Role of Hydrogen in Sensitivity to SCC of Advanced High Strength Steels in 5 NaCl Environment

摘要

Two AHSS grades with complex-phase (CP 1200) and dual-phase (DP 1000) microstructure were tensile tested under constant applied load in 5 % NaCl aqueous solution in order to evaluate their sensitivity to SCC and to clarify a possible role of hydrogen and steel microstructures in the fracture. Plane sub-sized tensile specimens with both-side notches about of 0.1 mm radius were tested using a constant load mode with 5 steps of loading for 72 hours testing. Depending on the result does the specimen fracture or not within the testing time the next testing load is decreased or increased. The testing procedure allows defining of the critical applied stress, which result in SCC of the tested materials in the used environment, which was found for the chosen testing time to be about 850 MPa for CP 1200 steel and about 825 MPa for DP 1000 steel, respectively. The tested specimens were Zn-coated except for the machined notches providing electrochemical conditions for the local hydrogen evolution at the bare surfaces of the notches and its uptake in the areas of the stress localization. The hydrogen uptake is measured with thermal desorption method and possible effects of hydrogen on the SCC in AHSS is discussed in terms of hydrogen interactions with nonmetallic inclusions (NMI) and other strengthening factors of the studied steels. Fracture surfaces of the tested steels are studied in details with FEG-SEM and EDS in order to clarify role of NMI and trapped hydrogen in the SCC nucleation and development.