Within the range of 77K to 295K, the mechanical properties、hydrogen behaviour during plastic deformation and the mechanism of hydrogen induced fracture of two kind of microstructures of 18Ni maraging steel were studied by means of cathodically hydrogen charging、slow tensile test、Ion Microprobe Mass Analyzer、Scanning Electron Microscope and transmission Electron Microscope. The results of tensile test show that within the range of 77K to 295K, with the increasing of hydrogen concentration in solid solution specimens, the strength (σ_(a2), σ_b)、and plasticity (ψ,δ) decreasing gradually. This conclusion is also true to aged specimens. Under same condition (temperature and hydrogen content), the strength (σ_(a2), σ_b) and the susceptibility to hydrogen embrittlement of aged specimens are higher than those of solid solution specimens, but the plasticity (ψ,δ) of former's are lower than that of latter. The study of Ion Microprobe Mass Analyzer indicate that hydrogen also release from high dislocation density material 18Ni maraging steel (～10~(12)/cm~2) during deformation. The release rate reaches the maximum around yield point and decreasing gradually with the increasing of deformation. The hydrogen evolution rate in high vacuum (5 * 10~(-7) torr) is 10-15%/hr and the hydrogen diffusion coefficient is 10~(13)cm~2/sec. The Scanning Electron Microscope observation reveal that when hydrogen concentration is about 5.90ppm, the fracture surface are brittle intergranular fracture in 295K and 223K while the fracture surface are transgranular fracture in 153K and 77K. When hydrogen concentration is below 5.90ppm (3.50ppm, 1.60ppm), the the fracture surface in the range of 77K to 295K are transgranular fracture, but the size and depth of the dimples are different from those of hydrogen concentration is 5.90ppm specimens. Transmission Electron Microscope observation make it clear that the structure of solid solution is high dislocation density martensite and there appear dislocation cell (diameter is about zero point 5 micron in average) after deformation. The separated out particles are mainly Ni Mo and it is about 200A in average. Twin were not found. By the sue of hydrogen trap theory and the mechanism of dislocation transportation. We studied the hydrogen release law of 18Ni maraging steel and make clear how movable dislocation、inmovable dislocation and separated out particles affect hydrogen distribution. The mechanism of hydrogen induced fracture were also discussed. Hydrogen restrained the formation of twin. Dislocation cell and separated out particles are the place where micro-crack of transgranular fracture formed and these micro-crack link up results in fracture. In 295K, hydrogen segregated on grain boundaries is the cause of hydrogen induced intergranular fracture.
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