Microstructural Modeling During Multi-Pass Rolling of a Nickel-Base Superalloy

Superalloys are metallic alloys used for high temperature applications such as encountered in the aircraft industry and where resistance to deformation is a primary requirement. Alloy 718 and waspaloy are few examples of Nickel-base superalloys that resist deformation at elevated temperatures and are therefore difficult to hot work. The major hotworking operations are forging, extrusion, and rolling. In the case of rolling and forging, the alloys might undergo multiple deformations in several passes associated with or without reheats between deformations. For a given composition of alloy, the high temperature flow stress is influenced to a large extent by the grain size of the microstructure. In the case of rolling, the correct working forces, which relate to gauge and shape control as well as to power requirements, can be estimated accurately only if the microstructure relevant to the specific pass of rolling is known. In addition, the microstructure present at the end of the rolling and cooling operations controls the product properties. Coarse grains (22 μm 90 μm) favor creep strength and crack-growth resistance while a fine grain structure (3 μm 11 μm) favors improved low-cycle fatigue life and tensile yield strength. Control of grain size is an increasingly important characteristic in any hot-working. The narrow temperature range for hot working of Nickel-base superalloys makes the grain size control more difficult. For example, Alloy 718 is hot worked between 980 C and 1040 C due to the presence of Titanium and Aluminum at higher percentages. Modeling the dynamic microstructural events is important in designing the rolling process. Tremendous amount of time and effort is needed in carrying out experiments and establishing the constitutive models for the microstructural events. In addition, industrial trials are expensive, difficult to control, and are necessarily constrained within the capabilities of the existing plant. Laboratory simulation tests are unable to reproduce all the conditions of industrial multi-pass rolling. Therefore, numerical methods are resorted to because of the complexity of the variables of industrial multipass rolling. One such popular numerical technique, Finite Element(FE) Method can predict process variables such as strain, strain rate and temperature for the deformation process. In general, microstructural modeling relates those process variables to microstructural evolution. During microstructural modeling, constitutive equations describing the microstructural evolutions are developed using experiments. Then, the microstructural constitutive equations are implemented in any FE analysis package. Microstructural Models: During hotworking, superalloys undergo microscopic and mesoscopic events such as dynamic (DRX), metadynamic (MDRX) recrystallizations and recovery (REC) depending on the temperature, strain rate and retained strain. DRX occurs during the actual deformation, when the equivalent retained strain exceeds a certain critical strain. During MDRX, the partially recrystallized grain structure observed right after deformation transforms to a more fully recrystallized structure. It takes place by the growth of recrystallization nuclei formed during DRX. MDRX involves imposed strains greater than the critical strain. A fine-grained structure is generated primarily during DRX and MDRX. Microstructure modeling predicts the recrystallized average grain size and the volume fraction recrystallized at the end of a forming process. During the cooling or heat treatment process, grain growth is calculated on an average sense. Empirical relationships (G. Shen, Ph.D. Dissertation 1994) between the grain size from various microstructural processes and the macroscopic variables such as temperature (T ), effective strain (ε̄) and strain rate ( ̇̄ ε) are developed. The commonly used equations for grain size developed during DRX, MDRX and REC are listed below. • Dynamically recrystallized grain size: ddrx = a ̇̄ ε b exp (