Tensile Strength

Wrought pipes are made much thicker and stronger than is necessary to withstand the internal pressures to which under ordinary conditions they are subjected. This additional thickness and strength is necessary to withstand the various stresses incident to cutting and threading pipes and screwing them in place, and the severe strains that pipe lines must withstand when subjected to alternate contraction and expansion. Additional thickness is advantageous also in situations where active corrosion is to be expected.

The tensile strength of a pipe is the resistance it offers to the fiber of its metal being torn apart. Tensile strength of pipe varies with the material of which it is composed, and it would naturally follow that the material which possesses the greatest tensile strength, all other qualities being equal, would make the best pipe material. The tensile strength of soft steel, such as is used in the manufacture of pipe, is about 61,000 pounds per square inch, and the tensile strength of wrought-iron is about 34,000 pounds per square inch, taken transversely in each case. It follows, therefore, that for pipes of equal size and thickness, steel pipe will withstand a working pressure of almost double that of wrought-iron pipe, and so far as the strength of the two metals is concerned, it is the better pipe material.

It might be inferred from the fact that steel pipe possesses almost double the tensile strength of wrought-iron pipe, that the walls of steel pipe could be made proportionately thinner, thus saving considerable in the material and weight of steel pipe. Other considerations, however, require that there be no appreciable difference between the thickness of walls of pipe made from the two metals; for instance, as steel pipe is about twice as strong as wrought-iron pipe, the loss from corrosion or other cause of a certain thickness from the walls of a steel pipe would weaken it almost twice as much as would the loss of an equal thickness from the walls of a wrought-iron pipe.

Strength Of Seam - the tensile strength of a pipe metal cannot be taken as the actual strength of the pipe, for just as a chain is only as strong as its weakest link, so a pipe is only as strong as its weakest part of the seam. The strength of a welded seam varies with the quality of metal and the skill of the workman who makes the weld. In the case of wrought-iron pipe the strength of the seam varies from 49 to 84 per cent., and will average about 70 per cent, of the tensile strength of the metal. On the other hand the strength of the welded seams of steel pipe varies from about 50 to about 93 per cent., and will average about 72 per cent, of the tensile strength of the metal.

So far, then, as the ratio between the strength of seam and tensile strength of the metal is concerned, there is but slight difference between that of wrought-iron and steel; it should be remembered, however, that the tensile strength of steel is almost double that of wrought-iron, consequently the actual strength of a weld in steel pipe is about double that in a wrought-iron pipe. It may safely be assumed, therefore, that for all purposes a certain size and weight of steel pipe possesses about twice the strength of an equal size and weight of wrought-iron pipe, and will sustain almost double the working pressure, besides withstanding almost double the torsional stresses without opening at the seam.