Soldering is the use of filler to join two pieces of metal together at temperatures of less than 427 degrees Celsius. The craftsman heats the metal until it is hot enough to melt solder when it touches the surface. At this point, the joint between the two pieces of metal is bathed in solder, creating a metal adhesive between the two newly-joined pieces. Most common metals can be soldered, but some are more difficult to work with than others.
Copper and its alloys (brass and bronze) are easy to solder. They heat well, which makes it easy to apply silver solder to the metal elements. Copper is also one of the most commonly soldered metals, with applications ranging from electrical work to plumbing. Mild steel, stainless steel and chromium can also be soldered. These metals have higher heating temperatures than copper, so large objects made from these materials can be a challenge. Chromium and stainless steel both require the use of a flux, which allows the soldering alloy to wet the metal. Aluminium can be soldered with the use of specially designed cleaning agents and fluxes. This is difficult to do, though, because the flux must be applied correctly so that the solder won't be inhibited by aluminium oxidation.
Soldering is occasionally used as a colloquialism that actually covers multiple processes. Another process that is sometimes called soldering is "brazing." Brazing is the use of a molten filler to bind two pieces of metal at temperatures of greater than 427 degrees Celsius. In most cases, the process requires the use of a torch, where a soldering iron or soldering gun is used to heat the metal for soldering. Because steel is difficult to solder or braze, welding is a more common method of joining pieces of steel together. The higher temperature brazing process is used by jewellers to attach precious metals together. They use a jeweller's torch, which can reach temperatures in excess of 0-8.333 degrees Celsius. They also use a brazing alloy to match the metal from which they make the jewellery.
Soldering and Brazing Alloys
Soldering and brazing alloys are carefully selected according to the types of metal to be joined. The selection process is especially challenging when joining two different types of metal together. Silver is the most common type of solder and is likely to be the variety available at the local hardware store. It can be used to join silver, gold, nickel, brass and copper. Most jewellers prefer to use gold to braze gold pieces because the colour of silver solder doesn't match. For platinum, a platinum alloy is typically used for filler.
A flux chemically cleans the area to be soldered and removes the oxidation from the surface of the metal so that when the solder makes contact, it bonds well with the metal. Fluxes are either of an acidic type or a rosin type. Rosin fluxes are non-conductive and non-corrosive, and are appropriate for electrical work. The acidic type is both conductive and corrosive. Many solders contain a core of flux. Solid solders (without a core) require the use of an external flux. It is important to match the type of flux to be used with the soldering or brazing project.