The mechanisms of ductile fracture in pressure vessel steels
The micromechanisms by which ductile fracture extended from a pre-existing crack was experimentally observed for two classes of forged SA 508 pressure vessel steel. The micromechanisms were related to the measured values of fracture toughness characterised by the resistance to crack initiation and growth. This comparison was only possible with the aid of an accurate fracture resistance test technique which could determine the crack growth toughness from a single specimen. An unloading compliance test system was developed and was used for the construction of crack growth resistance curves. Microstructural parameters determined from a specimen were related to the toughness measured on that specimen and this proved invaluable in isolating the controlling parameters. The effect of orientation and location on the toughness of the materials was assessed. The crack growth resistance was sensitive to the orientation of the crack with respect to the maximum hot working direction and the bands of segregation associated with elongated manganese sulphide inclusions. The toughness was high when the crack plane was perpendicular to the segregation bands and low when the crack plane was parallel with the bands. The location of the crack-tip through the thickness of the forging had a minor effect on the crack growth resistance. A limited study of test temperature, strength level and isothermal ageing was undertaken. Testing within the dynamic strain ageing regime of temperature had a marked effect and reduced the crack growth resistance to below the value at room temperature. Increasing the strength level of one steel by re-heat treating had no effect on the crack growth resistance. Subsequent isothermal ageing treatments also had no effect on the resistance curves. The magnitude and extent of void formation around growing cracks was studied and related to the applied loading. The size, shape and distribution of inclusions was characterised for the materials and orientations used in the fracture tests. Correlations between inclusion parameters and toughness revealed the important microstructural parameters controlling initiation and crack growth. Simple models for initiation and crack growth resistance were developed which take the controlling parameters into account. These models are shown to agree reasonably well with some experimental data.