Influence of hydrostatic tension on cleavage fracture of bainitic pressure vessel steel

Abstract

Experiments have been performed to investigate the influence of hydrostatic tension on the cleavage fracture of steel to ASTM Standard A508 Class II, a bainitic pressure vessel steel. Material given either of two heat treatments was studied, one giving a fully tempered granular bainitic microstructure and the other an as-transformed upper bainitic microstructure. Single-edge notched bend specimens with different notch angles were tested over a range of temperatures to locate the temperature at which fracture was coincident with general yielding. A slip line field theory analysis showed the maximum tensile stress present below the notch at fracture to be largely independent of notch angle and of temperature. It is concluded that the cleavage fracture of bainitic steels, like that of mild steel, is tensile-stress controlled. Thus it may be possible to apply a recently developed micromechanistic model of cleavage fracture to investigate the effects of temperature, strain rate, warm prestressing, strain aging, and irradiation on the fracture toughness of bainitic pressure vessel steels. The micromechanism of cleavage in bainitic steels is discussed in the light of this conclusion. Metallographic evidence supports the view that cleavage fracture is not nucleated by cracked carbide particles in these bainitic microstructures.