Thermite welding

Exothermic welding, also known as exothermic bonding, thermite welding (TW), and thermit welding, is a welding process that employs molten metal to permanently join the conductors. The process employs an exothermic reaction of a thermite composition to heat the metal and requires no external source of heat or current. The chemical reaction that produces the heat is an aluminothermic reaction between aluminum powder and metal oxide.

1. THERMITE WELDING

2. INTRODUCTION Thermite welding (TW) is a process that produces coalescence of metals by heating them with superheated molten metal from an aluminothermic reaction between a metal oxide and aluminum. Filler metal is obtained from the liquid metal . Thermite welding is a thermochemical process and highly exothermic. This exothermic process was discovered in 1898, in Germany , by Dr. Hans Goldschmidt of Goldschmidt AG . It should be noted that the standard term for this process is often listed as " thermit " welding . However, Thermit is a registered trademark of Th. Goldschmidt AG of Essen, Germany.

3. This welding process generally does not require an external power source , with the exception of a gas torch which may be necessary for preheating under certain conditions . In addition , preparatory cutting of the workpieces and completion procedures (such as shearing and grinding of the thermite weld), may require a generator or hydraulic pump . In the basic process, however, the aluminothermic reaction negates the need for an external power source to perform the weld. This makes thermite welding appropriate for use in remote areas where access to the welding site is limited and electric power may not be available.

4. PRINCIPLE OF OPERATION The thermochemical reaction that produces a thermite weld takes place according to the following general formula: The exothermic reaction between aluminum powder and a metal oxide can be initiated by an external heat source. The reaction is highly exothermic , and consequently self-sustaining once started . The reaction can be started and completed only if the oxygen affinity of the reducing agent (aluminum) is higher than that of the metal oxide to be reduced. The heat generated by this exothermic reaction results in a liquid product (slag) consisting of metal and aluminum oxide. If the density of the slag is lower than that of the metal , as is the case with steel and aluminum oxide, they separate immediately. The slag floats to the surface and the molten steel drops into the cavity to be welded. Metal oxide + aluminum (powder) → aluminum oxide + metal + heat

5. A few typical thermochemical reactions and the thermal energies produced are as following: 3Fe 3 O 4 + 8Al → 9Fe + 4Al 2 O 3 (3088⁰C) + 719.3kcal Fe 2 O 3 + 2Al → 2Fe + Al 2 O 3 (2960⁰C) + 181.5kcal 3CuO + 2Al → 3Cu + Al 2 O 3 ( 4865⁰C ) + 275.3kcal Cr 2 O 3 + 2Al → 2Cr + Al 2 O 3 ( 3977⁰C ) + 546.5kcal Aluminum is the reducing agent in these reactions. Theoretically, magnesium, silicon, and calcium also could be used; however, magnesium and calcium have limited use for general applications. Silicon often is used in thermite mixtures for heat treatment, but it is rarely used in welding. In some cases, an aluminum-silicon alloy is used as the reducing agent . The first of the reactions listed in the equation above is the one most commonly used as a basic mixture for thermite welding. The proportions of these mixtures usually are about three parts by weight of iron oxide to one part of aluminum. The theoretical temperature created by this reaction is about 3100°C . The addition of non-reacting constituents, heat loss to the reaction vessel, and radiation reduce this temperature to about 2480°C . This temperature is about the maximum that can be tolerated, because aluminum vaporizes at 2500°C . However, the minimum temperature should not be much lower because the aluminum slag (Al 2 O 3 ) solidifies at 2040°C .

6. This exothermic reaction is extremely violent if only the metal oxide and aluminum reducing agent are used. Pellets of ferroalloy are added to cool this reaction from a typical temperature of 3090 °C to 2480 °C . These additions also are used to produce the desired chemistry . Other alloying elements additions are used to increase the fluidity and lower the solidification temperature of the slag. Heat loss is highly dependent on the quantity of thermite being reacted. When large quantities are reacted, the heat loss per pound of thermite is considerably lower and the reaction more complete than when small quantities of thermite are reacted. The thermite reaction is non-explosive and requires less than one minute for completion, regardless of quantity . To start the reaction, a special ignition powder or ignition rod is required; both can be ignited with an ordinary match. The ignition powder or rod will produce enough heat to raise the thermite powder in contact with the rod to the ignition temperature, roughly 1200°C. The workpieces should be aligned properly and the weld interface should be free of rust , loose dirt, moisture, and grease . A proper size root opening must be provided at the weld interface, the size depending on the width of the joint. Wider joints normally require a larger root opening. A mold, which maybe built up on or premanufactured to conform to the workpieces , is placed around the joint to be welded.

