Abstract: Welding fume has become a major occupational disease in the 21st century, which seriously jeopardizes the health of welding workers and has a huge impact on the environment. All in all, solving welding fumes has become the key to the development of welding technology. This paper briefly introduces the influencing factors of welding fume from two aspects of welding materials and welding process, and introduces some major welding fume purification methods at this stage.
Welding fume is generated by the condensation of high-temperature steam generated during the welding process. The welding fumes mainly come from the liquid metal and slag at the end of the electrode or wire. The amount of dust generated by the welding material accounts for 80-90% of the total amount of welding fumes, and only part of it comes from the base metal. Scientific research and health surveys show that there are a large number of inhalable substances (such as manganese oxide, hexavalent chromium, and potassium and sodium oxides) in welding fumes. Once these substances enter the human body, they will cause great harm to the human body. Welding operators who work in welding fume environments for a long time have a significantly higher proportion of respiratory diseases such as chronic bronchitis than others, and inhalable substances are deposited in the bones and blood of the human body, causing decreased sexual ability and even cancer. In particular, in recent years, with the use of a large number of welding materials containing chemical components such as Cr, Mn, Ni, etc., the influence of welding fumes on occupational health and environmental load has become increasingly serious.
International research on welding hygiene began in 1920 with a focus on the effects of harmful nitrogen oxides. From the 20th to the 60th of the 20th century, the harmful effects of welding fume were studied. In the 1970s, there were mainly all-dust determination of welding rods, recommendations for welding hygiene standards and the establishment of welding rod sanitary signs; research on the mechanism and influencing factors of welding fume.
I. Influencing factors of welding fume There are many influencing factors of welding fume. The main factors include welding materials and process: the material refers to the composition of the electrode coating, the welding steel strip, the chemical composition of the powder, and the protective gas composition; Process refers to the selection of welding methods and the setting of process parameters. NJ HUDSON et al. pointed out that welding fumes mainly composed of mixed metal oxides and spinel structures generated during low-alloy MIG welding are mainly composed of electrode materials, alloy materials to be welded, process control, protective gas components, and voltage. And the current value is determined.
1. Welding material The effect of welding materials on welding fume is mainly reflected in the following two aspects:
(1) Welding materials are the source of welding fumes. The composition of welding materials directly affects the amount of welding fumes and the chemical composition of welding fumes.
(2) The welding material affects the welding arc physics, and the content of the metal vapor generated during the welding process is controlled by changing the droplet transfer mode.
Shi Yuxiang et al. found that the welding fumes of single-component and low-component welding rods found that marble, fluorite and other materials produced high smoke, and the smoke of the composition of the welding rod was also high; the slag and other substances themselves produced low smoke, and their composition The smoke of the welding rod is also low; the material itself such as rhombohedral soil has higher smoke, but it is not necessarily high when it is composed of the electrode. The coating material not only affects the welding fumes, but also has a complicated relationship with each other. It is possible to achieve the purpose of dust reduction by changing the proportion of the coating components.
Zimmer and Biswas studied the aerosols produced during gas metal argon arc welding and found that the alloy composition to be welded had a significant effect on the size distribution of the welding soot particles and the morphology and chemical structure of the welding fumes. In addition, the distribution of weld soot particle size is polymorphic and aerodynamic changes occur over time.
Xiao Lin and others found through experiments that the amount of dust generated by CO2-protected flux-cored wire is proportional to the carbon content of the steel strip used for the wire, such as reducing the carbon content of the steel strip from the usual 0.08% to less than 0.045% (preferably 0.02). %), the amount of dust generated can be reduced by about 30%, basically the same as the dust rate of the solid wire. For flux-cored wire, if the carbon content in the steel strip and lubricant is reduced, the dust generation rate can be greatly reduced.
JHDennis et al. found that the addition of 1% Zn to the welding material used in the FCAW welding process significantly reduced the formation of Cr6+ and ozone.
Wang Zhihui of Beijing University of Technology and others studied the effect of the amount of iron powder added on the dust emission rate of the wire. It was found that the iron oxide in the dust accounted for about 50%. When the iron content in the flux-cored wire is 19.5% to 28.7% (mass fraction), the amount of dust generated by the wire is 5.5226 to 8.2326 g; the proportion of iron and manganese oxides in the dust is large. Under the test conditions, the smaller the proportion of Fe powder in the core, the less the amount of dust generated per unit mass of the wire; the iron powder produced by different manufacturers and different processes has no obvious influence on the amount of dust generated. In addition, they also made an environmental load impact assessment on the electrode core and some auxiliary materials. Through data acquisition, environmental impact factor determination, and results analysis, the environmental load of the welding wire and the welding tape was found to be large, and the environmental load of the mineral powder production process was higher. small.
2. Process factors Welding parameters affect the dust generation rate of welding fumes. The choice of different welding methods, changes in polarity, changes in current and voltage, and wire feed speed all have an effect on the amount of dust and the chemical composition of the welding fumes.
When studying the welding fumes generated during the Al-Mg alloy DC and AC pulse inert gas shielded welding, it was found that the use of AC pulse inert gas shielded welding can substantially suppress the generation of soot.
In the study of the influence of arc voltage and wire feeding speed on the dust emission of welding wire and metal core welding material in GMAW, it is found that for the solid wire of f 1.4mm in the case of DC positive connection, the heat input exceeds the critical value. When the value is 9.5kW, the incidence of soot is a minimum; in the case of DC positive connection, the solid wire and the metal core wire have the same welding fume incidence rate, but the metal cored wire obtains the lowest soot incidence rate and the most suitable arc stability adjustment. The range is large. For DC reverse welding, for a metal cored wire, a higher wire feed speed can be used to obtain a satisfactory amount of welding dust.
