Upgrading the mechanical properties of marine structural steel by enhancement of acicular ferrite formation

Abstract

This work reports the results of an investigation of the effect of magnesium addition on microstructure evolution, acicular ferrite formation together with the change in the mechanical properties of micro-alloyed steel plates. The investigated steel is subjected to high heat input (4.5 kJ mm(-1)) simulating the thermal cycle of heat affected zones, conducted by induction heating in a computer controlled weld thermal cycle simulator. Both magnesium and magnesium free steel samples have carbon and aluminum contents of 0.08 wt.% and 0.009 wt.%, respectively, are investigated. The change in microstructure, hardness and crack tip opening displacement fracture toughness properties due to magnesium addition is evaluated, and compared with the other steel having the same chemical compositions but without Magnesium. The fracture toughness tests reveal the increase of magnesium steel's crack tip opening displacement values. In contrast, the hardness measuring presents the decrease of its hardness due to the decrease of martensite volume fraction and reduction in ferrite grain size. Optical and transmission electron microscopy observations detect some microstructural morphology that can affect the mechanical properties of such steels. Hence, the improved toughness of magnesium containing steel are attributed to both the generation of acicular ferrite nucleated within the matrix, refined microstructure and the bainite dominated microstructure in steel.