Modeling of Hydrogen Diffusion in 25% Cr Super Duplex Stainless Steel
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Over the last years, several failures in duplex and super duplex stainless steel structures in subsea applications are reported to be a result of hydrogen induced stress cracking. One main source of hydrogen is found to be cathodic protection. To control and predict hydrogen cracking of duplex stainless steels, knowledge of the diffusion properties and the hydrogen distribution is essential. Local stress and strain concentrations are known to cause hydrogen accumulation, which under appropriate conditions could lead to fracture. In this work, the diffusion of hydrogen in a 25 % Cr duplex stainless steel structure has been studied by finite element simulations in ABAQUS Standard. Finite element models containing both austenite and ferrite phases have been applied, with diffusion and material properties representative for each phase. The effect of stress and plastic strain caused by embedded and surface defects on hydrogen accumulation has been analyzed, and the fracture susceptibility of these defects has been evaluated through implementation of hydrogen dependent user defined cohesive elements in the crack paths. All stress and environmental conditions have been chosen to best represent subsea operation conditions and cathodic protection.Diffusion parallel to elongated austenite grains was found to give the highest diffusion coefficient. Over all, the calculated diffusion coefficients are in good agreement with reported diffusion coefficients found in literature and from previous research projects on duplex steels. For all results, trapping due to plastic strain was found to be the dominating effect in influencing the hydrogen distribution near the notch tip. Stress concentrations caused by defects were found to give a maximum hydrogen concentration of 1.33 and 1.1 times the initial concentration, for an inner surface and embedded defect respectively. Concentrations of plastic strain were found to give a maximum hydrogen concentration of 36.6, 6.6 and 7.6 times the concentration without influence of plastic strain, for an inner surface defect and both notch tips of an embedded defect respectively. Threshold stress intensity factors of 19.2 MPa/m^0.5, 20.8 MPa/m^0.5 and 23.3 MPa/m^0.5 were found for the outer surface defect and the first and second notch tips of the embedded defect, respectively. The results indicate that surface defects are more susceptible to hydrogen fracture, due to a combination of higher stress and higher plastic strain. For all cases of fracture, crack initiation occurred at the crack tip surface. Hydrogen concentration was found to be the dominating factor in controlling hydrogen cracking in the ferrite phase, while stress level was found to be the dominating factor in the austenite phase.For further work, valid parameters and relationships for hydrogen diffusion and cracking under the influence of stress and plastic strain should be established for both the austenite and ferrite phases in a duplex steel.