52

METALS USED IN CONSTRUCTION.

ally reduced till the castings are cooled. The requires to be gone through, in all its stages, wi care, that it is necessarily an expensive one. could be made cheaper, there is no doubt but t articles heated by it would become more exte used than they are, comparatively extensive as The cast-iron heated by this process takes a fine not easily affected by exposure to atmospheric c It is capable of being worked with the usual tools; it can be rolled, drawn, or hammered, being heated, and in small pieces welded, althou property in large pieces can scarcely be said to to it; it can, however, be easily brazed with It is capable of standing a high degree of heat, being case-hardened, and this to a better degr even wrought-iron.

(17.) As a means of strengthening cast-iron, M Colburn proposes to use a modification of the Be process, hereafter to be described (see paragraph so far that, in place of subjecting the iron to the ai'r for twenty, he would use it only for the s three, four, or five minutes. The objection mi macle, he says, to this, that if the Bessemer pro to be used at all, why not use it so that the iron at on ce be changed into steel; but Mr Colburn

this by pointing out several reasons why this should not be done. In the first place, a much higher or more superior quality of iron is required to make steel by the Bessemer process, and, generally, a quantity of Spiegeleisen is required to be added to it; further, the steel is usually run into ingots, and not into the form of goods or castings.

(18.) M. Gaudin obtains an exceedingly hard iron by heating a quality tolerably free from carbon with a small quantity of boron; and that, by adding the ordinary cast-iron in a state of fusion, a quantity of phosphate of iron, and peroxide of manganese, the same effect is produced. By adding tungstein to cast-iron a still greater degree of hardness is given to it; so much so, that he says it surpasses everything previously known as a material for tools for cutting rocks, and that crystals of it will cut glass as readily as the diamond.

(19.) THE conversion of cast-iron into mallea wrought iron is effected by getting rid of the d by exposing it in a melted state to the action atmosphere in reverbating furnaces, treating while to a mechanical process called "puddling;" after, after it becomes agglutinated, taking it out furnace and subjecting it to the process called " ling," and finally working it up with a steam-w hammer. The iron thus treated is found to be de

f its carbon, and is capable of being wrough ammered; or, in other words, from a brittle, changed into a malleable condition. In additi the capability of being hammered and drawn out out fracture, which iron in this condition posses has the even more valuable property, by virtue of two separate pieces can be joined together by the

cess of "welding." Familiar as most of us are with the process, it is somewhat singular that we should know comparatively little as to the principles upon which it is based, and that we are therefore at a loss to give some reasonable explanation of it. The process itself is well known, and consists in shaping, in a way dependent upon the form, the ends of the articles to be welded together; then heating them in a smith's fire or furnace to a white, or what is called technically a welding, heat; then taking them out, scattering a few grains of sand on the ends; then striking them on the anvil so as to throw off some of the scales or evaporation; then bringing the ends together in contact; and finally uniting them by two blows from the hammer of the smith, followed by repeated and well directed blows of the hammers of the striker or strikers. The process in practice is more quickly performed than the description of it here given; and, as before stated, its principles, or rather what is welding, are or is not fully known. The best temperature at which the pieces are to be joined has not yet been determined. The sand which is thrown upon the highly-heated surface unites with the oxide formed upon them, and it appears also to aid the close contact of the surfaces. Mr Colburn thus explains the phenomena of welding :-" In all

welding we first employ a degree of heat sufficient to overcome so much of the cohesive force between the atoms of ore iron, so as to allow of sufficient motion among themselves to bring all, or most of the atoms. forming the surface, within cohesive range of those forming the opposite surface." A writer on the subject, in the Mechanics' Magazine, in commenting upon this explanation, remarks very suggestively,

According to this, heat in welding would appear to have a double function-that of cleaning the surface, and that of rendering the less pressure necessary. But does this explanation meet the normal and chemical influences at work in the operation of welding iron? Why is it so difficult to weld burnt' iron? Why is it so difficult to weld steel? Why is pure iron-the iron of the chemist and the laboratory, and not that of commerce-so difficult to weld? These, amongst many other similar questions, are yet very far from solution, and they will probably remain so for many years. It is, however, somewhat strange that more of the obvious qualities of iron are not definitely known." With this suggestive statement we now proceed to the consideration of parts connected with the conversion of cast into wrought iron.

(20.) The cast-iron to be converted into malleable