Factors influencing as-deposited strength, microstructure, and toughness of manual metal arc welds suitable for C-Mn steel fabrications

Abstract

Published work relating to the influence of several important elements on the strength, toughness, and microstructure of as-deposited weld metal is reviewed. Published data for manual metal arc (MMA) welds has been supplemented by the incorporation of pertinent data for other welding processes. The factors affecting the strength of as-deposited weld metal include the influence of alloying (where the effects are larger than can be accounted for by solid solution hardening alone) and of grain size. Beginning in 1976, considerable advances have been made in understanding the key role which oxygen-containing inclusions play in as-deposited weld metal. It is now recognized that large inclusions can initiate cleavage fracture, so that a low weld metal oxygen (and inclusion) content is important for high toughness. However, it is also recognized that oxygen-containing inclusions can have a strong influence on as-deposited microstructure, and thus on toughness, since they provide nucleation sites for the fine-grained acicular ferrite which is generally associated with high toughness. Thus, the optimum oxygen content is not zero, and optimum levels have been found also for several other elements, including carbon, manganese, molybdenum, nickel, titanium, boron, and possibly silicon. While a number of strong deoxidants are among the additions which can be made to the coatings of MMA consumables, commercial secrecy has limited the volume of pertinent published work. One vital role of deoxidants in the coating is to reduce TiO₂ and thus introduce small amounts of titanium into the weld where, as a constituent of the oxygen-containing inclusions, it exerts a strong influence on as-deposited microstructures and on toughness. Impurities, which generally have a deleterious effect on toughness, are discussed. The foremost of these is nitrogen, introduced from the parent plate, the electrode core wire, and the surrounding atmosphere (if the gaseous shield is inadequate) and this element has a particularly strong detrimental effect. Through-thickness variations in strength and toughness, and the influence of procedural variables, including heat input, preheat, and restraint, on toughness are also reviewed. Some observations on possible routes for improving as-deposited weld toughness are included.