Performance of hydrogen trapping and phase transformation in hydrogenated duplex stainless steels

Duplex stainless steels (DSS) (SAF 2507) have outstanding mechanical properties and high levels of corrosion resistance (especially to SCC in chloride-containing environments). However in spite of this they are susceptible to hydrogen embrittlement. The interaction of hydrogen with the microstructure and defects in DSS (SAF 2507) was investigated in order to gain an in-depth understanding of the behaviour of hydrogen in this material. Hydrogen was introduced into the metal by means of electrochemical cathodic charging in acid solution. Using X-ray diffraction and SEM microscopy, the phase transformation was studied. γ→γ* was the major hydrogen-induced phase transformation in duplex steel. The pseudo hydride phase of γ* was unstable—disappearing during ageing, over a period of 24h at room temperature, and after thermal desorption (20–450°C). This is as a result of hydrogen evolution. A negligible amount of ɛ-martensite was further observed immediately after charging. Hydrogen desorption induced surface tensile stress provoking the phase transformation γ→ɛ→αM. A linear model was used to analyze thermal desorption of hydrogen, through the TDS spectrum, and resulted in the identification of trapping types in DSS. These include: the strain regions around the dislocations with activation energy of about 20kJ/mol; grain boundaries (22.5–28.5kJ/mol); the dislocations core (34.8–40.3kJ/mol), vacancies and austenite–ferrite interfaces (50.2–57.4kJ/mol). The relation of initial hydrogen concentration and heating rate to the thermal desorption process is discussed.