Re: What metal is the core of rods used for electric arc welding?

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.