Factors affecting the strength and toughness of ultra-low carbon steel weld metal

Abstract

The factors that affect strength and toughness often ultra-low carbon steel weld samples (HSLA-80 and HSLA-100), welded using the gas metal arc welding (GMAW) process and new ultra-low carbon consumable electrodes, were studied. The analysis was confined only to the weld metal, and the base metal was not considered. Analysis methods included optical microscopy, scanning electron microscopy, and transmission electron microscopy. Energy dispersive x- ray analysis was performed in the transmission electron microscope to analyze the chemical composition of non-metallic inclusions. The microstructure was found to be primarily granular ferrite with some primary ferrite, bainite, and martensite. Very little acicular ferrite was found (< 18 %). Because of this, to get the best mechanical properties in the weld, the size and volume fraction of non-metallic inclusions needs to be minimized. This can be accomplished by minimizing the amount of oxygen while increasing the amount of titanium and aluminum in the weld metal. EDX analysis revealed that the non-metallic inclusions were multi-phase particles with two predominant phases: a TiO-MnO phase and a MnO-SiO2-Al2O3, phase. Copper-sulfide caps were also found on the surface of some inclusions. This inclusion chemistry is typical of what is found in welding HSLA steel