BACKING STRIP FOR PLUG WELDING AND HOLDER OF BACKING STRIP

PROBLEM TO BE SOLVED: To obtain stable welding quality without the outflow of a deposited metal even in the state the small-grained deposits existing on the rear surface of base metals and to improve working efficiency by forming the surfaces of a backing strip made of ceramics to projecting spherical surfaces, thereby assuring the easy contact of the surface with the root of a hole. SOLUTION: The compsn. of the backing strip 10 for embedding the through- hole bored in a steel sheet by welding is made of the ceramics within a range consisting of SiO2 of 35 to 45%, Al2 O3 of 45 to 55% and MgO of 2 to 7%. More preferably, both front and rear surfaces of the main body 10 having a disk shape are formed to the projecting spherical surfaces 11. The reason thereof lies in that even if foreign matter adheres to the base material surface the occurrence of a spacing between the root of the through-hole and the backing strip is prevented. When the one surface wears on use, the application of the projecting spherical surface to the other surface side on the bottom of the through-hole to be welded is made possible simply by turning the backing strip inside out.