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| Research Focus & Application: |
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Gray (or grey) iron is an iron-carbon-silicon alloy system, with 1.7 to 4.5% carbon and 1 to 3% silicon, by volume. Its low tensile strength and ductility, and relatively high hardness, makes it useful for castings such as engine blocks, valve bodies, electrical boxes, and pump housings. In these applications, the tensile strength of the material is not critical, but the overall stability of the part is still important.
In gray cast iron, most of the carbon in the alloy appears as graphite flakes, which help to dissipate energy. This is especially useful in environments were vibration is a concern, as the graphite tends to relieve such internal friction. When gray iron fractures, it tends to do so along the graphite flakes, leaving a gray fracture surface (hence the name “gray iron”). In two dimensions, such as when viewing a polished sample under a microscope, the graphite flakes manifest as wavy gray lines.
These gray cast iron samples were prepared using equipment and consumables from Allied High Tech Products, Inc. For sectioning, a resin bonded aluminum oxide blade was used with the TechCut 5™ precision sectioning machine. Aluminum oxide is recommended for sectioning ferrous materials, while the resin bond breaks down during sectioning, exposing new abrasive to allow for a cool and even cut.
Prior to mounting, the samples were ultrasonically degreased in GP cleaning solution, rinsed with isopropyl alcohol, and heat dried to remove any oil/debris; this improves the adhesion of the mounting material. The samples were mounted in the TechPress 2™ hydraulic-pneumatic automatic mounting press using black glass-filled epoxy. This hot mounting material offers optimal edge retention and hardness properties.
The mounted cast iron samples were ground and polished using the MetPrep 4™ with power head and individual pressure functionality. Bulk material removal was performed with a series of silicon carbide abrasive papers ending with 600 grit. Intermediate polishing was carried out with 6 μm and 1 μm polycrystalline diamond suspensions on Gold Label and White Label polishing cloths, respectively, while final polishing was achieved with 0.03 μm colloidal alumina suspension on a Final A polishing cloth.
The AD-5™ automatic fluid dispenser provided automatic dosing of the polishing suspensions and lubricant, allowing for unattended and consistent sample preparation.
Between grinding and polishing steps, and before imaging, the samples were cleaned with micro organic soap to remove micro contaminants and polishing solutions from the sample surfaces. This helps to reduce particle contamination from one cloth to the next, which results in scratches and a poor surface quality. The samples were then rinsed in isopropyl alcohol and gently dried using Aero-Duster compressed air.
| Microscopy and Imaging Methods: |
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These images were taken using EC Epiplan Neofluar objectives on a Zeiss AxioObserver D1m inverted microscope with Brightfield, Darkfield, Circular-DIC, and Polarized light capabilities. An AxioCam MRc 5 digital camera and AxioVision 4 imaging software were utilized for the image capture and annotations. A 2% Nital solution was used as an etchant for the gray cast iron samples. | |
Image 1: Gray cast iron after mounting and polishing; originally taken with a 10x EC Epiplan Neofluar objective using polarized light on an AxioObserver D1m inverted microscope and an AxioCam MRc 5 digital camera.
Image 2: Gray cast iron after mounting and polishing; originally taken with a 50x EC Epiplan Neofluar objective using polarized light on an AxioObserver D1m inverted microscope and an AxioCam MRc 5 digital camera. |