Enhancing impact fracture toughness and tensile properties of a microalloyed cast steel by hot forging and post-forging heat treatment processes

In the present work, the effects of hot forging and post-forging heat treatment on the impact fracture toughness and tensile properties of a microalloyed cast steel were investigated. Mechanical tests were used to evaluate the room temperature impact fracture toughness and tensile properties of the steel. The resulting microstructures were analysed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The extraction replica technology was used to investigate the characterisation of complex precipitates formed during heat treatment. The obtained results showed that the coarse-grained microstructure of the forged specimen was significantly refined after post-forging heat treatment. Presence of complex precipitates had a favourable effect on the formation of refined austenite grains, and consequently refined final microstructure. Hot forging was beneficial to enhance the impact fracture toughness and tensile properties of the microalloyed cast steel. After 920 °C-treatment followed by air cooling, the impact energy of the forged specimen was significantly increased from 19.3 to 208.3 J, and further enhancement in tensile properties was obtained. The enhanced impact fracture toughness and tensile properties of the microalloyed cast steel after hot forging and post-forging heat treatment were closely related to the refined and homogenised ferritic–pearlitic microstructure.

► Complex precipitates are studied by using extraction replica technology.
► Grain refinement shows little effect on the increment of UTS.
► Post-forging heat treatment is very important to enhance the fracture toughness.
► Refined and homogenised microstructure is responsible for the enhanced properties.