Interactions Between Dynamic Softening and Strengthening Mechanisms During Hot Forging of a High-Strength Steel

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.

Constitutive Equation for Flow Stress in Superalloy 718 by Inverse Analysis under Hot Forming in a Region of Precipitation

The purpose of this study is to obtain a constitutive equation of high-accuracy flow stress in superalloy 718, which allows fabrication of highly reliable disks for gas turbine engines. Hot compression tests using superalloy 718 at deformation temperatures from 850 to 1100 °C, a 67% height reduction, and strain rates of 1, 10, and 25 s−1 were performed to investigate the flow stress behavior, which excludes environmental effects during hot working by inverse analysis. The effects of dynamic recrystallization and strain-induced dynamic precipitation on the flow stress were also investigated. The dynamically precipitated δ phases deformed at 1050 °C and γ″ phases deformed at 950 °C might affect the increase in the plastic modulus F1 and the decrease in the critical strain εc, deteriorating the accuracy of regression in terms of, for example, the strain rate sensitivity m and the temperature sensitivity A. A constitutive equation for a generalized flow curve for superalloy 718 is proposed by considering these effects.

Microstructure Evolution and Mechanical Properties of Mg-10.37Gd-3.66Y-2.27Zn-0.52Zr Alloy during Reciprocating Upsetting-Extrusion

The homogenized Mg-10.37Gd-3.66Y-2.27Zn-0.52Zr alloy was subjected to multi-passes reciprocating upsetting extrusion (RUE) deformation with variable temperature. The microstructure evolution and mechanical properties of as-homogenized and RUEed samples were investigated. The results showed that the area fraction of DRX grains gradually increased via the continuous consumption of coarse grains containing lamellar LPSO, and the content of the bulk LPSO phases gradually decreased due to continuous fragmentation. After three passes deformation, the microstructure was almost composed of completely DRXed grains. The LPSO phases with different morphologies were coordinated deformation by kinking, tearing, etc during RUE process. It is worth noting that after four passes, the lamellar LPSO phase did not disappear, but mixed with the fine DRXed grains together. In addition, a mass of particles were produced after each low temperature deformation, indicating that reducing the deformation temperature is beneficial to the dynamic precipitation. The yield tensile strength (TYS), Ultimate tensile strength (UTS) and fracture elongation (FE) of four passes deformed alloy reached 372.6 MPa, 320.8 MPa, 8.1%, respectively. The improvement of mechanical properties is attributed to the two main strengthening mechanisms: grain refinement and LPSO strengthening.