In the accompanying sections, some of the properties of iron and
steel, as employed in the construction of boilers, are given. It is,
therefore, desirable that the meanings applied to the various terms
used should be clearly understood. The definitions necessary are, then,
briefly as follows:—
=Tensile strength= is equivalent to the amount of force which, steadily
and slowly applied in a line with the axis of the test piece, just
overcomes the cohesion of the particles, and pulls it into separate
parts.
=Contraction of area= is the amount by which the area, at the point
where the specimen has broken, is reduced below what it was before any
strain or pulling force was applied.
=Elongation= is the amount to which the specimen stretches, between two
fixed points, due to a steady and slowly applied force, which pulls and
separates it into parts. Elongation is made up of two parts: one due to
the general stretch, more or less, over the length; the other, due to
contraction of area at about the point of fracture.
=Shearing strength= is equivalent to the force which, if steadily and
slowly applied at right angles, or nearly so, to the line of axis of
the rivet, causes it to separate into parts, which slide over each
other, the planes of the surface at the point of separation being at
right angles, or nearly so, to the axis of the rivet.
=Elastic limit= is the point where the addition to the permanent set
produced by each equal increment of load or force, steadily and slowly
applied, ceases to be fairly uniform, and is suddenly, after the point
is reached, increased in amount. It is expressed as a percentage of the
tensile strength.
=Tough.=—The material is said to be “tough” when it can be bent first
in one direction, then in the other, without fracturing. The greater
the angles it bends through (coupled with the number of times it
bends), the tougher it is.
=Ductile.=—The material is “ductile” when it can be extended by a
pulling or tensile force and remain extended after the force is
removed. The greater the permanent extension, the more ductile the
material.
=Elasticity= is that quality in a material by which, after being
stretched or compressed by force, it apparently regains its original
dimensions when the force is removed.
=Fatigued= is a term applied to the material when it has lost in
some degree its power of resistance to fracture, due to the repeated
application of forces, more particularly when the forces or strains
have varied considerable in amount.
=Malleable= is a term applied to the material when it can be extended
by hammering, rolling, or otherwise, without fracturing, and remains
extended. The more it can be extended without being fractured, the more
malleable it is.
=Weldable= is a term applied to the material if it can be united, when
hot, by hammering or pressing together the heated parts. The nearer the
properties of the material, after being welded, are to what they were
before being heated and welded, the more weldable it is.
=Cold-short= is a name given to the material when it cannot be worked
under the hammer or by rolling, or be bent when cold without cracking
at the edges. Such a material may be worked or bent when at a great
heat, but not at any temperature which is lower than about that
assigned to dull red.
=Hot-short= is when the material cannot be easily worked under the
hammer, or by rolling at a red-heat at any temperature which is higher
than about that assigned to a red-heat, without fracturing or cracking.
Such a material may be worked or bent at a less heat.
=Homogeneous= describes a material which is all of the same structure
and nature.
A homogeneous material is the best for boilers, and it should be of
suitable tensile strength with contraction of area and elongation
best suited for the purpose, having an elastic limit that will insure
the structure being reliable; it should be tough and ductile, and its
elasticity fairly good, and be capable of enduring strains without
becoming too quickly or easily fatigued. The material should be
malleable and in some cases weldable; that which is of a decidedly
cold-short or hot-short nature should be avoided.