HYDROGEN EMBRITTLEMENT OF DUPLEX STEEL 22/05

The presented paper evaluates hydrogen embrittlement of a duplex steel 22/05 with ferritic-austenitic microstructure. Tensile tests on specimens with diameter d(0) = 4 mm were used for the evaluation of steel resistance to hydrogen embrittlement. Tensile specimens were electrolytically hydrogen charged in 0.05 M solution of H2SO4, without or with addition of 1 g of KSCN per 1 liter of solution, for 24 hours or 168 hours. The current density for hydrogen charging varied from i = 1 to 5 mA.cm(-2). Tensile tests with a low strain rate were performed immediately after hydrogen charging. Impact of hydrogen on mechanical properties of the duplex steel during tensile test was confirmed by decreasing of plastic properties, especially reduction in area. Neither the yield strength R-p0,R-2, nor the tensile strength R-m were affected by the presence of hydrogen. Degree of embrittlement was expressed by the index of hydrogen embrittlement F. Hydrogen charging at current density i = 1 mA.cm(-2) caused almost none embrittlement of the steel studied, index of hydrogen embrittlement F being 0.8 %. Some decreasing of reduction in area was observed for increased current density 5 mA.cm(-2), but the index of hydrogen embrittlement still remained quite low, F = 3.2 %. When KSCN was added to the solution, for keeping of previous conditions, hydrogen embrittlement index increased three times approximately. Extending of hydrogen charging time to 168 hours led to pronounced decreasing of reduction in area and the index of hydrogen embrittlement reached value F = 23.6 %. Fractographic analysis showed that failure of specimens in the middle of fracture surface corresponded to transgranular ductile fracture with a dimple morphology. Regions of quasicleavage fracture were observed on the circumference of electrolytically hydrogen charged specimens. These regions were penetrating deeper for prolonged time of hydrogen charging. Diffusion coefficient of hydrogen in the studied steel could be estimated on the basis of fracture characteristics. Duplex steel seems to be much more resistant to hydrogen embrittlement in comparison with steels having predominantly bcc lattice.