STEEL.

CHAPTER XI.

PROPERTIES AND ALLOYS OF STEEL.

THE definition of the term "Steel" presents many difficulties, seeing that authorities differ considerably as to the precise meaning which is to be attached to the word, but all agree that it forms the connecting link between malleable- and pig-iron, and also that carbon is an essential constituent. The International Commission of the Philadelphia Exhibition in 1876 recommended the following terms for adoption :

(a) Weld-iron, for all malleable compounds of iron (containing the ordinary ingredients) prepared in any form other than the liquid state, and which cannot be hardened and tempered.

(b) Weld-steel, for all bodies similar to (a) which can be hardened and tempered.

(c) Ingot-iron, for all compounds of iron (containing the ordinary ingredients) which have been melted and cast, and which cannot be sensibly hardened and tempered.

(d) Ingot-steel, for all compounds similar to (c) which are capable of being hardened and tempered.

Steel may be defined, simply, as a compound of iron

with from 15 to 1.8 per cent. of carbon, which latter may be partially replaced by manganese, and to a small extent by other elements, malleability being essential. Sir Joseph Whitworth proposed to designate iron or steel according to its tensile strength, in tons per square inch of section, and the amount of its elongation before fracture, when a standard test-piece is employed.

Properties.-Steel, in the soft state, has a white colour with a bluish tinge, and becomes whiter when hardened, sometimes almost pure white. The lustre is similar to that of iron. When freshly broken the fractured surface affords some indication of its quality, being generally finely granular or crystalline, uniform in structure, and destitute of fibre; but these will vary with the mode of breaking, and with the amount of carbon and other elements which the steel contains. Much carbon makes steel close-grained and lustrous. When mild steel is broken by a sudden blow the surface is crystalline, but when broken with progressive stresses, the appearance inclines to the fibrous state. The tenacity of good steel is very high, exceeding that of any other metal, and is increased by cold-rolling, or by wire-drawing. Steel possesses the valuable property of being hardened by quick cooling after heating, and hard steel may be annealed or softened by making it red-hot and cooling it slowly. The hardness of hardened steel may be reduced, by gradually raising it in temperature up to a certain point, when it becomes highly elastic; this reduction is termed "tempering." Steel requires to be welded at a lower temperature than iron, and its power of being welded diminishes with the increase of carbon. The melting-point of steel is probably from 1600° to 1800° C., the fusibility increasing with the amount of carbon and other elements present. The specific gravity of steel varies from 7.6 to 7-8, being slightly less in the hardened than in the softened state, in consequence of the increase in bulk caused by this process. Steel resists the

influence of magnetism more than iron, but when magnetised the property is permanent; the presence of much manganese prevents the acceptance of magnetic power. Steel is less readily oxidised by exposure to air than iron, but the presence of other metals, such as manganese and chromium, increases its liability to oxidation.

EFFECT OF VARIOUS ELEMENTS ON STEEL.

Phosphorus.-The presence of this element is much more prejudicial in steel than in iron, and the more phosphorus a steel contains the more readily does it lose its characteristic properties by repeated heatings, becoming finally incapable of being tempered. Phosphorus hardens steel more than carbon does, makes it cold-short, more fusible, more brittle, more rigid, and less elastic, especially when the carbon is high. Gruner states that two or three parts of phosphorus in a thousand makes steel strong and elastic, but diminishes the tenacity. Steel with 5 per cent. of carbon must not contain more than 04 per cent. of phosphorus, or it will not roll well, so that a greater proportion than this can only be used by keeping the carbon low. This is effected by the use of ferro-manganese in the place of spiegeleisen, in the manufacture of Bessemer and Siemens steel.

Silicon. This element appears to harden steel to a less extent than phosphorus, and causes the metal to be red-short, brittle, and less tenacious, but a very small quantity may improve the steel. Silicon readily oxidises, forming silica SiO2, and this being an acid, unites with bases, such as manganese oxide, iron oxide, etc., to form a slag; for this reason silicon only occurs in steel in small quantities. It also lowers the proportion of combined carbon.

Sulphur. The presence of sulphur in steel in notable quantity makes it red-short, more fusible, brittle, and

less tenacious. It also prevents steel from welding, but is not so injurious in cold working.

Manganese. This metal hardens iron, but in a less degree than carbon. When present in small quantity it improves the quality of steel, increasing its tenacity and elasticity. The bodies known as ferro-manganese and spiegel-eisen, used in steel making, have been already described. The use of manganese is valuable, for the refining influence its oxide exerts on the impurities contained in the iron, such as silicon. phosphorus, sulphur, etc., removing them as oxides. It also neutralises the effect of the small quantities of these elements retained in the steel. Of late years Mr. Hadfield has introduced steels, containing very little carbon, and from 7 to 22 per cent. of manganese, which are extremely hard, tough, and non-magnetic. The strongest alloy contains 14 per cent. of manganese.

Chromium gives whiteness and brilliancy to steel, which then possesses great hardness and strength. Chromium has a remarkable influence in increasing the resistance of steel to compression, and when present in small quantity increases the ductility and tenacity.

Tungsten, when present in steel to the extent of 1 to 3 per cent., is said to make tough and ductile alloys, which are also very hard, with a fine crystalline structure. Such steels possess great coercive force, powerfully retaining their magnetism after being once excited.

Titanium may exist in steel in small quantity, but although much of this element may be present in the pigiron, it is usually removed in refining, like silicon.

Copper is often present in small amounts in steel, usually not exceeding 03 per cent. Its effect is to make iron and steel very hard, and in small quantity does not seriously affect the mechanical properties of steel. Mr. F. Stubbs considers that per cent. of copper in steel gives to it the peculiar property of resisting oxidation at high temperatures. The bad effect generally attributed

to copper in steel is probably due to sulphur, which usually accompanies copper in iron ores. Swedish iron is practically free from this element.

Tin alloys with steel, making it red-short and unweldable, even when present in small quantity.

Carbon. The effect of carbon on iron has been discussed (p. 36). Carbon exists in steel in two forms, corresponding to its condition in white and grey castiron. When the carbon is present in the combined form the steel is hard, and when present in the freestate the steel is soft, so that the relative hardness of two samples of steel will depend upon the amount of carbon in combination in each case. Sir F. Abel, after an exhaustive series of experiments on this subject, draws the following conclusions:

*

1°. "That in the annealed state, carbon exists as a carbide (Fe,C), uniformly diffused through the mass." 2°. "Cold rolled samples are in a similar condition to No. 1."

3°. "Hardening by sudden cooling prevents the separation of carbon as a carbide, its condition being the same as when the steel is in the fused state. Imperfect hardening will allow some carbide to separate out."

4°. "In tempered steel the condition of the carbon is intermediate between that of hardened and annealed steel, a blue temper being about half-way between these extremes."

Hardening. Steel is hardened by compression, and hammering hardens more than rolling, being more efficient. In the various methods of hardening, the quantity of liquid used, its specific gravity, conducting power, specific heat, boiling point, and heat of vaporisation influence the final result.

Take four liquids-mercury, water, oil, and coal-tar; raise four pieces of the same steel to the same tem

* This is what is generally described as "free carbon."

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