Metallographic preparation of cast iron

The main characteristics of cast iron

Iron is one of the most diverse metals, and it has been an important construction material for centuries, used in many buildings from train stations and market halls to offices and homes.

When alloyed with carbon and other elements, iron offers an enormous variety. The term cast iron refers to iron-carbon-silicon alloys that contain 2.5-4 % carbon and usually 1-3 % silicon. In grey irons, the carbon is present as graphite; in white cast iron it is present in the form of iron carbide and alloy carbides.

Cast iron has good castability and machinability, as well as moderate mechanical properties. Because of its economic advantages, cast iron is used for many applications in the automotive, manufacturing and processing of metals industries. In addition, specific cast irons are the material of choice for sea water pump housings, rolling mill rollers and parts for earth moving equipment.

Fig. 1: Austempered ductile iron, Beraha color etch  (Mag. 500x, DIC)

Difficulties in the metallographic preparation of cast iron

The main challenge when preparing samples of cast iron is to retain the graphite in its original shape and size to ensure a correct representation of the cast iron’s microstructures.

Fig. 4: Grey iron with flake graphite, insufficient polish (Mag: 200x)
Fig. 5: Grey iron with flake graphite, showing correct polish (Mag: 200x)

In the microscope, the image of the graphite is seen two-dimensionally. However, it is actually three-dimensional. This means that a certain percentage of graphite is cut very shallow during grinding and polishing, with only a weak hold in the matrix. Therefore, there is always a possibility that the graphite cannot be completely retained, especially very large flakes or agglomerations of flakes. As a result, the graphite phase cannot always be retained or polished well.

In malleable cast irons, graphite exists in the form of rosettes or temper carbon. This is a friable form of graphite and it can be particularly difficult to retain during metallographic preparation.

A common preparation error is the insufficient removal of smeared matrix metal after grinding, which can obscure the true shape and size of graphite. This is particularly prevalent in ferritic and austenitic cast irons, which are prone to deformation and scratching. For these materials, a thorough diamond and final polish is very important.

Most standard microscopic checks of cast irons are done with a magnification of 100x, which makes the graphite appear black. However, higher magnifications are required to verify if the carbon is completely retained, as well-polished graphite is grey.

Fig. 6: Insufficient polish leaves graphite nodules covered with smeared metal, etched 3 % Nital (Mag: 200x)
Fig. 7: Correct polish shows shape and size of graphite nodules suitable for evaluation, etched 3 % Nital (Mag: 200x)

Fig. 8: Well-polished graphite flakes (Mag. 500x)

Recommendations for the cutting and mounting of cast iron

• Alloyed white cast irons are very hard (HV 600) and can be difficult to cut, especially large sections. Despite this, diamond cut-off wheels are not suitable for cutting white cast iron. Instead, a cubic boron nitride wheel is recommended.
• For large sections, automatic cutting is more efficient than manual cutting.
• For cutting cast irons with graphite, it is recommended to select an aluminum oxide wheel according to the hardness of the cast iron.

• Quality control samples are usually prepared unmounted.
• For failure analysis samples, it is best to use hot compression mounting.
• For soft to medium-hard cast irons, a phenolic resin (MultiFast) is recommended – for optimum edge retention use LevoFast (melamin resin with mineral and glass filler).
• For harder cast irons, a reinforced resin (DuroFast) is suitable.

• Find more knowledge, expertise and insight in our cutting and mounting sections.
• See our range of cutting and mounting equipment.
• Find cutting consumables and accessories and mounting consumables and accessories.

Recommendations for cleaning of cast iron

• As many cast irons tend to corrode, cleaning must be fast and should always be carried out with cold water. However, the sample must not be left in contact with water.
• Thorough rinsing with ethanol and fast drying with a strong stream of warm air is recommended. If corrosion still occurs, clean and rinse with water-free alcohol only.

Summary

Cast irons are ferrous alloys with mostly 2.5-4 % carbon and 1-3 % silicon. The main difficulty in the metallographic preparation of cast iron is to retain the true shape and size of the graphite in its flake, nodular or tempered form. In particular, cast irons with a soft ferritic matrix tend to smear and are prone to deformation and scratching.

During grinding, the matrix is smeared over the graphite and, unless it is followed by a very thorough diamond polish, the graphite is not shown in its true form. Therefore, plane grinding with silicon carbide foil/paper is recommended, followed by fine grinding and polishing with diamond. A brief final polish with colloidal silica is optional.

Get insight into other materials

If you would like to learn more about the materialography of other metals and materials, check out our materials page.

Get the full application note

All images by Birgitte Nielsen, Application Specialist, Denmark
For specific information about the metallographic preparation of cast iron, contact our application specialists.