Pulsed power welding is a conventional welding technique used for wire welding operations. A recent modification to pulsed power welding, called pulsed power inverter (PPI) technology, involves the “inversion” of DC power to AC power. PPI is reported by equipment vendors to provide a better overall welding performance. Additional benefits include reductions in emissions of some welding fume components.

Department of Defense (DoD) craftspeople perform welding operations at most DoD facilities and, in particular, high volume welding is carried out at facilities dedicated to the major overhaul and repair of large military equipment (e.g., ships, tanks, armored personnel carriers, weapons systems, etc.).  Therefore, a welding technology that can reduce emissions of fume particles and metal constituents while maintaining the quality of welds would be of great benefit to DoD.

The objective of this demonstration project was to measure fume components and emission rates from both conventionally powered pulsed power welding and PPI power sources to determine what differences may exist. Atmospheric emissions of particulates, metal oxides, nitrogen oxide (NO_x), carbon monoxide (CO), and ozone (O₃) in welding fumes for both sources were measured, as was the relative exposure of welding personnel to these components. The level of ultraviolet (UV) radiation was also measured. To the extent possible, these parameters were measured relative to the welding parameters utilized (e.g., length of weld, amount of welding wire/rod used).  Given how critical the quality of welding is for military applications, the welded test plates were also evaluated to determine their metallurgical properties (e.g., tensile strength, yield strength, toughness).

This demonstration compared emissions from PPI power sources to the power sources used during existing welding processes, typically flux core arc welding (FCAW) and shielded metal arc welding (SMAW). Welding tests were performed on mild steel (<0.5% Cr), HY-80 steel (1.0-1.9% Cr), and “chrome-moly” 4130 steel (nominally 1% Cr) test plates at four DoD facilities (two Navy, one Marine Corps, and one Army). It should be noted that DoD equipment maintenance and repair operations do not typically weld stainless steel products in large quantities. The test plates were also evaluated for weld quality to determine if PPI provides comparable integrity compared to existing technology. 

Particle size distribution data show that emitted particle size for PPI power sources was predominantly in the sub-micron diameter range. Typically, more than 50% of the particles (by weight) were less than 0.8 micron in size. The only metals present in the PPI welding fumes at significant concentrations (above about 5%) were iron, manganese, and magnesium. Aluminum, zinc, and barium were also present, but they are believed to be an artifact of the cascade impactor substrate filter material. Other metals that appear in the 1-5% range were arsenic, nickel, strontium, and copper. Total chromium appeared in the 1% range only during welding operations on chromium-molybdenum (Cr-Mo) 4130 steel at the Southwest Regional Maintenance Center (SWRMC). Otherwise it was typically less than 0.1%. For almost all sampling events levels of Cr⁶⁺ in the samples did not exceed EPA or OSHA regulations, including OSHA’s requirement of 5.0 micrograms per cubic meter (μg/m³) for Cr⁶⁺ exposure (71 FR 10100 – 28 Feb 2006). The exception was welding 3140 Cr-Mo steel where Cr⁶⁺ emissions averaged 2.59 μg/m³, with the highest daily value being 8.60μg/m³. Unlike the other welding sites, the SWRMC welder wore a positive air pressure {assisted} respirator (PAPR).

For most of the PPI wire welding operations, O₃ emissions increased to more than 100 parts per billion by volume (ppbv) compared to background concentrations (below about 30 ppbv), and there appeared to be no significant difference between PPI and conventional pulsed power welding. NOx may also evolve during welding operations, but it is more difficult to determine quantitatively because of interferences from local fossil fuel combustion devices (e.g., trucks, forklifts, water heaters). Some welding operations did not show significant increases in NOx or O₃ concentrations compared to background. CO emissions do not appear to significantly increase above a background as a result of the welding operations. However, the presence of fossil fuel combustion sources, e.g., forklifts and nearby unit heaters in the test area, did result in elevated CO emissions.

There do not appear to be any obvious differences in any of the emission parameters from PPI versus conventional pulsed power sources. It appears, however, that SMAW produces a greater particulate emissions loading per amount of welding rod used when compared to wire welding techniques, confirming a well-known finding. Test plates were evaluated for weld quality to determine if PPI provides comparable integrity compared to conventional power sources. The quality of PPI welds, in terms of tensile strength, yield strength, and Charpy V Notch (CVN), was equivalent to conventional pulsed power sources.

The results of this demonstration do not indicate a significant difference for any of the emissions studied between PPI and conventional pulsed power welding. Weld quality also was comparable. A bead of filler material, however, can be laid down faster with PPI when compared with the conventional welding process, and the welder could potentially produce more work in a day. All welders said they felt comfortable with the PPI equipment after using it for a couple of days. No additional supplies or hardware are needed to operate the PPI equipment as compared to conventional equipment.