Fume problems during manual metal arc weld-surfacing

TWI Bulletin, January 1987

By Janet Moreton and Dick Sharples

Janet Moreton MA, is a Principal Research Chemist and Dick Sharples, BA, was a Research Metallurgist, both in the Materials Department.

Because of the high alloy content of manual metal arc hardsurfacing consumables, the resultant fume contains a higher concentration of potentially toxic components than is normally found in mild steel welding. Problems encountered in manual metal arc weldsurfacing operations are discussed below.

The application of a surface coating on to steel is often a cost effective method of improving a component to withstand wear or corrosion. Although there are many other ways of applying coatings, such as thermal spray, electroplating or vapour deposition, surfacing by welding remains the most common for wear resistance. All conventional welding (i.e. joining) processes for metals may be used for weld-surfacing, often after modifications, and these applications have been reviewed by Gregory.^[1] The present article, however, is concerned only with the manual metal arc (MMA) technique, one of the most commonly used methods for weld-surfacing because of the low cost of equipment, versatility of operation, availability of a wide range of materials and the existence of a wealth of industrial welding experience.

A qualitative summary of some of the present categories of surfacing materials is shown in Table 1 organised according to relative cost and resistance to wear and corrosion. It is apparent that surfacing consumables are generally more highly alloyed than welding consumables. Surface coatings are mostly iron-, nickel- or cobalt-based, chromium being present in most types in appreciable quantities, and there is also a range of copper based corrosion resistant materials. Depending upon the alloy, manganese, tungsten, molybdenum, copper, silicon, vanadium and niobium are also commonly present, although generally in smaller quantities than chromium.

It is the presence of significant quantities of these alloying elements which necessitates fume control measures during MMA surfacing. Table 2 gives details of electrode composition type and the corresponding application areas for the 10 MMA surfacing electrodes (A J) whose fume data are discussed. The fume problems encountered are considered against a background of considerable experience of airborne pollution derived from MMA consumables in general.

Table 2 Typical electrodes for surfacing by MMA

Quality and composition of fume

In general, both particulate fume and pollutant gases (carbon monoxide, oxides of nitrogen, ozone and organics) are produced during open arc welding. However, with the MMA process, very little pollutant gas is formed^[2] except from cellulosic electrodes, which are rarely used for surfacing; therefore MMA surfacing electrodes produce almost entirely particulate fume.

Quantity of fume produced

The amount of fume evolved during welding is known as the fume emission rate (FER). It can be measured either in mg fume per sec of arcing time, or in mg fume per g of deposited metal. Tests are made in a fume box (Fig.1)^[3] and are applicable to a range of welding processes. The fume box consists essentially of a hollow cone on a box at table height. There is an opening in the front of the box to allow insertion of the consumable and the input of replacement air. The consumable is welded in the flat (horizontal-vertical position) on a plate using the required welding conditions. The particulate fume emitted during welding is drawn up the cone by a draught created by a pump unit, and the fume is collected on a large diameter filter. Sampling is continued after arcing has ceased to ensure that the fume in the chamber is collected. The filter is weighed before and after welding, and the sampling time, metal deposit weight and welding parameters are recorded, allowing the FER to be calculated.

Figure 2 shows, in histogram form, the FER for 4mm diameter surfacing electrodes superimposed on data for typical MMA electrodes. On average, the FER is between 10 and 20 mg/g or 5 and 15 mg/sec for both the surfacing electrodes and for the general population of MMA electrodes. Therefore, it is clear that surfacing electrodes produce on average no greater quantity of fume then conventional MMA welding electrodes, but, as will be seen below, the composition of that fume is considerably more toxic.

Composition of fume produced

Welding fume is a variable mixture of substances, mostly metal oxides, whose composition depends on both the core and the coating of the consumable and, to a lesser extent, on the baseplate composition and welding conditions.

The welding and surfacing of steel produces fume consisting of the oxides of iron and alloying additions (Si, Mn, Cr, Ni, etc) derived from the electrode core, and from flux elements (Na, K, Ca, Ti, F, etc) derived from the coating. Some of these elements are said to be of 'occupational hygienic significance', in other words, the Health and Safety Executive gives guidance that airborne concentrations of these elements to which a worker is exposed in a workshop should not exceed certain limits, called 'occupational exposure limits' (OELs).^[4,5] Table 3 shows some elements (Fe, Mn, Ni, Cr, F, Co, Cu) of occupational hygiene significance (present mostly as oxides) in typical fume from surfacing electrodes. Other elements derived from the coating will be present, e.g. Na, K, Ca, Ti, but these are not important from an occupational hygiene viewpoint. Elements such as Mo, V, W, although they may be present in the consumables, are not generally transferred to the fume in more than trace amounts. Some Si may be present in the fume, but it presents no hazard because it is found as amorphous silicates. Although the composition of the fume depends on the nature of the consumables, it is important to appreciate that there is no direct relationship. For example, consumable F, which has 10%Ni in the weld metal (Table 2), produces only 1%Ni in the corresponding fume (Table 3). On the other hand, consumable J, with only a few per cent of fluoride-bearing compounds in the electrode coating, produces up to 30% fluoride in the resultant fume.

