Microstructure‐Impact Toughness Evolution of Ultrahigh Strength Steel: Influence of Quenching and Secondary Low‐Temperature Tempering

Both favorable strength and impact toughness have important implications for the equipment prepared by ultrahigh strength steel (UHSS) under harsh environments. This study explores the effect of austenitizing and secondary low‐temperature tempering temperature on the microstructure and mechanical properties of high‐Co–Ni secondary hardening steel. The heat treatment process and performance are improved by analyzing the synergistic effects of phase content and distribution, dislocation density, high‐angle grain boundaries, and carbides during quenching and secondary low‐temperature tempering. The martensite laths in the UHSS exhibit relatively smaller dimensions with increase of high‐angle grain boundary density as the quenching temperature ranges from 980 to 1020 °C. The UHSS with ultrahigh strength, good elongation, and impact toughness is achieved when austenitized at 1020 °C, primary tempered at 500 °C, and secondary low‐temperature tempered at 240 °C. Under this heat‐treating operation, the best mechanical properties are achieved: tensile strength of 2239 MPa, yield strength of 1925 MPa, elongation of 10.3%, and impact energy of 38.7 J cm −2 . The impact toughness increased by 38.2% after secondary low‐temperature tempering compared to the specimens processed by single quenching‐tempering.