Modelling and simulation of hydrogen redistribution in a heterogeneous alloy during the cooling down to 200 °C

Hydrogen embrittlement induced by internal hydrogen has been identified as a critical issue for heavy components manufactured from either plain or hollow ingots made of 16MnNIMo5 through 20MnNiMo5 alloys that have the particularity to be heterogeneous at various scales. Based on available data of solubilities, diffusivities and temperature dependent phase transformations for the matrix and segregated bands, a model of hydrogen distribution in such a heterogeneous microstructure has been developed. Additionally, penny-shaped thin cavities have been introduced to account for the presence of incoherent MnS inclusions leading to a potential decohesion at MnS/matrix interface. Pressure increase in such cavities has been evaluated using ideal gas behavior for hydrogen. A unidimensional Finite Difference numerical approach has been implemented in order to simulate hydrogen redistribution among matrix, segregation_band and cavity. This model has allowed to quantitatively evaluate the redistribution of hydrogen at the scale of the microstructural heterogeneities during the cooling down to 200 °C: first, close to the component surface where important kinetic effects have been demonstrated due to high cooling rate and second, in the bulk, where hydrogen redistribution was close to the thermodynamic equilibrium during the whole cooling period. It has been shown that the final equilibrium pressure of hydrogen in the cavity at 200 °C is negligible even for a high initial hydrogen content.