Mathematical modeling of the kinetics of martensite transformation and the thermomechanical properties of a low alloy medium-carbon steel

Abstract

Heat treatment processes are essential for steels in order to achieve desirable mechanical properties for applications requiring important mechanical loadings. This is especially true for companies operating in the aerospace industry where the expected performance requirements are very high. However, heat treatments are not without detrimental effects. Indeed, they are also sources of rejected parts due to the introduction of unwanted and unquantified distortions exceeding tolerances that are often very tight. The presence of distortions is a problem for companies because productivity suffers, and additional costs may be needed to cover for the replacement of rejected parts. The prediction of distortions from a heat treatment is also very difficult, because it involves several physical fields with a multitude of interactions that needs to be assessed. To be able to quantify and predict these distortions, simulation is mandatory as it is the most cost-effective method in comparison to the trial-and-error practice. However, for accurate simulations, a reliable material data is required. Therefore, experiments must be caried out and models must be proposed based on the current manufacturing process of the company. For this project, the main objective is to provide accurate models for the kinetics of phase transformation and for the thermo-mechanical properties of a low alloy medium-carbon steel used in the manufacturing of a landing gear. The first part of this study focuses on the phase transformation of a quenching process. Due to the industrial size of the component with complex geometry and different thicknesses, nonuniform temperature distribution is observed. This also leads to a non-uniform phase transformation occurring at different moments and at different rates during the whole quenching process. For a better understanding of the microstructure evolution, the phase transformation has been characterized by dilatometry. The ongoing phase transformation can simply be observed by the change in dilatation. Different cooling rates are also used under the martensite start temperature, better representing the quenching conditions of a landing gear. The results showed a significant deceleration in the transformation which increases with decreased cooling rates. This rarely documented behaviour has been attributed to the carbon partitioning from fresh martensite to the remaining austenite during auto-tempering. Following this observation, a new mathematical model is proposed in the form of a routine. Although, there are already some existing models for the prediction of martensitic phase transformations, they are unable to accurately depict the deceleration. In the present model, equations describing the different mechanisms present during the phase transformation are implemented. The fraction of martensite is described by the Koistinen-Marburger equation. However, due to the underlying mechanism of carbon partitioning, the rate of transformation is affected and decreases with increase carbon concentration in the remaining austenite. The benefit of the proposed model is highlighted by its capability to predict the instantaneous factions of martensite for a wide range of anisothermal cooling routes as it is solely dependent on material parameters. The second part of the project covers the entire steps for the determination of the thermomechanical properties of austenite and martensite for the temperature range of the quenching process, 25°C to 875°C. The main mechanical properties are the Young’s modulus, viscous stress, kinematic hardening stress, isotropic hardening stress and the mathematical yield stress. The experiments are conducted on the Gleeble and similarly to the previous section, models are proposed for the material properties extracted.

Date	1 Dec 2023
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Mohammad Jahazi (Supervisor)