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

Abstract

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 is one such Nickel-base superalloy that resists deformation at elevated temperatures and is therefore difficult to hot work. One of the major hotworking operations is multi-pass shape rolling in which Alloy 718 undergoes multiple deformations in several passes along with reheating between passes. 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 shape rolling in which the cross section changes from circular to oval in alternate passes, 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. Control of grain size is an increasingly important characteristic in hotworking. The narrow temperature range (980°C and 1120°C [1]) for hotworking of Alloy 718 makes the grain size control more difficult. During hotworking, Alloy 718 undergoes microscopic and mesoscopic events such as dynamic recrystallization (DRX), metadynamic recrystallization (MDRX) and static grain growth (SGG) depending on the temperature, strain rate and retained strain. Modeling these microstructural events is important in designing the rolling process. Due to the tremendous amount of time, cost and effort associated with experiments and industrial trials, numerical methods are resorted to because of the complexity of the variables involved in multi-pass 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 these process variables to microstructural evolution. During microstructural modeling, constitutive equations describing the microstructural evolutions are developed using experiments, which can then be readily implemented in an FE package capable of modeling rolling processes.