Table 3 Typical composition in fume of elements of occupational hygiene significance

Table 4 shows the value of the OELs for the elements of Table 3. The smaller the value of the OEL, the greater the potential toxicity of the substance. Some elements are quoted in Guidance Note EH/40^[4] as having more than one value for the OEL, depending on the state of chemical combination. For example 'soluble' nickel has an OEL of 0.1 mg/m³, but 'insoluble' nickel has a value of 1 mg/m³. All fume examined so far from nickel-bearing consumables contain nickel in an insoluble form.^[6] Similarly, chromium appears with several different entries in Guidance Note EH/40.^[4] The species found in welding fume are the trivalent CrIII and the hexavalent CrVI, the latter being a known carcinogen in other industries (but not proven in welding).

Table 4 Occupational exposure limits relevant to fume from typical hardfacing electrodes

Notes: *insoluble as oxides, see HSE Guidance Note EH/40

It can be seen in Table 3 that some of the fume from surfacing electrodes contain significant amounts of one or more elements with low OEL values. It is useful to make an assessment of the overall hazard of the fume by combining the effects of the various pollutants but Guidance Note EH/40^[4] does not do this, on the grounds that the different components in a mixture, e.g. Mn, Cr, Ni, Co in fume, may attack the body in different ways and that therefore their effects may not be additive. The Swedish Standard for welding fume SS 06 2801^[7] does, however, use an additive formula to give an expression TLV_f (threshold limit value for fume), which combines the proportional components of the mixture. This combined value TLV_f has been calculated for fume from all MMA electrodes for which data are available, and for fume from surfacing MMA electrodes. A histogram of the results is shown in Fig.3, and illustrates that the average value of TLV_f for typical MMA welding electrodes is about 4 mg/m.³ However, the average value for MMA surfacing electrodes is less than 2 mg/m.³ In other words, the fume from surfacing electrodes is in general, considerably more toxic than that from ordinary mild steel MMA welding electrodes, although the emission rate of all types is roughly the same (Fig.2). The higher toxicity of surfacing electrode fume is a reflection of the high alloy content of such electrodes.

Control of fume: reducing the operator's exposure

Guidance Note EH/40 refers to the airborne pollutant that the worker is breathing, averaged over an 8hr day, or a 40hr working week. The so-called welder's 'breathing zone' fume concentration depends not only on the particular electrodes he employs, but also on arcing time, welding conditions, base materials, on-site conditions and ventilation. The background fume concentration in a welding shop may be typically 1-2 mg/m³, and, in the absence of any ventilation, the welder's breathing zone concentration may be highly variable (from 1 to 100 mg/m³).^[2,8]

When welding with mild steel electrodes, the total OEL for all particulate fume is 5 mg/m³, and to achieve this fume concentration in a workshop, local extraction is necessary. When welding with surfacing consumables (which are usually highly alloyed), not only is such local extraction essential, but in some cases, respiratory protection may also be required to ensure that the calculated exposure limit of <2 mg/m³ (Fig.3) is not exceeded in the breathing zone.

Both local extraction equipment and respiratory protection apparatus are readily available commercially. Devices which are specifically applicable to the welding situation should be selected.^[2] Orinasal respirators with disposable filter cartridges may be suitable; more recently, moulded paper masks with exhalation valves have become available. Fresh air supplied helmets are popular although more expensive, but they do not need the closeness of fit which is required by orinasal masks for maximum efficiency.

Recent publicity relating to highly alloyed electrodes

The hazards of chromium in welding fume produced during hardfacing and stainless steel MMA welding received considerable attention in the UK popular press during 1985.^[9] This notoriety arose when welders and management at a firm in Sheffield were first alerted to possible problems following the death of four works cats within a short period. (A post-mortem examination of one cat showed that it had, in fact, died of peritonitis following a gastric ulcer and that there was no trace of cancer, chromium or welding fume in its lungs, although chromium was found in blood and other tissues.^[10]) Subsequently it was reported that welders who were using electrodes containing 25%Cr, 12%Ni, had high blood chromium levels.^[9] The report was followed by the successful prosecution, by the Health and Safety Executive, of two UK electrode suppliers under the Health and Safety at Work Etc Act, 1974, Section 6.^[11] The case was based on insufficient safety labelling of electrodes, and the failure of the suppliers to provide back-up safety information relating to their products.

Since the prosecution reported in January 1985, UK electrode manufacturers and suppliers, under the auspices of the Welding Manufacturers' Association (WMA), have issued a revised edition of their publication No 236, 'Hazard from welding fumes'.^[12] Also WMA has collaborated with HSE in the provision of new electrode packet labels and safety information sheets relating to particular products. Leaflet No 236 relates to welding processes, but is equally applicable to surfacing carried out by welding techniques.

All in all, the publicity surrounding the case has led to a greatly enhanced awareness of the problems of MMA hardfacing and stainless steel welding.

Summary

Although the amount of fume produced during surfacing operations with MMA electrodes is no greater in quantity than that from typical MMA welding consumables, the fume is potentially more toxic, and may contain substantial amounts of the oxides of Cr, Ni, Co, Cu and Mn, depending on the electrode composition.

Recommendations

It is recommended that care is taken to use efficient local extraction at all times, while surfacing with MMA electrodes, and respiratory protection should be considered. The consumable supplier should be consulted regarding the nature of airborne pollutants to be encountered from any particular consumable, and for recommended conditions of safe working.

References