7. To weld a butt joint, the joint faces should be adequately preheated to promote complete fusion between the thermite deposit and the base metal. Even though it is a welding process, thermite welding resembles metal casting , in which proper gates and risers are needed to perform the following functions : 1 . Compensate for shrinkage during solidification, 2 . Eliminate typical defects that appear in cast metal, 3 . Provide proper flow of the molten steel, and 4 . Avoid turbulence as the metal flows into the joint .

8. Main Parts of Thermite Welding Crucible : It is that part which is capable to withstand in high temperature condition. In thermite welding process, the crucible contains thermite material. The exothermic process during the welding process takes place in the crucible. The molten iron present at the bottom of the crucible and slag of aluminum oxide floats over the molten metal. The crucible contains a taping device to discharge the molten metal to the mold for the welding. A hard refractory stone, often magnesite is used for crucible lining. Thermite mixture: It is a pyrotechnic composition of metal oxide, aluminum powder, and fuel. Mold : A mold is created around the section to be welded. The mold consists of runner, riser, slag basin, heating gate. The molten metal is poured into the mold for the joining of the metal. The mold must be adequately vented to facilitate the escape of moisture and gases during preheating and welding. The geometry of the mold between the weld and the rail must minimize stress raisers to prevent weld fatigue and allow optimal performance. To ensure proper functionality of the mold , the sand should be free of clay components that have low melting points. Taping device: It is device which is used to discharged or pour molten metal form the crucible to the mold.

9. THERMITE WELDING WITH EXTERNAL PREHEAT Premanufactured two-piece or three-piece molds designed for single use generally are preferred for the thermite welding of standard rail sizes. The mold is aligned so that the center coincides with that of the root opening , which typically is 25 mm (1 in.) between the rail ends. The molds are fitted together, and packed. The rail ends are preheated in the range of 600°C to 1000°C with a gas torch flame directed into the mold. A refractory-lined crucible containing the thermite charge is positioned above the mold halves after preheating is completed. The crucible also is designed for a single use, which minimizes size and weight for better ergonomics. The charge is then ignited to initiate the thermite reaction. Tapping takes place and the thermite reaction continues . Additional time is allowed for the steel and aluminum oxide to separate.

10. FIGURE-1: Cross Section of a typical Thermite Mold for Repair Welding With External Preheat

11. The molten steel flows into the joint while the residual oxide spills into slag pans, and the pour is complete. In most procedures, the metal is center-poured (fed into the middle of the root opening),and then diverted to the outer legs to enter and fill the root opening, beginning at the rail base by way of a diverter plug. A self-tapping seal (thimble) is used in the bottom of the crucible . Approximately 20 seconds to 30 seconds after the thermite reaction is complete, the molten metal melts the seal and pours out of the bottom of the crucible into the joint root between the two rail sections . The lower-density liquid slag floats to the top of the thermite metal in the crucible . The liquid slag does not reach the mold cavity until all of the molten steel has entered and filled both the cavity between the rail sections and the mold. The slag remains on top of the weld and solidifies in slag pans. When the metal has solidified, the mold halves are removed and discarded. The excess metal is removed by hand grinding or by hydraulic or manual shearing devices . Preheating times and temperatures may be reduced by using a larger thermite charge. The heat dissipated into the rails during welding requires a larger mass of molten steel .

12. Welding with SELF PreheatING Although not used extensively, the self-preheating method is designed to eliminate the variables associated with torch preheating and the equipment needed for the welding operation. The rail ends are preheated by a portion of the molten metal produced by the thermite reaction. The crucible and mold are a one-piece design. The thermite molds, commonly known as shell molds, are premanufactured of sand bonded with phenolic resins. They are very light, non-hygroscopic, and moisture-free. They have along shelf life, typically one to two years. After the thermite reaction is completed, the molten steel automatically flows from the crucible into the joint rather than passing through the atmosphere, as is the case with a separate crucible. A cross section of this thermite mold, have the shape of the cavity in which the molten filler metal flows. A hollow chamber in the mold under the weld area receives the first molten metal, called the preheat metal, and allows it to preheat the rail ends.

13. FIGURE-2: Cross Section of a Mold- Crucible with a Preheat Metal Chamber By the time the chamber is filled, sufficient molten metal should have passed over the rail ends to preheat them to the required temperature to assure complete fusion with the base metal. The portions of the thermite mixture for this process are about twice the size of those used for the external preheat method . The heat-affected zones (HAZ) in the adjacent rail sections are considerably smaller than when external preheating is used.

