Abstract
In this research, we designed one low alloy ultrahigh strength steel and performed an analysis of the mechanical properties and microstructures at different tempering conditions. The structure of the steel is lath martensite with a high density of dislocations combined with many carbides precipitated in the microstructures at specific conditions. Strength and hardness of the material were found to decrease, while toughness varied differently as the tempering temperature increased. The best combination of strength and toughness was obtained when tempered at 200 °C. Prolonging the holding time led to higher strength but lowered toughness. Tempered martensite embrittlement occurred at 300–400 °C. The ruptured surface indicated that the material had fractured in a quasi-cleavage manner at 100 °C, quasi-cleavage and microvoid coalescence fracture occurred at 200–400 °C, intergranular fracture at 500 °C.

Key words
low alloy ultrahigh strength steel, tempering temperature, holding time, mechanical properties, microstructure

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