The material should show a tensile strength not greater than
54,000 pounds per square inch and an elongation in _eight inches_ of
thirty per cent. The United States government requirements are that
steel rivets shall flatten out cold under the hammer to the thickness
of one-half their diameter without showing cracks or flaws; shall
flatten out hot to one-third their diameter, and be capable of being
bent cold in the form of a hook with parallel sides without cracks or
flaws. These requirements were thought at first to be severe, but the
makers of steel now find no practical difficulty in meeting these
specifications.
The forming of the head of rivets, whether of steel or iron, and
whether the heads are conical or semi-spherical, should not be changed
by the process of riveting. The form of the head is intended to be
permanent, and this permanent form can only be retained by the use of
a "hold fast," which conforms to the shape of the head. In the use of
the flat hold fast (in general use in a majority of boiler shops) the
form of the head is changed, and if the rivet, by inadequate heating,
requires severe hammering, there is danger that the head of the rivet
may be "punched" off. By the use of a hold fast made to the shape of
the rivet head, this danger is avoided and the original form of the
head is retained.
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