Microstructure modification and surface hardening of 12% chromium steels by pulsed gas plasma flows

Abstract

The changes in the microstructure and the surface hardening of chromium (12 wt % Cr) ferritic–martensitic steels in various initial states treated by high-temperature pulsed gas plasma flows (flux energy density Q = 17–78 J/cm², pulse duration τ_p = 15–20 μs) have been studied experimentally. Treatment of fuel-element pipes and monolithic specimens of 12% chromium steel under melting of near-surface layers is found to form a gradient structure–phase state with a submicrocrystalline (~130 nm) surface layer up to 10 μm thick. The parameters of the formed cellular submicrostructure and the modified layer thickness are found to weakly depend on the composition and the thermomechanical treatment of the steels. It is shown that treatment of fuel-element pipes made of 12% chromium steels by plasma flows leads to their surface hardening by 40–60% and by a factor of 1.7–1.9 upon surface liquid-phase alloying with aluminum and chromium irrespective of steel composition.