Steel for boilers is always of the kind known as low steel, or
soft steel, and is, properly speaking, _ingot iron_, all of its
characteristics being those of a tenacious, bending, equal grained
iron, and quite different from true steels, such as knife blades,
cutting tools, etc., are composed of. Steel is rapidly displacing
iron in boiler construction, as it has greater strength for the same
thickness, than iron; and, except in rare instances, where the nature
of the water available for feed renders steel undesirable, iron should
not be used for making boilers, careful tests having shown it to be
vastly inferior to steel in many important features.
Good boiler steel up to one-half inch in thickness should be capable
of being doubled over and hammered down on itself without showing any
signs of fracture, and above that thickness it should be capable of
being bent around a mandrel of a diameter equal to one and one-half
times the thickness of the plate, to an angle of 180 degrees without
sign of distress. Such bending pieces should not be less in length than
sixteen times the thickness of the plate.
On this test piece the metal should show the following physical
qualities:
Tensile strength, 55,000 to 65,000 pounds per square inch.
Elongation, 20 per cent. for plates three-eighths inch thick or less.
Elongation, 22 per cent. for plates from three-eighths to three-fourths
inch thick.
Elongation, 25 per cent. for plates over three-fourths inch thick.
The cross sectional area of the test piece should be not less than
one-half of one square inch, _i.e._, if the piece is one-fourth inch
thick, its width should be two inches; if it be one-half inch thick,
its width should be one inch. But for heavier material the width shall
in no case be less than the thickness of the plate.