REDUCTION OF HEXAVALENT CHROMIUM ^ CONCENTRATION IN FUMES FROM METAL CORED ARC WELDING BY ADDITION OF REACTIVE METALS

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1 0003-^87 (95)C Am. txxuf. Hjf-, VoL 40, No. 3, pp Hjevier Science Ltd Copyright C 1»6 BOHS. All nj u raemd Piuitcd in Grctt Brltsin /96 SI REDUCTION OF HEXAVALENT CHROMIUM ^ CONCENTRATION IN FUMES FROM METAL CORED ARC WELDING BY ADDITION OF REACTIVE METALS J. H. Dennis, M. J. French, P. J. Hewitt, S. B. Mortazavi and C. A. J. Redding Department of Environmental Science, University of Bradford, Bradford BD7 1DP, U.K. (Received 23 June 1995) Abstract Previous work has demonstrated that both the mass and composition of fumes produced during metal arc welding can be influenced by changes in the welding wire composition, the flux or gas shielding used and by changes in the process parameters, including voltage, adopted. The present paper describes modifications directed at reducing the concentration of hexavalent chromium [Cr(VI)] in welding fume by the addition of active metals zinc, magnesium and aluminium to metal cored arc welding wires containing 10% Cr. There were marked changes in both the Cr(VT) concentration in the fume and the fume formation rate and hence in the CrfVT) formation rate over the range of voltages used (18-24 V). Fume from wires containing the addition of 1 % zinc contained Cr(VI) concentrations in the fume below those in the control and in wires with 1% magnesium and wires with 1% aluminium additions. Also, at 18 V, the Cr(VI) formation rate was at a minimum compared to the other wires. This advantage was not sustained as the voltage was increased and above 21 V the CrfVT) formation rate for all the three wires containing active metal additions was higher than the control. These results demonstrate that at 18 V a significant reduction of Cr(VI) in welding fume can be produced by the addition of 1% zinc to the welding wire. Copyright 1996 BOHS. Published by Elsevier Science Ltd. INTRODUCTION Concerns with the potential health effects of Cr(VI) emissions during metal arc welding have prompted investigations of methods for reducing the concentration of Cr(VI) to which welders may be exposed, for example, during the welding of stainless steel. Both the fume formation rate (FFR) and the fume composition in metal arc welding processes can be influenced by a number of factors, and changes can be effected by process modification (Kobayashi et al., 1976; Tandon et al., 1983; Heile and Hill, 1975; Hewitt, 1994). Hewitt and Madden (1986) studying metal inert gas welding (MIG) demonstrated that Cr(VI) is produced by oxidation involving ozone produced during welding and that the composition of the shielding gas can influence the amount of Cr(VI) produced. Lunau (1967) drew attention to the considerable reduction in ozone concentrations during welding aluminium containing 5% magnesium compared with welding aluminium containing 5% silicon. On the basis of previous work and on preliminary studies in our laboratories, an investigation into the effects of introducing reactive metals which might reduce either ozone or Cr(VI) was undertaken. For the studies reported here a series of experimental metal cored welding wires was produced, consisting of a mild steel sheath with 10% Cr metal in the core with reactive metal additions. These wires were then used to weld under controlled laboratory conditions, with a shielding gas and a 339

