Effect of heat treatment on microstructure and mechanical properties of AG728 steel joints welded with ER90S-G wires

Abstract

Abstract This study utilised self-developed ER90S-G wires for butt welding tests on AG728 plates to investigate the influence of heat treatment on the microstructure and mechanical properties of welded joints. Furthermore, it aimed to analyse the microstructure and metallurgical mechanisms of these joints. The findings reveal that the microstructure of the welded joints primarily consists of acicular ferrite with a minor presence of granular bainite. Following heat treatment, there is a noticeable growth in acicular ferrite and the emergence of tempered sorbite and massive ferrite; concurrently, austenite grain boundaries vanish, carbide aggregation increases, the quantity of large angular grain boundaries diminishes, and the martensite–austenite constituent elements gradually disappear. The physical phase of the base material, the heat-affected zone (HAZ), and the welded joints uniformly transition to α-Fe. Additionally, inclusions measuring 0.5 μm, predominantly MnSiO3, evidently promote the formation of ferrite. The tensile strength and impact toughness experience a minor reduction while the hardness of the HAZ and weld seam significantly decreases. The micro-morphology of the welded joint’s tensile fracture is predominantly characterised by dimples, within which inclusions or precipitated phase particles are distinctly visible, indicating a microporous coalescence-type fracture. At −45 °C, the impact test of welded joints reveals that the HAZ impact fracture micro-morphology exhibits brittle deconstructive fracture patterns, which, through heat treatment, transition to quasi-cleavage characteristics with reduced and shorter river patterns. The base material and weld fracture micro-morphology are primarily defined by the presence of dimples.