Finite Element Analysis of Surface Hardening Treatments of Steels for Automotive Powertrain Applications

Surface hardening is a procedure that is usually performed on steel components, in order to impart in these the strength and surface hardness that are necessary for their correct operation, whilst maintaining a reasonable value of ductility in the overall component. The latter is usually composed of a sequence of heat treatments: carburizing, quenching and tempering. Each of these have different effects that, if correctly combined, can lead to the desired final mechanical properties in the component. Carburizing is a process that increases the amount of carbon concentration in the component surface. Quenching is a quick decrease in temperature that causes in the component different phase transformations and an increase in the surface hardness, causing however also a decrease in toughness. Lastly, tempering is the reheat of the component to a prescribed temperature for a certain amount of time, which leads to an increase in toughness with consequent decrease in hardness. All of these processes involve complex phenomena that are difficult to study and predict. However, the prediction of the mechanical properties of heat-treated components is very useful and important for large Automotive companies. A simulation tool is therefore created to predict the mechanical properties in automotive powertrain components after the carburizing-quenching-tempering sequence. This tool is designed in ABAQUS, with the addition of user-defined subroutines to include in the FEA software all of the metallurgy-related-effects that are not already present. The outputs of this simulation are microhardness and steel phase composition. They are then compared to experimental microhardness measurements and to microstructure images obtained from the real automotive components. The results from such a comparison show that the simulation tool is able to predict qualitatively the different steel phases that are present in the component at different locations, and their general trend as a function of depth. Furthermore, the simulation software is also able to predict the general trend of the microhardness profile found at the surface of the components at the end of the heat treatment sequence.