Гричин С.В., Ульянова О.В. - Английский язык для инженеров сварочного производства (1044906), страница 9
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Описывается обусловленностьвыбора различных источников электропитания при ручном иавтоматическом режимах сварки. В зависимости от выбранногоисточника, а также от полярности электрода и основного металла,показываются возможные особенности процесса сварки, что позволяетсделать правильный выбор указанных параметров для получениясварного шва.DВ тексте описаны 3 основных вида сварки с использованиемплавящегося электрода: дуговая сварка металлическим покрытым51электродом, дуговая сварка металлическим электродом в средеинертного газа и дуговая сварка под флюсом. Подробное описаниекаждой из данных разновидностей сопровождается перечислением ихпреимуществ и недостатков, пригодности использования в зависимостиот типа свариваемого материала, требований к квалификации сварщика,экономичности, скорости и других параметров.Skim the English texts (1-4)(Power Supplies, Consumable ElectrodeMethod, Non-consumable Electrode Method, Safety Issues), and matchthem and Russian texts (A-D) in the previous activity.1Power SuppliesTo supply the electrical energynecessary for arc welding processes, anumber of different power supplies can beused.
The most common classification isconstant current power supplies andconstant voltage power supplies. In arcwelding, the voltage is directly related tothe length of the arc, and the current isrelated to the amount of heat input.Fig. 1 A constant current weldingConstant current power supplies are mostpower supply capable of AC and DCoften used for manual welding processessuch as gas tungsten arc welding and shielded metal arc welding, becausethey maintain a relatively constant current even as the voltage varies. This isimportant because in manual welding, it can be difficult to hold the electrodeperfectly steady, and as a result, the arc length and thus voltage tend tofluctuate. Constant voltage power supplies hold the voltage constant and varythe current, and as a result, are most often used for automated weldingprocesses such as gas metal arc welding, flux cored arc welding, andsubmerged arc welding.
In these processes, arc length is kept constant, sinceany fluctuation in the distance between the wire and the base material isquickly rectified by a large change in current. For example, if the wire andthe base material get too close, the current will rapidly increase, which in turncauses the heat to increase and the tip of the wire to melt, returning it to itsoriginal separation distance.The type of current used in arc welding also plays an important role inwelding. Consumable electrode processes such as shielded metal arc weldingand gas metal arc welding generally use direct current, but the electrode canbe charged either positively or negatively. In welding, the positively charged52anode will have a greater heat concentration, and as a result, changing thepolarity of the electrode has an impact on weld properties. If the electrode ispositively charged, it will melt more quickly, increasing weld penetration andwelding speed.
Alternatively, a negatively charged electrode results in moreshallow welds. Non-consumable electrode processes, such as gas tungsten arcwelding, can use either type of direct current, as well as alternating current.However, with direct current, because the electrode only creates the arc anddoes not provide filler material, a positively charged electrode causes shallowwelds, while a negatively charged electrode makes deeper welds. Alternatingcurrent rapidly moves between these two, resulting in medium-penetrationwelds. One disadvantage of AC, the fact that the arc must be re-ignited afterevery zero crossing, has been addressed with the invention of special powerunits that produce a square wave pattern instead of the normal sine wave,making rapid zero crossings possible and minimizing the effects of theproblem.2Consumable ElectrodeMethodsOne of the most common types of arcwelding is shielded metal arc welding(SMAW), which is also known asmanual metal arc welding (MMA) orstick welding.
Electric current is used tostrike an arc between the base materialand consumable electrode rod, which ismade of steel and is covered with a flux that protects the weld area fromoxidation and contamination by producing CO2 gas during the weldingprocess. The electrode core itself acts as filler material, making a separatefiller unnecessary. The process is very versatile, requiring little operatortraining and inexpensive equipment.
