Hydrogen embrittlement in martensitic advanced high-strength steels for the automotive sector

Hydrogen embrittlement is one of the most impacting issues for the use of the martensitic advanced high-strength steels in the automotive industry. Hydrogen diffuses in the metal lattice and accumulates in trap sites, producing degradation of the mechanical properties of the material, both in terms of strength and ductility. Knowledge of how hydrogen diffuses and damages the material is of fundamental importance to prevent this phenomenon. This thesis focuses on the evaluation of the hydrogen diffusion and trapping material properties and the development of a hydrogen embrittlement model for the quantification of the hydrogen effects on martensitic advanced high-strength steels. Two commercial martensitic advanced high-strength steels of grade 1300 and 1500 MPa were selected. Thermal desorption spectroscopy and electrochemical permeation tests were used to study the diffusion and trapping behavior. Thermal desorption spectroscopy results revealed that there is one reversible kind of trap site, and dislocations are the primary trapping sites for the tested alloys, thus making it possible to characterize hydrogen interaction with the material using McNabb and Foster’s model. It is demonstrated that a complete characterization of the trapping material properties can be performed, once the lattice hydrogen diffusion coefficient is known, by a proper fitting of successive decay and build-up permeation transients using the solution of the McNabb and Foster’s equations. A procedure for evaluating the trap site number, the release and capture rates is presented. The derived estimations of the release and capture rates agree with the values deduced from thermal desorption spectroscopy. Uniaxial slow strain rate tensile tests on smooth specimens together with a post-tensile fractographic analysis was used to identify the hydrogen embrittlement mechanism and develop a damage model together with a fracture criterion for the highest steel grade. The model is validated by reproducing the uniaxial slow strain rate tensile curves of holed and notched specimens. The first principal strain at fracture was found as the most attractive parameter to quantify the hydrogen effect on the material.