Surface segregation in low carbon steels

We studied by Auger electron spectroscopy the surface segregation phenomena in two low carbon steels containing titanium or titanium and niobium. The free elements dissolved in the matrix can segregate to the free surface; these elements are phosphorus, sulphur and titanium for the steel without niobium. In addition to the previous elements, carbon is detected for the steel with niobium. The presence of dissolved carbon in one of the steels and its absence in the other constitutes the most important result of the study relating to the role of carbon in the different properties of these steels. Free enthalpies of surface segregation were evaluated for each element using the Guttmann approach for ternary or binary systems. We clearly revealed different kinds of interaction between segregated elements. Strong attractive interaction between Ti and P or Ti and S produces cosegregation of both elements. On the other hand, there is a repulsive interaction between C and P or S and P. This repulsion strengthens a site competition phenomenon between the latter elements (S and P).