There is a continuing trend toward the reduction of allowable worker exposures to a range of potentially harmful substances generated by arc welding processes. The substances present specifically in the welding environment depend on the composition of the consumable electrode alloy, the base metal being joined, and the presence of any surface materials or coatings on that base material. A few specific substances in welding fumes may have known or suspected health risks and these substances have appropriate occupational airborne exposure limits. Until recently, it was recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) that an exposure limit for welding fume particulate of 5 mg/m³; however, the exposure limit for a given individual substance in the fume may be many orders of magnitude lower. Therefore, there are certain substances that are of environmental health significance which can effectively drive the need to limit total welding fume exposures to very low levels in order to ensure that the exposure to these specific compounds remains below their respective applicable exposure limits. Typically, these compounds include forms of manganese, chromium, nickel, cobalt, cadmium, and copper.
It is imperative the welding community has a better understanding of what can be done to reduce the amount of these and other potentially harmful materials found in the workplace, especially because of the recent significant lowering of applicable worker exposure limits to manganese and hexavalent chromium. The challenges are significant when it comes to keeping fume exposures to very low levels. Careful selection of consumables and welding processes, as well as the proper use of ventilation and personal protective equipment is the only way to achieve and maintain low levels of exposure.
Both parts of this article are intended to demonstrate an approach to fume reduction. It is not all inclusive or reflective of every material or process selection that can be made. It is a suggested guideline to a methodology for exposure reduction in the workplace.
The Generating of Welding Fumes and Pollutant Gases
The melting and vaporization of the consumable electrode causes the solid particulate or fume produced during arc welding. About 90-95% of this fume is from the electrode while the balance is from the base material. Generally, the fume consists of fine particles predominantly of respirable size. Spherical in shape, these fume particles are comprised of fused clusters of these spheres. Although the chemistry of the fume particles is complex, the major constituents are oxides of metals and other elements and are, in general, similar to the consumable composition. Certain elements in welding fume, however, such as manganese, are enriched in the fume and found at a higher level than that of the actual consumable. Even with their complex chemistries, worker exposure limits for individual elements or pure oxides are applied to the components of welding fume for control purposes.
Gases are also generated within the arc environment, in addition to solid fume particulates. These may include carbon monoxide, carbon dioxide, various gaseous fluorides, oxides of nitrogen (primarily NO and NO2), and ozone. Carbon monoxide is formed by the decomposition of carbon dioxide, which may be present in shielding gas or as a by-product of components found in flux-bearing consumables (e.g., shielded metal arc electrodes and flux-cored wires). Gaseous fluorides are present in fumes from many types of flux-cored wires and some shielded metal arc electrodes. Generally, these fluorides result from the decomposition of solid fluoride compounds in the core or in the coating of these consumables. As a result of the thermal oxidation of nitrogen found in the arc environment, oxides of nitrogen are typically present. All these gases are usually contained within the fume plume rising from the arc during the welding or cutting.
The interaction of specific wavelengths of ultraviolet light with oxygen in or near the arc can cause ozone to be formed. Typical sources of this UV light are argon-based shielding gases and silicon from the consumable that is vaporized in the arc. Ozone generated during argon-shielded gas metal arc welding using 4000 series aluminum consumables can be present in measurable quantities.
Approaches to Reducing Welding Fumes
The following hierarchy of controls should be considered because the potential for exposure to welding fume needs to be addressed to ensure a safe work environment for the welder.
- Elimination: Even though it is not feasible in most scenarios, consideration should first be given to assessing the opportunity for deploying measures that avoid the need for arc welding. In some applications it may be feasible to use solid-state welding techniques (e.g., friction stir welding) or adhesives.
- Process Changes: It is often possible to consider utilizing another welding process that generates less overall fume per acceptable weld. Each welding process generates a range of fume levels depending upon the consumables used. Substitution of alternate process technologies, use of different welding parameters, or the change of welding materials, shielding gases, or alloys may offer a potentially effective fume reduction solution.
- Engineering Controls: In situations where options 1 and 2 are not feasible or sufficiently effective, engineering controls such as local exhaust ventilation can be deployed to collect or move fumes away from the welder’s breathing zone and the general area. It may be preferable, in cases where isolation is feasible, to provide separation between personnel and the welding fume source. This is often accomplished with enclosed automated or robotic welding.
- Personal Protective Devices/Administrative Controls: The last option is the use of personal respiratory protective equipment, safety procedures, or administrative controls such as job rotation. (Note: for some compounds such as hexavalent chromium, regulated by OSHA in the United States, job rotation may be prohibited as a control measure.)
The Reduction of Fume by Process Selection/Change
The amount of welding fume generated is characteristic of the welding process selected and is influenced by the type of shielding gas, the welding current and voltage, and the welder’s individual work practices. As a result, many variables must be evaluated as one considers ways to reduce welding fume generation. Reduction in welding fume exposure is a multistep process which requires interdependent decisions based on the needs of the specific application from a weld performance standpoint, availability of equipment needed, materials, and skilled labor, as well as the overall economics of a specific approach. A good understanding of the differences that process selection, choice of consumables within a specific process, and welding parameters utilized for a specific application can make to the fume generation rate will help guide those decisions.
