182 METALLURGICAL MICROSCOPE 



element. Carbon is present in iron-carbon alloys either as graphite, 

 or as the carbide FesC, which may be free or which may form with 

 iron a solid solution. I am not attempting at present to distinguish 

 between solid solutions and colloidal solutions or emulsions. I 

 believe that the hardening of steel is due to the retention of the 

 carbide FcaC in a solid solution, but I also believe that the solution 

 thus retained by rapid cooling is allotropically different from the 

 solid solution stable above the thermal critical stage. 



Phosphorus. — It is believed on good grounds that phosphorus 

 exists as FeaP in iron, but unless there is a considerable percentage 

 of carbon present one cannot under the microscope detect the presence 

 of that compound, owing to the fact that it forms with ferrite a 

 solid solution. A method^by which steel high in phosphorus could 

 be differentiated under iWe microscope from one low in phosphorus 

 would be of great service. To be sure, it is believed that segrega- 

 tion of phosphorus may be detected by the Stead's reagent or by 

 similar reagents, but in the light of recent research we are in doubt 

 ivhether the segregation which we have been in the habit 

 of attributing to the occurrence of phosphorus may not be 

 due in some cases to the presence of some other element or elements •, 

 for instance, to the presence of oxygen. Obviously better means of 

 identification are needed. 



Sulphur. — We have satisfactory ground for our belief that 

 sulphur in steel unites with some of the manganese present to form 

 particles of manganese sulphide distributed somewhat irregularly 

 in the metal, and that it may also form a sulphide of iron. These 

 can be detected quite readily under the microscope. It is not 

 certain, however, that the dove-coloured inclusions generally assumed 

 to be manganese sulphide contain no other constituents, nor do 

 we know positively that sulphur forms no other compound and that 

 it is not present in any of the other constituents detectable under 

 the microscope. 



Manganese. — We believe that some of the manganese present 

 in steel forms, as stated above, manganese sulphide, as well as 

 manganese carbide, and also that some of it is present in solild 

 solution in iron, but with the exception of manganese sulphide it 

 is not possible to detect the presence of manganese in any of its 

 other forms under the microscope. 



Silicon. — Silicon is generally supposed to be present as an iron 

 silicide dissolved in iron. It is not possible, however, to verify by 

 the microscope the accuracy of this belief. 



Special Elements. — Microscopical evidences of the form in which 

 special elements, such as nickel, chromium, tungsten, vanadium, 

 etc., occur in steel are lacking. 



I believe that the discovery of etching or other methods that 

 would permit a more thorough and more exact microscopical analysis 

 of iron and steel and of their inclusions would be of great assistance 

 in the further development of metallography. 



