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.

The effect of quenching and partitioning (Q&P) processing on strength, ductility and fracture toughness is considered in a 0.33% C-1.8% Si-1.44 Mn-0.58% Cr steel. The steel was fully austenitized at 900 °C and quenched to 210 °C for 30 s. Partitioning at 350 °C for 600 s produces a martensitic matrix with transition carbides, bainitic ferrite and film-like retained austenite (RA) that is stable against transformation to strain-induced martensite under tension. This processing provided the highest strength and fracture toughness but the lowest ductility and product of strength and elongation (PSE), σ_B·δ (MPa·%). Partitioning at 500 °C produced RA with a relatively low carbon content and low volume fraction of carbides. The steel after this Q&P processing exhibits the highest ductility and PSE but low YS and Charpy V-notch (CVN) impact toughness. High ductility and PSE correlate with the ability of RA to transform into strain-induced martensite, while high strength and impact toughness are associated with the high-volume fraction of transition carbides in the carbon-depleted martensitic matrix and a lack of transformation of RA to strain-induced martensite. The highest CVN impact energy was attained in the steel exhibiting transgranular quasi-cleavage fracture with the lowest effective grain size for brittle fracture. No correlation between strength, ductility and fracture toughness is observed in Q&P steels if these materials have distinct structural constituents.

Owing to the addition of Si, 0.33C-1.8Si-1.44Mn-0.58Cr steel exhibits a unique tempering behavior. The tempering takes place in two distinct sequential stages that are significantly different from those in steels containing 0.2–0.5 wt.% of Si. Stage I is associated with the precipitation of transition carbides in a paraequilibrium manner, can take place in temperatures ranging from ~200 to ~474 °C, and concurrently increases strength, ductility, and toughness. Stage II is associated with the decomposition of retained austenite to bainitic ferrite and transition carbides. As a result, no significant effect of overlapping of Stage I with Stage II takes place. Stage III does not occur at temperatures below ~474 °C, since the precipitation of cementite in a orthoequilibrium manner is suppressed by the addition of 1.8 wt.% of Si. It was shown that a major portion of carbon atoms redistributes to Cottrell atmospheres under quenching. During low-temperature tempering at 200–400 °C, the precipitation of transition carbides consumes a large portion of carbon atoms, thereby increasing the number of ductile fractures and improving the impact toughness without strength degradation. The formation of chains of cementite particles on boundaries takes place in Stage IV at a tempering temperature of 500 °C. This process results in the full depletion of excess carbon from a ferritic matrix that provides increased ductility and toughness but decreased strength.

The 44CrMn2Si2Mo steel heat treated by quenching and partitioning demonstrates a unique combination of strength characteristics: the yield stress σ_0.2 = 1140 MPa, ultimate strength σ_В = 1690 MPa, and elongation δ = 20.7%. Quenching and partitioning leads to the formation of a multiphase structure consisting of primary martensite, retained austenite, bainite, and secondary martensite. Primary martensite and bainite contain transition-metal carbides Fe₂C. The high ductility of the steel is due to the transformation of retained austenite into strain-induced martensite during tension, which ensures high strain hardening. Stable plastic flow is observed at low strain, when a significant fraction of retained austenite is transformed into strain-induced martensite. The plastic flow instability, which appears as the Portevin–Le Chatelier effect on deformation curves and plastic flow localization in deformation bands, occurs at higher strains and is associated with the transformation of film-like retained austenite. The velocity of deformation bands decreases with a decrease in the volume fraction of retained austenite. Localization of plastic flow in the neck and fracture occur when the transformation of retained austenite into strain-induced martensite cannot provide strain hardening, and deformation bands lose their mobility.