Investigation of microstructure and thermal stability of pulsed plasma processed chromium ferritic-martensitic steels

Abstract

Abstract. This paper presents results of the microstructural evolution and thermal stability of the promising Russian ferritic-martensitic steels (EP 823, EP 900, EK 181 and ChS 139) for the nuclear and fusion application after surface modification by high temperature pulsed plasma flows (HTPPF) treatment. Investigations of microstructure, topography and elemental content changes associated with irradiation by nitrogen plasma with energy density 19-28 J/ cm 2 and pulse duration 20 μs were carried out. Changes in microstructure and elemental content occurring in the modified surface layer were characterized by means of scanning electron microscopy (SEM) and X-ray microanalysis (EDS and WDS). It was shown that independently of initial microstructure and phase composition, HTPPF treatment of ferriticmartensitic steels leads to formation of ultrafine homogeneous structure in the near surface layers with typical grain size ~100 nm. Results of microstructure investigations after annealing during 1 hour demonstrates significant thermal stability of nanostructure formed by HTPPF treatment. Introduction12% chromium ferritic-martensitic steels are planned to be used as structural materials for novel fission (Generation IV) and fusion reactors. Development of these new facilities demands to increase the operation temperature range of the materials up to 650 °C. Operating 12% chromium steels at the elevated temperature leads to new requirements both to the bulk properties and to the surface which will ultimately determine the applicability of the materials under corrosion/erosion effects of the environment. Hence, the optimization of surface structure has become a feasible and effective method of improving the overall properties of structural materials. Relatively novel methods such as ultrasonic treatment [1], electron and ion beams [2,3], pulsed laser and plasma irradiation [4,5] have been admitted as a promising methods for surface modification over the last few years. Such treatment results in the evolution of dislocation structure of the material, grain refinement and increase of the grain misorientation angles, etc., and eventually allow to create nanocrystalline materials. Ultrafine grains and large density of grain boundaries of nanomaterials possess unique properties in particularly increased strength/hardness, enhanced diffusivity, improved toughness/ductility, corrosion and erosion resistance. Numerous papers have been focused on the investigation of the nanostructuring methods and unique properties of produced materials [1][2][3][4][5], though, their thermal stability for high temperature applications such as nuclear and fusion industry is far from being clarified.