Abstract
Duplex stainless steels with an austenite-ferrite microstructure have excellent mechanical properties and a high corrosion resistance. However, they are susceptible to hydrogen induced mechanical degradation. The transformation of ferrite to sigma phase in duplex steels, e.g. after a heat treatment or welding procedure, will also have an influence on the mechanical properties and hydrogen/material interaction. In this work, the impact of hydrogen on a 50/50 duplex stainless steel was investigated as well as the effect of the presence of the sigma phase. Two conditions were compared, i.e. the initial versus the annealed condition, where a heat treatment at 850°C during 24 hours was applied to transform ferrite to sigma phase.
Both the initial material and additionally heat treated duplex stainless steel were electrochemically charged with hydrogen until the saturation level was reached, as determined with melt extraction. Thermal desorption spectroscopy measurements were conducted to characterize the hydrogen trapping sites in both steels. These include dislocations, grain boundaries, austenite/ferrite phase boundaries and austenite/sigma phase boundaries. Both materials were also tensile tested (i) in air and (ii) after hydrogen pre-charging to evaluate the change in mechanical properties due to the presence of sigma phase and/or hydrogen. The presence of sigma phase had a beneficial effect on the hydrogen embrittlement degree since the annealed condition showed an improved resistance against hydrogen induced failure. This was correlated to the lower hydrogen content and linked to the deep trapping of hydrogen at the austenite/sigma interface. Evaluation of the fracture surfaces indicated a shift from a ductile fracture to a hydrogen induced brittle fracture, especially at the edges due to the low hydrogen diffusivity. Moreover, the presence of hydrogen induced phase transformation was quantified.