THE INDIAN INSTITUTE OF WELDING - MUMBAI
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Health, safety and accident prevention Health risks from fume and gases during welding Factors affecting composition and quantity of fume and gases The quantity and composition of welding fume and gases are influenced by the following: type of process welding consumable presence of any material coatings nature of MMA flux coating, or core of FCA consumable welding parameters shielding gas composition The two most important are the welding process and the choice of consumable. Welding processes Gas welding Gas welding fume contains pollutants formed by combustion of the fuel gas. When an oxidising flame is used, these will be carbon dioxide with oxides of nitrogen but, for a slightly reducing flame, carbon monoxide will also be present. Safe practice and accident avoidance Providing there is good ventilation, the levels of fume and gases generated when welding mild steel are normally well below the occupational exposure limits (OELs). No special precauations are required. Manual metal arc (MMA) An MMA rod has a suitable composition for the weld metal but the flux covering provides gases for the arc, additional alloying elements and slag for protecting the weld pool. The composition and quantity of the fume generated will depend on the process variables, for example, type of consumable, polarity, voltage and current. The toxic elements in the fume will be similar to those in the consumable although the proportions are likely to be different. Safe practice and accident avoidance For normal welding operations, fume exposure will generally be over the welding fume exposure limit of 5 mg/m3. As stainless steel fume, in particular, causes respiratory tract sensitisation (the welder becomes more susceptible to occupational asthma), special precautions should be taken to control exposure. Local fume extraction should be used to remove fume at source. Flux cored arc (FCA) Continuously fed wire in self-shielded FCA welding contains a flux which produces a large amount of gas for the arc and protection/deoxidisation of the weld pool. Self-shielded wires are normally used for welding outdoors. In gas-shielded wires, which are only used in the welding shop, an additional shield (CO2 or argon-CO2) is needed to protect the weld pool. As FCA is normally employed at high welding current levels and higher duty cycle, more fume will be produced compared with MMA. Safe practice and accident avoidance Providing sensible precautions are taken, self-shielded FCA welding taking place outside will not need any fume removal measures. For gas-shielded welding inside a building, similar precautions to MMA welding should be used, such as local fume extraction. Metal inert gas (MIG/MAG) MIG uses a solid wire and a separate gas to form the arc and shield the weld pool. The shielding gas is normally CO2 or a mixture of argon-O2-CO2 and argon can be partly replaced with helium. As well as the effect of the welding parameters, the mode of metal transfer has a significant effect on fume levels. Dip transfer mode operates at a low welding current level and has a characteristic short arc length. Fume levels are low. However, spray transfer mode which operates at much higher current levels and at a greater arc length generates higher fume levels. Pulsed transfer mode operates at similar low current levels to dip transfer but with a longer arc length and generates intermediate fume levels depending upon the welding parameters. Special mention must be made of the presence of ozone which is generated by the effect of ultraviolet radiation on the air immediately surrounding the gas shield as shown in the diagram. Welding of stainless steel and aluminium, in particular, can produce a significant level of ozone and exposures may exceed the recommended OEL. Safe practice and accident avoidance Use local fume extraction equipment to remove fume at source. As ozone can be generated away from the arc, additional general ventilation may be needed. If the fume is not adequately controlled by these methods, the welder must wear Respiratory Protective Equipment (RPE). Tungsten inert gas (TIG) and plasma The arc is formed between the tungsten electrode and the workpiece with an inert gas. As the filler metal is added directly to the weld pool, metal does not pass through the arc so there is considerably less particulate fume emission. In open workshop conditions, exposure to particulate fume will normally be below the OEL of 5 mg/m3. Welding of stainless steel and aluminium may generate unacceptable levels of ozone. Ozone is generated by the effect of ultraviolet radiation on the air immediately surrounding the gas shield. Safe