Influence of oxygen-rich inclusions on the ??? phase transformation in high-strength low-alloy (HSLA) steel weld metals

Abstract

Reducing the oxygen content of two submerged-arc, high-strength, low-alloy (HSLA) steel weld metals has been shown to depress γ→α transformation temperatures and produce a marked change in the resultant microstructures. In weld metal of composition 0.12wt% C, 1.35wt% Mn, 0.29 wt% Si, 0.03 wt% Nb reducing the oxygen content from about 300 ppm to about 60 ppm decreased the transformation initiation temperature by about 30° C and changed the microstructure from acicular ferrite to parallel lath ferrite. In weld metal of composition 0.1 wt% C, 0.8 wt% Mn, 0.1 wt% Si, 0.01 wt% Nb reducing the oxygen content from about 600 ppm to about 300 ppm decreased the transformation initiation temperature by approximately 20° C and favoured the development of ferrite side-plates and acicular ferrite at the expense of the polygonal ferrite microstructure. In both weld metals the depressed transformation temperature is thought to be due to the larger γ-phase grain size developed when the volume fraction of small de-oxidation products is reduced. The marked microstructural change from fine-grained acicular ferrite to parallel lath ferrite which occurred when virtually all the de-oxidation products were removed suggests that these small de-oxidation products may also be of fundamental importance to the nucleation of acicular ferrite.