The welding rolls are very short rolls, almost “sheaves” or wheels, with concave edges of exactly the outside diameter of the pipe to be formed. Between these two rolls, at the end of a long straight bar, is a mandrel or projectile-shaped ball of high-speed steel over which the white-hot tube must be pushed. The reheated, curved skelp is pulled from the furnace and the forward end forced into the rolls which shoot it through and over the mandrel at high speed, forcing together and welding under heavy pressure the overlapping edges of what was formerly the plate. Amid the noise and the shooting sparks an unsuspecting bystander is quite startled by the suddenness of it all. While still hot the pipes pass to “sizing” rolls which correct any variation in inside and outside diameters. The cross or straightening rolls next smooth and clean their surfaces while straightening the pipes or boiler tubes. After the first trip through the welding rolls, boiler tubes and certain other high grades of pipe go back into the furnace where they are reheated. They are again put through the welding rolls to make absolutely sure of a tight weld. After cooling, the ends of each pipe are cut off. Because of the “scarfing” of the edges and the great pressure of the rolls, it is difficult to tell where the welds occur, the thickness of the walls being practically uniform all around. Lap-welded pipe of as great as 36″ diameter has been made in this way. [Illustration: PIPES IN SIZING AND CROSS ROLLS ] The water-pressure test is given to all lap-welded pipe as are certain tests for tensile and torsional strengths, and for ability to flatten without breaking. In the case of boiler tubes, a piece is cut from each end of each tube, which must stand flanging or spreading “cold” and also must crush down endwise under the heavy pressure applied in the testing machine without fracture or opening of the welds. The pipe may be “threaded” to order or shipped as it comes from the testing bench. [Illustration: THE FINISHING END ] As remarked, both butt and lap-welded pipe is regularly manufactured from wrought iron and from steel. [Illustration: HYDROSTATIC TEST OF THE PIPES ] It was suggested during the discussion of the manufacture of wrought iron, that, owing mainly to high labor costs, wrought iron was with difficulty competing with the soft steels. Wrought iron is noted for its welding properties and it has always had its loyal admirers. Aside from its application as “bar iron” which always has been and still is in favor with many metal workers for miscellaneous purposes and its use as Swedish bar iron or low phosphorus melting bar by makers of crucible steel as a base for their product, wrought iron probably finds its next most favored place as a material for pipe as is shown by the table given in Chapter VI.