Modeling of grain size in superalloy IN718 during hot deformation

The microstructure and mechanical properties of IN718 forgings are highly sensitive to metallurgical technology and thermomechanical processing. To obtain the most favorable mechanical properties and microstructures in IN718 forgings, it is necessary to further understand the microstructure variation behavior and to be able to predict the grain size of superalloy IN718 during hot deformation. On the basis of isothermal constant-speed compression tests conducted using a computer-controlled MTS machine at temperatures from 960-1040 degrees C, with initial strain rates from 0.001-1.0 s(-1), to engineering strains from 0.1-0.7, the microstructure variation behavior of superalloy IN718 has been studied. Two different types of microstructures were found at deformation temperatures below and above 1000 degrees C. The effect of hot-deformation parameters on the microstructures was analyzed and a model was developed to describe the duplex microstructure and to express the relationship between the grain size and the hot-deformation parameters. (C) 1999 Elsevier Science S.A. All rights reserved.