This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
There is a quality of iron known as malleable cast-iron, which combines, to a certain extent, the qualities of both wrought and cast iron, being cast-iron of a semi-wrought-iron character, which can be forged. The forging, however, has to be done cold, not hot, as with wrought-iron. The process is described as embedding cast-iron in powdered hæmatite ore or in scales of oxide of iron, or other oxides, and raising it to a bright red heat in an annealing oven. Pieces of cast-iron not over one-half inch thick can be made entirely malleable by this process ; with thicker pieces, however, it is only practicable to render the outside skin malleable to a depth of about one-quarter inch, the depth, of course, depending on the length of treatment, the interior retaining its unaltered cast-iron character.
The manufacture of steel is still in its early development and as a result great uncertainty attends the results obtained, unless proper tests are made at every stage. Nor have the different methods of manufacture been tested sufficiently long to definitely establish the superiority of either.
Why weight per yard.
Nature of Wrought and Rolled Iron Similar.
Manufacture of Steel.
It is partly for this reason, and also on account of the many different kinds of steel produced - dependent on the varying amount of carbon it contains - that there are so many radically different processes of manufacture.
As already remarked, steel in its chemical composition is the intermediate between wrought and cast iron - wrought-iron being theo-retically free from carbon, while cast-iron has ordinarily a good deal more carbon than steel and holds it in a different form. Steel with but little carbon is called "mild steel," with much carbon "hard steel," the former being the more reliable of the two for structural work. It should be noted, however, that whereas in the earlier stages of the manufacture of steel its chemical synthesis was made the criterion, yet to-day the oft-mooted question as to whether a certain product is iron or steel is more often determined by reference to its physical characteristics than to its analysis. The physical characteristic of steel is that it may be cast into wholly malleable ingots, that is, the cast ingot or lump can be rolled or forged in its heated state without any intermediate process. This is not true as regards cast or pig iron which is entirely non-malleable, while, on the other hand, wrought-iron cannot be cast, not being fusible at temperatures obtaining in the arts.
The great importance and future of steel lies in the above property as also in its large tensile strength, its hardness, and its further advantages of being capable of "hardening" and of taking a "temper." It combines within one metal the ductility and weldability of wrought-iron with the hardness and fusibility of cast-iron, and this with a tensile strength greater than that of wrougbt-iron.
Hardening consists in raisins the steel to a red heat and suddenly cooling it in water ; this greatly hardens steel but does not affect wrought-iron, while cast-iron would fly to pieces under such treatment. Hardened steel can be softened by tempering, this is done by re-heating the hardened steel, but to a lower temperature than before, and again cooling it, but slowly, when it will be tempered or softened.
Steel retains magnetism greatly. When struck, steel rings out a sharper metallic sound than iron, but in physical appearance it often resembles wrought-iron to such a degree that it is difficult to distinguish between them by merely looking at their fractured parts. Steel is malleable at a lower heat than wrought-iron, and for this, (and other reasons, but principally on account of the danger of "burning" it,) - it is usually rolled at a much lower temperature than wrought-iron; being, as a result, much more solid, that is, further from the melting point, than is iron when passing through the rolls, it requires very much greater power and heavier machinery than the latter.
Characteristic of Steel,
The production of steel direct from the ore is perfectly feasible, but has not yet attained to any relative commercial importance.
All the different processes of manufacturing steel really narrow down to the two principal methods of either the addition of a certain percentage of carbon to pure or malleable iron, or the removal of part of the carbon from pig-iron. To give all the variations practised in these two principal methods would be impossible here, but under the first method we can cite the cementation process or the manufacture of the commercial "cast" steel; as an intermediate process the "Open Hearth" processes; and under the second method the well-known "Bessemer" process.
Of these, in its main features the Cementation process is the oldest. Briefly, bars of wrought-iron are packed in charcoal in airtight retorts or "pots" and subjected to a white heat for over a week's time. The wrought-iron absorbs the carbon or alloys with it, showing a blistered appearance - whence the name of "blister" steel- and becomes readily fusible.
These blister steel bars when piled and rolled once constitute the "single" shear steel of commerce, or if re-piled and re-rolled the "double" shear steel. If, however, they are broken up and melted in a crucible and cast into an ingot which is re-rolled, the quality and uniformity of the product are much improved and it is known as "cast" steel; the principal use of which is for cutlery and edge tools. Cast-steel is the hardest, strongest and densest steel made, but when raised beyond a red heat becomes so brittle that it cannot be forged. It cannot be welded and flies to pieces under a powerful blow. The Open Hearth process may be justly regarded as the intermediate process because it is either the carbonization of or adding of carbon to wrought-iron, or the decarbonization of or removal of carbon from pig-iron, both reactions usually existing in one and the same heat, as in the best known Sit mens-Martin process, which is a typical and probably one of the best open-hearth methods to-day.