Calculation of Gas Carburizing Kinetics from Carbon Concentration Profiles based on Direct Flux Integration

Abstract

Initiated by the need of industry for gas carburizing process control and optimization, this paper focuses on understanding the effect of the time, temperature and carbon potential on the mass transfer coefficient and carbon diffusivity in Austenite. A method for direct flux integration has previously been proposed to calculate these kinetic parameters from the experimental carbon concentration profiles. AISI 8620 steel discs were gas carburized at different levels of atmosphere carburizing potential for selected austenizing temperatures. Analyses of the carburized parts included experimental measurement of weight gain, surface carbon concentration and carbon concentration profiles. The time-dependent weight gain and surface carbon content measurements allowed calculation of the time average mass transfer coefficient, while carbon concentration profiles were used to calculate the concentration dependent carbon diffusivity for selected process parameters. Excellent agreement was found between the calculated mass transfer coefficient and carbon diffusivity values and those reported in the literature. The calculated values served as input in the previously developed carburizing model validating the predicted results by comparison with the experimental concentration profiles.

Multi-objective optimisation of gas carburising process in batch furnaces with endothermic carburising atmosphere

“…The two control parameters determining the final carbon concentration profile are: the mass transfer coefficient (b) defining the flux of carbon atoms from the atmosphere to the steel surface and the coefficient of carbon diffusion in steel (D) at the austenising temperature. The mass transfer coefficient is a complex function of the atmosphere gas composition, carburising potential and temperature, 7,8 while carbon diffusivity depends on the carburising temperature, carbon concentration and steel alloy composition. 7,9 Carbon concentration profiles and the effective case depth were modelled numerically using a carbon potential and temperature dependent mass transfer coefficient and the concentration dependent carbon diffusivity in austenite.…”

“…The mass transfer coefficient is a complex function of the atmosphere gas composition, carburising potential and temperature, 7,8 while carbon diffusivity depends on the carburising temperature, carbon concentration and steel alloy composition. 7,9 Carbon concentration profiles and the effective case depth were modelled numerically using a carbon potential and temperature dependent mass transfer coefficient and the concentration dependent carbon diffusivity in austenite. 5,7 The model is based on the finite difference approximation of the parabolic equation governing carbon diffusion in steel and a flux balance boundary condition accounting for the mass transfer coefficient in the atmosphere and the kinetics of the interfacial carburising reactions…”

“…Increasing carburising temperature enhances the mass transfer coefficient in the atmosphere b and carbon diffusivity in austenite D, and thus shortens the time necessary to achieve the desired case depth. 7 Such a solution, however, may decrease furnace life and promote excessive austenite grain growth. While increasing C P enables a greater rate of carbon transfer from the atmosphere to the steel surface, it also increases the tendency of such carburising atmospheres to soot when C P approaches the carbon saturation limit in austenite.…”

“…Optimisation of the gas carburising process parameters in this paper is the conclusive chapter in a series of experimental and theoretical investigations on gas carburising process control and optimisation. [5][6][7] The previously presented carburising model 5 was extensively used in this work to simulate the carbon concentration profiles and the corresponding case depth. A method of direct flux integration was developed which allowed calculation of the mass transfer coefficient and carbon diffusivity from the experimental data.…”

“…A method of direct flux integration was developed which allowed calculation of the mass transfer coefficient and carbon diffusivity from the experimental data. 6,7 These kinetic parameters were used to model the relationship between the controllable input parameters (carbon potential, temperature and time) and the carburising performance (surface carbon concentration and case depth). The optimisation strategy is based on:…”