GASES IN METALS 179 



non is known as "spitting" and is a common occurrence when casting 

 iron, copper, cobalt, and platinum saturated with hydrogen, and 

 silver saturated with oxygen. 



The magnetic properties of iron and its alloys are known to be 

 greatly affected by gases. Ciofii,"'' of Bell Telephone Laboratories, has 

 shown that the permeability of iron can be increased to 190,000 by 

 heat treating it in hydrogen at 1500° C. This effect is attributed to 

 the removal of carbon, oxygen, nitrogen, and sulphur. In this field, 

 also, Yensen has done interesting work, details of which are contained 

 in his publications.'* The permeability of iron can be greatly increased, 

 also, by vacuum melting. Here again the gaseous impurities and 

 those which react to form gaseous products are removed by the 

 treatment. 



The influence of oxygen on the carburization of steel is not clearly 

 understood, but its importance has been emphasized by many writers. 

 Grossmann ^ believes that steel absorbs oxygen along with carbon 

 during pack carburization and that this favors a solubility of cementite 

 in alpha iron. By this mechanism, he accounts for the phenomenon 

 of split cementite. Guthrie and Wozasek ^ found that the presence 

 of oxygen speeds up the process of gas carburizing, which indicates 

 that oxygen must affect the solubility and rate of solution of carbon 

 in austenite. 



The influence of nitrogen on the properties of steel is of commercial 

 importance. It was learned first that nitrogen in steel formed nitrides 

 which were dispersed in the metal and which caused brittleness. 

 Later, nascent nitrogen, obtained from the thermal dissociation of 

 ammonia, was found to react with certain constituents in steel to 

 form an exceedingly hard case. From this discovery, the modern 

 commercial nitriding process has been developed. 



Pfeil, Lea, and others have observed that small quantities of 

 hydrogen absorbed by iron during electrolytic pickling appreciably 

 affect its mechanical properties. Pfeil ^ found that the tensile strength 

 of mild steel rods is decreased from 18.34 tons per square inch to 

 16.69 tons and that the elongation in one inch falls from 62.5 per cent to 

 10.6 per cent. Normal properties are restored if the steel is allowed 

 to stand for sometime in the air. Lea,^ also studying mild steel, 

 confirmed Pfeil's elongation data but found only a slight effect on 



3 Cioffi, Phys. Rev., 39, 363 (1932). 



* Yensen, Metal Progress, June, 1932, p. 28; A. I. M. M. E. Tech. Pub. No. 185. 



s Grossmann, Trans. A. S. S. T., 16, 1 (1929); 18, 601 (1930). 



« Guthrie and Wozasek, Trans. A. S. S. T., 12, 853 (1927). 



^ Pfeil, Proc. Roy. Sac. London, 112, 182 (1926). 



« Lea, Proc. Roy. Soc. London, 123, 171 (1929). 



