High Temperature Deformation of Inconel 718Plus™

Abstract

Since its introduction in 2003, alloy 718Plus™ spurred a lot of interest owing to its increased maximum service temperature over conventional Inconel 718 (704°C vs 650°C), good formability and weldability together with its moderate cost. Understanding the high temperature deformation characteristics and microstructural evolution is still of interest to many. It is known that the service performance and hot-flow behavior of this alloy is a strong function of the microstructure, particularly the grain size. To develop precise microstructure evolution models and foresee the final microstructure, it is important to understand how and under which forming conditions softening and precipitation processes occur concurrently. In this work, the softening behavior, its mechanisms and the precipitation characteristics of 718Plus™ were investigated in two parallel studies. While cylindrical compression tests were employed to observe the hot-flow behavior, the precipitation behavior and other microstructural phenomena such as particle coarsening were tracked via hardness measurements. A PTT diagram was reported and modeling of the flow curves via hyperbolic sine model were discussed in the light of the PTT behavior. Both “apparent” approach and “physically based” approach are implemented and two different sets of parameters were reported for the latter. Finally, recovery and recrystallization kinetics are described via Estrin-Mecking and Bergstrom, and Avrami kinetics, respectively.

High-Temperature Deformation Behavior of 718Plus: Consideration of γ′ Effects

“…A TTT diagram of the alloy was previously presented by Xie et al [11]. A partially agreeing Precipitation Time Temperature (PTT) diagram was published by the current author [2]. According to these, γ precipitation nose is in the range of 900 • C -925 • C. Around 950 • C the transformation line can be accepted as horizontally asymptotic.…”

“…The selection of the deformation temperatures in Set 2 were based on TTT diagram of 718Plus [2,11,28].…”

“…Accordingly, the volume fraction and precipitate radius values are summarized on Table 2. [2]. Temperature dependency of the shear modulus is considered through Eq.…”

“…A study regarding the hot working response of 718Plus was published previously by the authors [2]. This study is focused on a precipitate-free initial microstructure with relatively large grains (herewith this initial state is referred to as 'baseline').…”

“…The material constants defining the work hardening of the material and detailed discussion of the general hot flow behavior is presented elsewhere[2]. Previously apparent values of Ω and U are given as a function of Z.…”