Cementite Distribiition in Steel 143 



will be noted that practically all the inclusions occur in the light ferrite 

 areas. 



Segregation about inclusions cannot be destroyed by ordinary heat 

 treatment, such as would break up ordinary grain formations. Besides 

 being areas of little or no strength they then act as obstacles to the 

 proper thermal treatment of the metal. 



Whether inclusions are the cause of the segregation of cementite 

 in hyper-eutectoid and ferrite in hypo-eutectoid steel or whether their 

 co-existence is the result of some common and deeper seated cause has 

 been the subject of considerable discussion. 



Stead' pointed out that in the case of hypo-eutectoid steel the in- 

 clusions surrounded by ferrite generally occur in areas rich in phosphorus 

 and he held that the phosphorus is the real cause of the ferrite segrega- 

 tion and that the presence of the inclusions is merely incidental. He 

 Lelieved that the inclusions themselves are not the cause of this segrega- 

 tion. 



Howe' expressed the opinion that the presence of the inclusions 

 within the ferrite or cementite areas is due to the tendency of the 

 steel in the process of cooling to reject both inclusions and the ferrite 

 or cementite to the grain boundaries. 



Brearly' and Ziegler^ suggested that the inclusions might act as 

 nuclei for ferrite or cementite crystallization in a manner similar to 

 a string suspended in a sugar solution. 



Work in this laboratory' has led to the conclusion that the cause 

 of ferrite and cementite segregation about inclu.':ions is due to the fact 

 that the inclusion itself or some reaction product cf it with the surround- 

 ing steel is soluble to a slight extent in the metal at high temperatures 

 and that this introduction of a third component into the solid solution 

 system of iron and iron carbide so alters solubility relations that 

 supersaturation is reached at a higher temperature in the metal im- 

 mediately surrounding the inclusion than in the rest of the steel. If 

 there is a zone, however narrow, lying about the inclusion and contain- 

 ing even a trace of any dissolved material from the inclusion, this 

 foreign material should alter the solubility of the excess ferrite or 

 cementite in the austenite next the inclusion and thus cause a local 

 condition of supersaturation first in this region. The excess constituent 

 then would separate here first. After separation had started it would 

 continue about these spots as nuclei as the steel cooled farther and 

 consequently the inclusions would be imbedded in the cementite or 

 ferrite. 



Under the direction of Dr. E. G. Mahin the writer undertook to 

 study the effect produced upon cementite distribution in hyper-eutectoid 

 steel when artificial inclusions of known composition were placed in the 

 metal. 



1 J. Iron Steel Inst.. 'J7, 287 (1918). 



- "The Metallography of Steel and Cast Iron", 280. 



3 Proc. Sheffield Soc. Eng. Metallurgists, 1919. 



'Rev. metall., S, 655 (1911). 



= J. Ind. Eng. Chem., 11, 739 (1919). 



Ibid., 12, 1090 (1920). 



