Soldering is a joint repair for two pieces of metal, like a chip to a circuit board using lead. Welding is taking two pieces of metal and heating them up to the same temperature and making a single piece of metal out of two separate pieces.
I was taught that soldering,as it relates to piping, is the heating of the base metal to a point to where the filler metal will melt and fill the gap between the two pieces of metal being soldered. The base metal is NOT melted, only the filler metal. Soldering is usually used to join non-ferrous metals. Welding is when the base metal is heated to a point where it will melt and , with the filler metal, becomes part of the weldment. In other words, the two pieces become one, and the resulting weld is now a blend of base metal and filler metal.
soldering is a very low heat (melting temperature of the soldering alloy is less than 840 degrees Fahrenheit), semi-adhesion process, final product: 2 base materials, and a filler material on top. welding is high heat fusion process, final product: a solid blended material