Pipes larger than 3″, and boiler tubes or other particularly high grade welded tubes of 2″ and over are usually “lap-welded.” This process gives a considerably more reliable product than does the butt-welding process, for reasons which are readily seen.
How Lap-weld Pipes Are Made
Bent Skelp for Lap-welding Being Charged into Furnace
Skelp for lap-welding is rolled in just the same way as is skelp for butt-welding except that the edges are “scarfed” or decidedly beveled so that the two edges can make a considerable lap without increase of thickness of that part of the wall. These pieces of skelp are charged into the heating furnace just as occurred in the butt-weld process and, after coming to a white heat, they are drawn through a sort of bell or die which curves them so that one edge considerably overlaps the other. Back they go into the furnace to regain any heat that has been lost, for, to weld properly, the skelp must be hot enough that any scale which had covered it drips off.
The Lap-welding Rolls with Mandrel in Position
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.