Optimizing Cryogenic Toughness in Steel: The Role of Cerium as a Nickel Substitute

Significance 

The use of nickel (Ni) in steel alloys is prevalent due to its effectiveness in reducing the ductile-to-brittle transition temperature, thus enhancing cryogenic toughness. Moreover, there is increasing demand for materials that can perform reliably at cryogenic temperatures, particularly in applications such as liquefied natural gas storage and transportation, where materials must withstand the brittleness that accompanies low temperatures. However, the economic and resource challenges associated with Ni have led to the exploration of alternatives. Rare earth (RE) elements, abundant in China, present a viable option. Previous studies have indicated that the addition of RE elements, particularly cerium (Ce), can refine grains, purify the steel matrix, and alter inclusions’ morphology, thereby improving mechanical properties at low temperatures. To addresses the critical issue of substituting Ni with Ce to achieve desired mechanical properties while mitigating costs and resource limitations, a new study published in Steel Research International and led by Professor Qing Liu from University of Science and Technology Beijing and conducted by Dr. Liping Wu from Inner Mongolia University of Technology alongside Junxiong Huang, Jianguo Zhi, Huisheng Wang, the authors investigated the effect of replacing  Ni  with  Ce in cryogenic steels, particularly focusing on the cryogenic toughness and the microstructural evolution of the steel.

The team prepared 7Ni steel plates with varying contents of Ce. A control sample without RE addition was used for comparison. They produced the under controlled conditions in an induction furnace, where specific amounts of Ni and Ce-Fe alloy were added to the melt before casting. After casting, the steel samples underwent a specific heat treatment regimen, which included high-temperature quenching followed by tempering. This process was important for achieving the desired microstructure conducive to high toughness at cryogenic temperatures. The authors conducted the Charpy impact tests at cryogenic temperatures of -150°C and -196°C to measure the energy absorbed by the material during fracture, which is a direct indicator of its toughness. They also employed a range of sophisticated techniques to characterize the microstructure and inclusion morphology within the steel including field-emission scanning electron microscopy for high-resolution imaging of the microstructure, electron-probe microanalysis for detailed chemical analysis at micro-regions, particularly focusing on the distribution of Ce and Ni, x-ray Diffraction for phase identification within the steel matrix and transmission electron microscopy for an in-depth examination of crystal structures and fine inclusion particles.

The authors’ found that a small addition of Ce (0.0026 wt%) in place of Ni maintained similar cryogenic toughness to that of the 7Ni control steel. This indicates that Ce can effectively replace Ni up to a certain extent without compromising the steel’s performance in cryogenic conditions. Moreover, increasing the Ce content beyond the optimal level (to 0.0265 wt%) resulted in a noticeable decline in cryogenic toughness. They also found that excessive addition of Ce to deteriorate the steel’s mechanical properties at low temperatures, highlighting the importance of precise control over Ce content.

The researchers’ microstructural analysis showed that appropriate levels of Ce could mimic the beneficial effects of Ni, such as refining the steel’s microstructure and promoting a ductile fracture mode. However, excessive Ce led to coarser grains and more pronounced impurity particle formation, which in turn resulted in a transition towards quasi-cleavage fractures, indicative of reduced toughness. Additionally, the inclusion analysis demonstrated that Ce additions altered the morphology and composition of inclusions within the steel matrix. Optimal Ce levels resulted in fine and evenly dispersed inclusion particles, contributing to the steel’s toughness by facilitating plastic deformation around these inclusions. In contrast, higher Ce concentrations led to the aggregation of larger, detrimental inclusion particles that act as stress concentrators and crack initiation sites. In conclusion, the pioneering research of Professor Qing Liu and colleagues conclusively demonstrated that while Ce has the potential to replace Ni in cryogenic steels, achieving the desired mechanical properties requires careful control over the Ce content. Optimal Ce addition can maintain, if not enhance, the cryogenic toughness of steel by refining its microstructure and optimizing inclusion morphology. However, the detrimental effects of excessive Ce highlight the need for a balanced approach to alloy design, aiming for a synergy between cost-effectiveness and mechanical performance in engineering materials for cryogenic applications.

Optimizing Cryogenic Toughness in Steel: The Role of Cerium as a Nickel Substitute - Advances in Engineering
Figure 1. EPMA images of typical impurity particles in 0.0265 wt.% Ce steel and their elemental surface scanning results: a) secondary electron images, b) distribution of Ce elements, c) distribution of C elements, and d) distribution of Ni elements
Optimizing Cryogenic Toughness in Steel: The Role of Cerium as a Nickel Substitute - Advances in Engineering
Figure 2. TEM micrographs and corresponding EDS mapping images of inclusions in the experimental steel with 0.0265% Ce: a) nanoscale inclusions b) micrometer-scale inclusion
Optimizing Cryogenic Toughness in Steel: The Role of Cerium as a Nickel Substitute - Advances in Engineering
Figure 3. The back cover picture related to the article “Effect of Rare Earth Cerium Replacement of Nickel on Cryogenic Toughness of 7Ni Steel” in Steel Research International.

About the author

Professor Qing Liu

University of Science and Technology Beijing, China

Prof. Qing Liu received his PHD degree of the University of Science and Technology Beijing in 2002 and has been working in Iron and steel metallurgy field at University of Science and Technology Beijing ever since. He is the academic leader of metallurgical intelligence and equipment serving on the State Key Laboratory of Advanced Metallurgy in University of Science and Technology Beijing (CHINA), and selected as foreign member of the Russian Academy of Natural Sciences in2023.

His current research interests include continuous casting process of steel, metallurgical process engineering and intelligence, metallurgical process simulation and optimization, focusing on the cooling control and the characterization of second phase particle precipitation during the solidification process of steel in continuous casting process, metallurgical process modelling and multi-process collaborative manufacturing, secondary metallurgy modeling based on metallurgical mechanism and machine learning, etc.

In 2022, he won the Gold Medal of the 18th Seoul International Invention Fair in Korea, the Silver Medal  of the 74th Nuremberg International Invention Fair in Germany, and the Silver Medal  of the 48th Geneva International Invention Fair in Switzerland in 2023. Besides, Prof. Qing Liu has also won the Gold Awards (10th World Scientist Grand Awards, WSA) and (9th World Invention Innovation Contest, WIC) by IFIA and KINEWS in Korea due to the work on multi-scale modeling in steelmaking plants with collaborative manufacturing and LF refining steel in 2023.

About the author

Dr. Liping Wu

Inner Mongolia University of Technology, China

Wu Liping received his bachelor’s in 2015 and Master’s degrees in 2017 in Metallurgical Engineering from Inner Mongolia University of Science & Technology. Then, he studied in University of Science and Technology Beijing and got his doctor’s degree in 2023. And he had been an engineer in Inner Mongolia Baotou Steel Union Co., Ltd. form 2018-2023. After that, he went to Inner Mongolia University of Technology to teach and served as the first young editorial board member of Special Steel Magazine.

His research interests are the application of rare earth in steel materials and focus on the innovation and theoretical analysis of the action mechanism of rare earth in advanced steel materials under modern metallurgical conditions. He devoted to achieve the replacement of rare and precious alloy elements with high abundance rare earth, and focusing on accelerating the industrialization application of rare earth materials.

Reference

Liping Wu, Junxiong Huang, Qing Liu, Jianguo Zhi, Huisheng Wang. Effect of Rare Earth Cerium Replacement of Nickel on Cryogenic Toughness of 7Ni Steel. Steel Research International 2023, 94, 2200883.

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