General Process Considerations
Together, the shielded metal arc (SMA), gas metal arc (GMA) and flux cored arc (FCA) welding processes account for about 85-90% of the arc welding done to join carbon steels. The amount of fume generated by these processes can be relatively high. Gas tungsten arc (GTA) and submerged arc (SA) welding, by contrast, generate significantly less fume than other techniques because of the way in which material is transferred from the consumable electrode or rod to the weld pool. Fume reduction can be significant where these processes can be used.
A number of factors can determine the amount of fume generated by any arc welding process, and each can have a significant impact on fume production. Fume generation can be expressed several different ways including: as the amount of fume generated per unit of time (generally a minute), or the amount of fume generated per amount of weld metal deposited (expressed as a percentage). The conditions of comparison used will determine the value of each.
For example, it would be useful to know how much welding fume is generated in a fixed period of time (grams of fume/min of arc time) when estimating the size of a fume extraction system that might be required for a specific manufacturing space. It is possible to affect the amount of fume generated in a fixed amount of time by selecting one process over another. It may be useful, when looking at the economics of process selection, to consider the amount of fume generated per amount of weld metal required to complete a specific joint/application (g fume/weight of weld metal deposited). Fume levels may not increase linearly as the deposited rate for a process increases.
For the major arc welding processes, fume generation rates can overlap. Factors which influence the fume generation rate for a given process include welding parameters, consumable composition and diameter, and mixture of gases used for shielding (if any is used).
Specific Process Considerations
SMAW
Different electrode formulations/types lead to variations in fume generation rate. The electrode formulation can significantly affect the amount of fume generated for a fixed set of conditions since the rate greatly depends upon the magnitude of the welding current and voltage. For example, the level of voltage required for stable operation of the electrode and its resulting arc length can be affected by electrode coating formulation. In turn, this can influence the fume generation rate.
Fume generation rates and ratios of weight of fume to weight of weld metal may be somewhat higher for certain electrodes (such as E6010) than for others (such as E7018 or E7024). There is not a considerable difference in the fume composition when a variety of diameters of the same electrode type are tested. Although the amount of fume may vary with the diameter of the electrode, the composition of that fume remains relatively constant.
FCAW
The fume generation rate for gas-shielded flux-cored wires can vary considerably from one classification to another, and even within one classification, depending upon the desired performance characteristics and the required weld properties produced using that wire. Depending upon the specific formation and manufacturing practice used to produce that wire, it will also vary from one manufacturer to another of the same wire classification. Generally, fume generation increases with electrode diameter, but when a weight of fume per amount of weld metal deposited comparison is used, there is no real change of fume level with changing electrode diameter. Electrodes of the “T-5” classification type exhibit the highest fume generation rate for gas-shielded flux-cored wires. Fume generation for self-shielded flux-cored wires vary widely and are generally significantly higher than for gas-shield wires. It is generally possible, by referring to manufacturers literature for flux-cored wires, including their safety data sheet information, to select an electrode that generates less fume while still remaining suitable for a specific application and amount of deposited weld metal. If a change is made to a lower-fume product, it is then necessary to ensure that such a product meets the performance criteria for the applications in which it will be used.
GMAW
Typically, solid wires used with a wide range of shielding gases generate low levels of welding fumes regardless of the operation parameters. The composition of the fume from a particular classification of solid wire varies little from one manufacturer to another. With the composition of gases used for shielding and the type of metal transfer (short-circuiting, globular, conventional spray, pulsed spray) used with the solid wire welding process, the fume rates will vary. A higher level of fume than conventional or pulsed spray transfer will be generated with the arc ignition/reignition cycle associated with short-circuiting transfer and the large, irregular, and unstable droplets constituting globular metal transfer. Significant changes in total fume emission can occur as a result of small alterations of the voltage within the spray transfer mode.
Additional potential for the reduction of welding fume has been realized with the introduction of inverter power supply technology and its application to pulsed metal transfer. Quick detachment of the metal droplets formed at the tip of the wire electrode have been allowed by the rapid “current rise” times and high frequency of pulsing. In turn, this minimizes the time fume is generated at the wire tip, and thus decreases overall fume levels.
GTAW
Using either mild steel or stainless steel consumables, the fume generation rate for the GTAW process is very low. The temperatures to which the consumable is exposed are inherently less in GTAW as the arc is used to melt — not vaporize — the filler metal. GTAW fume generation rates are 5-15 times lower than typical levels for the SMAW, FCAW, and GMAW processes.
SAW
Submerged arc welding uses a blanket of fusible, granular mineral compounds or flux to provide shielding for the welding arc. The arc is struck between the workpiece and a bare electrode wire, the tip of which is covered by the flux. Although the arc is not visible beneath the flux, metal transfer occurs with minimal spatter and generation of fume.
