Interactions between diffusible hydrogen and the microstructure of a FeCMnSi third generation steel exhibiting a TRIP effect

Details

Supervisors

Jacques, Pascal

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en chimie et science des matériaux

Abstract

The automotive industry is interested by producing steels with good mechanical properties from a chemical composition with little alloying elements and using a low-energy manufacturing process. Several processes are good candidates to achieve such constraints. Most of them take advantage of a TRIP effect enhancing the ductility of the steels. While the third generation steels already show good promises they have the drawbacks of begin susceptible to hydrogen embrittlement. This work focuses on the Q&P process and its variations, and the interactions between hydrogen and the microstructures produced by this process. The first objective was to design Q&P treatments that yield good mechanical properties and possess a characteristic microstructure. Different models were studied to help narrow the range of the three parameters of the thermal treatment: the quenching temperature, the partitioning temperature and the partitioning time. The first treatments were unsuccessful as the models were too simplified to represent the complex interactions between martensite, bainite and austenite occuring during the heat treatment. After trials and errors, five candidates were selected using dilatometry, X-ray diffraction, SEM and EBSD. Dilatometry was key to design the heat treatments as it provides in-situ data on phase transformations, and it does not require much preparation. The selected candidates are two one-step Q&P, two carbide-free bainitic steels, and one two-step Q&P. The carbide-free bainitic steel at high partitioning temperature as well as the two-step quenching and partitioning steel exhibit very good mechanical properties with a good combination of ductility and strength. The candidate microstructures show a lot of promise with good mechanical properties but are heavily aected by hydrogen embrittlement. The effects of hydrogen on the microstructure were investigated using thermal desorption analysis and slow-strain-rate testing. Activation energy associated to trapping sites were identified as corresponding to single iron-vacancy, grain boundaries and dislocation strain field.