The material used for plates, rivets, braces, and all other parts on which the structural strength of a high-grade boiler depends are made of a low-carbon open-hearth steel in which is allowable only very small quantities of phosphorus and sulphur. Nickel is often alloyed with this steel to improve its tensile and elastic strength.

The elastic strength, rather than the tensile strength of the material, is of first importance, as the permanent safety of the boiler depends upon all stresses remaining within the elastic limit. A good margin between the elastic and the final strength of the material provides a ductility or elongation which will many times save actual and disastrous disruption under pressure by allowing the material to bulge out or otherwise stretch greatly before it breaks.

Specifications for high-grade boiler plate require an elastic strength of about 1/2 the tensile strength, a tensile strength of about 70,000 pounds, and an elongation, when pulled apart, of about 25% in a test bar 8 inches long. Rivets, bolts, and material for boiler braces are required to exceed slightly the requirements specified for plates. Plates which are to be flanged for parts of the boiler structure must be ductile to a high degree, and specifications usually require the test piece to show a slightly greater per cent of elongation and allow slightly less tensile and elastic strength than for other plates.

Fig. 220.