Fume Reduction by Consumables Selection
If it is not present in some form in the original welding consumable, the baseplate, or the coatings/contaminants on the plate surface, the particular element or material cannot be found in welding will not be found in welding fume. They provide no simple guidance as to how much of each element is present in the fume even when the concentrations of elements in the consumable are known. Based on their accelerated vaporization rates, some elements become concentrated in the fume. Determining an applicable exposure limit for the fume generated in a specific welding application largely depends on a function of the pressure of low exposure limit [threshold limit value (TLV) or Permissible Exposure Limit (PEL)] constituents (e.g., Cr (VI), Mn, Ba) in the fume.
Plain Carbon (Mild) Steel Consumables — Focus on Manganese
The most common fume constituent possessing a low TLV (PEL is significantly higher), when joining plan carbon steels using mild steel consumables, is manganese (Mn). The factors that need to be considered when comparing different welding consumables on the basis of potential manganese exposure include: the %Mn in the consumable, the fume generation rate of the process, the % electrode converted to fume, and the total amount of arc time for a given period of time (operator efficiency factor).
It is important to consider fume generation potential when considering the welding processes that might potentially be used in a specific joining application. The lowest Mn generation rates (MnGr) tend to be produced in the fume with submerged arc welding and gas tungsten arc welding using mild steel consumables. Generally, gas metal arc welding is the next lowest, especially when high-argon-content shielding gas mixtures are used. The ranking of MnGrs can vary widely among the SMAW, FCAW, and metal cored arc welding (MCAW) processes because of the differing constituents present in the consumables used for each process type.
The typical ranges of fume generation rates for various mild steel welding processes and their manganese generation potential are determined by multiplying the typical manganese content of the fume by the average fume generation rate of that material. These are simply estimates and can be used to compare processes with respect to the amount of manganese to be expected. Flux-core wires with modified manganese levels are also included here to show how formulation changes can affect potential manganese exposure. There will be variations in fume and manganese generation rates depending upon the slag system, shielding gas composition, electrode diameter, and welding parameters.
A general rule is that cellulosic SMAW electrodes (EXX10 and EXX11) tend to be lower in manganese than low-hydrogen electrodes (EXX10 and EXX16). Generally, variations in FCAW electrode formulation are responsible for the varying fume levels produced using these electrodes.
The higher the level of carbon dioxide in the shielding gas, generally the greater amount of fume generated. For example, a weld made using 100% CO2 will produce more fume than one made using Ar/25%Co2, which will produce more fume than one using Ar/2% O2. Flux-cored electrodes are usually formulated for specific shielding gases or ranges of shielding gas compositions. it is important to ensure the products are designed to be used with those compositions if higher argon mixtures are selected.
Depending on electrode diameter and welding current, the fume and manganese generation rates will vary. A general rule is that larger electrodes and higher currents mean higher fume generation rates. The relationship is not always linear, however. In some cases there is a “sweet spot” where the fume generation rate may actually decrease at higher amperage and voltage. This is due to the fact that most consumables have a set of operating parameters that produces the greatest arc stability with optimized metal transfer.
Low-Alloy Steel Consumables — Focus on Manganese and Hexavalent Chromium
Low-alloy steels usually contain higher levels of manganese and other alloying elements that may have low exposure limits. Hexavalent chromium (Cr (VI)) is the most common of these. All the recommended methods discussed for manganese reduction when using plain carbon steel consumables work similarly for low-alloy consumables.
Although the manganese level of the fume is directly related to the amount of manganese in the electrode/rod, the amount of hexavalent chromium in the fume is a result of a more complex relationship between chromium and the other constituents present in the electrode. While hexavalent chromium is not present in the electrode, it is created in the arc environment as chromium reacts with other fume constituents. With the amount of chromium present in the weld, the percentage of hexavalent chromium in the fume increases. The specific percentage of Cr (VI) in the fume will also be a function of the type of electrode/process used.
Stainless Steel Applications and Consumables — Focus on Hexavalent Chromium and Manganese
The percentage of hexavalent chromium in the fume increases as the amount of chromium increases in a stainless steel weld. Processes with flux constituents will generate a higher level of hexavalent chromium in the fume than nonflux processes. This means for a given amount of chromium in the fume, a higher percentage of that chromium is present in hexavalent form with SMAW and FCAW than with gas-shielded GMAW and GTAW processes.
It is also important to consider the fume generation rate of each electrode type, as potential hexavalent chromium exposure from the fume is also a function of this rate. The SMAW and FCAW processes are most subject to these variations in Cr (VI) production.
High Alloyed Applications and Consumables, including Ni Grades and Hardfacing Types
Chromium, nickel, and manganese are major fume constituents of concern. Guidelines similar to those indicated previously for manganese and for chromium fume control will pertain here as well.
This is just part one of a guide to reduce welding fumes. Stay tuned for the part 2 which will discuss other material factors that influence fume generation, the use of engineer controls, and other methods for reducing the exposure to contaminants. For more information feel free to contact us at BWM Services with the link below!
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