LAUSR

Two different types of strength‐toughness relations are investigated in tempered steel (tempered at 25–420 °C). It is found that there is a trade‐off relation between strength‐fracture toughness and becomes invariable relation between strength‐impact toughness. The effect of microstructure on the materials toughness is related to the manner of energy consumption, which should be attributed to the notch root radius of the specimen. The coarse carbides and dislocation density inside the martensite lath have distinct effects on the energy for crack initiation (impact toughness) and crack growth (fracture toughness). Additionally, the mechanism of coarse carbides on the tempered martensite embrittlement is elaborated deeply.