Effect of surface roughness on kinetics of mass transfer during gas carburising

Abstract

Despite its worldwide application, gas carburising faces certain challenges in control and variability. No current carburisation model accounts for the effect of surface roughness and is able to predict observed case depth variations. An investigation of the effect of surface roughness on gas carburising performance, with the aim of developing a functional relationship between surface roughness and the mass transfer coefficient, is reported. A further objective was to model surface roughness effects on the carbon concentration profile and corresponding case depth variations. A scanning laser microscope was used to measure 3D roughness parameters and surface area of AISI8620 steel samples subject to various surface preparation operations. Carburising response was analysed in terms of weight gain, microhardness and carbon concentration profiles. The weight gain and surface carbon concentration were used to calculate the mass transfer coefficient. The carburising kinetics was found to be directly proportional to the surface roughness and was a function of surface area available for carbon transfer. The calculated mass transfer coefficients and mass transfer coefficients from the literature were used as input to the carburisation model. Experimentally determined concentration profiles validated the model prediction. It is concluded that the model can be used to estimate initial surface conditions to improve carburising performance and for effective process control.

Keywords:

• GAS CARBURISING
• SURFACE AREA
• MASS TRANSFER COEFFICIENT
• CARBON FLAX
• KINETICS
• MODELLING