Steel embrittlement mystery solved

Editorial

Rebecca Pool

Monday, January 13, 2020 - 22:30
Image: Atom probe tomography reveals how hydrogen (red) at carbon-rich (blue) dislocations in steel. [University of Sydney]
 
In a breakthrough for a future hydrogen economy, Australia-based researchers have discovered new evidence of how hydrogen embrittlement in high-strength steel takes place, and importantly, how to prevent it.
 
While hydrogen atoms tend to diffuse and accumulate at dislocations and grain boundaries in a steel, cryogenic atom probe microscopy results, published in Science, have shown exactly how niobium carbide clusters within its microstructure help to trap the hydrogen, reduce embrittlement and prevent steel fracture.
 
The latest results point the way to the design of embrittlement-resistant steels for stronger steel tanks and pipelines in hydrogen environments.
 
Illustration highlighting the concentration of hydrogen atoms (red balls) at the crystal boundaries and dislocations in steel. [University of Sydney]
 
Hydrogen embrittlement of high-strength steel is a well-known obstacle for using these steels in sustainable energy production.
 
While the phenomenon is known to involve the interaction of hydrogen and defects, issues around precisely locating hydrogen atoms throughout the steel microstructure have hindered investigations.
 
As Dr Yi-Sheng Chen from the Australian Centre for Microscopy and Microanalysis at the University of Sydney writes in Science: “To understand both embrittlement and trapping, we require information about the position of hydrogen atoms in relation to microstructural features at the nanometre-length scale.”
 
“However, the combination of the low level of interaction between an electron and hydrogen, and the extremely fast diffusion of hydrogen in steels renders it extremely difficult to experimentally determine the location of hydrogen by electron microscopy,” he adds.
 
With this in mind, Chen and colleagues, turned to cryo-transfer atom probe tomography to pinpoint the location of hydrogen at microstructural features in a niobium-bearing low-carbon steels and also study its interaction with second phases such as carbides.
 
Developing a custom cryo-transfer process, they were able to study needle-shaped steel samples in a CAMECA Local Electrode Atom Probe (LEAP) 4000X Si, under cryogenic conditions.
 
Analyses confirmed past embrittlement models, which indicate that hydrogen collects at carbon-rich dislocations and grain boundaries.
 
And importantly, results showed hydrogen pinned at incoherent interfaces between niobium carbides and the surrounding steel, providing direct evidence that these boundaries can act as trapping sites.
 
Atom probe image showing accumulations of hydrogen (red) at carbon-rich (blue) dislocations in steel. [University of Sydney]
 
“These findings are vital for designing embrittlement-resistant steel,” says Chen. “The carbides offer a solution to ensuring high-strength steels are not prone to early fracture and reduced toughness in the presence of hydrogen.”
 
"Hydrogen is a low carbon fuel source that could potentially replace fossil fuels, but there are challenges with the use of steel, the world's most important engineering material, to safely store and transport it,” adds Chen's colleague, Professor Julie Cairney. “This research gives us key insights into how we might be able to improve this situation.”
 
Research is published in Science.
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