One of the many ways to reduce the energy required for transportation is to make vehicles lighter. High-strength (HS) steels are perfect candidate materials for this purpose, as their higher weight-to-strength ratio allows for the use of less metal to achieve a similar structural integrity. Many automobile companies believe HS steels will be an essential component of various types of cars in the future. However, for this to become a reality, there is a glaring problem that needs to be solved.
When HS steel is exposed to rainwater (H2O) or hydrogen, a phenomenon known as hydrogen embrittlement occurs. Hydrogen atoms diffuse into the lattice structure of the material and progressively weaken it, making it more prone to failure under mechanical stress. Scientists have observed that hydrogen embrittlement in HS steel often leads to intergranular (IG) fractures, which occur along the grain boundaries of the crystalline lattice. Unfortunately, the underlying mechanisms behind this particular type of fracture are difficult to study in isolation in HS steel because other types of fracture tend to occur alongside it.
Against this backdrop, a research team including Professor Kenichi Takai from Sophia University, Japan, recently conducted a study to address this problem. Thanks to an ingenious alternative to the conventional tensile tests used to assess the mechanical properties of materials, they managed to produce almost-pure IG fractures on embrittled HS steel samples. In turn, this enabled them to study these fractures with unprecedented detail. The study, which was co-authored by Dr. Takahiro Chiba of Graduate School of Sophia University (now at Nippon Steel Corporation), was made available online on October 1, 2022 and got published in Volume 223 of Scripta Materialia on January 15, 2023.
In conventional tensile tests for metals, a dog-bone-shaped sample is put under increasing tension until it breaks. As stated above, this causes multiple types of fracture besides IG fractures, such as quasi-cleave fractures, dimples, and shear lips. To prevent this, the researchers came up with an original mechanical test involving repeated loading and unloading of 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,” explains Prof. Takai.
As confirmed by scanning electron microscopy (SEM) images, the proposed load reduction test successfully produced pure IG fractures. The team believes this happens because, after each unloading step, hydrogen atoms are given enough time to fill up the new cracks generated in the material to keep advancing the fracture exclusively along the grain boundaries.
To gain insight into the lattice defects present right below the fracture, the researchers carefully extracted small pieces of the broken sample very close to the fracture surface and used them for lower-temperature thermal desorption spectroscopy (L-TDS). This technique involves observing the rate of desorption of a gas (hydrogen, in this case) from the material at different temperatures, which in turn provides information about the number and types of defects present in it. “L-TDS enabled us to distinguish hydrogen trapping sites on the atomic scale,” remarks Prof. Takai, “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.”
In a final set of experiments, the team performed various analyses on SEM images to determine whether the formation of the vacancies and vacancy clusters observed via L-TDS involved plasticity. These analyses revealed that some vacancies coalesced into nano-voids, and that the martensite laths and blocks in the regions around these voids were heavily distorted and difficult to tell apart. This suggests that local plastic deformation occurs right below the IG fracture caused by hydrogen embrittlement.
Overall, the findings of this study will help materials scientists understand hydrogen embrittlement in HS steel better. With luck, this will pave the way to new methods to suppress it and enable the safe use of HS steel in hydrogen-powered vehicles.
【Title of original paper】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
【Authors】Takahiro Chiba1*, Tetsushi Chida2, Tomohiko Omura2, Daisuke Hirakami2, and Kenichi Takai3
【Affiliations】1Graduate School of Science and Technology, Sophia University, 2Nippon Steel Corporation, 3Department of Engineering and Applied Sciences, Faculty of Science and Technology, Sophia University *(now at Nippon Steel Corporation)
About Sophia University
Established as a private Jesuit affiliated university in 1913, Sophia University is one of the most prestigious universities located in the heart of Tokyo, Japan. Imparting education through 29 departments in 9 faculties and 25 majors in 10 graduate schools, Sophia hosts more than 13,000 students from around the world.
Conceived with the spirit of “For Others, With Others,” Sophia University truly values internationality and neighborliness, and believes in education and research that go beyond national, linguistic, and academic boundaries. Sophia emphasizes on the need for multidisciplinary and fusion research to find solutions for the most pressing global issues like climate change, poverty, conflict, and violence. Over the course of the last century, Sophia has made dedicated efforts to hone future-ready graduates who can contribute their talents and learnings for the benefit of others, and pave the way for a sustainable future while “Bringing the World Together.”
About Professor Kenichi Takai from Sophia University
From 1990 to 1999, Kenichi Takai conducted research at Nippon Telegraph and Telephone Corporation before joining Sophia University. Today, he is a full Professor of the Department of Engineering and Applied Sciences at the Faculty of Science and Technology of Sophia University. His research interests include the determination of hydrogen trapping sites in metals, the clarification of the hydrogen embrittlement mechanism, and the development of high-strength steels with high resistance to hydrogen embrittlement. He has over 100 papers published to his name and has received awards from the Ministry of Education, Culture, Sports, Science and Technology, the Japan Society of Corrosion and Corrosion Protection, the Iron and Steel Institute of Japan, and the American Institute of Mining, Metallurgical and Petroleum Engineers (AIME), among others.
This research is supported by Grant-in-Aid for Scientific Research (B) of Japan Society for the Promotion of Science (JSPS) , New Energy and Industrial Technology Development Organization (NEDO), and The Iron and Steel Institute of Japan.