Commencing with a countersunk head as the strongest form of head, the greater the fillet permissible under the head of a rivet, or bolt, the greater the strength and the decrease in liability to fracture, as a fillet is the life of the rivet.

If rivets are made of iron, the material should be strong, tough, and ductile, of a tensile strength not exceeding 54,000 pounds per square inch, and giving an elongation in eight inches of not less than twenty-five per cent.  The rivet iron should be as ductile as the best boiler plate when cold.  Iron rivets should be annealed and the iron in the bar should be sufficiently ductile to be bent cold to a right angle without fracture.  When heated it should be capable of being flattened out to one-third its diameter without crack or flaw.

[Illustration:  Fig. 15.  Solid Die Rivet.]

[Illustration:  Fig. 16.  Open Die Rivet.]

If rivets are made of steel they must be low in carbon, otherwise they will harden by chilling when the hot rivets are placed in the cold plates.  Therefore, the steel must be particularly a low grade or mild steel.  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.  This feature of the use of proper rivet tools in boiler shops has not received the attention it deserves.  Practical use of the above named hold fast would soon convince the consumers of rivets of its value and efficiency.