Microstructure and toughness of the simulated heat-affected zone in Ti- and Al-killed steels

Abstract

Abstract The microstructure, transformation behaviour and toughness of the simulated welding heat-affected zone in titanium- and aluminium-killed steels were investigated. Over the temperature range studied, the titanium-killed steel exhibited a larger austenite grain than the aluminium-killed steel. At peak temperatures higher than 1200°C, the larger austenite grain (greater than 50 μm) and the existence of titanium oxide in titanium-killed steel provided a favourable environment for the formation of intragranular ferrite laths which formed at a temperature slightly higher than the formation temperature of bainite in aluminium-killed steel. The intragranular ferrite originated mainly from titanium oxide and subsequently grew by an edge-on-face sympathetic nucleation mode. The resultant microstructure was characterized by highly interlocked groups of parallel ferrite laths with aligned M-A-C. The interlocked groups of ferrite sectioned the original austenite into small colonies, which were very effective in resisting brittle fracture, and thus possessed a superior toughness to the bainite in aluminium-killed steel. The brittle fracture surface of intragranular ferrite revealed a quasi-cleavage fracture surface with fine facets. A linear relation between the inverse square root of the fracture facet size and the impact transition temperature was obtained. The formation of intragranular ferrite became less pronounced with decreasing peak temperature because of the small original austenite grain size. At peak temperatures below 1100°C, the absence of intragranular ferrite and the larger final grain size inherited from an initially coarser austenite grain made the toughness of titanium-killed steel inferior to that of aluminium-killed steel. It was concluded that the difference in toughness between titanium- and aluminium-killed steels is mainly attributable to the difference in initial austenite grain size and the tendency towards intragranular nucleation.