No. 1d shows a typical section of wrought iron cut lengthwise of the rolled bar. It will be noted that, as in photomicrograph No. 99b, there are no graphite flakes.
In the process of wrought iron manufacture practically all of the carbon of the original pig iron is burned out, leaving little besides the iron itself and some viscous cinder or slag. In the rolling or hammering out of the resulting white-hot “bloom,” the slag enclosures which remain after the squeezing process are extended lengthwise through the bar. Upon observing with the microscope any prepared section of wrought iron which has been cut lengthwise of the bar the filaments of slag may be plainly seen, all parallel or practically so. When such filaments of slag can be discerned in a longitudinal section it is practically an absolute indication that the material in question is wrought iron.
No. 22c. Steel with .50 Per Cent of Carbon
No. 36a. Steel with 1.98 Per Cent of Carbon
Photomicrograph No. 3b is that of mild steel containing .10 per cent (⅒ per cent) of carbon. Here we have neither the graphite flakes of No. 198 nor the slag filaments of No. 1d. We can plainly see the boundary lines of the grains. The irregular dark patches which are evenly distributed throughout are the defining features of steel. In what might be called a chemical-mechanical combination, these dark patches contain all of the small percentage of carbon which gives to carbon steel its definite properties.
During the manufacture of this alloy all but a small amount of carbon is eliminated by burning it out, as happens with wrought iron. But the steel is molten or fluid when finished and the slag which has been formed floats on top and is also eliminated, which does not occur with wrought iron, which is thick and pasty at its finishing temperature.
No. 74. Gray Cast Iron. The Crooked Black Lines Are Graphite Flakes