Gray Cast Iron is made from foundry pigs. No. 1. the darkest in colour, contains a large proportion of free carbon ; is soft, very fluid when melted, and useful for very delicate castings. No. 2 is lighter in colour, less fluid, but is harder than No. 1 when cold, and good for casting girders, etc. No. 3 is of a still lighter colour, harder, more brittle, and adapted for heavy castings.
Mottled Cast Iron contains both gray and white, which can easily be distinguished on a fresh fractured surface.
The Structure of Cast Iron is highly crystalline ; a bar broken across shows no sign of fibre - nothing but crystals close together.
The shape given to castings is important. There should be no sudden changes of thickness, or sharp angles as in Fig. 408, but the thickness should change gradually and the angles be rounded off as in Fig. 409. If these precautions are not attended to the casting will crack at the angles, or at any rate have a tendency to do so.
All castings should be smooth in surface, free from air bubbles or flaws, with perfect edges.
Chilled Iron is a very hard substance like white cast iron : it is produced on parts of castings which are required to be especially hard by placing pieces of cold iron against those parts when the metal is being run in.
Thus the running surface of a cast iron wheel may be chilled and made hard, the rest of the wheel being of a tough gray cast iron.
This is done for small castings by imbedding them in oxide of iron and raising to a red heat.
Iron so heated is softened to a certain depth all over the surface, and can be hammered or bent to a certain extent.