Micromechanistic study of hydrogen embrittlement in pipeline steels

Abstract

Hydrogen embrittlement, which causes the premature failure of steel pipelines, poses a long-standing challenge to hydrogen energy utilization. Ferrite-pearlite steels dominate the in-service hydrogen pipelines market. Yet hydrogen embrittlement mechanisms for the highly susceptible pearlite phase have remained inconclusive since the complicated microstructures in the bulk ferrite-pearlite steels interfere with categorizing the contribution of pearlite to hydrogen-induced failure. Here we provide a protocol combining in-situ micromechanical testing and ex-situ electrochemical hydrogen charging to successfully examine the effects of hydrogen on the mechanical behavior of pearlite and ferrite micropillars.

In this project atom probe tomography with cryogenic-transfer technique was conducted on hydrogen-charged pearlite samples and observed hydrogen is trapped in the cementite lamellae rather than at the ferrite-cementite interfaces. The introduction of hydrogen reduces the yield strength of pearlite micropillars to a narrow range, which means that hydrogen weakens the anisotropic yielding of pearlite. Slip occurs at the ferrite-cementite interface for uncharged micropillars with inclined lamellae but after hydrogen charging it takes place in the ferrite matrix. Shear deformation dominates in micropillars with vertical and horizontal lamellae, where fracture occurs in the presence of hydrogen. Unlike pearlite, hydrogen only slightly reduces the yield strength of ferrite but has a greater impact on plasticity. Hydrogen softens ferrite micropillars and weakens intermittency during plastic deformation. These phenomena are attributed mainly to the hydrogen-enhanced local plasticity mechanism.

This thesis also provides a new scanning electron microscope-based protocol to test the effect of hydrogen on the mechanical behavior of ferrite-pearlite steels that can facilitate fundamental studies on the interactions between hydrogen, microstructure, and deformation behavior.