The welding process results in the formation of high concentrations of nano-sized particles loaded with toxic metals such as hexavalent chromium (Cr(VI)), nickel (Ni), and manganese (Mn). Tightened occupational standards require an exposure reduction of at least 90% that is not satisfied by current control technologies. The objective of this project was to demonstrate two innovative technologies for controlling hazardous air pollutants (HAPs) generated during certain welding operations.
At the University of Florida (UF), the team demonstrated an innovative silica precursor technology that limits the oxidation of Cr (chromium) by quenching oxygen species and coating metal particles in welding fumes with a thin, amorphous silica layer, and assessed the benefit of increased particle size distribution. The demonstration verified the feasibility and practicality of implementing silica precursor technology into Department of Defense (DoD) welding operations. At Ohio State University (OSU), the team demonstrated new Cr-free welding consumables (ENiCuRu, a shielded metal arc welding (SMAW) electrode alloyed with Ni, copper (Cu), and ruthenium (Ru); and ERNiCuRu, a solid wire gas-metal arc welding (GMAW) electrode alloyed with Ni, Cu, and Ru) for DoD application. The objective was to reduce Cr(VI) and hazardous air emission during welding by 90% with the Cr-free SMAW ENiCuRu and GMAW ERNiCuRu electrodes.
The silica precursor technology has been demonstrated to be an effective means of controlling metal emissions in welding fumes. The two-fold approach of limiting oxidation potential and coating metal particles with an amorphous silica layer goes beyond previous control technologies by addressing all the toxic metals, regardless of their oxidation state. This project demonstrated, through both a laboratory study and field tests, the benefits of adding silica precursor during the welding process. The new Cr-free SMAW and GMAW consumables were developed as a replacement for the conventional Types 308 and 316 stainless steel welding consumables. SMAW and GMAW have been demonstrated to provide almost a 100-fold reduction of Cr(VI) in the welding fume and produce welds with comparable corrosion resistance and mechanical properties relative to the conventional stainless steel consumables.
Silica Precursor Technology (University of Florida) The laboratory study showed that use of an insulated double-shroud torch to inject vapor-phase silica precursor tetramethylsilane (TMS) into the welding operation reduced Cr(VI) exposure by more than 90% and satisfied the Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) of 5 micrograms per cubic meter (μg/m3). The calculated silica coating efficiencies gave quantitative evidence of the encapsulation of metals inside the silica shell, and the transmission electron microscopy images provided visual evidence. Scanning mobility particle size data showed the particle size distribution shifted to a larger size range, and the mode size of fume particles increased to 180-300 nanometers (nm) from 20 nm. The E. coli study provided a preliminary result supporting the reduced biotoxicity of welding fume particles using the novel silica precursor technology. The results of the field study further confirmed the capability of this technology to reduce Cr(VI) and to encapsulate toxic metals such as Mn, Ni, and Cr. Two different sampling approaches were used in the field demonstration (low- and high-flow samplings). The results from the low-flow sampling were limited due to insufficient fume mass collection. The concentration of Cr(VI) in most samples was lower than the OSHA PEL (5 μg/m3), regardless of whether they were baseline or TMS-injected samples. The results of high-flow sampling clearly showed the silica precursor technology was capable of reducing Cr(VI) exposure below the OSHA PEL with > 90% Cr(VI) reduction efficiency, and resulted in about 31.8% of the metals by mass sealed inside the silica shell. The mechanical quality test suggested there is room for improving the TMS technology to achieve a higher weld quality. The weld qualities resulting from both baseline and TMS technology were lower than the minimum required by the standard for uniform metals, indicating that problems could have been partially due to welder error. Cr-Free SMAW and GMAW Consumables (Ohio State University) Cr(VI) and hazardous air emission during welding were reduced by 90% with the newly developed Cr-free SMAW ENiCuRu and GMAW ERNiCuRu electrodes. The ENiCuRu electrode provided reduction in Cr(VI) exposure of more than 92% compared to the OSHA PEL and more than 94% compared to the conventional E308L-16 electrode. The ERNiCuRu electrode provided reduction in Cr(VI) exposure of more than 71% compared to the conventional E308L-16 electrode. The fume content of Cu and Ni was up to two orders of magnitude higher than in the conventional ER308LSi and single measurements exceeded the OSHA PELs. Such behavior is expected since ERNiCuRu is a Ni-based welding consumable with a high alloy content of Cu. A possible solution for reduction of these Ni and Cu emissions would be using this electrode with a low heat input GMAW process such as cold metal transfer. Welds of both Cr-free consumables met the performance objectives of 70,000 pounds per square inch tensile strength and successfully passed the bend test. During the OSU laboratory demonstration, the ENiCuRu and ERNiCuRu electrodes produced high quality welds free of defects. During the OSU field demonstration, some of the ENiCuRu welds lacked fusion defects and did not pass the X-ray test. Lack of fusion, lack of penetration, and undercut defects were found in welds made with the ERNiCuRu electrode. Similar defects were found in welds of conventional E308L-16 and ER308LSi electrodes. Particular defect-free welds of both the ENiCuRu and ERNiCuRu consumables met the performance objective of 30% minimum elongation (El). Defect containing welds of both the Cr-free consumables and the conventional reference electrodes had El less than 30%. Both Cr-free welding consumables demonstrated good welding operability and arc stability, comparable to conventional Ni-based welding consumables.
Interviews with welders in the UF field demonstration showed that the use of TMS had no significant impact on welding operations. While TMS technology does not significantly deteriorate the mechanical quality of the welds, optimization of the different parameters to achieve the expected mechanical tensile parameters will be helpful. The cost assessment showed that use of TMS mixed at the nozzle and commercially available TMS cylinder gas did not significantly increase the overall cost of the welding operation. Also, it could potentially reduce the costs of retrofitting ventilation systems needed to meet new OSHA regulations. One implementation issue considered was safe handling of the TMS. A worst-case scenario was used to estimate the maximum possible TMS concentration. The TMS concentration in the case of a complete leak was still lower than the safety threshold value. In addition, there were no incidents caused by the TMS additive during the laboratory study and field demonstration. While TMS technology does not significantly deteriorate the mechanical quality of the welds, optimization of the different parameters to achieve the expected mechanical tensile parameters will be helpful. One issue related to the implementation of the Cr-free ENiCuRu and ERNiCuRu welding consumables demonstrated at OSU may be the absence of an OSHA PEL for Ru in welding fume. In fact, no published occupational exposure limits for Ru was found in any of the literature. This issue can be addressed by conducting related studies at particular National Institute for Occupational Safety and Health (NIOSH) or DoD laboratories. Also, additional training is required for welders who have no experience working with Ni-based welding consumables.