Steel varies very much in its characteristics according to the amount of carbon it contains.
Thus Mild or Soft Steel contains from 2 to .5 per cent of carbon. When more carbon is present it becomes Hard Steel.
Speaking generally, the following are the characteristics of steel.
Other characteristics of steel are its sharp metallic ring when struck, its great elasticity, and its retention of magnetism.
Steel is generally made by adding carbon to pure wrought iron (see p. 247).
Blister Steel is produced by heating bars of the purest wrought iron with charcoal (carbon).
It has a crystalline structure, is covered with blisters and full of cavities, which render it unfit for edge tools, and it is used chiefly for conversion into better descriptions of steel.
Shear Steel is made by piling short lengths of blister steel and welding them together under the hammer, which closes the cavities, removes the blisters, and produces a more uniform material known as Single Shear Steel. A repetition of the piling and welding produces Double Shear Steel. Shear steel is used for large knives, plane irons, shears, etc.
Crucible Cast Steel is made by melting blister steel in crucibles, or by melting wrought iron with the addition of the necessary carbon in the form of charcoal. It is used for the best tools and cutlery.
Bessemer Steel is produced direct from pig iron which, when melted in a "converter," is deprived by a blast of air through it of all its carbon, the amount necessary to convert it into steel of the softness required is then added in the form of spiegeleisen, a variety of cast iron rich in carbon. The resulting metal is run out into ingots, which are hammered, rolled, and worked to the forms required.
Bessemer steel is much used for rails and for the tyres of wheels, also for large roofs and bridges, boiler-plates, etc.
The Basic Process is somewhat similar to Bessemers, but that the converters are lined with material which deprives the pig iron of some of its impurities, thus enabling iron from the less pure ores to be converted.
The Siemens-Martin Process consists in melting pig iron in a regenerative furnace and then adding various substances, so that the molten metal may contain the exact amount of carbon necessary to produce the description of steel required. Steel made by this process is much used for rails, tyres, bridges, roofs, boiler-plates, etc.
Puddled Steel is made by stopping the puddling process before all the carbon has been removed. It is a poor material, used chiefly for making inferior plates.
Case-hardening is a process by which the surface of wrought iron is turned into steel. This is effected by red-heating the article to be case-hardened when immersed in bone dust, which adds carbon to the surface and turns it into steel to the depth of from 1/16 to 3/8 inch. The parts required to be hardened are then quenched. The process is useful for keys, and other articles where a hard surface is required to be combined with toughness.
The remarks made at p. 250 with regard to the tests for wrought iron, and the fractured surface, apply also to steel, except that in the case of steel the forge tests are much more important than for iron.
A recent specification for a large steel bridge requires that the bars and plates must have a tensile strength of not less than 28 tons or more than 31 tons per square inch, an elongation of not less than 20 per cent and a limit of elasticity of 15 tons. Besides this there are tests as to welding and tempering, too elaborate to be described here.
The ultimate tensile stresses to which iron and steel are subjected when tested are the breaking stresses. When, however, they are used in structures it is so arranged that the members of iron and steel should be subjected only to safe working stresses such as certainly will not cause fracture.