No. 30. Very Soft Cast Iron. Note Large Graphite Flakes
No. 31. Medium Hard Cast Iron
The “combined carbon” is in the roundish, dark parts. It is the “combined carbon” that increases the strength of cast iron and steel.
The “precipitation” of graphite which is necessary for softness is brought about mainly through the influence of silicon, which we before termed the “softener.” Other conditions being equal, the higher the silicon (if not above 4 per cent), the higher will be the graphite and the lower the combined carbon; and vice versa, the lower the silicon the lower will be the graphite and the higher the combined carbon. It is mainly due to the “combined carbon” which is left after precipitation of the graphite that the alloy has greater strength, hardness, and closer grain. So, just as the steels are stronger and harder as the carbon increases (in steel all the carbon is combined), so, other conditions being equal, the strengths and hardnesses of the cast irons, within usual limits, increase as the combined carbon increases.
No. 92d. Semi-Steel. A Closer Grained and Yet Stronger Cast Iron
No. 33e. Mottled Cast Iron
So-called because it is a mixture of white and gray iron.
Just here it is interesting to remember that from the standpoint of metallography cast irons are simply steels in which there is what we might call an impurity or an adulterant, graphite crystals. It will be seen at once that could these graphite crystals be removed from the cast irons shown in photomicrographs No. 74, No. 92d, No. 30 and No. 31, we would have alloys quite similar in appearance to the steels shown in photomicrographs No. 3b and No. 22c which appeared on pages [77] and [78].[[7]]
[7]. Magnification 70 diameters.