Abstract
Electrochemical H charging, hydrogen permeation, and hydrogen-induced cracking (HIC) behavior of 690 MPa grade steel substrate and different heat-treatment states (annealed, quenched, normalized, tempered) are investigated by cyclic voltammetry (CV), hydrogen permeation, electrochemical H charging, and slow strain rate tensile test (SSRT). The results show that hydrogen diffuses through the steel with the highest rate in base metal and the lowest rate in annealed steel. The hydrogen-induced cracks in base metal show obvious step shape with tiny cracks near the main crack. The cracks of annealed steel are mainly distributed along pearlite. The crack propagation of quenched steel is mainly transgranular, while the hydrogen-induced crack propagation of tempered steel is along the prior austenite grain boundary. HIC sensitivity of base metal is the lowest due to its fine homogeneous grain structure, small hydrogen diffusion coefficient, and small hydrogen diffusion rate. There are many hydrogen traps in annealed steel, such as the two-phase interface which provides accommodation sites for H atoms and increases the HIC susceptibility.
Highlights
Hydrogen in microstructures can be roughly divided into two forms, diffusing hydrogen and trapping hydrogen [1]
Pressouyre [2] classified hydrogen traps as reversible and irreversible traps according to the desorption activation energy of hydrogen, which can be measured by thermal desorption spectroscopy
The experiment began with two consecutive cyclic voltammetry (CV) scans as described above, and a linear scan was performed in NaOH solution with and without thiourea at a scanning rate of 2 mV/s from open circuit potential (OCP) to −1.6 V
Summary
Hydrogen in microstructures can be roughly divided into two forms, diffusing hydrogen and trapping hydrogen [1]. The HIC sensitivity of steels with different microstructures is different, because of the difference in the distribution of phases, grain size, and defects, which can affect hydrogen diffusion and accommodation [9]. If only martensite exists in the steel microstructure, this does not necessarily mean high hydrogen embrittlement sensitivity Many other factors, such as inclusion, alloy content, second phase precipitation, grain structure, and martensite type, can affect the hydrogen embrittlement sensitivity in martensitic steel [11]. The rate of hydrogen diffusion in austenite steel, which has a dense lattice structure, is about three to four orders of magnitude lower than that in martensite steel, which means that it takes a longer time for hydrogen to accumulate in the austenite and reach its critical value. The influence of microstructure on HIC sensitivity was verified by the HIC test and slow strain rate tensile test
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