Tempering behavior of an ultra-high-strength steel with 1.6 wt% Si at low to medium temperatures.

Tempering behavior of a 0.53%C-1.6%Si-0.9%Mn-0.76%Cr-0.14%V-0.05%Nb steel was examined. Water quenching produced lath martensite structure with a 10.5% volume fraction of retained austenite (RA) with film-like shape. Low temperature tempering (LTT) leads to precipitation of transition carbides and carbon partitioning between martensite and RA. The following precipitation sequence was found: martensite →non-stoichiometric η-carbide→stoichiometric η-carbide→Fe 3 C. Highest density of non-stoichiometric Fe 2 C η−carbides was found after isochronal tempering at 280 °C that provided attractive combination of high yield stress (YS) (1890 MPa) with a ductility of ∼6% and a Charpy V-notch (CVN) impact energy of ∼10 J/cm2. At 400 °C, the replacement of non-stoichiometric η-carbide by the stoichiometric one decreases strength and increases ductility due to 30% decrease in its volume fraction. No tempered martensite embrittlement was found due to the fact that Si effectively suppresses the precipitation of cementite at T ≤ 400 °C. The replacement of transition η-carbide by boundary cementite after isochronal tempering at 500 °C leads to considerably decrease in YS down to 1360 MPa, while elongation to failure increases up to 9.3%. Effect of decomposition of RA on strength, ductility and fracture toughness is insignificant. • Low-temperature tempering of a ultra-high-strength steel with 1.6wt.% Si leads to precipitation of transition η-carbides. • Carbon content in transition η-carbides strongly affects volume fraction of carbides and mechanical properties of steel. • High volume fraction of non-stoichiometric η-carbides in steel promotes the best combination of yield stress and ductility. • Replacement of non-stoichiometric η-carbides by stoichiometric ones decreases strength and increases ductility. • Si effectively suppresses the precipitation of paraequilibrium cementite at T ≤ 400 °C. [ABSTRACT FROM AUTHOR]