As would be expected from the greater tensile strength of steel and from the greater strength of a steel weld, steel pipe will withstand a much greater degree of torsional stress, without failing at the weld or being twisted off, than will equal weights and sizes of wrought-iron pipe. The greater torsional strength of steel pipe can be seen by comparing Tables LXVI and LXVII, which show the results of torsional tests of iron and steel pipe made by the National Tube Works of Pittsburgh, Pa.

In the following tables of tests each of the fifteen 1/2-inch steel pipes was subjected to a torsional strain that twisted it an average of fifteen complete turns. Under this stress only six pipes broke, no welds failed at the seams, and nine of the pieces underwent the test without failure. On the other hand, of the fifteen wrought-iron pipes subjected to a torsional strain that twisted them each an average of five and three-quarter turns, every one of the fifteen failed, five by breaking off and the remaining ten failed in the weld.

In the larger sizes of pipe, the ratio 1 to 2.6 between the number of twists of wrought-iron and steel would not hold true. For instance, in the case of 3/4-inch pipe the ratio is as 1 to 2.3 and in 1-inch pipe the ratio is 1 to 1.66 and decreases proportionately as the size of the pipe increases.

On account of the greater pliability of steel pipe which permits of its being bent and twisted without opening at the seam or in other ways failing, it makes a better material for installations where numerous pipe bends are to be made. It might be well to note that in the bending of wrought pipe there is less liability of its failing at the seam if the pipe is held so the weld will be at the side and not at the top or bottom.

Table LXVI - Torsional Tests Of Steel Pipe. 1/2-Inch Steel (National Tube Company)

Table LXVII - Torsional Tests Of Wrought-Iron Pipe 1/2-Inch Pipe (National Tube Company)