Modeling the changes in flow stress of structural steels during deformation under hot-working conditions
[ jeudi 04-06-2015 13:00
| français ]Modeling the changes in flow stress of structural steels during deformation under hot-working conditions
School of Metallurgical Engineering and Materials Science, Faculty of Engineering, Universidad Central de Venezuela
Former Chaire International, Université Lille Nord de France, USTL,
Laboratoire de Mécanique de Lille (LML)
The computation of loads, torques and power consumption required in industrial hot-working operations of structural steels is significantly dependent not only on deformation conditions, i.e., deformation temperature and strain rate, but also on the microstructural changes that occur during plastic deformation, particularly work-hardening, dynamic recovery and dynamic recrystallization. In this seminar, the main results of the investigations that have been carried out in the past few years between the University of Lille (LML) and the University of Valenciennes (LAMIH), on the analysis of the work-hardening and work-softening transients present on the flow stress curves of structural steels deformed in a wide range of temperatures and strain rates and particularly, under transient loading conditions, will be presented and discussed. Emphasis will be placed on the determination of five important stress parameters: yield, critical, peak, saturation and steady-state flow stress, as well as on the description of their temperature and strain rate dependence by means of the Sellars-Tegart-Garofalo model (STG), employing for this purpose the experimental stress-strain curves of the material. Subsequently, an expression for the time required to achieve 50% dynamic recrystallization as a function of the deformation conditions will be established, as well as the computation of the Avrami exponent of the material. It is further shown that all the above information can be subsequently employed in the description of the flow stress of the material as a function of deformation conditions and microstructure. However, given the fact that strain is not a valid state parameter, since it cannot represent the microstructure of the material in any way, an original constitutive description in differential form, which combines a work-hardening and dynamic recovery term with an additional softening expression, which involves the Avrami relationship in differential form, has been proposed. It is further shown that the work-hardening and dynamic recovery of the material should be described by the differential expression of an exponential saturation law, such as those earlier advanced by Voce, Sah et al. and Estrin and Mecking, which rules out the possibility of using the Johnson-Cook relationship for this purpose. Finally, it is shown that the advanced evolution equations are entirely independent of the strain applied to the material, which allows a satisfactory description of the flow stress during transient loading conditions as a consequence of changes in strain rate or deformation temperature, regardless if the material undergoes dynamic recrystallization during such a transient. Contrary to many different models reported in the literature, the approach here proposed is also independent of the peak parameters exhibited on the flow curves.