Microstructural Limits of Applicability of the Master Curve
Authors: Kirk MT , Natishan ME , Wagenhofer M 
Published: September 2009

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.