Microstructure of Hardened Steel at High Temperature and High Strain Rate

In high-speed machining, hardened steel materials are subjected to high temperatures and high strain rates. Under these conditions, the composition and microstructure of the material may change, and phenomena, such as thermal softening, emerge. These effects are difficult to detect by only observing the chip morphology. Here, using a microscopic detection method, the dynamic mechanical behavior and microstructure of SDK11 hardened steel (62 HRC) is investigated at high temperature and high strain-rate, and the relationship between strain hardening, thermal softening, and strain-rate strengthening is determined. The metallographic phases of specimens treated using a split-Hopkinson pressure bar, and "chips" generated during high-speed machining at high temperature and high strain rate state are compared. The results indicate that the phase composition at low temperature and low strain rate differs from that at high temperature and high strain rate. It is further concluded that shear slip occurs at high temperature and high strain rate, and the shear behavior is more pronounced at higher strain rates.