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Strength of Boiler Materials for Construction of

iron and plate

STRENGTH OF BOILER MATERIALS FOR CONSTRUCTION OF The crushing strength of cast iron is high, varying from 50,000 to 75,000 pounds per square inch ; its tensile strength is low, varying with the chemical and physical properties of the iron from about 15,000 to 22,000 pounds per square inch.

Wrought-iron plates having a tensile strength of from 50,000 to 60,000 pounds, with an elongation or ductility of from 20 per cent to 30 per cent, are suitable for boiler work. Boiler iron may be tested in the following ways if testing machines are not available : Cut from the plate a strip about 2 inches wide and bend it cold, down upon itself ; if it shows no fracture on the outside curve, it is satisfactory. This is, however, a severe test, and only the best flange iron will stand it ; on the other hand, any iron which, when heated to a cherry red and bent, shows cracks or fracture on the outer curve, is unfit for use in boiler construction. When wrought iron was used for boiler plates it was customary to give the plate what is called the hammer test. The plate was suspended clear of the ground and struck with a hammer at intervals of three or four inches over its surface ; a clear, ringing tone indicating a sound plate, while a dull sound indicated with fair certainty a defect such as internal unsoundness.

Mild steel has a tensile strength of from 55,000 to 65,000 pounds per square inch, with an elongation of 25 per cent. A test piece cut from a plate 3/4 inch thick or less should stand bending double, when hot or cold, and not show any cracks ; thicker plates should be capable of being bent at a small radius to a large angle without showing any cracks. Steel should never be worked at a blue beat, as in this state it is very brittle. It is also mechanically tested by being heated to a cherry red, quenched in water at 82 F. then bent in a curve of small radius ; if it cracks, it has become tempered, and it is therefore unsuitable for this work. If the tensile strength of the steel is under 70,000 pounds per square inch, it is sufficiently tough and ductile and can be easily worked.

In general, boiler materials are carefully tested for the following qualities : Tensile strength, to resist rupturing strains. Also in order that the plates may be thin.

Toughness and elasticity, to resist corrosion and the wear and tear of manufacture.

Ductility, so that the boiler may change its shape slightly without rupture. This is a more important quality.