Duplex Stainless Steels (DSS) were developed as an alternative to austenitic and ferritic stainless grades in large fields of industry, mainly in petroleum and gas applications due to the combination of high corrosion and superior mechanical properties compared to austenitic and ferritic steel grades. In order to obtain a better performance of DSS, Super Duplex Stainless Steels (SDSS) were developed and are essentially a Fe–Cr–Ni alloy with higher content of alloying elements where the microstructure presents a dual-phase austenite/ferrite. As such, SDSS are widely employed in petroleum industry for subsea equipment in deep-water exploration. The aim of this work is to study the susceptibility to hydrogen embrittlement of two types SDSS: Hot Isostatic Pressed (HIP) and as forged. The hydrogen permeation as a function of temperature in two different processed SDSS was determined. In addition, the microstructural characterization by SEM analysis, X-ray diffraction (XRD) analysis, tensile tests in as-received and hydrogenated samples in different conditions, thermal desorption spectroscopy (TDS) analysis and gas hydrogen permeation tests were conducted in this work for both alloys.
It was observed that the ductility is affected by the hydrogen concentration absorbed in each microstructure. Although, the loss of ductility is important when the samples are exposed to hydrogen under elastic stress. Tensile tests under slow strain rate were applied on both alloys. The forged alloy is more susceptible to hydrogen embrittlement than samples from the HIP alloy. This behavior can be explained by the space between austenite grains, which control the hydrogen resistance of this materials.
XRD diffraction analysis of the HIP duplex stainless steel, after electrolytic hydrogenation, exhibits peaks which can be attributed to hydrogen induced formation of epsilon phase in this alloy.
Tensile tests in low fugacity hydrogenation showed a decrease in the yield strain and in the ultimate tensile stress with increasing current densities for HIP condition. Results obtained from tensile tests in high fugacity of H2 and at slow strain rate (ε ~ 10-6 s-1) conditions showed that hydrogenated samples presented a decrease in ductility, influenced by the dislocation transport of hydrogen mechanism and hydrogen saturation at the metallic surface for both alloys.
TDS showed the presence of two peaks, for the HIP steel hydrogenated at high temperature, which are attributed to the austenite-ferrite interface and the hydrogen trapping in the austenite phase respectively.