detailed observation
• Special illumination methods (dark field, interference, etc.) can be used for special needs
• When adjusting focus, first gently turn coarse adjustment to bring objective and observation surface close, then focus through eyepiece, then gently turn fine adjustment until image is clear. Avoid collision between objective and specimen surface during adjustment to prevent lens damage
• After use, promptly remove objective and eyepiece, store in lens case, then disconnect power

Maintenance:

• Working location must be dry, dust-free, low vibration; avoid dark and damp places; avoid direct sunlight
• Keep away from volatile, corrosive chemicals to prevent corrosive environment
• Remove residual liquids and oil stains from samples; if lenses are accidentally contaminated, immediately clean with cotton; clean oil lenses with xylene immediately after use
• Microscope illumination bulbs must be connected to 6V transformer; never directly plug into 220V power to avoid burning out
• Turn coarse and fine adjustment wheels slowly; report malfunctions immediately; never force turning to avoid damaging mechanism
• Objectives and eyepieces should generally be stored in dry conditions; use air blower to remove dust, then clean with lens paper
• Damp air is very harmful to microscopes, causing rust and damage; all parts should be protected during rainy season
• Do not disassemble mechanical parts casually; regularly add lubricating grease to ensure normal operation

Precautions:

• Laboratory should have three protection conditions: shock-proof (away from vibration sources), moisture-proof (use air conditioning, dehumidifier), dust-proof (floor with flooring); Power: 220V±10%, 50Hz; Temperature: 0°C-40°C
• Be careful when focusing not to let objective touch specimen to avoid scratching
• Do not switch objectives when stage垫 piece round hole center is far from objective center
• Avoid frequent brightness changes or excessive brightness, which affects bulb lifespan and eyesight
• All function switching should be gentle and in place
• Reduce brightness to minimum when shutting down
• Non-professionals should not adjust illumination system (filament position, etc.) to avoid affecting image quality
• Be careful of high temperature when replacing halogen lamps to avoid burns; do not touch halogen lamp glass directly with hands
• When not in use, adjust objective to lowest state through focusing mechanism

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Section 3: Quantitative Metallography

3.1 Image Analyzer

Microscope + CCD + Image analysis software = Image analyzer

Functions: Image acquisition → Image processing → Image feature extraction → Scale measurement → Calculation → Result output

Features: Scale calibration, system scale loading, image brightness/contrast adjustment, image annotation, field setting, geometric measurement, magnified printing

Functions: Length measurement, binarization, binary image processing, morphology toolbox, area content determination, graphite size rating, graphite length rating, particle analysis

3.2 Geometric Measurement

Length measurement: Select measurement items (e.g., length, width, grain diameter, etc.) before measurement. Can generate total length and average values.

3.3 Grain Size Measurement

Automatic method: Suitable for images with clear grain boundaries that can be extracted by binarization

1. Open an image, load correct system scale
2. Binarize to extract grain boundaries and apply

Semi-automatic measurement: Suitable for all images, operation similar to length measurement

• Choose different measurement line modes based on different conditions
• Displays real-time results
• Generates reports

Rating chart comparison method: Suitable for all images, system compares with rating charts

3.4 Particle Analysis

Can analyze particle size distribution, count, content percentage, equivalent diameter, etc.

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Section 4: Microhardness Tester

4.1 Hardness Overview

Hardness: The ability of a material’s local surface to resist being pressed into by another object. It is one of the most commonly used mechanical performance indicators for metal materials. Hardness testing is one of the most convenient and practical testing methods and is widely used in metallographic analysis.

4.2 Brinell Hardness

Range: 8-650 HB. K value is specified as 30, 15, 10, 5, 2.5, 1 (ratio of test force F to ball diameter D squared).

Ball diameter: φ10mm, φ5mm, φ2.5mm, φ1mm — using cemented carbide (tungsten carbide) ball HBW.

Application scope and advantages: Cast iron, non-ferrous metals and their alloys, most steels after annealing or tempering in as-delivered condition. High measurement accuracy, good reproducibility and representativeness. Disadvantages: longer operation time, requires changing indenters and measuring indenters for different hardness materials.

4.3 Rockwell Hardness

Hardness scales include HRA, HRB, HRC, commonly written as hardness value + scale letter, e.g., 60 HRC.

Uses three test forces and three indenters, totaling 9 scale combinations covering almost all commonly used materials per GB/T230.1-2018 standard.

• HRA: For thin steel strips, thin steel plates after hardening treatment, test force 60Kg, range 20-88 HRA
• HRB: For medium hardness materials, cast iron, various brass and most bronze, indenter type 1.5875mm ball, F=100Kg, range 20-100 HRB
• HRC: For quenched and low-temperature tempered carbon steels, indenter 120° diamond cone, test force 150Kg, range 20-70 HRC

Precautions:

• Specimen surface must be smooth, flat, clean, free of oil and dust
• Specimen should be placed stably without displacement or deformation during testing
• Calibrate with standard hardness blocks close to test specimen (at least 3 points) before testing

4.4 Vickers Hardness

Proposed by British scientist Vickers, uses a pyramid diamond indenter with 136° opposite face angle pressed into material surface; after holding for specified time, measure indentation diagonal length and calculate hardness using formula. Test standard: GB/T4340.1-2009 “Metallic materials — Vickers hardness test — Part 1: Test method.”

Vickers hardness tester classification:

• Standard Vickers: Maximum load 10-50 kg
• Low-load Vickers: Maximum load 5 kg
• Micro Vickers: Maximum load 1 kg

优势 Many test forces available, wide application range, suitable for specimens with rough surfaces.

4.5 Leeb Hardness

Proposed by Swiss Dr. LEEB in 1978, a new hardness measurement method. An impact body of specified mass is accelerated by elastic force to impact the specimen surface. The value is calculated from the ratio of rebound velocity to impact velocity at 1mm from the specimen surface. Test method: GB/T17394.1-2014 “Metallic materials — Leeb hardness test.”

Application scope:

• Installed machinery or permanently assembled components
• Pressure vessels, steam turbine generators and other equipment failure analysis
• Large workpieces that are difficult to move
• Multiple testing points needed in narrow spaces

4.6 Microhardness Testing Steps

• Indenter: Vickers pyramid indenter, Knoop indenter
• Common loads: 10gf – 1000gf
• Standard: GB/T14342-1991 “Metallic materials — Microhardness test”
• Main steps: Sample preparation → Test force selection → Adjust instrument zero position → Standard sample calibration → Load holding → Test indentation → Report results

Requirements:

• Specimen surface must meet roughness requirements
• Two diagonal difference should not exceed the average of diagonals
• Holding time: as specified
• Result: average of three test points

Hardness value expression: Hardness value before symbol, followed by selected test force value, holding time (10-15s not labeled). Example: 640HV0.5 means Vickers hardness value of 640 measured at test force of 4.903N for 10-15s.

Causes of abnormal indentations:

• Unequal-sided rhombus indentation, regular one-way asymmetry: specimen surface not parallel to bottom or load axis indenter not parallel to workbench
• Indentation diagonal intersection not at one point, or diagonals not in one line: tip or edge damaged, adjust to “zero position” after changing indenter
• Spring plate supporting load axis is loose; spring plate severely twisted

4.7 Hardness Tester Maintenance

• Environment: dry, free of harmful gases
• Level placement, no vibration
• Keep instrument clean
• Regularly check and calibrate
• Regularly add lubricating oil to moving parts

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