Austenite Reconstruction Elucidates Prior Grain Size Dependence of Toughness in a Low Alloy Steel
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TRODUCTION
THE importance of steels in current technology is undeniable. Their wide scope of application justifies substantial effort to understand how microstructural properties affect macroscopic mechanical properties such as toughness, hardness, and ductility. These parameters are especially important in steels used to extract and transport pressurized fluids, which need to withstand high pressure and possible wide ranges of temperature without fracture, and as such there has been much work to characterize their microstructural evolution.[1–4] The effect of, e.g., austenitizing temperature, tempering temperature, and cooling rates on Charpy v-notch impact energy (CVN energy) has been empirically measured for many different steel alloys,
CHASEN RANGER is with the Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213-3890. Contact e-mail: [email protected] VAHID TARI and ANTHONY ROLLETT are with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 152133890. SUSAN FARJAMI and MATTHEW J. MERWIN are with the U. S. Steel Research and Technology Center, Munhall, PA 151205044. LIONEL GERMAIN is with the Laboratoire d’Etude des Microstructures et de Me´canique des Mate´riaux (LEM3), UMR 7239, CNR /Universite´ de Lorraine, 57045 Metz, France and also with the Laboratory of Excellence on Design of Alloy Metals for low-mAss Structures (DAMAS), Universite´ de Lorraine, 57045 Metz, France. Manuscript submitted December 4, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
and, in general, higher austenitizing temperatures tend to result in lower toughness. This can be understood as an increase in parent grain size and, in some cases, stronger texture (see, e.g., Reference 5). Clearly, crystal orientations in the martensitic state are related via the orientation relationship(s) to the austenitic state, which introduces a connection between the parent (austenite) texture and the mechanical properties in the product.[4–9] Strongly textured austenite with large grain size leads to strong textures and larger domain sizes in the martensite with deleterious effect on mechanical properties. A larger austenite grain size translates into larger packets, i.e., group of six crystallographic variants sharing the same (111) habit plane.[10] These are known to cause a decrease in toughness.[11] If all possible variants allowed by the operative orientation relationship occur in the martensitic state and are well dispersed throughout the volume of a prior austenite grain, then the task of reconstructing the parent orientation would be straightforward. In real life, however, not all variants appear and twin-related pairs of parent grains are common, giving rise to non-trivial ambiguities. Therefore, it is necessary to reconstruct the austenite microstructure using probabilistic methods in order to overcome problems of noise, incomplete sets of variants, and twin-related ambiguities.[12–15] Such reconstructions are particularly useful because the relationship between physical propertie
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