WELDING RESEARCH Table 4 — Chromium in Welding Plumes Produced by the ESAB A2 Machine No. Flow Rate Time Welding Total Cr Soluble Cr(VI) Insoluble Cr(VI) Fraction of Total Cr L/min min Plume Mass Conc. Mass Ratio Mass Ratio Mass Ratio Soluble Insoluble mg mg/m3 mg mg/mg mg mg/mg mg mg/mg 1 3.5 10 24.2 691 4.2 0.174 2 3.5 10 28.2 806 3 3.5 12 30.7 731 Combined 58.9 0.7 0.01 2 0.03 0.07 0.195 4 3.5 11 21 545 4.2 0.2 5 3.5 11 27.6 717 6 3.5 11 27.4 712 Combined 55 0.5 0 2.5 0.05 0.05 0.225 7 3.5 9 19.6 622 3.3 0.168 8 3.5 9 18.1 575 9 3.5 8 17.1 611 Combined 35.2 0.5 0.01 2 0.06 0.08 0.339 10 3.5 12 21.1 502 3.8 0.18 11 3.5 6 17.8 848 12 3.5 6 18.2 867 Combined 36 0.6 0.02 1.6 0.04 0.09 0.244 geometric mean 676 0.182 0.01 0.04 0.07 0.245 geometric standard deviation 1.19 1.08 1.31 1.24 1.38 1.26 MARCH 2016 / WELDING JOURNAL 91-s measures are necessary, this despite the small quantity of Cr in aluminum alloys. The maximum concentration of other forms of Cr would be 0.683 64 = 44 mg/m3 for Cr(0) + Cr(III). This concentration is considerably less than the 8-h TLV (500 mg/m3) for these forms of Cr in the welding plume as stated in Table 2. These results apply specifically to GMAW processes using argon shielding. Further experimental work is needed to assess the situation for GTAW processes. Discussion Given the potential for chromium to be present in many forms, including those for which specific TLVs exist, the uncertainty intrinsic in these results provides more basis for suggesting the need for control measures than for suggesting the opposite. However, this cannot be proven conclusively from the type of analysis performed here. This requires air sampling to determine welder exposure for the specific operation. Workers exposed to the welding plume and grinding dust routinely wore respiratory protection approved by NIOSH containing P100 filtration. These products significantly reduce exposure to chromium metal and compounds contained in particulates. The Fabrication Building has a high-velocity, low-volume (HVLV) local exhaust system containing many inlets to connect hoses and collector hoods. Welders may not have used this system effectively during this work. If the system was used effectively, local exhaust ventilation would have provided major benefit in the control of exposure to Cr(VI) compounds that have extremely low exposure standards. Any system of control of exposure to emissions from argon-shielded GMAW processes must address two realities. The first is reduction of emissions through effective collection and the second reality is preservation of the shielding gases. The HVLV system installed in the Fabrication Building was almost impossible to utilize for several reasons. The first reason was the inability of welders to see the location of the welding plume for effective positioning of the collector hood. This is a consequence of the dark shade of the lens used for viewing the arc. Positioning the collector hood or a holding device onto metal surfaces was also highly problematic because the nonferrous nature of aluminum precluded magnetic attachment and the considerable variety of geometric configurations required many clamping configurations. In either situation, in order to be effective for collecting and removing the hot plume, the welder must position the collector hood above and to the side or front away from the face. The hot plume contains the shield gas(es) as well as atmospheric gases entrained during collection. Maintaining the integrity of the gaseous shield around the arc is essential for obtaining welds that meet requirements for quality. Overly aggressive collection of the plume will destroy the gaseous shield on which weld quality and livelihood of the welder depend. Positioning the hood of the local exhaust equipment in a location that does not destroy the shield through induced turbulent motion is an acquired skill requiring intuition and attention to detail. Notes: Conc. is concentration. Soluble denotes solubility in water. Insoluble denotes insolubility in water. Ratio is calculated from (mass of the form of Cr)/(mass of material collected in the plume) and has units of mg/mg.
Welding Journal | March 2016
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