2 340 J. H. Dennis et al. series of voltages typically used for such welding, such that the total fume produced and the Cr(VI) concentration could be determined. MATERIALS AND METHODS Rolls of four experimental metal cored wires, 1.6 mm dia., were produced by ESAB U.K. to allow continuous gas shielded welding in the purpose built equipment described below. The wire sheath was mainly iron with 1-2% manganese and a core containing iron powder and 10% Cr metal. The control wire contained no additional reactive metals while each of the three other wires contained additions of 1 % zinc, magnesium or aluminium, replacing some of the iron powder in the core. The shielding gas used was Argoshield 20 (BOC Gases Ltd), containing 78% argon, 20% carbon dioxide and 2% oxygen. The equipment and methods used to generate, collect and analyse fumes were essentially similar to those described by Hewitt and Hirst (1993) (Fig. 1). Welding took place onto a variable-speed rotating base plate above which a welding torch (BOC MG5/MXA503) wasfixedso that a uniform weld of high quality could be achieved. The base plate composition was mild steel. The height of the torch above the base plate could be adjusted by a screw to which the torch holder was attached. The welding generator used was a constant voltage MIG rectifier (BOC SMR500) with a BOC TF2.0S wire feeder. This allowed wire from a roll to be fed at a chosen speed between 1 and 18 m min~' (which for a typical 1.2 mm dia. wire corresponds to a consumption rate of approximately g min~'). Shielding gases of chosen concentration and flow rate could be used. The equipment has been developed in order to allow accurate monitoring of all important welding parameters and to allow electronically controlled automatic welding to be carried out under controlled conditions. Adjustable torch holder \ Welding torch To centrifugal pump Filter holder Fume chamber Access hole Fig. 1. Equipment used for generation and collection of welding fume.

3 Reduction of Cr(VT) concentration in fume 341 The welding torch and base plate were in a large conical fume box of similar construction to that recommended by the American Welding Society (AWS, 1979) in which an extractor fan drew the welding fumes to the top of the chamber where they could be collected on a Whatman GFA glass fibre filter. Welding typically lasted s and the extractor fan was left operating for a further period of about 90 s so that the fume collection efficiency was greater than 95%. Direct weighing of the GFA filter before and after welding allowed the calculation of fume formation rate (FFR) expressed in grammes per unit time. The fume generation coefficient (FGC), the amount of fume generated per unit mass of welding wire, could then be calculated. Fume samples were fused with potassium hydrogen sulphate and analysed using atomic absorption spectrometry. Cr(VI) concentrations were determined by extracting the fume with 2% sodium hydroxide-3% sodium carbonate, which has been demonstrated efficiently to separate Cr(VI) from other Cr compounds (Pedersen et al, 1987) followed by atomic absorption spectrometry. The collected fume was analysed as required for metal constituents including Cr(VI). The results for a particular experiment were expressed in terms of percentage Cr(VI) in fume or, from a knowledge of the FFR, as the Cr(VI) formation rate. RESULTS AND DISCUSSION The mass of fume produced and its composition is known to be strongly influenced not only by the welding wire composition but by the variables, including voltage, chosen to carry out the weld. Gray and Hewitt (1982) demonstrated that changes in voltage in MIG welding over the range V could result in the fume formation rates changing by a factor of 5. Such changes with voltage are not monotomic since voltage is a significant factor influencing the mode of metal transfer. At 12 V, dip transfer is the dominant mode resulting in low fume formation. Progressive increase in the voltage over the range used in the experiments described in this present paper, results in the metal transfer becoming predominantly of the globular mode, whereby droplets grow at the tip of the welding wire and become detached before falling into the welding pool. The rise in fume formation with increasing voltage over this range is steep and can be influenced by the ionization potential of the various metals produced in the welding arc and by shielding gas and other factors. Evaporation of elements from the tip of the wire according to their respective vapour pressures occurs, but further major contributions to fume result from the ejection of spatter fractions from the electrode tip which are subsequently oxidized to form fume in general, a more stable arc produces less spatter. In addition, the lowest voltage which can be used to produce a good weld will also yield the minimum fume. Figure 2 shows that for all the experimental metal cored wires used in this present study there was an increase in fume formation rate with voltage in the range V. The 1% aluminium wire gave the lowest fume formation rate over the voltage range tested. The respective boiling points for the active metals are as follows: aluminium 2467 C, magnesium i090 C and zinc 907 C (CRC, 1976). The presence of 1% aluminium, with the highest boiling point in the series, produces the lowest fume

4 342 J. H. Dennis et al Fig. 2. Fume formation rate against voltage for metal cored wires containing reactive metals = 0.20 u \ \ B ** o ^ D \ IT 1 0 _ ^ l%mg l%zn 0 Control < ^ D Volts Fig. 3. Cr(VT) concentration in fume against voltage for metal cored wires containing reactive metals.

