The steel used for making rivets is similarly tested, and sam ples of the finished rivets are also taken and are hammered into various shapes (some hot and some cold) to ensure that the metal is soft and ductile and suitable for the work.
The testing of ship steel is practically always carried out under the supervision of one of the Classification Societies, and their principal requirements in this respect may be tabulated as follows (Figures given represent stress in tons per sq. in.) The testing is carried out at the steel works, and if the material is passed each plate and bar is stamped with the brand of the Classification Society and with an identification mark which would enable it to be traced back to its origin should it prove unsatisfac tory in course of being worked in the shipyard.
The stem and sternframe are generally made of forged iron, but are also made of cast steel. Castings are tested by being let fall on hard ground and are then slung in chains and hammered all over, flaws being detected by the sound produced To test the quality of the steel in the casting, small pieces which are cast on for the purpose are removed and tested in the same manner as the samples cut from the plates and bars ; these test pieces should have about the same tensile strength as those cut from plates, but a little less ductility may be permitted.
The last few years have witnessed the introduction of a new type of material known as Special Quality Steel, which possesses elastic properties superior to ordinary mild steel. This material can withstand a stress of about 15 tons per sq. in. without suffering any permanent deformation, as compared with a much lower fig ure—say 7 or 8 tons per sq. in. for mild steel. The superior prop erties of the new material are obtained chiefly by exercising great care to see that in the course of manufacture the temperature of the steel does not fall below a given critical temperature before the operation of rolling is completed. On account of its superior properties certain reductions in scantlings are allowed, which re sult in a saving of weight of steel in a moderate sized vessel of about i o%. Several ships have been constructed of this material, and have proved quite satisfactory, although the experience available does not yet permit of any definite conclusions.
The sections of the steel bars in common use are named as follows : A Angle D H-bar G Half round B Tee E Bulb Plate H Cope Iron C Channel F Bulb Angle The vertical portion of the H, T, channel and bulb sections is called the web and varies from about 3 inches to 12 or even 17 inches in depth ; the horizontal parts are called the flanges, this term being applied to both branches of an angle. The flanges of channels and bulb angles vary from 21 inches to 4 inches in breadth and in an angle bar from 21 inches to 8 inches. The thick
ness varies from 4 inch to a inch. These dimensions taken to gether are known as the scantlings of the material. The thick nesses of the plates in common use generally lie between 4 inch and I inch. Thinner or thicker plates or bars are obtainable, but are not often used. Plates are of varying sizes as required but in general vary from about 4 feet to 8 feet in breadth and from 20 to 3o feet in length, the actual size used depending upon the dimensions of the ship and the facilities for working in the ship yard. Bars are supplied in lengths of from 20 to 8o feet as re quired or as may be limited by the means of transport between the steel works and the shipyard. The various plates and bars in a ship are connected together by rivets. Types of rivets are dis tinguished by the names of the heads and points as follows: A. Countersunk head B. Snap head C. Pan head D. Pan head with conical or swelled neck E. Countersunk point F. Rough hammered point G. Snap point hand work H. Snap point machine work The pan head rivet with swelled neck (D) is the most commonly used, as it is convenient to handle and gives sound work. The snap head (B) and the pan head (C), without cones under the heads, are used only for small work, while the countersunk head is only employed where a flush surface is desired. The countersunk point (E) is used on the outside of the shell and other places where a flush surface is required ; elsewhere the rivet is finished off with a rough hammered point (F). Where riveting is done by hydraulic machinery, rivets having snap heads (B) and snap points (H) are used. Rivets vary in diameter from about inch to i inch, depending on the thickness of the plates to be connected, the diameter being usually about a inch more than the thickness of the separate plates. The lengths of the rivets are as required to go through the total thicknesses of the plates and leave enough material properly to form the points. The distance from centre to centre of the rivets is spoken of as the pitch, and is usually ex pressed in diameters. For connecting plating to framing or beams the pitch is usually 7 diameters; for securing edges which must be watertight, the pitch is from 41 to 5, or if the edges are to be oiltight 31 diameters. In the butts and edges of shell plating the pitch varies from 31 to 41 diameters. In some positions rivets like the above cannot be drawn into place and properly hammered up; resort is then made to rivets which have screwed points, called tap rivets. The rivet is screwed up by means of the square head which is chipped off after the rivet is hove up tight.