However, weld times are rather slow,since the consumable electrodes must be frequently replaced and becauseslag, the residue from the flux, must be chipped away after welding.Furthermore, the process is generally limited to welding ferrous materials,though special electrodes have made possible the welding of cast iron, nickel,aluminum, copper, and other metals. The versatility of the method makes itpopular in a number of applications, including repair work and construction.Gas metal arc welding (GMAW), also known as metal inert gas (MIG)welding, is a semi-automatic or automatic welding process that uses acontinuous wire feed as an electrode and an inert or semi-inert gas mixture toprotect the weld from contamination. Since the electrode is continuous,53welding speeds are greater for GMAW than for SMAW.
However, becauseof the additional equipment, the process is less portable and versatile, but stilluseful for industrial applications. The process can be applied to a widevariety of metals, both ferrous and non-ferrous. A related process, flux-coredarc welding (FCAW), uses similar equipment but uses wire consisting of asteel electrode surrounding a powder fill material. This cored wire is moreexpensive than the standard solid wire and can generate fumes and/or slag,but it permits higher welding speed and greater metal penetration.Submerged arc welding (SAW) is a high-productivity welding methodin which the arc is struck beneath a covering layer of flux.
This increases arcquality, since contaminants in the atmosphere are blocked by the flux. Theslag that forms on the weld generally comes off by itself, and combined withthe use of a continuous wire feed, the weld deposition rate is high. Workingconditions are much improved over other arc welding processes, since theflux hides the arc and no smoke is produced. The process is commonly usedin industry, especially for large products.3Non-Consumable Electrode MethodsGas tungsten arc welding (GTAW), ortungsten inert gas (TIG) welding, is amanual welding process that uses a nonconsumable electrode made of tungsten,an inert or semi-inert gas mixture, and aseparate filler material.
Especially usefulfor welding thin materials, this methodis characterized by a stable arc and highquality welds, but it requires significantoperator skill and can only be accomplished at relatively low speeds. It canbe used on nearly all weldable metals, though it is most often applied tostainless steel and light metals. It is often used when quality welds areextremely important, such as in bicycle, aircraft and naval applications. Arelated process, plasma arc welding, also uses a tungsten electrode but usesplasma gas to make the arc. The arc is more concentrated than the GTAWarc, making transverse control more critical and thus generally restricting thetechnique to a mechanized process.
Because of its stable current, the methodcan be used on a wider range of material thicknesses than can the GTAWprocess, and furthermore, it is much faster. It can be applied to all of the samematerials as GTAW except magnesium, and automated welding of stainlesssteel is one important application of the process. A variation of the process is54plasma cutting, an efficient steel cutting process. Other arc welding processesinclude atomic hydrogen welding, carbon arc welding, electroslag welding,electrogas welding, and stud arc welding.4Safety IssuesWelding, without the proper precautions, can be a dangerous andunhealthy practice.
However, with the use of new technology and properprotection, the risks of injury and death associated with welding can begreatly reduced. Because many common welding procedures involve an openelectric arc or flame, the risk of burns is significant. To prevent them, welderswear protective clothing in the form of heavy leather gloves and protectivelong sleeve jackets to avoid exposure to extreme heat and flames.Additionally, the brightness of the weld area leads to a condition called arceye in which ultraviolet light causes the inflammation of the cornea and canburn the retinas of the eyes. Goggles and helmets with dark face plates areworn to prevent this exposure, and in recent years, new helmet models havebeen produced that feature a face plate that self-darkens upon exposure tohigh amounts of UV light.
To protect bystanders, transparent weldingcurtains often surround the welding area. These curtains, made of a polyvinylchloride plastic film, shield nearby workers from exposure to the UV lightfrom the electric arc, but should not be used to replace the filter glass used inhelmets.Welders are also often exposed to dangerous gases and particulatematter. Processes like flux-cored arc welding and shielded metal arc weldingproduce smoke containing particles of various types of oxides. The size ofthe particles in question tends to influence the toxicity of the fumes, withsmaller particles presenting a greater danger.
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