Abstract
Open-die forging is a critical step in the manufacture of large numbers of components used in the transportation and energy industries. Dynamic recrystallization, dynamic transformation, and dynamic precipitation take place during the hot deformation process and significantly affect microstructure conditioning, which ultimately influences the service properties of the component. In the present work, using a Gleeble 3800 thermomechanical simulator, the open-die forging of a large-size ingot made of a modified AISI 6140 medium carbon high-strength steel is investigated. Deformation temperatures ranging from 950°C to 1,250°C and strain rates ranging from 0.01 to 1 s−1, representative of the actual process, are considered in the analysis. The generated true stress–true strain curves are used as a basis for the development of a constitutive model predicting the occurrence of softening and strengthening phenomena as a function of thermomechanical conditions. The corresponding activation energy is determined to be about 374 kJ mol−1 and is compared against the values reported in the literature for other high-strength steels. Dynamic recrystallization kinetics is studied using the t50 model, and the influence of temperature and strain rate is quantified and discussed. The interaction between dynamic precipitation and dynamic recrystallization is discussed, and the deformation conditions under which such interactions occur are determined. The thermomechanical results are validated by microstructure examination, including laser confocal microscopy, field emission scanning electron microscopy, transmission electron microscopy, and energy-dispersive spectroscopy. The present study focuses on reproducing the deformation cycle applied during the open-die forging process of a vanadium-containing high-strength steel used in the industry with special attention to the interaction between dynamic recrystallization and precipitation processes.
Highlights
High-strength medium carbon steels are characterized by their excellent mechanical properties along with good formability and low cost (Titov et al, 2017; Ebrahimi et al, 2017)
The present study focuses on reproducing the deformation cycle applied during the open-die forging process of a vanadiumcontaining high-strength steel used in the industry with special attention to the interaction between dynamic recrystallization and precipitation processes
The analysis shows that the critical condition for the onset of dynamic transformation (DT) is constant over the temperatures and strain rates used in the present investigation
Summary
High-strength medium carbon steels are characterized by their excellent mechanical properties along with good formability and low cost (Titov et al, 2017; Ebrahimi et al, 2017). Major microstructural changes are produced during the open-die forging process These include the closing down of the solidification porosities as well as the breakdown of the coarse columnar dendritic as-cast structure into much finer equiaxed grains through the recrystallization process (Chadha et al, 2016; Han et al, 2015). These steels are characterized by their high strength, which is mostly obtained through a judicious control of their composition and the formation of very fine precipitates that control the grain size and provide strong work hardening properties. The open-die forging process is essentially a thermomechanical process in which temperature, and strain and strain rates are key processing parameters that determine the evolution of the microstructure, recrystallization, precipitation, and phase transformation processes in the steels
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