Cementite Distribution in Steel 141 



THE EFFECT OF NON-METALLIC IMPURITIES ON 

 CEMENTITE DISTRIBUTION IN STEEL. 



G. B. Wilson, Purdue University. 



It is a well known fact that non-metallic inclusions in iron and 

 steel exert injurious and very undesirable effects upon the metal. Since 

 it has been impossible entirely to eliminate their presence they have been 

 the subject of much investigation and discussion. 



Their weakening effect upon the steel is much more pronounced than 

 can be explained by assuming that they are essentially the same in their 

 effect as would be produced by cavities of the same size. Inclusions 

 which may occupy a very small proportion of the cross sectional area 

 of a given piece of steel very often prove fatal, whereas the total effective 

 area of the metal lying between the inclusions would seem to be suffi- 

 cient to withstand the strain. Their importance to the manufacturer 

 and to the user of steel then is vastly greater than their size inight 

 at first indicate. 



When pure hyper-eutectoid steel (steel containing more than 0.87 

 per cent carbon and generally called high carbon steel) is heated above 

 its critical range it is composed of austenite. The upper limit of this 

 range varies from 725° to 112.5°C as the carbon content varies from 

 0.87 to 1.7 per cent. Austenite is essentially solid iron with carbon 

 in the fonii of iron carbide (Fe^C) dissolved in it. Iron carbide as a 

 constituent of steel is called cementite. If the steel is slowly cooled 

 nothing happens except that the austenite grains increase in size until 

 the first recalescent point Ar"" is reached. At this point the unsaturated 

 austenite of higher temperature becomes saturated with cementite and 

 the latter is thrown out of solution. As the temperature continues 

 to fall the austenite continues rejecting cementite until the second 

 recalescent point Ar, is reached when austenite of eutectic composition 

 remains and is changed bodily into pearlite. Pearlite then is the mixture 

 of eutectic composition stable below the Ar, point and consists of a 

 conglomerate of carbonless iron (ferrite) and iron carbide (cementite). 



Figures \a is a photomicrograph of an annealed hyper-eutectoid 

 steel which has been highly polished and etched with a solution of nitric 

 acid in alcohol in order to show the grain structure. Each irregular 

 cell-like area is a grain of this conglomerate, pearlite. The light network 

 and areas between the pearlite grains represent the excess cementite 

 above that required to form the eutectic and which was rejected to the 

 grain boundaries as the steel cooled. 



In the case of hypo-eutectoid steel (steel containing less than 0.87 

 per cent' carbon) the iron or ferrite instead of cementite is in excess 

 of that required for eutectic composition, consequently ferrite is thrown 

 from solution as it cools through the critical range and the cooled steel 

 consists of pearlite and ferrite. Figure Ifo, shows a hypo-eutectoid steel 

 in which the network and light areas are ferrite. The dark constituent 

 is again pearlite. 



"Proc. 38th Meeting, 1922 (1923)." 



