THE ROLE OF PRIOR DAMAGE BY HYDROGEN ATTACK IN INFLUENCING CYCLIC AND MONOTONIC CRACK ADVANCE IN PRESSURE VESSEL STEELS (INTERNAL VOIDS, GROWTH, DUCTILE FRACTURE)

A study was made of the influence of prior elevated temperature, elevated pressure hydrogen attack (HA) damage (with exposure conditions in the range 550(DEGREES)-600(DEGREES)C, 12.4-17.2 MPa up to 1480 hours) on fatigue crack propagation and ductile fracture in two commercial pressure vessel steels: quenched and tempered Mn-Mo-Ni (ASTM A533B Class 2) and normalized and tempered 2.25 Cr - 1Mo (ASTM A387 Class 2 Grade 22). Hydrogen attack was characterized in terms of SEM and optical microscopy observations of bubbles, density measurements, estimation of residual carbon content and changes in mechanical properties. In addition, the changes in morphology and constitution of carbides with the progress of HA were examined using the scanning transmission electron microscope (STEM). A533B steel, in which controlled amounts of damage characteristic of the principal HA regimes could be produced with judicious control of exposure conditions, was utilized as a 'model material' for more extensive study.; Ductile fracture toughness in the L-T and S-T specimen orientations was assessed in terms of valid J(,IC) (J-integral) values and J(,R)-(DELTA)(alpha) resistance curves obtained from mechanical tests. It was found that even mild degrees of damage (bubble sizes (TURN)0.1 (mu)m and almost no decarburization) characteristic of the incubation stages of HA promote gross reductions in ductile fracture toughness and tensile ductility. This effect is concomitant with a severe curtailment of the growth of 'primary voids' in the microstructure. With more severe exposure (bubble size (TURN)0.6 (mu)m and (TURN)30% reduction in strength), although toughness values deteriorate further, the effect on tensile ductility is more dramatic. This latter effect is accompanied by a complete change of fracture mode to intergranular bubble coalescence, presumably due to the localization of flow in the cavitated grain boundary regions.; In the fatigue study, the crack growth behavior over the entire range from (TURN)10('-3) mm/ cycle down to ultra-low growth rates approaching the so-called 'fatigue threshold' ((TURN)10('-8) mm/cycle) was examined. It was found that, even after severe degrees of damage, crack growth was influenced significantly only in the higher growth rate regime, concomitant with the inception of extensively intergranular fracture modes. The effect at near-threshold levels was, however, surprisingly small.; The rather distinct nature of microstructural factors that influence ductile fracture and fatigue are considered an important implication in alloy-design work for materials which must withstand both cyclic and monotonic loading during service. (Abstract shortened with permission of author.)...