Microstructural influence on hydrogen permeation and trapping in steels

Abstract

The microstructural influence on hydrogen permeation and trapping in pure iron and two ferritic-pearlitic steels, AISI 1018 and AISI 4340 is quantified. To this end, hydrogen is introduced into specimens of these materials through electrochemical charging and the total hydrogen content of the specimens are quantified following gas fusion analysis principle. Furthermore, a modeling framework based on Fickian diffusion equations including the relevant microstructural features, electrochemical charging conditions and three-dimensional geometry of the specimen affecting the overall diffusion behavior is adopted to describe the time-dependence of hydrogen content in the three materials. The approach quantitatively describes the hydrogen ingress into the three materials, as well as its distribution across various defects and microstructural features. Traps of two potencies are identified, dislocations and grain boundaries (trap 1), and ferrite/cementite interfaces (trap 2). The former are shown to be responsible for the trapped hydrogen at early stages of its ingress, whereas trap 2 is shown to gather trapped hydrogen at later stages. The ability to design microstructures to control hydrogen ingress and diffusion is discussed, showing how the framework presented here can be adopted for controlling hydrogen in commercial components, and how this can delay hydrogen-related embrittlement. Keywords: Hydrogen diffusion, Hydrogen quantification, Hydrogen embrittlement, Steels, Microstructure, Modeling