The approximate strength of single-riveted boiler plate is 55 per cent.; of double-riveted, 70 per cent. L. L. Bernier, in the Boiler Maker, gives the ratio of cost of acetylene welding with a generator, compared with double riveting, as seven to twelve. The cost of triple riveting is not given, but it would still further increase the discrepancy. Besides being cheaper, the acetylene-weld is absolutely leak proof, a great advantage over a riveted joint.

Fig. 54. - Top section of broken crank case with broken arms.

On the other hand, an apparently sound acetylene-weld may have a tensile strength of 25 per cent, instead of 95, and may be crystalline and brittle; whereas the riveted joint is of certain strength. In the present state of the art it would be a mistake to advocate the acetylene-welding of boilers of any size, though for small containers and tubes that can be rolled and annealed the riveted and brazed joint is being rapidly superseded by the acetylene-weld. But oxy-acetylene repair welds are now frequently made on cracked and corroded boilers. Cracks are first laid open by a partial heating, until their full extent is known. Then they are welded by the flame and a melt bar, working up from the bottom of the crack. Where the plate is full of cracks or is corroded deeply, a section of the plate is cut out with a cutting flame and a fresh plate patch is welded in. Boiler repairing requires very careful preheating, hammering of the weld and subsequent annealing of the plate surrounding the weld.

Fig. 55. - Engine crank case with welded arms.