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The influence of tempering on the microstructure and mechanical properties of HSLA-100 steel (with C-0.04, , and Nb-.038 pct) has been studied. The plate samples were tempered from 300 ЊC to 700 ЊC for 1 hour after austenitizing and water quenching. The transmission electron microscopy (TEM) studies of the as-quenched steel revealed a predominantly lath martensite structure along with fine precipitates of Cu and Nb(C, N). A very small amount of retained austenite could be seen in the lath boundaries in the quenched condition. Profuse precipitation of Cu could be noticed on tempering at 450 ЊC, which enhanced the strength of the steel significantly (yield strength (YS)-1168 MPa, and ultimate tensile strength (UTS)-1219 MPa), though at the cost of its notch toughness, which dropped to 37 and 14 J at 25 ЊC and Ϫ85 ЊC, respectively. The precipitates became considerably coarsened and elongated on tempering at 650 ЊC, resulting in a phenomenal rise in impact toughness (Charpy V-notch (CVN) of 196 and 149 J, respectively, at 25 ЊC and Ϫ85 ЊC) at the expense of YS and UTS. The best combination of strength and toughness has been obtained on tempering at 600 ЊC for 1 hour (YS-1015 MPa and UTS-1068 MPa, with 88 J at Ϫ85 ЊC).

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