IRON, NICKEL, COBALT 491 



Phosphide of iron makes steel brittle when cold (" cold short "). 

 Sulphide of iron makes it brittle when hot (" red short "), and 

 unsuitable for forging. Hence phosphorus and sulphur are 

 reduced to the lowest possible amounts. 



. Bessemer Process. Pig iron is melted and run into the 

 cojoverter_(Fig. 113), which is lined with fire-brick, measures about 

 15 by 8 feet, and holds 15 tons. An air-blast, entering through 

 one axle, blows through the metal 

 from tuyeres at the bottom. The 

 oxidation of the carbon and silicon, 

 which raises the temperature above 

 the melting-point of pure iron, is 

 over in 20 minutes. Sp^geleise^ 

 is then added to give the desired 



percentage of carbon and manganese, and the liquids, first the 

 slag and then the iron, are poured into ladles, and the metal is 

 cast. 



Sulphur and phosphorus are not removed by the air. If present 

 in too great amounts, they are removed by lining the converter 

 with basic material such as magnesium and calcium carbonates 

 (Thomas-Gilchrist process). The slag then contains phosphates, 

 and is valuable as a fertilizer. 



Bessemer steel is used for heavy and light machinery castings, 

 and is rolled into bridge and structural iron. It contains from 

 0.1 per cent (soft) to 1 per cent (hard) of carbon. 



jQffen-hearth (Siemens- Mar tin) Process. In this process 

 pig iron and scrap iron (uprEo 751ions) are melted on a hearth lined 

 with fire-brick and sand (Fig. 114). At a later stage haematite is 

 added to furnish oxygen (as in puddling). To secure economically 

 the temperature necessary to keep the pure product (iron) fused, 

 Siemens contributed the idea of preheating the fuel gas and air by 

 a regenerative device, The spent air and gas pass out through a 



