Cad welding / Caldwell thermit welding process:
Cad welding or Caldwell Welding process is a modification of the simple thermit welding process (exothermic welding process) in which controlled reduction of copper oxide and aluminium is used to produce molten copper and aluminium oxide slag. Exothermic is a chemical term describing a chemical reaction which gives off heat as the reaction takes place.
The reactions involved may be written as:
3CuO + 2Al = 3 Cu + Al2O3 + Heat
3Cu2O + 2Al = 6 Cu + Al2O3 + Heat
The reactions produce 98 percentage of pure copper. The heat of reaction and the pure copper thus obtained are used in the welding of copper to copper and copper to steel. Some specific applications are in the welding of heavy copper cable connections and in the welding of signal bonds to rails.
The cable welding equipment essentially consists of a combination of crucible mould machined in graphite material or carbon material. This mould is assembled into a “handle and clamp” assembly. The mixture is packed in cartridges and can be stored safely. The resulting welding tool is a compact assembly which is portable.
History of Cad Welding:
The process was developed in 1938 for welding copper alloy signal bonds to rails. Designed by Dr Charles Caldwell on behalf of ERICO, the process was named CADWELD.
Components of CAD Weld:
1. Mould to fit your conductors.
2. Weld Metal required by your mould including drop metal disk.
3. Handle Clamps or Frame.
4. Flint Ignitor (included with handle clamps and frames).
5. Lugs, sleeves, packing material listed on the page with the mould.
An exothermic chemical welding procedure in which the heat energy required is provided by an exothermic type of reaction as fusion welding. The combustion of the thermit, which is a mixture of aluminium powder and iron oxide in a 1:3 weight ratio, produces a temperature of 3000 degrees Celsius.
Preheating the thermite mixture to 1300 Degrees celsius is required before the reaction can be initiated.
The thermite mixture undergoes the following chemical reaction as a result of its composition:
8Al + Fe3O4 = 9Fe + 4 Al2O3 + heat
Thermite chemical reaction is based on the high affinity of aluminium for reacting with oxygen, which is the basis of the reaction. When pure steel is treated with ferric oxide, the pure steel settles to the bottom, whereas aluminium oxide floats to the top as slag. Everything happens as a result of a significant variation in their densities.
1. The insulation on the cables is removed, the conductors are positioned in the mould cavity and the welding tool is closed and clamped. The cartridge is emptied into the crucible.
2. The bottom of each cartridge is filled with a little inflammable powder for starting. This powder is ignited with a spark gun which sets off the reaction.
3. The molten copper melts the small steel disc preplaced at the bottom of the crucible, and the metal runs into the mould to weld the two conductors together.
4. The resulting cad welded conductor has the tensile strength of soft drawn copper. By deliberately increasing the cross sectional area of the weld, the current carrying capacity of the joint is made equal to or more than of the original conductor.
5. After the welding, a little aluminium oxide slag remains in the crucible of the unit which can be cleaned and the welding tool made ready for the next weld.
Caldwell Thermit Welding Steps for the Rail Construction
- Preparation of the rail edges that will be welded
- Choosing the appropriate weld gap
- Use a clamp to secure object.
- Mold should be fixed.
- Place the thermite mixture into the crucible and set aside.
- Preheating of the rail’s ends is necessary.
- Start the fire and let the steel flow.
- Time to get the mould out of the house.
- Eliminate any additional head metal
- Start the grinding process.
- Grinding to a high shine in the final stages
Advantages of CAD Welding:
1. Making electrical connections of copper to copper or copper to steel in which no outside source of heat is required.
2. For use in critical ground terminations where a high level of security is required
3. Also ideal for installation in heavy traffic areas
4. Suitable for both paving and hot tar applications