1 Edes, 86. 2 Holtzapffell.
3 Bloxam's Chemistry.
Many people think that they can judge of steel by the appear-ance of the fracture.
Mr. Kirkaldy found that "the conclusions respecting wrought iron are equally appropriate to steel, viz. - Whenever rupture occurs sloidy, a silky fibrous, and'when suddenly, a granular appearance, is invariably the result, both kinds varying in fineness according to quality.
" The colour is a light pearl gray, slightly varying in shade with the quality; the granular fractures are almost entirely free of lustre, and, consequently, totally unlike the brilliant crystalline appearance of wrought iron.1
The appearance of the fracture is, however, at the best but a vague and uncertain guide, and, without great experience on the part of the observer, almost useless.
The only certain test, however, for tensile strength' and ductility is by direct experiment.
The tensile strength of steel may be tested in the same way as that of wrought iron.
The varieties of steel are, however, even more numerous than those of wrought iron, and their strength differs accordingly. Moreover, it is greatly influenced by the treatment to which the steel has been subjected.
Plates for shipbuilding, bars, angles, angle-bulbs, tees, tee-bulbs or tee-bars, made by Bessemer and Siemens-Martin process.
Strips cut lengthways (or for plates either lengthways or crossways, or in round bars a piece from the bar), "to have an ultimate tensile strength of not less than 26, and not exceeding 30 tons per square inch of section, with an elongation of 20 per cent in a length of 8 inches."
"Such forge tests, both hot and cold, as may be sufficient in the opinion of the receiving officer to prove soundness of material and fitness, for the service."
Strips cut lengthwise (or for plates either lengthwise or crosswise) " 11/2 inches wide, or in round bars a piece from the bar, heated uniformly to a low cherry red and cooled in water of 82° Fahr., must stand bending in a press to a curve of which the inner radius is one and a half times the thickness of the steel tested.
"The strips are all to be cut in a planing machine, and to have the sharp edges taken off.
"The ductility of every bar is to be ascertained by the application of one or both of these tests to the shearings, or by bending them cold by the hammer.
"The pieces cut for testing are to be of parallel width from end to end, or for at least 8 inches in length."
A specimen bar of 2" diameter is taken, when required by the overseer, from every charge, or from every 50 bars or portions of 50, and subjected to a percussive test. The test for a bar of 2" diameter should be the fall of 15 cwt. through 30 feet, or 20 cwts. through 221/2 feet, whichever may be most convenient. Sample must stand at least one blow without injury, and the following facts must be noted.
a. The number of blows to break the bar.
b. The character of the fracture.
c. The reduction in diameter after each blow.
d. The reduction in sectional area at point of fracture.
e. The elongation in 8 inches and in the inch containing the fracture.
Sample pieces will be taken for testing the welding qualities of the steel, by welding two pieces together and bending it in the way of the weld when cold.
1 Kirkaldy's Experiments on Wrought Iron and Steel.
The steel plates used in ships to he classed in the register of Lloyd's Insurance Corporation have to stand a tensile stress of 27 to 31 tons per square inch, with 20 per cent elongation in 8 inches; the angles and heams 27 to 33 tons, with 16 per cent elongation, and the same tempering tests as required by the Admiralty.
Steel rails are sometimes tested by repeated loads, and generally by a falling weight The following extract is from a recent specification for a steel rail weighing 79 lbs. per yard, requiring both tests : -
"A length of 6 feet will be cut off from each sample rail and tested as follows -
"1st. A piece will be placed in the position it would assume for traffic, on solid supports 3' 6" apart in the clear, and equidistant from the ends, and a weight of one ton will be allowed to fall freely upon the centre of the rail from a height of 12 feet 6 inches. The rail must bear two such blows without showing the least sign of fracture, and the permanent set caused by the first blow must not exceed 2 inches.
"2d. A piece of rail is to be placed on supports as before, and a weight of 18 tons is to be applied at the centre, when the deflection must not exceed 3/16 of an inch."
The War Office percussive test for bolts is given at p. 283.
Steel for Bridges and Roofsl should have a high elastic limit which will enable it to endure a high working stress. Good steel for such purposes can be obtained having an ultimate tensile strength of 35 tons per square inch, a limit of elasticity of 20 tons, and with 20 per cent elongation on the best specimen of 8 inches length. Such a steel would endure a working stress of 8 tons to the inch.2
Recent specifications from the India Office for large steel bridges contain the following requirements: -
Steel Bars and Plates must weld perfectly, and not crack or crumble at all when hammered at a welding heat. The strips, 1 inch wide and 8 inches long, to have a tensile strength not less than 28 tons or more than 31 tons per square inch, an elongation of not less than 20 per cent, and a limit of elasticity of 15 tons per square inch. The same tempering tests as the Admiralty reqiiire, except that the radius of the curve to which the steel is bent is three inches instead of 11/2 inches.
Buckle Plates of Roadway to bear a concentrated load of 12 tons at centre without permanent set, and of 24 tons at centre without fracture.
Tensile strength 26 to 28 tons per square inch, in test pieces of 10 diameters, elongation not less than 25 per cent. A piece of bar heated to cherry red, quenched in water of 82° Fahr., to bear being doubled quite close without injury. A piece heated to full red or orange, dropped into a hole in a cast iron block, so that 11/2 to 2 diameters project, to bear having the end hammered out to a thin edge all round without showing signs of cracking.