Modeling the transition region fracture behavior of ferritic steels

The transition temperature of a material is that temperature at which the mode of fracture changes from brittle to ductile. Knowledge of the transition temperature allows one to design structures that can exist in variable temperature environments without being subject to catastrophic failure by cleavage fracture. Of the empirical methods available for determining a material's transition temperature, only the ASTM Master Curve (ASTM E1921-97) accomplishes this directly from fracture toughness measurements and the observation that all ferritic steels exhibit the same toughness temperature dependence. The Master Curve, however, cannot be applied to steels of varying composition or condition without considerable testing due to a lack of a rigorous, physical description of this temperature dependence. The current study examines the physical nature of the toughness temperature dependence and presents a newly developed expression for the work expended in causing unstable cleavage fracture at temperatures near the transition. This expression is evaluated for a C-Mn steel using relevant data obtained from the literature. Tensile and fracture toughness testing is performed on an A533B steel of the type used in nuclear pressure vessels to provide data for further validation of the expression. Comparison of these results to the largely empirical fracture work expression that forms the basis of the Master Curve shows excellent correlation. This indicates that assumptions made regarding the nature of deformation and fracture processes in BCC steels are accurately applied in describing the fracture work. Suggestions are made for additional work to allow direct determination of fracture toughness values.