Effects of hydrogen charging methods on ductility and fracture characteristics of AISI 9840 steel.

Two different methods were used in the tests: the premature fracture method and the tensile test under hydrogen charging method, on AISI 9840 steel stress-corrosion specimens. The aim of this work was to reveal the hydrogen effects on plastic deformation in tensile tests with or without simultaneous hydrogen charging. True stress-true strain curves have clearly shown the material's different behaviour in the two tests: in the case of premature fracture tests, during plastic deformation, dislocations glide and allow hydrogen to escape and the material to partially regain its properties more rapidly; on the contrary, in the case of tensile tests under hydrogen charging, the interaction between the penetrating hydrogen and the Cottrell clouds (pre-existing hydrogen) slackens dislocation glide, so that plastic deformation is greatly reduced, as is shown both from the diagrams and the SEM analyses.