14. VARIANT IN THERMITE WELDING- Pressure Thermite Welding In this process the superheated molten metal and the slag produced by the exothermic reaction of thermite are poured into a mold surrounding the metal pieces to be welded. This results in the heating of the workpiece ends which are then forced together under pressure to achieve a weld between them. Fig. 2.48 shows the mechanism of pressure thermite welding of pipes in which suitably designed clamp mechanism is employed to force the pipe ends to­gether. Pressure thermite welding is not much used possibly due to its higher cost and because of the availability and popularity of other methods.

15. THERMITE WELDING METALLURGY Metallurgical structures that are present in thermite welds depend on the chemical composition of the weld metal and on the cooling rate of the joint after pouring is completed. Figure-3 shows a typical macrostructure of a carbon steel rail thermite weld and the microstructure of the fusion zone, the fusion line, the end of the heat-affected zone (HAZ), and the unaffected rail. Typically, the weld is 100% pearlite with varying degrees of coarseness. This is due to the slow cooling rate that completes transformation before reaching the martensite start temperature and the formation of untempered martensite .

16. FIGURE-3: CARBON STEEL RAIL THERMITE WELD (A) MACROSTRUCTURE (B) WELD MATERIAL, 65 × (C) FUSION LINE AREA, 65 × (D) HEAT-AFFECTED ZONE, 65 × ( E) UNAFFECTED RAIL AREA, 65×

17. Type of Joints possible from thermite welding Horizontal Joints Vertical Joints Straight Joints Cross Joints Overlap Joints L-shape Joints T-shape Joints

18. Kinds of Thermite Welding commonly used For commercial welding purposes there are now produced three varieties of Thermite known as : Plain Thermite Railroad Thermite Cast-iron Thermite Plain Thermite is simply a mixture of aluminum and iron oxide, as previously stated, and is used in making pipe welds and welding necks on mill rolls and pinions where the Thermite is merely used as a heating agent to bring the pipe ends up to a welding temperature and the roll and pinion ends to a molten state. Railroad Thermite is plain Thermite with the addition of % nickel, 1 % manganese and 15 % mild-steel punchings. This grade is used in connection with steel welds . Cast-iron Thermite is plain Thermite with the addition of 3 % ferrosilicon and 20% mild-steel punchings, and is used, as its name implies, for welding cast-iron parts.  

19. Thermite Welding Safety Moisture is a major concern in thermite welding. The presence of moisture in the thermite mixture, in the crucible , or on the workpieces can lead to the rapid formation of steam when the thermite reaction takes place . Steam pressure may cause the violent ejection of molten metal from the crucible. Therefore, the thermite mixture and molds must be stored in a dry location. Tooling should be preheated prior to use, and the surrounding work area should be dry and free of combustible materials. Appropriate protective clothing, safety boots, gloves, safety glasses, or a full face shield with filter lenses should be worn by all personnel in the immediate vicinity.

20. APPLICATIONS OF THERMITE WELDING The welding of rail sections into continuous lengths is an effective method of minimizing the number of bolted joints in the track structure. Thermite welding is employed in the marine field for the repair of heavy sections of ferrous metal such as broken stern frames, rudder parts, shafts, and struts . Thermite welding without preheat provides a method of splicing concrete-reinforcing steel bars . Thermite welding can be used for heat-treating purposes. A thermite mixture of copper oxide and aluminum is used to weld joints in copper conductors. Hence this technique is employed to weld electrical conducting joints, particularly to provide electrical continuity for railroad signal systems. L arge-diameter rolls, shafts, ingot molds, and heavy mill housings can be successfully repaired by using the thermite welding process.

21. Limitations of Thermite Welding Thermite welding should not be used for austenitic stainless steel and duplex steel pipelines. Thermite welding should not be used for structures that contain or have contained flammable or combustible liquid.

22. ADVANTAGES OF THERMITE WELDING No external power source is required (heat of chemical reaction is utilized ). Very large heavy section parts may be joined . Thermite welding can be done at site where casting is impossible . Process setup is simple and extremely portable with minimal arrangements. Very low capital equipment costs.

23. DISADVANTAGES OF THERMITE WELDING It requires the supply of replaceable molds. Lack of repeatability. It cannot be used in wet conditions or bad weather when working outdoors . It is uneconomical for welding cheap metals and light parts . It is used for limited ferrous metals like iron and copper . Very slow rate of welding. Weld may contain gas (Hydrogen ) and slag contaminations.

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