Flow stress is the most important information for hot strip rolling as it affects the rolling force and the thickness of the rolled product. A high-speed compression test up to a strain rate of 300 s − 1 , which is the compression speed of 3 600 mm•s − 1 for a 12-mm-high cylindrical specimen, is necessary as a strain rate of 100-300 s − 1 is the normal rate in the production of hot-strip-rolled steel sheets. An experiment is conducted using a servo-hydraulic compression test machine, which enables a high compression speed and a high temperature, but the oscillation is observed in stress-strain curve at high strain rate over 50 s − 1. To determine the natural frequency of the compression test machine, the Savitzky-Golay filtering method is used for regression and the fast Fourier transformation (FFT) is adopted. To explain the mechanism of this phenomenon, a spring-mass-damper model is used and the results are compared with the FFT analysis result. After eliminating oscillation on the time versus load curve, a flow curve is obtained by inverse analysis, which compensates for the nonuniform strain rate, the inhomogeneous distribution of deformation, and the temperature increase during deformation.

No abstract

No abstract

This study aims to formulate a constitutive equation to accurately determine the flow stress of superalloy 718 for effective production of gas turbine disks. A hot-compression test with superalloy 718 at temperatures ranging from 900 to 1000 °C, a reduction rate of 67%, and strain rates of 0.1, 1, and 10 s −1 , respectively, was conducted to analyze the flow stress in the dynamic precipitation region. An accurate flow stress curve was obtained for each strain rate and initial temperature. The flow curves obtained at a deformation temperature of 900 °C and strain rates of 0.1 and 1 s −1 , represent a combination of work-hardening and dynamic recovery. Dynamic recrystallization (DRX) behavior was observed under other deformation conditions. At a deformation temperature of 950 °C and each strain rate, the strain at the onset of DRX ( c ) decreases, and DRX tends to occur rapidly. In addition, the steady-state stress at a strain rate of 1 s −1 was greater than that at a higher strain rate of 10 s −1 . The lowest steady-state stress among all the experimental conditions was observed at a strain rate of 10 s −1 . This may be attributed to the role of nucleation sites, precipitation hardening caused by dynamically precipitated " phases at approximately 950 °C and a strain rate of 1 s −1 , and dynamic softening effects due to significant heat generated by deformation at a strain rate of 10 s −1 . A new constitutive equation for the generalized flow curve of superalloy 718 was obtained by considering these metallurgical phenomena.

No abstract