The effect of tempering on the mechanical properties, structure, and dispersion of secondary phase particles is studied in 0.4%C-2%Si-1%Cr-1%Mo-VNb steel. This steel austenitized at 900 °C with subsequent water quenching exhibits a yield stress of 1445 MPa and a lath martensite structure with MX particles of ~40 nm located in matrix and boundary M₆C carbides of ~210 nm. Tempering in the temperature interval of 200–400 °C provides a yield stress of 1625 MPa due to the precipitation of ε-carbide and cementite within laths. The yield stress decreases to 1415 and 1310 MPa after tempering at 500 and 650 °C, respectively, due to the replacement of matrix carbides by boundary M₂₃C₆ carbide. A Charpy V-notch impact energy of ~12 J/cm² is almost independent from tempering temperatures of up to 400 °C and increases up to ~33 J/cm² after tempering at 650 °C due to decreased yield stresses and increased plasticity.

An effect of tempforming on the microstructure, the carbide precipitation, and the strengthening mechanisms of high-strength low-alloyed steel has been analyzed. The quenched steel was subjected to 1 h tempering at a temperature of 873 K, 923 K, or 973 K followed by plate rolling at the same temperature. Tempforming resulted in the formation of an ultrafine grained lamellar-type microstructure with finely dispersed carbides of (Nb,V)C, Fe₃C and Cr₂₃C₆. A decrease in tempforming temperature resulted in a reduction of the transverse grain size from 950 nm to 350 nm. Correspondingly, the size of Fe₃C/Cr₂₃C₆ particles decreased from 90 nm to 40 nm while the size of (Nb,V)C particles decreased from 17 nm to 4 nm. Refining the tempformed microstructure with a decrease in thetempforming temperature provided an increase in the yield strength from 690 MPa to 1230 MPa.

The effect of tempering after water quenching on the strength and fracture toughness of two steels with chemical compositions of 0.34%C-1.77%Si-1.35Mn-0.56%Cr-0.2%Mo-0.04%Nb-0.03Ti-0.002B and 0.44%C-1.81%Si-1.33%Mn-0.82%Cr-0.28%Mo was examined. The last steel exhibits quenching embrittlement in an as-quenched condition. At a tempering temperature of 280 °C, the precipitation of transition η–Fe₂C carbides in martensitic matrix leads to increasing fracture toughness and eliminates quench embrittlement in the steel with 0.44 wt.%C. Tempered martensite embrittlement at 400 °C appears as decreased values of the Charpy V-notch impact energy, ductility and the product of strength and elongation, σ_B×δ (MPa×%) and is attributed to increased effective grain size for fracture, mainly. The precipitation of boundary cementite takes place at tempering at 500 °C and provides increased ductility and fracture toughness despite a decohesion along carbide/ferrite interfaces. The low severity of TME in Si-rich low-alloy medium carbon steels is attributed to the suppression of boundary cementite precipitation at tempering temperatures ≤400 °C.

Quenching and partitioning (Q&P) treatments were applied to 0.25C steel to produce the microstructures that exhibit an improved balance of mechanical properties. The simultaneous bainitic transformation and carbon enrichment of retained austenite (RA) during the partitioning stage at 350 °C result in the coexistence of RA islands with irregular shapes embedded in bainitic ferrite and film-like RA in the martensitic matrix. The decomposition of coarse RA islands and the tempering of primary martensite during partitioning is accompanied by a decrease in the dislocation density and the precipitation/growth of η-carbide in the lath interiors of primary martensite. The best combinations of a yield strength above 1200 MPa and an impact toughness of about 100 J were obtained in the steel samples quenched to 210–230 °C and subjected to partitioning at 350 °C for 100–600 s. A detailed analysis of the microstructures and the mechanical properties of the steel subjected to Q&P, water quenching, and isothermal treatment revealed that the ideal strength–toughness combinations could be attributed to the mixture of the tempered lath martensite with finely dispersed and stabilized RA and the particles of η-carbide located in the lath interiors.

An Fe-0.44%C-1.8%Si-1.3%Mn-0.82%Cr-0.28%Mo steel was subjected to quenching followed by low-temperature tempering (Q&T) and quenching and partitioning (Q&P) processing after full austenitization. The Q&P treatment led to an increase in the volume fraction of retained austenite (RA) by factors ranging from 30 to 40 depending on the quenching temperature, T_q, and an additional precipitation of transition η-carbides in the martensitic matrix. The Q&P processing provided a decrease in the yield stress (YS) from 1730 to 1350 MPa and an increase in the ductility by a factor of 3; the product of strength and elongation (PSE) increased from 13.7 to 32 GPa·%. The novelty of the work lies in establishing the origin of the good ductility and high YS of Q&P steel. Blocky-type RA plays a vital role in the effect of Q&P processing on mechanical properties. The main feature of RA is a very high dislocation density proving the strength of ~1000 MPa of this structural component. The strength of RA controls the YS of the steel if its volume fraction is ≥25%. Ductility is provided by the almost full transformation of RA into strain-induced martensite under tension. The localization of plastic deformation in the form of deformation bands is associated with the γ→α′ transformation. Medium carbon Q&P steel with a high volume fraction of RA meets the requirements for advanced high-strength steel (AHSS) belonging to the third generation of AHSS due to the combination of the YS > 1050 MPa with the PSE > 30 GPa·%.

The effect of tempforming on the strength and fracture toughness of 0.4%C-2%Si-1%Cr-1%Mo-VNb steel was examined. Plate rolling followed by tempering at the same temperature of 600 °C increases yield stress by 25% and the Charpy V-notch impact energy by a factor of ~10. Increasing rolling reduction leads to the reorientation and elongation of grains toward the rolling direction (RD) and the development of a strong {001} (rotated cube) texture component that highly enhances fracture toughness. A lamellar structure with a spacing of 72 nm between boundaries and a lattice dislocation density of ~10¹⁵ m⁻² evolves after tempforming at 600 °C with a total strain of 1.4. Two types of delamination were found, attributed to crack branching and the propagation of secondary cracks along the rolling plane perpendicular to the propagation direction of the primary crack. Delamination toughness is associated with the nucleation of secondary cracks in RD and their propagation over a large distance. The critical condition for delamination toughness is the propagation of primary cracks by the ductile fracture mechanism and the propagation of secondary cracks by the brittle quasi-cleavage mechanism.