Steel has been denned by Dr. Percy as "iron containing a small percentage of carbon, the alloy having the property of taking a. temper; and this definition is substantially equivalent to those found in the works of Karsten, Wedding, Griiner, and Tunner." 1
On this point, however, there are many opinions, some of which will presently be briefly referred to.
The amount of carbon in steel, as used in engineering, varies from about "12 for very soft to 1.5 per cent for very hard steels.
Practically, steel often contains other substances besides iron and carbon.
These substances are generally got rid of, as far as possible, in the process of manufacture. When, however, they remain in the steel, they influence its characteristics in the manner described at p. 262.
In consequence of the practical existence of these impurities, and for other reasons, it is difficult to give an exact definition of steel.
Several definitions have been proposed, some depending upon the chemical composition, and some upon the physical characteristics of the material. None, however, has at present been universally accepted.2
M. Adolphe Grenier, of Seraing, has classified the irons and steels, according to the proportion of carbon they contain, in the following manner : - 2
Percentage of carbon.
0 to 0.15.
0.15 to 0.45.
0.45 to 0.55.
0.55 to 1.50, or more.
Series of the irons.
Steely irons or puddled steels.
Cemented steels. Styrian steel.
Series of the steels.
Extra soft steel.
Half soft steels.
Sir Joseph Whitworth has pointed out that a definition based upon chemical composition is unsatisfactory. He proposes to do away with all distinctive names such as blister steel, shear steel, cast steel, etc., and "to express what is wanted to be known by two numbers which should represent tensile strength and ductility. . . . He would suggest that the limit of tensile strength be taken at about 18 tons per square inch, so that the metal exceeding this strength should be called 'steel,' while any description of iron falling below this limit of tensile strength should be known as ' wrought iron' "1
1 Dr. Siemens' Address, Iron and Steel Institute, 1877.
2 Journal Iron and Steel Institute, 1873.
An International Committee sitting at Philadelphia in 1876 recommended a somewhat elaborate nomenclature for different descriptions of iron and steel. Dr. Siemens, alluding to this, says -
"Difficulties . . . have hitherto prevented the adoption of any of the proposed nomenclatures, and have decided engineers and manufacturers in the meantime to include under the general denomination of cast steel all compounds consisting chiefly of iron which have been produced through fusion and are malleable. Such a general definition does not exclude from the denomination of steel materials that may not have been produced by fusion, and which may be capable of tempering, such as shear steel, blister steel, and puddled steel; nor does it interfere with distinctions between cast steels produced by different methods, such as pot steel, Bessemer steel, or steel by fusion on the open hearth."3
In a paper on "Steel for Structures" Mr. Matheson said, "Steel for the purposes of the present paper is any variety of iron or alloy of iron which is cast while in the liquid state into a malleable ingot, and to go further, which will when rolled in a plate or bar, endure from 26 to 40 tons before fracture." 3
To the engineer some practical definition which would enable him to know exactly what material he would receive under a certain specification would be of great value.
In whatever way steel may be defined, it is of the utmost importance that the characteristic differences between it and iron, both cast and wrought, should be clearly understood.
Some of these will now be pointed out.
The characteristic difference between steel and pure wrought iron is as follows: -
Tempering is a characteristic of steel which distinguishes it from cast iron. If steel has been hardened by being heated and suddenly cooled, as above described, it may be softened again by applying a lower degree of heat and again cooling. This is known as tempering.
Cast iron, on the contrary, though it is hardened by the first process, cannot be softened by the second.
When a bar of steel is struck it gives out a sharp metallic ring, quite different from the sound produced by striking wrought iron.
Other characteristics of steel are its great elasticity and its retention of magnetism.
It has already been stated that the peculiarities of cast iron, wrought iron, and steel are caused by the difference in the amounts of carbon which they respectively contain.
Pure wrought iron contains no carbon. The wrought iron of commerce contains a minute quantity, steel contains more, while the largest percentage is found in the softer kinds of grey cast iron.
1 Proceedings Mechanical Engineers, 1875.
2 Dr. Siemens' Address, Iron and Steel Institute, 1877.
3 M.I.C.E., vol. lxix. p. 1.
The transition from one class to the other is so gradual and insensible that it is difficult to say where one ends and the other begins, but the following remarks bear with them the high authority of Dr. Percy.
"When the carbon reaches .5 per cent and other foreign matters are present in small quantity, iron is capable of being hardened sufficiently to give sparks with flint, and may then be regarded as steel. But in the case of iron perfectly free from foreign matters, not less than .65 of carbon is required to induce this property.
"Iron containing from 1.0 to 1.5 per cent is steel, which, after hardening, acquires the maximum hardness combined with the maximum tenacity.
"When the carbon exceeds the highest of these limits still greater hardness may be obtained, but only at the expense of tenacity and weldability.
"When the carbon rises to 1.9 per cent or more, the metal ceases to be malleable while hot; and 2 per cent of carbon appears to be the limit between steel and cast iron, when the metal in the softened state can no longer be drawn out without cracking and breaking to pieces under the hammer." 1
As a general rule it may be said that the varieties of steel containing the larger proportions of carbon are harder, stronger, more brittle, and more easily melted. Those containing less carbon are tougher, more easily welded and forged, but are weaker as regards tenacity.2