Metallographic testing, or metallography, is the art and science of preparing, examining, and interpreting the microstructure of metals and alloys. This process is fundamental in materials science for quality control, failure analysis, and research and development. The primary goal is to reveal the internal structure of a material, which directly correlates to its mechanical properties, processing history, and performance.
The process can be broken down into four main stages:
Sample Selection and Sectioning
Mounting and Preparation (Grinding and Polishing)
Etching
Microscopic Examination and Analysis
- Sample Selection and Sectioning
The first step is to obtain a representative sample from the component or material of interest. The location and orientation of the cut are critical, as the microstructure can vary significantly across a part.
Sectioning: The sample is cut to a manageable size, typically using an abrasive cutter, a precision saw, or wire EDM (Electrical Discharge Machining). It is crucial to use a coolant during cutting to prevent any heat-induced microstructural changes (like tempering or hardening) that would alter the true structure.
2. Mounting and Preparation
This is the most critical phase for achieving accurate results. A poorly prepared sample will yield misleading microstructural information.
- Mounting (Potting)
Small or irregularly shaped samples are mounted in a rigid polymer resin to facilitate handling during the subsequent grinding and polishing steps. The two most common types of resins are:
Thermosetting Resins (e.g., Phenolic): Hard and durable, cured under heat and pressure.
Thermoplastic Resins (e.g., Acrylic): Transparent, cured under pressure without heat (cold mounting).
- Grinding
The mounted sample is ground using a series of progressively finer abrasive papers (e.g., 120, 240, 400, 600, 800 grit SiC paper). The objective is to:
Create a flat surface.
Remove any damage from the sectioning process.
Eliminate scratches from the previous, coarser grinding step.
The sample is thoroughly cleaned and rotated 90 degrees between each grinding step to ensure scratches from the previous grit are completely removed.
- Polishing
After grinding, the sample is polished to a mirror-like, scratch-free finish. This is typically a two-step process:
Coarse Polishing: Uses a cloth impregnated with a diamond suspension (e.g., 9µm or 3µm). Diamond is the hardest material and is effective on all metals.
Fine Polishing: Uses a finer abrasive, such as alumina (Al₂O₃) or colloidal silica (0.04µm), on a soft cloth. Colloidal silica has a chemical-mechanical action that can simultaneously polish and lightly etch some materials.
A perfectly polished surface should be featureless and highly reflective under a microscope. Any features visible at this stage are non-metallic inclusions (e.g., sulfides, oxides).
3. Etching
To reveal the microstructure, the featureless, polished surface must be chemically or electrochemically treated in a process called etching.
Principle: Different microstructural constituents (e.g., grain boundaries, phases, precipitates) have different chemical activities. The etchant attacks these regions at different rates.
Result: This differential attack creates topographic relief (peaks and valleys) or alters the surface reflectivity. When light hits the etched surface, these variations appear as contrast under a microscope.
Common Etchants:
Nital: A nitric acid and alcohol solution, most common for carbon and low-alloy steels. It reveals ferrite grain boundaries and pearlite colonies.
Kalling’s Reagent: Used for stainless steels and high-nickel alloys.
Keller’s Reagent: Used for aluminum alloys.
Marcel’s Reagent: Used for titanium alloys.
The etching time is critical; over-etching can obscure details, while under-etching will reveal nothing.
4. Microscopic Examination and Analysis
The final, prepared sample is examined under a microscope.
Optical Microscope / Light Microscope: The primary tool. It uses reflected light and can achieve magnifications typically up to 1000x. Different illumination modes are used:
Bright Field: The standard mode; illuminated areas appear bright.
Dark Field: Illumination comes from the side; edges and defects scatter light and appear bright against a dark background. Excellent for seeing cracks and pores.
Polarized Light: Useful for examining non-cubic metals like Ti, Zr, and Be, and for identifying non-metallic inclusions.
Differential Interference Contrast (DIC): Enhances topographic contrast, making grain boundaries and phases appear 3D.
Scanning Electron Microscope (SEM): For higher magnification and greater depth of field. An SEM uses a focused electron beam to generate high-resolution images and can be coupled with Energy Dispersive X-ray Spectroscopy (EDS/EDX) for elemental analysis of specific features.
What We Analyze:
Grain Size: Measured according to ASTM standards (e.g., E112). Grain size affects strength and toughness.
Phase Distribution: Identifying and quantifying different phases (e.g., ferrite, austenite, cementite, martensite).
Inclusion Content: Rating the amount and type of non-metallic inclusions (e.g., using ASTM E45).
Case Depth: For surface-hardened components.
Porosity and Cracks: For quality control in cast or welded materials.
Evidence of Failure: Such as fatigue striations, overload, or corrosion.
Applications of Metallography
Quality Control: Verifying heat treatment processes, weld integrity, and material conformity.
Failure Analysis: Determining the root cause of a component’s failure in service.
Research and Development: Developing new alloys and optimizing processing parameters (e.g., casting, forging, heat treatment).
Material Identification and Certification.
In summary, metallographic testing is a systematic and meticulous process that transforms a piece of metal into a readable “map” of its history and properties, providing invaluable insights for engineers and scientists.