Hewitt et al. found through a series of tests that during the FCAW welding process, when the DC reverse connection is used and the voltage is 40V, the amount of dust is the largest; when the voltage is 29V, the DC is positively connected and 8% of the flux is used, and the dust emission is reduced. It is 1/3 of the original.
When studying the dust dusting rate of CO2 gas shielded welding of stainless steel flux-cored wire and the concentration of Cr and Cr 6+, it was found that there is a clear relationship between the total amount of soot and the input energy. Under the three heat input conditions of low, best and high, the corresponding dust generation amounts are 189-344 mg/min, 389-698 mg/min and 682-1157 mg/min, respectively. The Cr production rate is 3.83~8.27 mg/ Min, 12.75～37.25 mg/min, 38.79～76.46 mg/min; Cr 6+ production rates were 0.46～2.89 mg/min, 0.76～6.28 mg/min and 1.70～11.21 mg/min, respectively. The incidence of soot increased exponentially with the increase of heat input, and increased exponentially with the change of 1.75 with the increase of heat input. With the increase of heat input, the concentration of Cr in soot increased from 1.57% to 2.65% to 5.45%~8.13. %, the concentration of Cr 6+ increases from 0.15% to 1.08%, and the dissolved portion of Cr 6+ produced by FCAW is equivalent to 80% to 90% of all Cr 6+ .
Second, welding fume purification and ventilation Ventilation system mainly includes suction and blow ventilation system, local ventilation system, global ventilation system and individual protective mask. The selection principle of the ventilation system is portability, mobility and practicality. In foreign countries, welders use a large number of movable local ventilation devices. Foreign countries strongly advocate the principle of ventilation system, which is supplemented by comprehensive ventilation and local ventilation. Various welding soot collection and purification devices have been developed in the direction of completeness, combination, mobility, miniaturization and resource conservation, and have achieved good results. In particular, various movable welding fume purification devices are widely available in various countries. use.
1. Suction ventilation The blow-up ventilation is a ventilation device that effectively controls the spread of pollution sources. In the welding of the slabs, the suction hood is simply used to eliminate the large-area welding fumes, and there are problems such as unsatisfactory results and unreasonable methods. Therefore, the blowing ventilating device is used for the slab welding fume control.
The test found that the suction-type ventilation device was used in the welding of the slab, so that the welding dust emitted during the operation was effectively controlled. The average concentration of soot in the working belt near the slab was in compliance with the national sanitary standard (less than 6mg/m3). ), and because of the small amount of air required, it can reduce power consumption and save initial investment and operating costs of equipment. Therefore, compared with the suction hood, it is not only more reasonable and effective, but also has certain advanced nature. At the same time, it provides a reference method for the treatment of large-scale welding smoke pollution.
2. The pressure-ejection type local ventilation device is welded in a closed container, and the welding fume is continuously condensed, and the concentration is continuously increased, sometimes up to 800 mg/m3, which seriously jeopardizes the health of the welding worker. Partial ventilation of the closed container welding operation is necessary to improve the working environment. Closed containers generally have only 1 to 2 orifices, and the orifice area is small. Some closed containers have complex internal structures and small spaces. At the same time, in order to control welding fumes, ventilation facilities are required to be placed near the welding points and can be moved at any time. This requires local ventilation facilities not only to have good ventilation, but also to be light and simple.
The pressure-ejection type local ventilation device is mainly composed of an ejector, a tape wind cylinder and a magnetic fixed support. As an effective device for the purification of welding dust in closed containers, the advantages of pressure-exposed local ventilation devices are reflected in the following aspects compared with the dust removal unit:
(1) Safe and reliable.
(2) Small size, light weight and easy to carry.
(3) The effective range for controlling the spread of welding fumes is large, and it is not necessary to frequently move the vents.
(4) Dust-laden air is discharged to the outside of the closed container and further treated by the workshop ventilation facility. But there are also some deficiencies that generate noise and require a compressed air source.
3. The wind curtain set dust fan wind curtain set smoke dust fan uses its unique structure to create a reasonable flow field, that is, short-circuit flow field, effectively control and capture welding smoke, increase the effective suction of smoking dust, and remove the traditional solid used. Vacuum hood.
The technology has established a mathematical model of "short-circuit flow field" theoretically through the development of various types of prototypes and multiple smoke tests and a large number of test analyses. The actual soot test has verified its good smoke dust collection effect. Both the actual measurement and the theoretical calculations show that the short-flow field is effective in controlling the dust diffusion, and the effective suction stroke of the suction port is 2 to 3 times higher than that of the conventional method, and the dust collection rate is over 95%.
III. Conclusion Welding fume has made the hazards of occupational health and the deterioration of the ecological environment deepen. The following suggestions are proposed for solving the problem of welding fumes:
(1) Strengthen the research on the mechanism and influencing factors of welding flux of various flux-cored welding wires to make up for the shortcomings of the existing research.
(2) Continue research and development of low-dust, low-toxic welding materials, control the source of welding fumes, and achieve the goal of minimizing damage.
(3) Standardize welding practice specifications to eliminate human factors affecting welding fumes.
(4) Improve the level of mechanization automation of welding, speed up the research of new welding methods, and promote the use of welding robots. (20060716)
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