5 Reduction of Cr(VT) concentration in fume a l%mg a l%zn O \%A\ 0 Control Fig. 4. Cr(VT) formation rate against voltage for metal cored wires containing reactive metals. formation rate as expected. Aluminium also has the lowest ionization energy hence one would expect a more stable arc and a lower fume formation rate. Although the presence of zinc produces a high fume formation rate, especially as the voltage increases, its value as a reducing agent is apparent in Fig. 3 in which the mass of Cr(VI) is plotted against voltage. Over the voltage range tested, the Cr(VI) concentration in the fume produced from the 1% zinc wire is lower than that produced by the other wires and by the control. Combining Cr(VI) concentration and fume formation rate results allows the Cr(VI) formation rate to be plotted against the voltage (Fig. 4). From this it is clear that the lowest fume formation rate is produced at 18 V using the 1% zinc wire. However, because the fume formation rate of the 1 % zinc wire rapidly increases with voltage, its Cr(VI) formation rate rises above that of the 1 % aluminium wire and the control at higher voltages. The 1 % magnesium wire produced the highest Cr(VI) formation rate over the entire voltage range tested. In summary, the addition of 1 % of reactive metals to welding wires has been shown to influence both the mass of fume produced and the Cr(VI) concentration. Published reports of previous investigations in our laboratories, have drawn attention to the complex nature of fume formation in welding and have proposed mechanisms for formation of Cr(VI) in welding fumes. The presence of reactive metals can influence metal transfer mechanisms across the welding arc with the consequential effects on arc stability and fume formation. On this basis the increased fume formation rate resulting from the addition of 1% zinc to the wires can be explained. These studies have however pointed to significant advantages of zinc 24

6 344 J. H. Dennis et at. additions in reducing Cr(VI) concentration in the fume which merits further investigation. In particular, the percentage zinc chosen, the shielding gas composition and other parameters may be optimized to produce the minimum Cr(VI) emissions compatible with a good quality weld. REFERENCES AWS (1979) Laboratory methods for measuring fume generation rates and total fume emissions of welding and allied processes. Fl American Welding Society, Miami, Florida CRC (1977) Handbook of Chemistry and Physics (57th Edn). Chemical Rubber Company, Cleveland, Ohio. Gray, C. N. and Hewitt, P. J. (1982) Control of particulate emission* from electric arc welding by process modification. Ann. occup. Hyg. 25, Heile, R. F. and Hill, D. C. (1975) Particulate fume generation in arc welding processes. Weld. J. Res. Suppl. 54, Hewitt, P. J. (1994) Reducing fume emissions through process parameter selections. Occupational Hygiene 1, Hewitt, P. J. and Hirst, A. A. (1993) A systems approach to the control of welding fumes at source. Ann. occup. Hyg. 37, Hewitt, P. J. and Madden, M. G. (1986) Welding process parameters and hexavalent chromium in MIG fume. Ann. occup. Hyg. 30, Kobayashi, M., MaJri, S. and Ohe, I. (1976) Factors affecting the amount of fume generated in MMA welding. IIW Doc. VHI International Institute of Welding Publication, Abington Hall, Cambridge. Lunau, F. W. (1967) Ozone in arc welding. Ann. occup. Hyg. 10, Pedersen, B., Thomson, E. and Stem, R. M. (1987) Some problems in sampling analysis and evaluation of welding fumes containing Cr(VI). Ann. occup. Hyg. 31, Tandon, R. K., Crip, P. T, Ellis, J. and Baker, R. S. (1983) Variation in the chemical composition and generation rates of fumes from stainless steel electrodes under different AC welding conditions. Aust. Weld. J. Autumn,