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 eta-carbide -> stoichiometric eta-carbide -> FeC. Highest density of non-stoichiometric FeC eta-carbides was found after isochronal tempering at 280 degrees C that provided attractive combination of high yield stress (YS) (1890 MPa) with a ductility of similar to 6% and a Charpy V-notch (CVN) impact energy of similar to 10 J/cm. At 400 degrees C, the replacement of non-stoichiometric eta-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 degrees C. The replacement of transition eta-carbide by boundary cementite after isochronal tempering at 500 degrees 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.