Metallographic preparation of cast iron

The metallographic preparation of cast iron is a vital step in quality control, as the mechanical properties of the material are strongly affected by the morphology (e.g., shape, volume) of the graphite. The characterization of graphite in cast iron can be performed directly on the polished, unetched sample, usually using standard reference charts or image analysis methods. However, etching is required to reveal and assess the structure of the iron matrix itself. Precise metallographic analysis is essential not only for process control during production, but also for the ongoing development and optimization of cast iron materials.

Cutting and mounting

Due to its brittle nature and relatively high hardness, cast iron requires careful sectioning to avoid introducing microstructural damage such as cracks, overheating, or graphite pull-out. The cutting strategy depends on the type of cast iron:

• White cast iron is extremely hard due to its high carbide content, with hardness values exceeding HV 600. For reliable sectioning of this material, we recommend using resin-bonded silicon carbide cut-off wheel, such as the QATM FS-E, available in diameters from 250 to 600 mm.
• Gray cast iron and other graphite-containing types (e.g., ductile iron, CGI) are somewhat easier to cut, although still relatively hard. For these materials, we recommend using cut-off wheel type FS-D or alternatively type FS-E for optimal performance.

To ensure clean and damage-free cuts, it is important to use a precision cut-off machine with adequate coolant flow to prevent overheating. Apply moderate feed pressure to avoid microcracks or chipping, and make sure the sample is firmly clamped to prevent movement or vibration during cutting.

Mounting is recommended for cast iron samples that require high edge quality and consistent preparation, such as in failure analysis or detailed microstructural evaluation. However, for large specimens or routine quality control, unmounted preparation is commonly used to save time and resources.

When mounting is necessary, hot mounting is the preferred method due to its speed and durability. We recommend using EPO BLACK a high-performance, mineral- and glass-filled epoxy resin. It offers very low gap formation, excellent edge retention, and high parallelism. The high filler content also improves machinability, especially important for harder types of cast iron. For standard applications Bakelite is a cost-effective alternative.

Etching

When evaluating the graphite morphology in cast iron, i.e. the shape, size, and distribution of graphite, etching is not necessary, as graphite is already visible on the polished, unetched surface. However, to study the metallic matrix and phases within the microstructure in more detail, chemical etching is required. An overview of the most common etchants for cast iron is given in the table below:

Safety Notice: Acids must be used with caution. Wear protective equipment and follow lab safety guidelines.