Effect of surface roughness on the kinetics of mass transfer during gas carburizing *

Abstract

Abstract Despite its worldwide application, gas carburizing faces certain challenges in control and variability. No current carburization 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 carburizing 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 AISI 8620 steel samples subject to various surface preparation operations. Carburizing response was analyzed 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 carburizing 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 carburization model. Experimentally determined concentration profiles validated the model prediction. It is included that the model can be used to estimate initial surface conditions to improve carburizing performance and for effective process control. ▪