China Nonferrous Metallurgy

The close combination of Babbitt alloy and bearings (carbon steel) is crucial for the efficiency of bearing operation, and currently iron-tin compounds are mainly used as intermediate materials to improve bearing life. At present, there is no report in relevant literature on the effect of carbon atom content in carbon steel on the diffusion of tin atoms into iron. A series of experiments concerning hot-tin plating on the surfaces of 25 and 35 steel matrix have been carried in this paper. The key indicators involving Fe-Sn equilibrium time diffusion coefficient of Sn atoms and diffusion thickness have also been analyzed, which could be used to estimate the binding quality during the reaction process at 310℃. The results indicate that the distance from tin atoms diffused into the surface of carbon steel increases with the increasing of holding time, and the diffusion coefficient of tin atoms entering 25 steel is 6.6×10-9, which is about twice that of 35 steel at 310℃. The diffusion distance ratio of carbon atoms is inversely proportional to the square root of the carbon concentration quotient in the surfaces of 25 and 35 steel. In addition, the calculated bonding energy of Fe₃C is -133eV/atom, while the bonding energies of FeSn and Fe₃Sn₂ are only -7.4 and -6.9eV/atom, respectively. It is obvious that carbon atoms are easily combined with iron atoms, that is, the high carbon content would hinder tin atoms to diffuse into the steel. This is the fundamental reason why tin atoms are easily diffused towards the surface of 25 steel. As a result, the matrix material of 25 steel is identified as an ideal babbitt alloy bearing material.