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Investigation of low oxygen HSLA steel weld metal
White, Drew
White, Drew
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2018
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Hot-wire gas tungsten arc welding is a process gaining more popularity in industry today due to increased deposition rate, from the resistively heated filler metal introduced by an external wire feed system. A modified version of this system, which incorporates an oscillation mechanism to modify the frequency with which the filler metal enters the molten weld pool was utilized in this work and the effect of varying process parameters on microstructure, weld bead morphology, and inclusion size and distribution are characterized. Hot-wire amperage, wire feed speed, frequency of wire oscillation and heat input were varied on seven wire consumables to determine their effects on acicular ferrite. Optimal microstructural development in high strength low alloy steel weldments is mainly dependent on acicular ferrite, which nucleates on oxide inclusions. However, with the gas tungsten arc process, it is difficult to manipulate weld pool oxygen content to achieve the needed level for optimal acicular ferrite formation. The oxide inclusion population is investigated to determine the morphology, size and spatial distribution and whether they served as nucleation sites for ferrite formation. The microstructure was primarily affected by heat input and composition, with increasing heat input reducing the total acicular ferrite volume percent. These effects directly correlated to inclusion population, with three distinct distributions demonstrated between inclusion radii in the range of 0.1 to 0.75 micron. Composition effects were tied to both an increase in Molybdenum, 0.1 wt. pct. to 0.25 wt. pct., a Titanium peak at 70 ppm and Oxygen content. The oscillation affected weld bead morphology, improved weld wetting but with little effect on final microstructure. Two experimental wire compositions were identified to move forward for additional mechanical testing.
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