Abstract
Hydrogen embrittlement of high strength steels is largely studied in order to understand the relationship between the metallurgical defects and the diffusion and segregation of hydrogen. Different models have been proposed to explain the hydrogen embrittlement that could lead to premature or delayed fracture. In the case of delayed fracture, the role of the different hydrogen species is not well understood, and the impact of the redistribution of hydrogen that may occur during the baking process on the fracture mode is still under consideration. The objective of our study is to give more precise information on the impact of baking process (temperature, time) on the hydrogen distribution that may control the delayed fracture of martensitic steels. It was observed that the increase of baking temperature shifted the kinetic release of hydrogen towards shorter times. It was reported that a part of the diffusible hydrogen is moved towards deeper traps. Evolution of the mechanical resistance during a tensile test after different baking times at 20°C shows a progressive loss of ductility during the first hours associated with a partial brittle facture, then a total recovery is obtained at longer baking time. This loss of ductility could be correlated with the release kinetic of the diffusible hydrogen during the baking process.
Introduction
Hydrogen embrittlement (HE) could induce a premature or delayed rupture leading to the total loss of the integrity of the metal structures when stress is applied. This phenomenon affects many structural elements in the industry and in particular the fastening elements in automotive and aerospace industries where martensitic steels are currently used. To improve corrosion resistance, a protective coating is commonly applied by electroplating. However, martensitic steels can be susceptible to severe internal hydrogen embrittlement in relation with co-deposition of hydrogen during electroplating. To prevent the HE, a "baking" process, where a material is held at a specific temperature for a given time to desorb hydrogen, is generally performed, according to the standard ASTM1 STP 96,1 after specific surface preparation or electroplating.
However the efficiency of this method can be questioned because it depends respectively on microstructures of the steel and the coating. Additionally, this latest can act as a hydrogen source and/or hydrogen diffusion barrier which complicates the evaluation of the impact of the baking process on hydrogen ingress, hydrogen desorption and trapping as a function of time and temperature.
