Modeling and Simulation of Thermochemical Heat Treatment Processes: A Phase Field Calculation of Nitriding in Steel.

In order to provide greater improvements in material performance with respect to hardness, fatigue, wear resistance and corrosion resistance, among other mechanical properties, nitrogen is commonly introduced on steel surface at elevated temperature. Such enhancement, referred to as nitriding, results in the diffusion of nitrogen into the steel with development of continuous layers of iron-based nitrides and precipitation of alloying-element nitrides, depending on the nitriding kinetics.Up till now, the simulation of nitriding process is done only under restricted conditions by using classical models which describe the diffuse nitriding layers with sharp interfaces. However, the classical models lead to difficulties in the computation of the free boundary problem, since the position of the interface has to becalculated explicitly.This work provides a generalized approach for studying the evolution of the microstructure during nitriding process in steel. It presents the application of a phase field model to describe the nitriding of steel in order to numerically deal with the moving free boundary. The treated nitriding layers are described in terms of a phase field parameter which represents a property of the system that is non-zero in a distinct region of the steel surface and 0 otherwise. A partial differential equation is formulated to govern the time evolution of the phase field parameter and it is coupled to other equation thatdetermines the relevant field of transport phenomenon. This allows the whole domain to be treated simultaneously. In particular, the interface is not tracked but is given implicitly by the value of the phase field parameter as a function of time and space.The set of coupled evolution equations - the concentration field and the phase-field model equations, is solved by using a finite element software. The approach is applied to compute nitriding of a low-carbon steel and evolution of compound layer formation. The numerical calculations correlate with experimental measurements at different times.