1. The mechanical properties of nodular cast iron depend on the shape and size of graphite in cast iron and the matrix structure of cast iron. Metallographic examination is necessary to study the shape, size and matrix structure of graphite.


2. Sample preparation


1) The quality of metallographic samples of ductile iron mainly depends on the three key processes of rough grinding, polishing and corrosion;


2) The sample shall be intercepted at a representative position;


3) Generally, the metallographic cutting machine is used for sampling in the laboratory. When cutting with the grinding wheel, it is necessary to avoid overheating, which will lead to the change of the matrix structure of the sample. There should be no bluing phenomenon on the surface during cutting. If there is bluing phenomenon, it indicates that the structure has changed;


4) When sampling, the size of the sample should be considered. Too small a sample size is conducive to the subsequent polishing work. Thermal inlay is often required. The sample size is too large and it is difficult to grind flat in the later polishing process. It is generally recommended that the sampling area is 10mm*10mm. It is recommended to select metlab cut-10 for the grinding wheel cutter, and qmaixs grinding wheel cutting blade is generally selected for the grinding wheel cutting blade


3. Grinding and polishing of sample: grinding and polishing is the most important stage in the whole sample preparation process. The purpose of sample grinding and polishing is to grind the section of the sample to be smooth and flat, so as to meet the requirements of observing the structural characteristics under the optical microscope. The grinding process of the sample is divided into two steps: coarse grinding and fine grinding

1) Rough grinding: the purpose is to trim the surface of the sample to make the surface flat. Qmaixs carbipaper silicon carbide metallographic abrasive paper is generally selected as the consumable


2) Fine grinding: the purpose is to eliminate the deep and coarse scratches left by rough grinding and prepare for polishing. Qmaixs carbipaper silicon carbide abrasive paper is generally selected as consumables.


4. Polishing of the sample: after fine grinding, the polishing of the sample is followed. The purpose of polishing is to remove the scratches left by grinding, improve the reflectance of the surface and improve the resolution


1) Selection of polishing fluid: ductile iron is generally polished by mechanical polishing, which is to polish the metallographic sample into a smooth mirror by the grinding effect of polishing fluid.

2) Selection of polishing cloth: there are various forms of graphite in the ductile iron sample. When polishing, the sample should not be scratched, but also ensure that the graphite is not polluted, does not fall off, does not drag the tail, and correctly reveal its shape, size and color. Generally, qmaxis nyloncloth series metallographic polishing cloth with monodia6 micron diamond polishing fluid and chemocloth with aluminaprep0.05 alumina suspension are selected for polishing;


5. Corrosion of the sample: if the corrosion of ductile iron is too shallow or too raw, and the surface is not clean, it is easy to produce spots. This will make the microstructure characteristics not obvious and increase the difficulty of evaluation. The corrosive solution is 2%-5% nitric acid alcohol solution