Carbon in cast iron-the reason we need “white cast”

When analyzing granular metals by spark OES it is normal to use a “pre-spark” or “pre-burn” before the actual measurement. This pre spark consists of several thousand high energy sparks whose purpose is to melt and thereby homogenize the sample surface before the measurement itself. Unfortunately, if free graphite inclusions are present, the pre-spark results in sublimation of some or all of the graphite before the actual measurement takes place; leading to depressed carbon values. This effect can be visualized by looking at the pre-spark area under the microscope.

For the sampling of molten metal samples, first of all, it should be ensured that the analysis sample can represent the average chemical composition of the melt or the sampled product, that is, the elements of the sample should have good uniformity; secondly, the sample should remove pollutants such as coatings, moisture and dust during processing, and should also avoid pores, cracks, looseness, burrs, folds or other surface defects; finally, when the sample is cooling, its chemical composition and metallographic structure should be kept consistent. For example, cast iron samples should pay attention to the problem of white cast. White cast means that under rapid cooling, the carbon element in cast iron exists in the metallographic structure in the form of cementite (Fe3C carbide), and its fracture is bright white, so it is called white cast iron structure. After white cast iron is treated, its carbon element exists in the metallographic structure in the form of carbide (Fe3C), without graphite carbon; the carbon element is distributed more evenly in the white cast iron structure in the form of carbide (Fe3C), without segregation, which meets the requirements of photoelectric direct reading spectroscopy for samples.

If the degree of white cast is poor, the percentage of C and S analyzed will be higher. Therefore, the better the degree of white cast, the more accurate the analysis of the components in the sample will be.

When the molten iron is poured into the copper mold, it begins to solidify. Once the sample is completely solidified, it can be demoulded and placed in water to cool. After the sample is cooled, it can be taken out and polished, and then analyzed on the spectrometer.

It should be noted that:

1. "Quick cooling" refers to the short time it takes for molten iron to solidify. Copper mold should be used, and copper plate is the best for the bottom plate.

2. The sample needs to be successfully cast in one go.

3. Because the "white cast" sample is extremely hard, when grinding the sample, sandpaper, sanding disc or grinding wheel with larger particle size, harder texture and better adhesion should be used.