V - Fragilisation par l’hydrogène de l’acier inoxydable duplex Z2CND2205 chargé en hydrogène à 200°C

Hydrogen embrittlement of duplex stainless steel Z2CND2205 hydrogen charged at 200°C

Abstract

Ageing the duplex stainless steel below 500°C, the main and usually phenomenon is the unmixing of the δ-FeCr solid solution by spinodal decomposition (or precipitation of α‘-Cr at higher temperatures). Various intermetallics are detected such as fine dispersed (Ni, Si, Mo-rich) G-phase.

At higher ageing temperatures (between 600°C and 1050°C), a variety of secondary phases may form, such as γ₂, σ R χ and Π. The precipitation of carbides M₂₃C₆, M₇C₂ and Cr₂N are also detected.

This paper is devoted to study the interactions between hydrogen atoms and these phases in a 2205 duplex steel. Hydrogenation had been achieved at 200°C in a molten salts bath, during 3 hours (tempering time also) with an applied cathodic potential equal to -1.5 V/Ag/AgCl. After hydrogen charging, samples have been out- gassed under vacuum at 600°C to measure the quantity of hydrogen Q_H. The ageing temperature was varied between 200°C and 1050°C. The hydrogen embrittlement factor was determined by performing 10^-6 s^-1 tensile tests at room temperature.

The results show that the variation of U.T.S. is sensitive to the microstructure change in absence and in presence of hydrogen. The relationship between λ (U.T.S._Hyd/U.T.S._Air) and Q_H is linear; λ is low (i.e. high embrittlement) in presence of a phase with low Q_H level, while, in presence of the other phases, even with high Q_H, λ is relatively high (low embrittlement).