Suppression of Intergranular Brittle Fracture in Pure Iron by Reducing Grain Boundary Segregation of Sulfur with Oxides

Studies were made of the effect of alloying elements on the grain boundary embrittlement of commercially pure iron (from electrolytic iron; C content approximately 0.002 mass%, O content approximately 0.015 mass%, S content approximately 0.006 mass%) by Charpy tests and AES analysis of impurities at grain boundaries. Special reference was made to oxygen as oxides. It was found that the improvement of the embrittlement by the addition of C, Al, Mn and Ca is due to the decrease of S concentration at grain boundaries. As a result of the reaction with Mg which is a trace impurity in molten iron, (Al, Mg) sub(x)O, possibly MgO multiplied by Al sub(2)O sub(3) of the cation vacancy type, was formed in the specimens containing Al. On the basis of this result, it was deduced that the diffusion of Mg and Al by cation vacancies easily occur and the preferential nucleation of MgS and Al(N, S) on the outermost MgO multiplied by Al sub(2)O sub(3) results in the suppression of grain boundary segregation of S. A similar analysis were made on the effect of Mn and Ca.