IRON, NICKEL, COBALT 493 



Open-hearth steel is used for the better class of rails, for railway 

 bridges, for shafts, armor-plate, and heavy guns, and wherever 

 the steel is subject to much vibration. 



Steel Alloys. We must distinguish between manganese, 

 aluminium, silicon, or titanium added in small amounts (" med- 

 icine ") to purify the iron, and passing (in combination) into 

 the slag, as described in preceding sections, and the present 

 subject, which concerns metals added so as to produce regular 

 alloys. 



Manganese steel (7 to 20 per cent Mn) is exceedingly hard even 

 when cooled slowly. It therefore does not lose its temper readily 

 when heated by friction. It is used for the jaws of rock-crushing 

 machinery and for burglar-proof safes. 



Chromium-vanadium steel (1 per cent Cr, 0.15 per cent Va) has 

 great tensile strength, can be bent double while cold, and offers 

 great resistance to changes of stress, and to torsion. It is used for 

 frames and axles of automobiles, and for connecting rods. 



Tungsten steel (tungsten 8 to 20 per cent, and chromium 3 to 

 5 per cent) is used for tools in high-speed metal turning, as it can 

 become red hot (from friction) without loss of temper. 



Nickel steel (containing 2 to 4 per cent of nickel) resists cor- 

 rosion, and has a very high limit of elasticity and great hardness. 

 It is used for armor-plate, wire cables, and propeller shafts. The 

 alloy with 36 per cent nickel, called invar, is practically non- 

 expansive when heated and is valuable for meter-scales and pen- 

 dulum rods. 



Duriron or tantiron (15 per cent Si) is rustproof and is not 

 attacked by sulphuric, nitric or acetic acid, hot or cold, dilute or 

 concentrated. Vessels made of this alloy are therefore used 

 industrially in acid concentrations. 



Properties of Pure Iron. Pure iron may be made by 

 electrolysis, or by reduction of a pure salt by hydrogen. It has 



