490 SMITH'S INTERMEDIATE CHEMISTRY 



basic slags of high oxidizing power until perfectly pure, and then 

 by suitable additions to give it any required final composition. 



Properties of Steel. Cast iron can be melted and cast, is 

 hard when chilled, but can not be forged or rolled. Wrought 

 iron is slag-bearing and malleable, and is not hardened by quench- 

 ing from a high temperature. It is never cast. Steel is free 

 from slag, being cast from an originally liquid condition. If its 

 carbon content is high enough, it can be hardened by quenching. 

 Steel has also greater tensile strength* than wrought iron, and it 

 can be permanently magnetized. In addition, high carbon steel 

 can also be tempered to the required degree of hardness. 



Tempering. To understand the last fact, it must be noted 

 that carbon dissolves readily in molten iron, and is partly 

 converted to a carbide of iron (Fe 3 C, 6.6 per cent C by weight) 

 in solution. As the temperature falls, the solubility of carbon in 

 iron diminishes. When white hot steel (up to 2 per cent C) is 

 suddenly chilled, there is no tune for any changes to occur during 

 the cooling, and a supersaturated solid solution is obtained which 

 is very hard and brittle. When, however, the cooling is slow, 

 some of the carbon separates in minute crystals of cementite, 

 carbide of iron Fe 3 C, until at about 700 there remains only 0.9 

 per cent of carbon in solution. At this temperature the solid 

 solution breaks down into a mechanical mixture of pure iron 

 which is soft, and carbide of iron which is hard. Steel is thus 

 a mixture, and not homogeneous, when slowly cooled. When 

 therefore hard, chilled steel is heated once more for the purpose 

 of tempering, the extent to which the softer material is formed 

 depends on the temperature reached, and on the rate and dura- 

 tion of the cooling permitted. By varying these the degree of 

 hardness allowed to remain can be adjusted. 



* Tensile strength or tenacity is measured by the weight (in kilograms) 

 required to break a wire of the metal 1 sq. mm. in section. Lead 2.6, copper 

 51, iron 71, steel 91. 



