Abstract
The absorption of hydrogen in metals can result in hydrogen embrittlement, particularly in high-strength steels. One of the potential approaches to counteract hydrogen embrittlement is to incorporate hydrogen traps into the material's microstructure, so the absorbed hydrogen cannot participate in the embrittling process under high loads. Concerning that weak hydrogen traps may not be able to withhold hydrogen effectively, their usefulness for increasing embrittlement resistance is questionable. As such, our goal is to develop steels that have the microstructure incorporating strong hydrogen traps. In addition, the steels need to have excellent mechanical properties that are meaningful for engineering applications.
With the goal in mind, we shed light on how to utilize lattice defects as strong hydrogen traps, which can be easily produced in abundance and increase materials strength effectively. Specifically, we examined the role of tangled dislocations at cementite-ferrite interfaces in a strained pearlitic steel for hydrogen trapping and embrittlement resistance. We conducted slow-strain mechanical testing on hydrogen-charged steel samples in various extents of strain. We found the high-strained steel having the tangled dislocations, verified by using transmission electron microscopy (TEM), are less susceptible to hydrogen embrittlement in term of ductility loss. We also used thermal desorption spectrometer (TDS) to characterize the hydrogen trapping at macroscale, confirming the presence of strong hydrogen traps in the pearlitic steels.
We then used atom probe tomography (APT) to assess hydrogen trapping behaviours at nanoscale, aiming to clarify what microstructural feature originates strong hydrogen trapping. We conducted the APT hydrogen observation with the use of a cryogenic sample transfer workflow that can preserve the charged hydrogen, both weakly and strongly trapped, in the samples. We also conducted the observation without cryo-transfer, where only strongly trapped hydrogen can be observed. By comparing the results, we conclude that tangled dislocations at cementite-ferrite interfaces are strong hydrogen traps. Our finding suggests that this type of lattice defect can trap hydrogen effectively, which, as importantly, can increase steel strength and be produced cost-effectively.