Hardbanding is the process of depositing a layer of super hard metal onto drill pipe tool joints, collars and heavy weight pipe in order to protect both the casing and drill string components from wear associated with drilling practices.

From the Petroleum Industry Glossary:
Hardbanding is the process of laying on a coating or surface of [super-hard metal] on a [softer metal part] at a point or on an area of severe wear or abrasion - putting a hard surface on a softer metal by welding or other metallurgical process. Where it may be impractical or prohibited by structural constraints to manufacture a part from the harder metal, hard-surfacing of the part is a practical solution. Hardbanding is the term given to hardfacing drill pipe – hardbanding is a specific form of hardfacing."