Speaker
Description
Hydrogen is nowadays viewed as a key element for developing low-carbon energies, but it diffuses in metals and alloys and segregates in crystalline defects, reducing the ductility of the material. This phenomenon is well-known and called hydrogen embrittlement (HE). In this work, we aim to reduce the negative impact of hydrogen by adding boron at interfaces to improve the resistance against HE.
Boron mostly segregates into prior austenite grain boundaries (PAGBs) before the martensitic transformation in steels. This segregation naturally improves resistance to HE compared to boron-free steels by avoiding the intergranular fracture at PAGBs [1]. The competition between boron and hydrogen at PAGBs has been investigated further using thermal desorption spectroscopy (TDS) experiments and ab initio calculations. The latter shows that even if the interaction between hydrogen and the structural defect is attractive, it becomes repulsive when boron is present in the grain boundary. This behaviour has also been observed using TDS measurements, with the disappearance of one peak when boron is incorporated into the microstructure.
The hydrogen concentration partitioned in the microstructure has then been quantified from peak integration of the TDS measurements and compared to the hydrogen concentration at thermodynamic equilibrium. The latter concentration has been evaluated using the Langmuir-McLean approximation with trapping energy deduced from the TDS measurements and permeation tests. This thermodynamic model shows that all traps are filled identically when the total hydrogen concentration is low for boron-free steel. However, when it increases, traps of the lowest segregation energies (mostly PAGBs) are firstly saturated, which promotes failure initiation at this defect type. This finding partially explains why PAGBs are the weakest microstructure feature when martensitic steels are exposed to hydrogen-containing environments.
However, HE is still observed when the hydrogen content is increased due to a fracture at martensite boundaries, interfaces where boron is not located [1]. Additional heat treatments have been performed to activate the mobility of boron and carbon in the solid solution which can segregate in other martensitic interfaces (packets, blocks, and lath boundaries). This heat treatment improves the resistance of boron-doped steels to avoid a cleavage fracture by reducing the hydrogen uptake. The newly developed microstructure is analysed through synchrotron experiments and atom probe tomography, which confirms the segregation of boron at interfaces without any change in the stress states (dislocation density and crystalline size).
References
[1] H. Shi, G. Hachet, P.T. Sukumar, D. Ponge, B. Sun and D. Raabe, Internal communication, 2024
[2] G. Hachet, A. Tehranchi, H. Shi, M. Prabhakar, D. Ponge and D. Raabe, Communication submitted, 2023
