What is Metallography?
Metallography is the general description of the study of a metal material’s physical structure and components (phases and micro-constituents), typically examined after preparation by microscopic techniques. Particularly in metal alloys, a material with the same chemical composition can show varying metallography depending on several different factors, primarily being solidification rates and post solidification heat treatments that are designed to alter an alloy’s physical and mechanical properties. Metallography is often used in comprehensive examination of materials to correlate physical and mechanical properties, and is key to understanding the behavior of the alloys under certain conditions and external stresses.
Steps of a Metallographic Examination
Step 1: Define the Examination Plane
Metallography is a process, and therefore requires different steps and definitions to complete a metallographic examination. The standard in the industry for preparation of metallographic specimens generally referred to is ASTM E3. First, the disposition of the material or part for which a metallographic specimen is to be extracted must be understood. Orientation of the examination plane is the first critical determination that must be made, which requires knowledge of the part formation. The directionality of the examination plane is typically described in terms such as: longitudinal (along the forming or lengthwise axis), transverse (normal to the forming or lengthwise axis), axial, radial, and tangential in the case of circular or ring parts. These specified directions are critical in the features analysis that will occur in the later stages, and can lead to incorrect results if left ambiguous or to “best guess” directions.
Step 2: Mounting the Specimen
Once the examination plane has been defined, the extraction cuts can be made to isolate the specimen from the bulk part. The specimen is then loaded into a mounting medium to prepare a circular “puck” encapsulating the specimen such that the resulting specimen can be further control prepared. The mounting media is typically a thermoset media that encapsulates under high temperature and pressure or simple two part epoxies or acrylic when temperature sensitivity of the specimen and/or target feature is known to be present. The examination plane is typically set to the bottom of the mount by gravity such that it is the visible portion of the mount when the mount is completed.
Step 3: Grinding & Polishing
Once the specimen has been mounted, it will then progress to grinding and polishing techniques. The typical process is to utilize serial abrasive media progressing from a rough to fine grain size.
This is often a labor intensive process of starting as large as 80 grit sandpaper to as fine as 600 grit sandpaper, paying close attention to the directionality of each successive sanding. The methodology might also define this “rough” sanding portion of the process to be done wet. Once the rough sanding is done up to the specified plane, the preparation will continue to a fine polish, which will additionally be done with abrasive media on wet grinding wheels to in some cases submicron suspension media. After polishing to the level needed for the examination, the surface is cleaned of preparation media and is ready for microscopic examination and typically consists of a mirror like finish.
Step 4: Microscopic Examination
Microscopic examination of the “as-polished” condition will typically be utilized for features like inclusion content ratings, micro-cleanliness ratings (ASTM E45), bulk micro-constituent features such as nodularity or flake distribution in cast matrices, or surface feature investigations such a coating thicknesses and surface roughness. Several different microscopic techniques also exist for analysis of specific features such as bright and dark field reflectance microscopy (the primary standard for routine features analysis), polarized light microscopy (PLM), phase contrast microscopy (PCM), differential interference contrast microscopy (DIC), and scanning electron microscopy (SEM). Each microscopic technique possesses it’s own set of strengths and weaknesses for certain features examinations.
Etching to Examine Micro-features of a Specimen
Certain features which aren’t readily apparent in the as-polished condition can be revealed when applying “etching” techniques to the examination plane. Etching simply will target a particular micro-feature and introduce contrast to the image such that the feature will be revealed. The list of available etchants is numerous however can be classified in three distinct categories: chemical etchants, where a select chemical is applied to the polished surface; electro-etchants, where a current is passed through the specimen from a cathode to an anode; and oxidative etchants, where a heat treatment is applied to the specimen in an oxygen rich atmosphere.
The etching techniques are often specified for the particular alloy or feature and are generally described in ASTM E3 and numerous ASM citations, however, many laboratories also maintain modifications to etching procedures by trial and error and therefore consider proprietary. Micro-features that are generally revealed by etching include such features as: grain size (ASTM E112), case depth (SAE J423), and depth of decarburization (SAE J419).