|MadSci Network: Chemistry|
The metal core of welding rods is likely to be the same as the metal that is being welded. Often however, it is desirable to make the weld from a stronger metal to ensure that the weld doesn't act as a weak point. In this case, materials such as Nickel-based alloys are used to make the weld as they are stronger and more corrosion-resistant than most steels. The core essentially supplies a source of filler metal. When you make a weld, most of the time there is a space which must be filled (you can think of the metal core as a "glue" when you stick two pieces of plastic together). Some welds do not require extra material, and they are called autogeneous welds. What happens in a weld is that the heat of the arc melts the ends of the two pieces of metal you wish to join. At the same time it melts the core which flows into the space between the two pieces of metal, mixing with their melted ends, and then solidifying when you take the arc away. The coating serves many purposes. It may contain extra amounts of metal to increase the amount deposited in the weld, it may contain chemicals such as titanium dioxide (aka Rutile, TiO2) which helps to form a crust over the top of the weld to protect it whilst it solidifies. This help to reduce the amount of gas bubbles in the weld, making the weld as strong as possible. It also contains a binder, which will help to keep the coating on the rod. This binder is vapourised by the heat of the arc and will help to keep the weld dry and moisture free by shielding it from the atmosphere as the weld is being made. The crust may also help by acting as a trap for hydrogen, minimising the amount of Hydrogen (H2) left in the weld which may cause the weld to crack. A good site to get more information on welding is The Welding Institute website (http:\\www.twi.co.uk) as they have online tutotrials about all aspects of welding.
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