IRON, COBALT, AND NICKEL 381 



engaged on heating. This galvanic deposition of iron is used for 

 making galvanoplastic cliches, which are distinguished for their great 



these latter contain it because their ores contain titanium, which combines directly with 

 nitrogen. Hence the part played" by nitrogen in steel is but an insignificant one. Ifc 

 may be useful here to add some information taken from Caron's treatise concerning the 

 influence of foreign matter on the quality of steel. The principal properties of steel are* 

 those of tempering and annealing. The compounds of iron with silicon and boron have 

 not these properties. They are more stable than the carbon compound, and this latter 1 

 is capable of changing its properties; because the carbon in it either enters into 

 .combination or else is disengaged, which determines the condition of hardness or softness 

 of steel, as in white and grey cast iron. When slowly cooled, steel splits up into & 

 mixture of soft and carburetted iron ; but, nevertheless, the carbon does not separate 

 from the iron. If such steel be again heated, it forms a uniform compound, and hardens, 

 when rapidly cooled. If the same steel as before be taken and heated tPlong time, then, 

 after being slowly cooled, it becomes much more soluble in acid, and leaves a residue of 

 pure carbon. This shows that the combination between the carbon and iron in steel 

 becomes destroyed when subjected to heat, and the steel becomes iron mixed with 

 carbon. Such burnt steel cannot be tempered, but may be corrected by continued 

 forging in a heated condition, which has the effect of redistributing the carbon equally 

 throughout the whole mass. After the forging, if the iron is pure and the carbon has 

 not been burnt out, steel is again formed, which may be tempered. If steel be re- 

 peatedly or strongly heated, it becomes burnt through and cannot be tempered or 

 annealed ; the carbon separates from the iron, and this is effected more easily if the 

 steel contains other impurities which are capable of forming stable combinations with 

 iron, such as silicon, sulphur, or phosphorus. If there be much silicon, it occupies the 

 place of the carbon, and then continued forging will not induce the carbon once 

 separated to re-enter into combination. Such steel is easily burnt through and cannot 

 be corrected; when burnt through, it is hard and cannot be annealed this is tougb, 

 steel, an inferior kind. Iron which contains sulphur and phosphorus cements badly, 

 combines but little with carbon, and steel of this kind is brittle, both hot and cold. 

 Iron in combination with the above-mentioned substances cannot be annealed by alow 

 cooling, showing that these compounds are more stable than those of carbon and ironfc 

 and therefore they prevent the formation of the latter. Such metals as tin and zino 

 combine with iron, but not with carbon, and form a brittle mass which cannot be 

 annealed and is deleterious to steel. Manganese and tungsten, on the contrary, ajfe, 

 capable of combining with charcoal ; they do not hinder the formation of steel, but even 

 remove the injurious effects, of other admixtures (by transforming these admixed sub- 

 stances into new compounds and slags), and are therefore ranked with the substances 

 which act beneficially on steel ; but, nevertheless, the best steel, which is capable of 

 renewing most often its primitive qualities after burning or hot forging, is the purest. 

 The addition of Ni, Cr, W, and certain, other metals to steel renders it very suitable for, 

 certain special purposes, and is therefore frequently made use of. 



It is worthy of attention that steel, besides temper, possesses many variable 

 properties, a review of which may be made in the classification of the sorts of steel 

 (1878, Cockerell). (1) Very mild steel contains from 0'05 to 0'20 p.c. of carbon, breaks 

 with a weight of 40 to 50 kilosjper square millimetre, and has an extension of 20 to 

 80 p.c.; it may be welded, like wrought iron, but cannot be tempered ; is used in sheets 

 for boilers, armour plate and bridges, nails, rivets, &c., as a substitute for wrought iron ; 

 (2) mild steel, from 0'20 to 0'35 p.c. of carbon, resistance to tension 50 to 60 kitos, 

 extension 15 to 20 p.c., not easily welded, and tempers badly, used for axles, rails, and 

 railway tyres, for cannons and guns, and for parts of machines destined to resist bending 

 and torsion ; (3 hard steel, carbon 0'85 to 0'50 p.c., breaking weight 60 to 70 kilos per 

 square millimetre, extension 10 to 15 p.c., cannot be welded, takes a temper ; used for 

 rails, all kinds of springe, swords, parts of machinery in motion subjected to friction, 



