Final answer:
The process of heating copper tube joints to temperatures above 840°F and adding filler metal is known as brazing (option c) , which differs from welding as it does not melt the base metals.
Step-by-step explanation:
The process you're referring to, where pipe joints in copper tube are heated to temperatures greater than 840°F and filler metal is added, is known as brazing. This method uses a filler metal that has a lower melting point than the base metals being joined, but higher than what is used in soldering. Unlike welding, where the base metal is melted, in brazing, only the filler metal is melted, which then flows into the joint and bonds the pieces of metal together upon cooling.
Welding is a different process where the base metals are melted, usually with the use of a filler material, to join them. This process uses various sources of heat such as gas, electricity, or laser. Forge welding, a technique used by blacksmiths, involves the joining of iron and steel when at high temperatures without a filler material.