An investigation of the effect of aged structures on diffusivity, permeability and solubility of hydrogen in 18Ni maraging steel has been carried out at room temperature to 320 ℃ by the technique of electrochemical permeation and gas phase permeation. The apparent diffusion coefficient and solubility of hydrogen was measured at low temperature below 453K, and their temperature dependence deviated from the Arrhenius relation. There was a maximum when the steel was aged at 500 ℃ for a hours, which can be interpreted in terms of the difference between hydrogen trapping effects of various lattice imperfections, such as grain boundary, the interface between the precipitates and the matrix of martensits. The activation energy for diffusion through 18Ni maraging steel is 27.695 KJ/mol at 303-353K by using electrochemical transient technique, in reasonable agreement with the result at 353-453K by gas phase permeation. A new simplified equation was obtained to describe trapping behavior from McNabb-Foster's model. The trop binding energies and the fraction of occupied traps in the grain boundaries and the precipitate interfaces were found to be 6.976, 5.532 KJ/mol, and 6.158 * 10~(27) exp (21.186 KJ/mol)/(RT), 2.265 * 10~(27) exp (15.509 KJ/mol)/(RT) atom. H/m~3Pa~(1/2), Respectively, steady state hydrogen transport was virtually independent of processing history, therefore, the permebility is little related to the hydrogen trapping. The interaction of hydrogen with the grain boundaries and the interface between the precipitates and the matrix of martensites in 18 Ni maragin steel was studied by thermal hydrogen evolution analysis technique using gas chromatograph as hydrogen detector. The amount of hydrogen evolved from trap site was measured and its relation with activation energy was studied. The evolution rate peaks appeared both at 405K and 451K with 3k/min heating rate and 0.55 mm specimen thickness. It is due to the hydrogen release from the grain boundaries and the precipitate interfaces. The trap activation energies needed to escape from these were obtained from the relation between peak temperature and heating rate were 14.218 KJ/mol, and 23.161 KJ/mol, respectively. The energy level of the hydrogen around the trap site was estimated from the above values. The trapping behavior of the precipitate interface was discussed in detail by a trapping energy spectrum which was obtained from the experiments.
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