The development of nuclear power energy has increased globally due to its vast potential, especially in the future. Unfortunately, safe operation of nuclear facilities has remained a great challenge in their development attributed to structural materials failure. Therefore, researchers have been looking for alternative structural materials especially for fusion reactors and have identified martensitic steels as promising candidates. Martensitic steels exhibit unique properties such as irradiation resistance, adequate toughness, and strength.
Interestingly, despite the potential applications of martensite steels, their continuous use, in the long run, may result in mechanical properties degradation like the irradiation embrittlement. Thus, an increase in the ductile-brittle transition temperature (DBTT) will occur. DBTT is the temperature below which steels start to exhibit brittle fracture and low toughness hence it is a crucial consideration in accessing embrittlement properties of materials. Currently, the focus of researchers has been directed towards developing effective methods for accurate prediction of DBTT.
However, the several developed methods for DBTT evaluation like Charpy impact testing method, for example, are destructive especially for inadequate irradiate specimen for longevity reactors and expensive experimental cost. Therefore, the need for non-destructive evaluation (NDE) techniques for detecting DBTT for nuclear applications materials is on the rise.
Because the magentic properties is very sensitive to micro defects of materials, recently, significant efforts have been made towards the investigation of monitoring the change in mechanical properties of steels by utilizing magnetic methods taking into consideration that both magnetic and mechanical properties of the materials depend on their microstructure. Consequently, DBTT is controlled by the dislocation mobility which leads to the change in the magnetic and mechanical properties in the ductile and brittle temperature regions. In this way, the magnetic method will nondestructively detect the DBTT temperature of the materials.
A team of researchers at the Chinese Academy of Sciences, Institute of Solid State Physics led by Professor Tao Zhang developed a nondestructive approach for evaluation of the DBTT of martensitic steels. The approach was based on the correlation between DBTT and the magnetic properties including magnetization and the coercive field. They purposed to qualify the materials embrittlement especially for nuclear reactor applications for safe operation. Their work is now published in the journal, Journal of Nondestructive Evaluation.
The authors carried out Charpy impact tests and measured magnetic properties of various martensitic steel materials at different temperatures. They observed a consistent variation trend in the coercive field (Hc) and magnetization (M) with an increase in the temperature as depicted in the temperature curves. The results agreed to the corresponding DBTT evaluated by Charpy impact method thus confirming the accuracy of the developed method.
The research team successfully showed that the magnetic approach can be used for effective nondestructive detection of DBTT in martensitic steels. This was attributed to the different pinning strength of dislocations and domain walls in the brittle to ductile temperature regions. Therefore, it is a promising method with high sensitivity and better reproducibility in the evaluation of DBTT of martensitic steels and thus will advance the safety in the nuclear applications such as nuclear power generation.
Ding, H., Zhang, T., Fang, Q., Gao, R., Wang, X., & Liu, C. (2017). Nondestructive Evaluation of Ductile to Brittle Transition Temperature for Martensitic Steels Based on Magnetic Measurements. Journal of Nondestructive Evaluation, 36(2).Go To Journal of Nondestructive Evaluation