ASTM Standard Test Method E 1921-97, “Test Method for the Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range, addresses determination of T0, a fracture toughness reference temperature for ferritic steels having yield strength ranging from 275 to 825 MPa. E 1921 defines a ferritic steel as: “Carbon and low-alloy steels, and higher alloy steels, with the exception of austenitic stainless steels, martensitic, and precipitation hardened steels. All ferritic steels have body centered cubic crystal structures that display ductile to cleavage transition temperature. This definition is not intended to imply that all of the many possible types of ferritic steels have been verified as being amenable to analysis by this test method.” The equivocation provided by the final sentence was introduced due to lack of direct empirical evidence (i.e., fracture toughness data) demonstrating Master Curve applicability for all ferritic alloys in all heat treatment/irradiation conditions of interest. This question regarding the steels to which E 1921 applies inhibits its widespread application for it suggests that the user should perform some experimental confirmation of Master Curve applicability before it is applied to a new, or previously untested, ferritic steel. Such confirmations are, in many cases, either impractical to perform (due to considerations of time and/or economy) or impossible to perform (due to material unavailability). In this paper we propose an alternative to experimental demonstration to establish the steels to which the Master Curve and, consequently, ASTM Standard Test Method E 1921 applies. Based on dislocation mechanics considerations we demonstrate that the temperature dependency of fracture toughness in the fracture mode transition region depends only on the short-range barriers to dislocation motion established by the lattice structure (body-centered cubic (BCC) in the case of ferritic steels). Other factors that vary with steel composition, heat treatment, and irradiation include grain size/boundaries, point defects, inclusions, precipitates, and dislocation substructures. These all provide long-range barriers to dislocation motion, and so influence the position of the transition curve on the temperature axis (i.e., T0 as determined by E 1921-97), but not its shape. This understanding suggests that the myriad of metallurgical factors that can influence absolute strength and toughness values exert no control over the form of the variation of toughness with temperature in fracture mode transition. Moreover, this understanding provides a theoretical basis to establish, a priori, those steels to which the Master Curve should apply, and those to which it should not. On this basis, the Master Curve should model the transition fracture toughness behavior of all steels having an iron BCC lattice structure (e.g., pearlitic steels, ferritic steels, bainitic steels, and tempered martensitic steels). Conversely, the Master Curve should not apply to untempered martensitic steels, which have a body-centered tetragonal (BCT) lattice structure, or to austenite, which has a FCC structure. We confirm these expectations using experimental strength and toughness data drawn from the literature.
In this paper we trace the development of transition fracture toughness models from the landmark paper of Ritchie, Knott, and Rice in 1973 up through the current day. While such models have become considerably more sophisticated since 1973, none have achieved the goal of blindly predicting fracture toughness data. In this paper we suggest one possible way to obtain such a predictive model.
An abundance of empirical data supports the use of the Master Curve, as proposed by Wallin, Saario and Törrönen, to describe the fracture toughness transition behavior of ferritic steels, particularly the notion of a curve shape that is invariant with steel microstructure (other than lattice structure). However, nuclear surveillance programs do not always contain samples of the steel that most limits reactor operations, making direct measurement of fracture toughness impossible. This suggests that a purely empirical argument cannot define the limits of applicability of the Master Curve or validate its use for all conditions of interest. In previous papers a microstructural basis for the existence of a single “Master” fracture toughness transition curve for all ferritic steels was established and limits of applicability have begun to be explored from a theoretical viewpoint. These previous papers established that all steels with the same lattice structure and cleavage fracture mechanism should be expected to adhere to transition behavior that can be defined by a single curve shape with variations in microstructure accounting only for a shift in the transition temperature. In this paper we explore the basis for “Master Curve” validity for irradiated steels by exploring how irradiation affects the microstructure and fracture mode and using the Zerilli-Armstrong constitutive model as the basis for predictions of irradiated steel behavior.
A program was undertaken to develop a predictive model of the scatter in toughness of a structural steel across the wide temperature range of ductile-to-brittle transition based on physical understanding of deformation and fracture behaviors. The initial model was focused on microcrack initiation and includes criteria to describe the propagation of particle-sized microcracks into the surrounding ferritic matrix. Parametric studies using this model found that the temperature dependence for fracture toughness, derived from microcrack, was insufficient to describe the temperature dependence observed in measured KJc toughness values. A microcrack propagation model was developed to account for additional barriers associated with transgranular crack propagation to failure. This propagation model accounts for the temperature dependence of crack propagation across grain boundaries and is therefore expected to increase the degree of temperature dependence. This paper summarizes the initiation model and discusses the approach taken in developing a microcrack propagation model component and discusses preliminary results from a Monte Carlo simulation.
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