70 JOURNAL OF THE [July, 



subject. Similar conditions generally apply to other grades of 

 steel, so far as the texture is concerned. 



The fact of the great forces of crystallization present in molten 

 steel prevents finding steel after casting to be a homogeneous, 

 structureless compound, but it is composed of several, with more 

 or less structural detail. In other words, our steel ingots are 

 built up of great numbers of mineral aggregates, and to bring the 

 steel into the highest physical condition we must reduce or 

 render the mass as homogeneous as possible and give toughness 

 to the metal as well. 



The wonderful and important property of carbon uniting with 

 iron at temperatures below fusion has long been known, espe- 

 cially in regard to tool steels ; another property being that this 

 union lowers the melting point of iron and renders it possible to 

 cast it as iron, or, when the carbon is present in lesser quantities, 

 to cast the product as steel. 



The first-mentioned property of carbon forms new structure 

 and compounds at different temperatures, or diffuses the carbon 

 in a different manner, and can hoi I the structure of different tem- 

 peratures when suddenly cooled. 



Another useful property of the compound of carbon and iron 

 is that of hardening upon being heated to a cherry red and sud- 

 denly quenching in water, oil, molten lead, and several other 

 similar media. Certain degrees of hardness can be given to steel, 

 according to the use the steel is to subserve. 



The same fact seems to be true of the open-hearth and Bes- 

 semer steels, and advantage is now taken of it to improve the 

 quality of large masses of steel by heat treatment, as well as de- 

 pending upon chemical composition and mechanical work. This 

 heat treatment consists in raising the temperature of the steel to the 

 degree which gives the desired structure, and then fixing this struc- 

 ture, which will be hard but more or less brittle, by cooling quickly 

 in some media. To give toughness to the steel it is then annealed 

 at a temperature below quenching. This permits some rearrange- 

 ment of the compounds. A part of the hardening carbon is con- 

 verted into cement carbon, which lessens the hardness and brit- 

 tleness of the remaining hardening carbon. 



The results already reached of improving the physical properties 

 of steel of the same chemical composition by heat treatment are 



