Microstructure of the near-surface layers of austenitic stainless steels irradiated with a low-energy, high-current electron beam

The surface morphology, chemical composition and microstructural evolution of the near-surface (up to 0.5 μm) layers of austenitic stainless steels (SS) 304L and 316L irradiated with a pulsed (2.5 μs) low-energy (20-30 keV), high-current (up to 30 kA) electron beam (2-10 J/cm²) have been studied. It was revealed that the cratering on irradiation caused by the local overheating followed by explosive ejection of the material at the sites of localization of low-melting-point second-phase (e.g. FeS₂) particles having a low-thermal-conductivity occurs. The plate rolling stock SS 304L cratered far less than the rod stock one. Multiply repeated pulsed melting of SS 304L (plate) and 316L (rod) almost completely suppresses cratering and reduces the surface roughness compared to the untreated surface. Surface smoothing is accompanied by cleaning of the near-surface (≈50 nm) layer from O, C and N. As a result of fast (∼10⁹ K/s) quenching from the melt, in the near-surface (≈0.5 μm) layer, a single-phase (γ) microstructure is formed with a grain size of 0.2-0.6 μm, which is almost two orders of magnitude lower compared to the untreated SS. The formation of such substructure allows to considerably enhance the electric strength of vacuum insulation and corrosion resistance of SS.