| The relations between fracture toughness and three microstructural parameters were investigated for a copper-bearing high-strength low-alloy steel. Resistance to opening mode ductile and cleavage failure, to impact failure, and to mixed mode (I/III) shear failure were measured for six heat treatment conditions which were designed to vary ferrite grain size, and niobium carbonitride and copper precipitate sizes. The highest toughness values and lowest transition temperatures were obtained by overaging to produce a large ((TURN)45 nm) copper precipitate size. Specimens with overaged copper precipitate distributions also showed increased resistance to local shear instabilities in tension. Higher strength values were obtained by ageing to peak strength and reducing the ferrite grain diameter to (TURN) 2 (mu)m, without significant toughness drop below that of the as-rolled and aged plate. Refining the grain size while lowering the pearlite content reduced the ductile-to-brittle transition temperature. Inclusions were also seen to have a significant effect on fracture toughness, especially in mixed mode shear failure. |
