Atmosphere gas carburizing is an essential heat treating process used in industry. High carbon potential in the furnace enables carbon flux from the atmosphere to the steel surface and across the gas-solid interface. This establishes carbon gradient in the steel and provides a driving force for carbon diffusion into the bulk of the material. The goal of this work is to understand the effect of flow rate, and therefore, mean residence time, on the furnace carburizing potential. A thermodynamic model has been developed and programmed in MATLAB to predict equilibrium gas composition of both endothermic gas and carburizing atmosphere after natural gas enriching. The atmosphere development during the enriching stage has also been modeled using Reaction Engineering Lab in COMSOL. The predicted profiles show good agreement with the experimental data and can further be used to perform “what if” scenarios.