Hot working processes often induce high levels of strain at high strain rates, and impose very complex multi-axial modes of solicitation. These processes are essentially limited by the apparition and development of plastic instabilities. These may be the direct cause of rapid crack propagation, which lead to a possible final rupture. The complexity of strain modes and the simultaneous intervention of several parameters have led many researchers to develop various criteria, with different approaches, to predict the occurrence of defects and to optimize the process control parameters. The aim of the present paper is to summarize the general characteristics of some instability criteria, widely used in the literature, for the prediction of plastic instabilities during hot working. It was considered appropriate to divide the work into two parts. Part I presents the phenomenological criteria for the prediction of plastic instabilities, based on descriptive observation of the microscopic phenomena of the strain (strain hardening and strain rate sensitivity), and discusses the continuum criteria based on the principle of maximum rate of entropy production of irreversible thermodynamics applied to continuum mechanics of large plastic flow. Also, this part provides a bibliographical discussion among several authors with regard to the physical foundations of the dynamic materials model. In Part II of the work, a comparative study has been carried out to characterize the flow instability during a hot working process of a medium carbon micro-alloyed steel using phenomenological and continuum criteria.
In the study of hot working processes, the knowledge of interaction between microstructural behaviour and control process parameters such as temperature, strain rate, and strain is very important. In the last decades, processing maps have been developed to design, control, and optimize the hot strain of various metallic materials. In this work, to study the hot workability of medium carbon microalloyed steel, during hot compression tests, a comparative study between two types of processing maps constructed using phenomenological and thermodynamic continuum criteria has been carried out. The analysis of the maps indicates that the studied steel does not undergo any type of plastic instability. However, the maps corresponding to the strain of ¿ = 0.6 reveal a domain of dynamic recrystallization, considered as the more efficient domain within the ‘safe’ region process. This domain is centred at 1150°C and 10 s- 1. Also, the comparative study of the obtained results shows the difference between the positions of plastic strain domains predicted by the two criteria.