In situ observations on effects of hydrogen on deformation and fracture of A533B pressure vessel steel

External hydrogen gas atmospheres enhanced dislocation motion, multiplication of dislocations, and dis-location source activation under applied loading during in situ high-voltage electron microscopy (HVEM) observations of A533B pressure vessel steel. However, in both vacuum and hydrogen atmos-pheres, fracture occurred in a ductile manner in specimen areas transparent in the 1000-keV HVEM. The principal effect of the hydrogen atmosphere was to decrease the stress required for deformation near the crack tip and for crack propagation. Deformation at the crack tip was highly localized in both atmos-pheres, and a yielding strip plastic zone, analogous to the Dugdale-Barenblatt model for crack growth, formed ahead of the crack tip. The crack tip plasticity was confined to this strip. Inside the yielding strip, final cracking occurred through a sliding-off mechanism in the thin areas of the HVEM specimen. In the thicker areas of the specimen, where the yielding strip ahead of the crack was no longer transparent, crack tip blunting and void/microcrack formation ahead of the main crack tip could be observed directly. Crack tip blunting occurred by a two-corner mechanism, and further crack growth initiated by strain lo-calization at one of the crack tip vertices. Also void/microcrack formation ahead of the main crack tip was operative and resulted in coalescence into the main crack tip along the anticipated shear bands. Frac-tography of the thicker areas showed a ductile, dimpled fracture mechanism both in vacuum and hydro-gen atmospheres. Technical Research Centre of Finland, Metals Labo-ratory, SF-02150 Espoo, Finland