MICROSTRUCTURAL MODELING DURING MULTI-PASS ROLLING OF A NICKEL-BASE SUPERALLOY
1 online resource (124 pages) : PDF
University of North Carolina at Charlotte
Microstructure present at the end of rolling and cooling operations controls the product properties. Therefore, control of grain size is an important characteristic in any hot-working. The narrow temperature range for hot working of Alloy 718 makes the grain size control more difficult. In the current work, a systematic numerical approach to predict the microstructure of Alloy 718 during multi-pass rolling is developed. This approach takes into account the severe deformation that takes place during each pass and also the possible reheating between passes. In order to predict the grain size at the end of rolling process, microstructural processes such as dynamic recrystallization (DRX), metadynamic recrystallization (MDRX), and static grain growth need to be captured at every deformation step for superalloys. Empirical relationships between the average grain size from various icrostructural processes and the macroscopic variables such as temperature, effective strain, and strain rate form the basis for the current work. The empirical relationships considered in this work are based on Avrami equations and utilize data taken from various forging analyses. The macroscopic variables are calculated using the Finite Element Method (FEM) by modeling the rolling process as a creeping flow problem. FEM incorporates a mesh re-zoning algorithm that enables the analysis to continue for several passes. A two-dimensional transient thermal analysis is carried out between passes that can capture the MDRX and/or static grain growth during the microstructural evolution. The microstructure prediction algorithm continuously updates two families of grains, namely, the recrystallized family and strained family at the start of deformation in any given pass. In addition, the algorithm calculates various subgroups within these two families at every deformation step within a pass. As the material undergoes deformation between the rolls, recrystallization equations are invoked depending on critical strain and strain rate conditions that are characteristics of Alloy 718. This approach predicts the microstructural evolution based on recrystallization kinetics and static grain growth only. Precipitation of phases such as γ’, γ”, and δ are not considered. Modeling this complex precipitation is difficult and requires a more detailed understanding than is presently available. Nevetheless, comparisons of the grain sizes from the proposed numerical models with experimental results for 16-stand rolling process show very good agreement.
ALLOY 718GRAIN GROWTHMICROSTRUCTUREMULTIPASS ROLLINGRECRYSTALLIZATIONSUPERALLOY
Minisandram, RameshBose, KingshukCai, WeiBiswas, Animikh
Thesis (Ph.D.)--University of North Carolina at Charlotte, 2009.
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