Abstract
Quenched and partitioning (Q&P) steels show high ductility in combination with high strength attributed to their multiphase microstructure, consisting of tempered martensite and austenite. The interaction of the Q&P microstructure, particularly of the metastable retained austenite and its interface, with hydrogen is investigated using a systematic joined experimental and numerical study.
The experimental part (Part I) presents permeation and thermal desorption experiments investigating the diffusion and desorption behaviour of samples in unstrained, pre-strained and in-situ strained condition during hydrogen charging and microstructure analysis. In the simulation part, (Part II) numerical trapping and diffusion models based on irreversible thermodynamics are compared with the measurement data of permeation and thermal desorption experiments to discuss the changes in diffusion and trapping behaviour in more detail.
In a first step, the trap densities in ferrite, austenite and the interface are estimated from crystallographic considerations accounting for microstructure, volume fractions and morphology. In ferrite, traps sites, such as grain boundaries, sub-grain boundaries, dislocations, and vacancies are considered. Bulk traps are considered in austenite. In all cases, local thermodynamic equilibrium between lattice and traps is assumed. In combination with existing literature data, this yields the starting point to model TDS and permeation experiments considering multiple sorts of traps. The exact combination of trap densities of traps attributed to different defects is determined for the different testing conditions. Changes in trap density with different pre-strain and differences in the results between permeation data and TDS are discussed.
It is also investigated, how the interface between austenite and ferrite affects the kinetics at which austenite bulk can be charged and discharged. A novel formulation for the interface condition of the flux is applied. In existing formulations, the interface is not considered as trap. In the presented work, the interface can act as trap and/or diffusion barrier demanding same chemical potentials on either side of the interface. The effect of the interface properties on the austenite charging kinetics are studied using a submodel, representing a single, austenite island embedded in ferrite.
It is shown that plastic pre-strain during charging leads to higher number density of traps with lower binding energies, slowing down the diffusion. The presence of mechanical load during charging alters the trapping behaviour. Strong trapping at the interface would affect the charging of the austenite for low hydrogen concentrations. The binding energy of the interface, however, is more likely to be similar to large angle grain boundaries.