Exothermic welding , also known as exothermic bond , thermit welding ( TW ), and thermit welding , is a welding process that uses molten metal to permanently join the conductor. This process uses an exothermic reaction of the thermite composition to heat the metal, and does not require a source of heat or external currents. The chemical reaction that produces heat is an aluminothermic reaction between the aluminum powder and the metal oxide.
Video Exothermic welding
Ikhtisar
Dalam pengelasan eksotermik, debu aluminium mengurangi oksida dari logam lain, paling sering oksida besi, karena aluminium sangat reaktif. Besi (III) oksida umumnya digunakan:
Its products are aluminum oxide, iron free element, and large amount of heat. The reactants are generally powdered and mixed with the binder to keep the solids and prevent the separation.
Generally the reacting composition is five parts of a red iron oxide powder (rust) and three parts of heavy aluminum powder, which is ignited at high temperatures. Exothermic (heat-generating) reactions occur through reduction and oxidation resulting in a healty mass of white iron and an insufferable aluminum slag slag. Liquid iron is the actual welding material; aluminum oxide is much denser than liquid iron and floats to the top of the reaction, so welding installation should take into consideration that the molten metal is actually located at the bottom of the container and covered by a floating slag.
Other metal oxides can be used, such as chromium oxide, to produce the metal given in its elemental form. Copper thermite, using copper oxide, is used to create electrical connections:
Thermit welding is widely used to weld railroads. One of the first rails to evaluate the use of thermite welding was the Delaware and Hudson Railroad in the United States in 1935 The weld quality of a pure chemical thermite was low due to low heat penetration into the joining metal and very low carbon and alloys. the contents in a pure iron melting. To obtain a sound train track, the thermite-welded rail end is preheated with a torch to orange heat, to ensure that the melted steels are not cool during casting. Since the thermite reaction produces relatively pure iron, not a much stronger steel, some pellets or rods of high-alloy carbon metal are included in the thermite mixture; this alloy material melts from the heat of the thermite reaction and mixes with the weld metal. The composition of the alloy beads will vary, according to the welded rail alloy.
The reaction reaches a very high temperature, depending on the metal oxide used. The reactants are usually given in powder form, with reactions triggered using a spark from a lighter flame. The activation energy for this reaction is very high, and initiation requires either the use of "reinforcing" materials such as magnesium metal powders or extremely hot flame sources. The resulting aluminum oxide slag is removed.
When welding a copper conductor, this process uses a semi-permanent crucial graphite mold, in which the liquid copper, produced by the reaction, flows through the mold and above and around the conductor to be welded, forming a conductive electric weld between them. When the copper cools, the mold is damaged or left in place. Alternatively, graphite crucibles can be used. The advantages of this crucibles include portability, lower cost (because it can be reused), and flexibility, especially in field applications.
Maps Exothermic welding
Properties
Exothermic welds have higher mechanical strength than other forms of welding, and excellent corrosion resistance. It is also very stable when exposed to repeated short circuit pulses, and does not experience an increase in electrical resistance during the installation. However, the process is relatively expensive compared to other welding processes, requiring replaceable mold supplies, suffering from lack of repetition, and may be hampered by wet or bad weather conditions (when performed outdoors).
Apps
Exothermic welding is commonly used for welding of copper conductors but suitable for welding of various metals, including stainless steels, cast iron, ordinary steel, brass, bronze, and Monel. This is very useful for joining different metals. This process is marketed under various names such as APLIWELD (in tablet form), American Rail Weld, ULTRASHOT Harger, ERICO CADWELD, Quikweld, Tectoweld, Ultraweld, Techweld, TerraWeld, Thermoweld, Ardo Weld, AmiableWeld, AIWeld, FurseWeld and Kumwell.
Due to good electrical conductivity and high stability in the face of short-circuit pulses, exothermic welding is one of the options specified by the United States National Electrical Code Ã,§250.7 for grounding conductors and bonding jumper. This is the preferred method of bonding, and indeed it is the only acceptable way to bind copper to a galvanic cable. NEC does not require such exothermic welding connections to be listed or labeled, but some technical specifications require that exothermic welding be inspected using X-ray equipment.
Welding rails
Typically, the rails tip is cleaned, flat and correct, and within 25 mm (1 inch). The gap between the end rails for welding is to ensure consistent results in pouring molten steel into welded molds. In the case of welding failure, the rails can be cut into a 75 mm (3 inch) slot, removing the ends of the melted and damaged rails, and new welds are tried with special molds and larger thermite loads. Two or three parts of a hardened sand mold are clamped around the end of the rail, and a suitable heat-capacity torch is used for heating the rails and the inside of the mold. The right amount of thermite with alloy metal is placed in a refractory container, and when the rail has reached sufficient temperature, the thermite is ignited and allowed to react to completion (allowing time for each alloying metal to thoroughly melt and mix, producing the desired steel or melt alloy). The reaction vessel is then tapped at the bottom. The modern melting plate has a self-tapping shot in the flowing nozzle. The molten steel flows into the mold, joining the end of the rail and forming the weld. Slag, which is lighter than the last flowing steel from the container and overflows the mold into a steel catching basin, to be discharged after cooling. All settings are allowed to cool. The mold is removed and the weld is cleaned with hot sculpting and grinding to produce a smooth connection. The typical time from the start of the work until the train can run on the tracks is about 45 minutes to over an hour, depending on the size of the rail and ambient temperature. In any case, the steel rail shall be cooled to less than 370 ° C (700 ° F) before it can maintain the weight of the railroad locomotive.
When the thermite process is used for the track circuit - the cable ties to the rails with copper alloys, the graphite mold is used. Graphite molding can be used repeatedly, since copper alloys are not as hot as the steel alloys used in rail welding. On signal bonds, the volume of liquid copper is quite small, about 2 cm (soup) 3 (0.1 inches) and the mold is clamped lightly onto the side of the rail, also holding the signal wire in place.. In rail welding, the welding charge can weigh up to 13 kg (29 pounds). Heavy and heavy hardened sand molds must be securely locked in a very specific position and then experience great heat for several minutes before firing a load. When the rails are welded into long strings, the elongated expansion and steel contraction must be taken into account. English practice sometimes uses a sort of shear connection at the end of a long continuous welded rail run, to allow for some movement, though by using heavy concrete sleepers and the amount of extra weights on the end of the bed, the tracks, which will prestress according to the ambient temperature at the time of installation, will develop a compressive stress in ambient temperature, or tensile stress in cold environment, its strong attachment to heavy sleeping pads prevents buckling or other deformation. The current practice is to use welded rails along high-speed lines, and the expansion joints are kept to a minimum, often just to protect the intersections and crossings from excessive pressure. American practice seems very similar, physical restraint of the railroad. The rails are prestressed, or considered "stress neutral" at a certain ambient temperature. This "neutral" temperature will vary according to local climatic conditions, taking into account the lowest winters and warmest summer temperatures. The rails are physically secured to bonds or sleep with anchor rails, or anti-propagation. If the ballast track is good and clean and its bond is in good condition, and the track geometry is good, then the welded rail will withstand normal environmental temperature changes to the region.
Remote welding
remote exothermic welding is a type of exothermic welding process to join two electrical conductors from a distance. This process reduces the inherent risks associated with exothermic welding and is used in installations that require welding operators to permanently join the conductors from a safe distance from super super copper alloys.
This process combines both igniter for use with standard graphite prints or sealed seal welded metal cartridges, semi permanent graphite prints, and ignition sources that bind to cartridges with cables that provide secure remote ignition.
See also
- Rails length
References
exothermic welding
Source of the article : Wikipedia
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