High temperature carburizing of a stainless steel with uranium carbide

Abstract

Diffusion theory (DT) and activated complex theory (ACT) were applied to the carburizing process of austenitic stainless steel 1.4988 with uranium carbide by sodium bonding at 773, 873, 973 and 1073K for 1000h. Microhardness profiles of the carburized steel specimens were obtained. Diffusion coefficient (D) of carbon atoms into the steel were calculated for each temperature by using the microhardness values instead of the carbon concentrations in the approximate solution of the second Fick’s equation. Arrhenius equation for the carburizing process was found as: D=2.6×10−11exp (−66753/RT) inm2s−1. Equilibrium constant (K#) and enthalpy of activation (ΔH#) for the formation of an activated complex calculated for each temperature from the Eyring equation using the diffusion coefficient instead of the reaction rate constant. Temperature dependence of the ΔH# and change in heat capacities (ΔCp#) by the carburizing were graphically determined as: ΔH#=66741 − 8.3T in Jmol−1 and ΔCp#=8.3JK−1mol−1. Temperature dependence of lnK#, Gibbs energy of activation (ΔG#) and entropy of activation (ΔS#) were found respectively as follows: lnK#=−66741/RT=(8.3/R)lnT −48.3, ΔG#=66741+8.3T lnT+48.3RT in Jmol−1, and ΔS#=−409.9−8.3 lnT inJK−1mol−1. Based on these results, formation, size, and composition of the activated complex were discussed.