practice and accident avoidance Good ventilation does not require fume extraction but when welding stainless steel and aluminium, local extraction to control ozone may be needed. Submerged arc Submerged arc welding uses a bed of granulated flux to cover the arc. As the arc is not exposed, there is negligible emission of fume and gaseous pollutants. Safe practice and accident avoidance No special precautions are needed but it should be remembered that dust may be produced when filling the hopper with flux. Exposure to welding fume and gases The effect of process, consumable and parent metal composition on exposure assessment is summarised in the Table. The Indication of fume levels assumes that no control systems are being used. The generic term Occupational Exposure Limit (OEL) is used and may apply to one or all of the individual constituents of the fume. The Assessment indicators summarise information published in EH 54 'Assessment of exposure to fume from welding and allied processes.' The Welding Manufacturers Association has produced a standard format for hazard data sheets to enable manufacturers to comply with their legal obligations under the Health and Safety at Work Act 1974 Section 6. The hazard sheet contains the following information: chemical analysis of substances present in the fume produced by the consumable the appropriate OELs For safe use of consumables, reference should always be made to the manufacturers' or suppliers' hazard data sheets. It should be noted that in addition to particulate fume, where appropriate, risk assessment must also include composition of shielding gases which may be toxic or asphyxiant. Gases are also produced by the action of heat on the welding flux or slag (carbon dioxide and carbon monoxide) and heat or ultraviolet radiation on the atmosphere surrounding the welding arc (nitric oxide, nitrogen dioxide and ozone). Effect of process, consumable and parent metal composition on the assessment of exposure Process Material Indication of fume level Nature of fume - assessment indicators Gas welding Mild steel Below OELSs except in poorly ventilated or confined space Carbon dioxide, carbon monoxide (reducing flame) nitrogen dioxide Manual metal arc (MMA) Mild steel and low alloy steels Stainless steel Aluminium, copper, nickel alloys, cast iron, hardfacing 3 mg/m3 -30 mg/m3 in open workshop conditions, depends on operator variables 3 mg/m3-30 mg/m3 in open workshop Generally in excess of OELs Iron oxide flux particulates eg fluorides Consumable particulates, may contain up to 8% chromium, the majority present as hexavalent chromium (CrVI) Consumable particulates, ozone in aluminium welding TIG and plasma arc Mild and low alloy steels Stainless steel alloys Less than OELs Less than OELs for particulate fume, greater than OELs for gases Particulate fume from consumable - shielding gas can constitute a hazard in confined spaces by reducing the available oxygen to a level which will not support life Ozone, shielding gases in confined spaces Metal inert gas/metal active gas (MIG/MAG Mild and low alloy steels Stainless steel Aluminium and aluminium alloys Generally in excess of 5 mg/m3 Greater than OELs Greater than OELs for gases and fume Particulate iron oxide, ozone, shield gases. High fume levels are obtained with CO2 shielding than with argon Particulate fume from consumable, ozone and shield gases. Process related values will apply to particulate fume Particulate aluminium oxide fume. Ozone levels can be very high particularly with aluminium/silicon alloys some distance from the arc Flux-cored arc Mild and low alloy steels Stainless steel Greater than 10 mg/m3 Greater than 10 mg/m3 Particulate iron oxide and flux materials, some consumables may give rise to soluble barium in fume Consumable and flux particulates, chromium VI likely to be present Publications and relevant standards EH 40 Occupation Exposure Limits, HSE Books. EH 54 Assessment of exposure to fume from welding and allied processes, HSE Books, 1990. EH 55 The control of exposure to fume from welding, brazing and similar processes, HSE Books, 1990. Welding Manufacturers Association, Leaflet 236, Hazards from welding fume. This article was prepared by Bill Lucas, E-mail: wlucas@twi.co.uk in collaboration with Roger Sykes, Health & Safety Executive. Further information and technical data on fume limits can be obtained from Graham Carter at TWI, E-mail: gjcarter@twi.co.uk Note: When assessing fume risk, the suppliers' data sheets and hazard information must be fully consulted. Copyright by TWI, 1999 About the IIW / Mumbai Branch / Other Branches / Coming Events / Technical Lectures Mumbai Weldnet / Trends in welding / Related Websites / IIW Forum / Feedback / Home