Plasma-induced surface degradation in 304 stainless steel used for TRIAM-1M limiter

Surface degradation in a 304 stainless steel limiter of TRIAM-1M by long-pulse discharge during long period operation has been examined by means of X-ray diffraction, scanning electron microscopy and dynamical microindentation tests. Particular exfoliation and hardening of the surface of the electron drift side were observed. These result from the formation of α′ martensite induced by hydrogen in the plasma. The stability of the martensitic phase has been studied by annealing experiments on the cathodically hydrogen charged 316 stainless steel by X-ray diffraction. Both ε and α′ martensites were formed by 22 h cathodic charging. The former reverts to γ-phase and/or converts to α′ martensite below 723 K and the latter reverts to γ-phase below 923 K, re pectively.