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The ductile-brittle transition in ferritic steels is reviewed using data from pressure vessel steels A533B, A508, BS1501-271, and their associated weld metals and heat-affected zones in a variety of conditions. A schematic model is presented which provides a rational basis for understanding the phenomenon and making predictions. This model, which is consistent with the predictions obtained from physical models of cleavage fracture, contains the following important features. 1. If in the transition regime, KIc is defined as the fracture toughness at the onset of the brittle mode of fracture and KIJ is defined as the fracture toughness at the onset of ductile crack growth, then KIc > KIJ. 2. The ductile crack extension obtained between the initiation of ductile crack growth and the onset of brittle fracture is defined by the elastic-plastic crack growth resistance curve. 3. KIc cannot be reached without generating the appropriate amount of ductile crack extension.
It follows that, when performing a failure analysis in this temperature regime, the choice of KIJ as a failure criterion is unnecessarily pessimistic and inconsistant with the choice of KIc as a failure criterion below these temperatures. A procedure is presented which allows the permissible extent of crack growth to be obtained.
ductile-brittle transition, ferritic steels, phenomenological model, fracture toughness, J, R, -curve, ductile crack extension, permissible extent of crack growth, elastic-plastic fracture
Research officer, Central Electricity Research Laboratories, Central Electricity Generating Board, Leatherhead, Surrey
Project leader, Schlumberger Cambridge Research, Cambridge,