Fracture toughness improvement of austempered high silicon steel by titanium, vanadium and rare earth elements modification

The microstructure of austempered high silicon (AHS) steel before and after treating with a modifier containing titanium, vanadium and rare earth elements (so-called Ti–V–RE modifier) and austempered at different temperatures has been investigated. The plane strain fracture toughness of the steel in room temperature and ambient atmosphere has been examined. The microstructure was characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and optical metallography and correlated to the fracture toughness of the steel. The results show that the primary austenite grains are refined, the dendritic austempered structure is eliminated, and the volume of blocky shaped retained austenite is reduced by the addition of Ti–V–RE modifier. Modification with Ti–V–RE modifier can prompt the bainitic ferrite transformation and reduce the volume fraction of retained austenite. High fracture toughness is obtained for AHS steel with the addition of Ti–V–RE modifier when austempered between 350 and 385 °C with a retained austenite of 30–35% and the carbon content in the austenite is about 1.9–2%. The fracture toughness of AHS steel by the modification treatment can increase 10–40% than that of unmodified, an optimum value of 85 MPa m1/2 was obtained when austempered at 385 °C.