The effect of temperature, specimen size, and geometry on the fracture toughness of a 3 Pct NiCrMoV low pressure turbine disc steel

The variation of fracture toughness with temperature and specimen size of a 3 pct NiCrMoV LP disc steel has been investigated over the temperature range -100 to +1000C using compact tension and single-edge-notched bend geometries. A number of large 'half-disc' three point bend specimens were also tested. Toughness increased up to a transition temperature coinciding with the onset of stabl ductile tearing prior to instability. Below this temperature fracture could be described by established linear elastic or post yield fracture analyses. Above this temperature failure was by plastic collapse. The transition temperature decreased with decreasing specimen size, and at similar thicknesses was lower for the bend geometry than for the compact tension so that it was not possible to predict th fracture behavior of the full size service component from small scale tests in the transition region. A further complicating feature was the extreme scatter of some duplicate test results below the transition temperature. The implications for toughness testing in the transition region are discussed. The data obtained in this work have been combined with published data for similar steels to derive an equation which describes the variation of fracture toughness with temperature for steels of this type