1 Introduction

Material data and constitutive equations together with software processors and validation procedures are the three elements that equally contribute to a successful application of process simulation technology. The accuracy of numerical simulations of hot forming operations strongly depends on the quality in describing the rheological behaviour of the material under deformation.

The material response to hot deformation conditions is affected not only by the current values of the process parameters strain, strain rate and temperature but also by the previous thermal and mechanical cycles before the current deformation step, as well as by the variations of temperature and strain rate inside the deformation step.

The constitutive equations currently implemented in commercial FE codes e.g. Norton-Hoff or Arrehnius laws correlate the material flow strength only to the instantaneous values of the process parameters. These equations usually give proper description of the material rheological behaviour only when strain hardening phenomena are dominant. Instead, they do not give reliable results when the thermally activated softening phenomena prevail over the hardening and when the history of deformation cannot be neglected, such as in multi-step hot forging operations.

The influence on flow strength of prior thermo-mechanical histories has been investigated in several hot deformation studies [ALT 97], [BAR 98], [MAR 80], [OH 95], [RAO 96] [YOS 95], [YOS 94]. Most of them refer to stationary processes such as rolling and extrusion and are primarily aimed at...