The history of iron may for convenience be divided into three periods: a first in which only the direct extraction of wrought iron from the ore was practised; a second which added to this primitive art the extraction of iron in the form of carburized or cast iron, to be used either as such or for conversion into wrought iron; and a third in which the iron worker used a temperature high enough to melt wrought iron, which he then called molten steel.
And it was the lengthen ing of the forge, and the length and intimacy of contact between ore and fuel to which it led, that carburized the metal and turned it into cast iron.
For instance, following Krupp's formula, the side and barbette armour of war-vessels is now generally if not universally made of nickel steel containing about 3.25% of nickel, 0.40% of carbon, and 1.50% of chromium, deeply carburized on its impact face.
These two things are done simultaneously by heating and melting the ore in contact with coke, charcoal or anthracite, in the iron blast furnace, from which issue intermittently two molten streams, the iron now deoxidized and incidentally carburized by the fuel with which it has been in contact, and the mineral matter, now called " slag."
Thick are carburized and so converted into high carbon " blister steel," by heating them in contact with charcoal in a closed chamber to about 1000° C. (1832° F.) for from 8 to ii days.
The many steel objects which need an extremely hard outer surface but a softer and more malleable interior may be carburized superficially by heating them in contact with charcoal or other carbonaceous matter, for instance for between 5 and 48 hours at a temperature of 800° to goo° C. This is known as " case hardening."
The impact face is thus carburized to a depth of about 14 in.
In the United States the charge usually consists chiefly of wrought iron, and in melting in the crucible it is carburized by mixing with it either charcoal or " washed metal," a very pure cast iron made by the Bell-Krupp process (§ 107).