Achieving 2.7 GPa tensile strength in ultrastrong high-carbon steel through prolonged low-temperature tempering

The mechanical properties of high‑carbon martensitic steel are improved by low-temperature tempering. This is due to carbon diffusion and carbide precipitation, which depend on temperature and time. To further improve mechanical properties and develop optimum tempering conditions of the conventional high‑carbon martensitic steel, long-term tempering was carried out at approximately 175 °C and 250 °C. Excellent mechanical properties, which had not been achieved before, were obtained after tempering samples at 175 °C for over 40 d, resulting in tensile strength of 2.7 GPa, yield strength of 2.3 GPa and total strain of 2.6%. Similarly, yield strength values exceeding 2.4 GPa were observed in samples tempered at 250 °C for 5 d. To understand the remarkable outcomes achieved, a detailed investigation into the tempering stages, with the use of Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD) analyses, was essential. The microstructure contained the typical tempered martensite matrix embedded by ultrafine carbides and retained austenite which decomposed slowly with increasing tempering time at 175 °C, contributing to a noticeable increase in strength. Conversely, no pronounced effect of retained austenite was observed on the strength evolution of tempered specimens at 250 °C. At the initial stage of long-term tempering, the primary strengthening mechanism was carbon solid solution strengthening with a value exceeding 1000 MPa. The solid solution strengthening weakened with tempering time, while precipitation strengthening increased with the growth and increasing volume fraction of carbides. The interplay of their effects and the softening of retained austenite led to a trend in which the total yield strength increased, followed by a decrease with increasing holding time. The resulting nuanced mechanical properties and microstructure obtained from long-term tempering offer strategies for synergistic strengthening and ductility of high‑carbon steels, thereby facilitating the expansion of their potential applications.