Although high-strength (HS) steel is a potential material for next-generation automobiles, it has a mechanical performance drawback due to hydrogen embrittlement. Recent investigations on fractures caused by embrittled HS steel specimens were meticulously carried out by researchers.
Their discoveries provided insight into the types of atomic-scale flaws present in the crystalline structure of fractured HS steel. This information will improve the current understanding of hydrogen embrittlement and could open up possibilities to find solutions for its suppression.
Making automobiles lighter is one of the numerous strategies to lower the energy needed for transportation. High-strength (HS) steels are the ideal candidate materials for this purpose since they utilize less metal to produce a similar level of structural integrity due to their significant weight-to-strength ratio.
Many car manufacturers are of the opinion that HS steels will be a crucial part of a wide range of automobiles in the future. However, there is an obvious issue that has to be resolved for this to become a reality.
The phenomenon of hydrogen embrittlement happens when HS steel is exposed to rainwater (H2O) or hydrogen. Hydrogen atoms infiltrate the material’s lattice structure, gradually weakening it and increasing the probability of it fracturing under mechanical stress.
Scientists have noticed that intergranular (IG) fractures, which occur at the grain boundaries of the crystalline lattice, are frequently caused by hydrogen embrittlement in HS steel.
Unfortunately, because other forms of fracture frequently occur simultaneously with this one, it is challenging to examine the underlying causes for this specific type of fracture in HS steel.
To combat this issue, a research team led by Professor Kenichi Takai of Sophia University in Japan recently performed a study. They were able to create almost-pure IG fractures on embrittled HS steel samples by devising a clever replacement for the standard tensile tests used to evaluate the mechanical characteristics of materials.
They were then able to examine these fractures in unprecedented detail. Dr. Takahiro Chiba of the Graduate School of Sophia University, who is currently at Nippon Steel Corporation, co-authored the study, which was published on January 15th, 2023, in Volume 223 of Scripta Materialia and made available online on October 1st, 2022.
In conventional tensile testing for metals, a sample in the form of a dog bone is stretched to its breaking point. As mentioned above, this can result in fractures other than IG fractures, including quasi-cleave fractures, dimples, and shear lips.
To avoid this, the researchers developed a novel mechanical test that involved repeatedly loading and unloading the sample during hydrogen charging.
Our load reduction test was designed to progressively reduce the material’s ultimate tensile strength (UTS). We achieved this by repeatedly removing the load applied to the specimen immediately after the tensile stress reached the UTS under hydrogen charging and the re-applying it.
Kenichi Takai, Department of Engineering and Applied Sciences, Faculty of Science and Technology, Sophia University
The proposed load reduction test effectively created pure IG fractures, demonstrated by images taken using scanning electron microscopy (SEM).
The research team hypothesizes that this occurs because, following each unloading step, hydrogen atoms are given ample time to fill in the new fractures that are created in the material, allowing the fracture to advance entirely along the grain boundaries.
The researchers carefully removed small pieces of the damaged sample close to the fracture surface and utilized them for lower-temperature thermal desorption spectroscopy (L-TDS) to get insight into the lattice defects present directly below the fracture.
With this approach, the number and types of defects in a material are determined by tracking the rate at which a gas (in this example, hydrogen) desorbs from it at various temperatures.
Takai added, “L-TDS enabled us to distinguish hydrogen trapping sites on the atomic scale. Obtaining such basic knowledge about the lattice defects formed in the local area just below an IG fracture surface will provide important clues to understand and potentially suppress hydrogen embrittlement in HS steel.”
To ascertain if plasticity was involved in the creation of the vacancies and vacancy clusters detected by L-TDS, the researchers conducted a final set of experiments that comprised several studies on SEM images.
According to these studies, certain vacancies solidified into nano-voids, and the martensite laths and blocks in the immediate area of these voids were severely deformed and challenging to distinguish from one another. This implies that local plastic deformation takes place just below the IG fracture caused by hydrogen embrittlement.
The results of this study will help scientists in their understanding of hydrogen embrittlement in HS steel. Hopefully, this will provide new avenues for its suppression and make it possible for HS steel to be used in hydrogen-powered automobiles safely.
Chiba, T., et al. (2023) Preparation of an overall intergranular fracture surface caused by hydrogen and identification of lattice defects present in the local area just below the surface of tempered martensitic steel. Scripta Materialia. doi:10.1016/j.scriptamat.2022.115072.