Anti-aging treatment of nuclear power plant steel

Recent developments with regard to practical metallic materials have been focusing on dealing with non-equilibrium or metastable states of the resultant alloys. Presently, it is well recognized that a majority of engineering alloys are usually in a non-equilibrium state at the time of production, and evolve towards the equilibrium state during service. Application of such materials is highly desirable owing to their superior mechanical properties. However, the non-equilibrium to equilibrium transformation results in material degradation, such as aging. This leads to alteration or loss of material properties thereby increasing possibility of failure. For a long time, scholars have persistently been attempting to predict the structural transformations and even develop strategies to control and retard the transformation. Thermodynamically, reversal of structure transformation is possible if sufficient negative free energy flux were injected into the system.

Nuclear power plants are high safety facilities and any deterioration of crucial components would result in catastrophic failure. Cast duplex stainless steel (DSS) has been utilized in many nuclear power plants globally. Unfortunately, it loses impact toughness quickly during its service at nuclear power plant station as pipe and boiler due to such transformation.

Recently, a team of researchers: Yan Zhao, Binyan He and Professor Rongshan Qin from the School of Engineering & Innovation at The Open University in the United Kingdom In collaboration with French scientists: Sébastien Saillet, Christophe Domain, and Patrick Le Delliou at EDF R&D Département Matériaux et Mécanique des Composants and also Michel Perez at University of Lyon conducted a study with the objective of regenerating the microstructure and mechanical properties of DSS which was caused by the early stage aging. To be precise, they used electric current to regenerate the microstructure and properties of duplex stainless steel. Their work is currently published in the research journal, Materials Science & Engineering A.

Foremost, the researchers commenced by casting the desired steel under deliberated processing conditions to consist of ferrite and austenite phases without the formation of any precipitates. Five samples were then obtained and aged at Electricité de France (EDF) on laboratory furnaces at defined aging conditions. In order to check the microstructure evolution in electropulsing treatment, samples before and after the electropulsing treatment were prepared and subjected to several subsequent tests. Eventually, two electropulsing facilities were implemented to treat the samples for regeneration purpose.

The authors observed that one hour of electropulsing treatment resulted in regeneration of more than 83% of the aged microstructure in terms of thermoelectric power characterization. This observation was made using thermoelectric power measurements. In addition, they observed that following the elctropulsing treatment, Charpy impact toughness and Vickers micro-hardness were recovered significantly. Moreover, the regeneration was noted to not change the microstructure in grain scale.

In summary, the study by Professor Rongshan Qin and research team demonstrated a fast regeneration method applicable for treatment of aged duplex stainless steel Charpy test specimens. In general, the regeneration was seen to be due to the electric-current-induced reverse of system free energy sequence. Altogether, electropulsing-enhanced diffusivity enables the anti-aging processing to be completed quickly.