Water Well Journal
Issue link: http://read.dmtmag.com/i/705618
an electric arc between the base metal and a continuously spool-fed consumable electrode. Shielding gas is supplied externally; hence, the electrode typically has no flux coating or core. Although GMAW requires a higher electrical current than shielded metal arc welding, GMAW produces fewer fumes since the electrode has no fluxing agents. However, due to the intense current levels, GMAW produces significant levels of ozone and nitrogen oxides. Flux-cored arc welding (FCAW) Used for carbon steels, low alloy steels, and stainless steels. This welding process has similarities to both shielded metal and gas metal arc welding. The consumable electrode is continuously fed from a spool and an electric arc flows be- tween the electrode and base metal. The electrode wire has a central core containing fluxing agents and additional shielding gas may be supplied externally. This welding process gener- ates a substantial amount of fumes due to the high electrical currents and the flux-cored electrode. However, FCAW gener- ates little ozone, nitric oxide, and nitrogen dioxide. Gas tungsten arc welding (GTAW) Also known as tungsten inert gas (TIG) welding, is used on metals such as aluminum, magnesium, carbon steel, stainless steel, brass, silver, and copper-nickel alloys. This technique uses a non-consumable tungsten electrode. The filler metal is fed manually and the shielding gas is supplied externally. High electrical currents are used, which causes GTAW to pro- duce significant levels of ozone, nitric oxide, and nitrogen dioxide. Submerged arc welding (SAW) Another common welding process used to weld thick plates of carbon steel and low alloy steels. In this welding process, the electric arc flows between the base metal and a consumable wire electrode. The arc, though, is not visible since it is submerged under flux material. This flux material keeps the fumes down and since the arc is not visible, there is little ozone, nitric oxide, and nitrogen dioxide generated. The major airborne hazard with SAW is the fluoride compounds generated from the flux material. Fume Generation Rates The primary source of information when determining the components likely to be in the fumes is the safety data sheet (SDS) of the consumable welding electrode/wire. About 90% to 95% of the fumes are generated from the filler metal and flux coating/core of consumable electrodes. Since the base metal weld pool is much cooler than the electrode tip, the base metal contributes only a minor amount of the total fumes. However, the base metal may be a significant factor of the fume exposure if the metal or surface residue contains a highly toxic substance (lead, cadmium). Also, the American Welding Society has conducted and published studies that provide the concentrations of selected metal constituents in the fumes produced by various welding electrodes. In addition to the welding technique, studies have shown the fume generation rate is also influenced by the fol- lowing factors. • Electrical current: In general, the fume generation rate is exponentially proportional to the current. • Arc voltage: The fume generation rate generally increases when the arc voltage increases. • Electrode diameter: The electrode diameter has a modest effect on the fume generation rate because of the differences in voltage and current. In general, a small diameter electrode generates more fumes than a large diameter electrode. • Electrode angle: The angle of the electrode to the work piece has a slight (but unpredictable) effect on the fume generation rate. • Shielding gas: In gas-shielded arc welding, the fume generation rate tends to be greater when carbon dioxide (as opposed to argon) is used as the shielding gas. • Speed of welding: As the welding rate increases, the fume generation rate obviously increases. • Steady/current pulsed current welding: Tech- nology has advanced to power sources that have pulsing capabilities. Recent studies have shown that using a puls- ing current during welding generates fewer fumes than under a steady current welding process. In general, flux-cored arc welding produces the greatest fume generation rate (for mild steel welding) followed by shielded metal, gas metal, and gas tungsten arc welding. Since most of the fumes are attributed to the welding con- sumables, there has been a drive to develop a number of "low-fume" consumables. The focus appears to be in the reformulation of the flux-cored wire to low-carbon strip mate- rials for the tube and less mineral compounds for the core. This information should be considered when performing an initial exposure assessment. The welding process and compo- sition of the material (primarily the ingredients in the elec- trode unless the steel is coated) should be the basis of categorizing similar exposure groups. High alloy materials tend to contain metals with lower occupational exposure limits (such as chromium, nickel, copper). The similar exposure groups can be further defined by the specific task, the position of the work piece (in relation to the welder's breathing zone), the presence or absence of local exhaust ventilation, and other work-related factors. WWJ SAFETY from page 32 34 August 2016 WWJ Learn Safety in NGWA Webinar Columnist Jerome Spear, CSP, CIH, will lead a one-hour webinar on January 19, 2017 titled "OSHA's New Final Rule on Crystalline Silica" at 12 p.m. ET. Another Safety Matters columnist, Gary Ganson, CSP, CIH, will lead a one-hour webinar on "How to Conduct an Incident Investigation" on November 3 from 11 a.m.-12 p.m. ET. Find out more about both sessions at www.NGWA.org/Events- Education. waterwelljournal.com Jerome E. Spear, CSP, CIH, is president of J.E. Spear Consulting and has more than 22 years of experience helping organizations prevent injuries and illnesses, control losses, and achieve regulatory compliance. He held the positions of technical services manager with XL Specialty Risk Con- sulting and corporate industrial hygiene manager for Chicago Bridge and Iron Co., a worldwide steel fabricator and construction company.