The pig iron can be devided into two groups, white and gray pig iron.
The former is not only poor in the Silicon and free carbon (grapbite) contents, in general, but the carbon content remains in the combined form of cementite Fe3C with iron.
The fracture is quite lustrous, white, and freak, It is very hard and brittle. The latter is quite softer and stronger than the white pig and used for the castings. It is remarkably rich in Silicon and graphite and the colour of the fracture is gray.
The writer's experiments in the Electric reduction of the magnetic sand have shown the existence of the critical temperatures for the formation of both kinds of iron, white and gray. The temperature ranges are 1, 450-1, 500°C at the formation or the smelting zone. The deviations of the temperatures are chiefly due to the slag compositions. The gray iron can never be made under nor the white iron above the temperatures. Referring to these facts, he suggests of the principle of the produc-tion of the low silicon, and high carbon pig iron in the blast furnace process.
The writer made the explanation of the mechanism of the graphitization of the pig iron during its formation in the furnace. He says that the reduction of the silicon above the critical temperature diminishes the solubility of the cementite in iron and consequently cementite decomposes into iron and colloidal graphite. The distribution of the graphite is quite uniform throughout the molten iron. The state of the graphite after cooling is totally depending upon the composition, temperature of the melt, and the rate of cooling. The effects of the quantity of Si and Mn in the iron are also shown micrographically.