Effects of Austenite Phase Content on Susceptibilities to Environmental Cracking in a Duplex Stainless Steel

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Abstract
The effects of austenite (γ) content in ferrite (α) matrix on anodic polarization curves, hydrogen permeability and susceptibilities for both hydrogen embrittlement and stress corrosion cracking (SCC) were studied for a 25Cr-6Ni-1.8Mo-0.1N duplex stainless steel (SUS 329J1) with the different α/γ ratios prepared by isothermal heat treatment. The results obtained are as follows: (1) The anodic polarization curves of (α+γ) duplex structures in 0.5kmol/m3 H2SO4+0.5kmol/m3 NaCl and 0.5kmol/m3 H2SO4+0.01kmol/m3 KSCN solutions showed the separated two current peaks in the active region. The current peaks corresponded to preferential anodic dissolution of α- and γ- phases; the less noble peak to α-phase and the noble peak to γ-phase. (2) The diffusion coefficient of hydrogen in (α+γ) duplex structures containing 0 to 28% γ was almost independent of the γ-content and was 4×10-12m2/s which was the same as that of Fe-25Cr alloy with α-structure. The hydrogen content after holding at 673K in 30MPa hydrogen increased linearly with γ-content. (3) The time to failure of (α+γ) duplex steel due to hydrogen embrittlement under cathodic charging in H2SO4-NaCl solution increased remarkably with γ-content. (4) The time to failure of (α+γ) duplex steel due to SCC in boiling 42% MgCl2 solution increased as the γ-content increased and the SCC resistivity attained the highest at about 40%γ. The high SCC resistance which was produced by the coexistence of γ-phase in α-matrix was explained on the basis of the keying effect of the γ-phase.

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