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uncertain ; it will also be very expensive in fuel, labour, and waste of metal. With iron such as that in (5), where the mean tensile strength was increased from 1 to 18.26 = 5.6 = 3.26 at the 4th melting, it would no doubt be commercially advantageous: in such a case experiments should be specially made on the iron intended to be used (87).

(8.) By maintaining cast iron in a state of fusion for lengthened periods, the tensile strength is greatly increased: thus with iron twice re-melted and kept in fusion for 0 1

2

3 hours the tensile strength was = 15,861 20,420 24,383

25,733 lbs. per square inch. In another set of experiments, the time being =

1
11

2 hours
the tensile strength =
17,843
20,127
24,387

34,496 lbs. (9.) Cold-blast iron is considerably stronger than hot-blast iron; taking the former = 1.0, that of the latter was found at Lowmoor = .831; at Dowlais, .835: at Ystalyfera, 802. The deterioration in strength appears by the American experiments to be proportional to the temperature of the blast; thus the strength of cold-blast iron being 1.0, it is reduced to .865 with the blast at 150°, and to .807 at 250°, &c.

WROUGHT IRON AND STEEL. (10.) The strength of wrought iron increases, as might be expected, with repeated working in the fire and under the hammer. Mr. Clay found that the strength of a puddled bar being 1.0, it becomes 1.36 when piled three or four times, and 1.41 when piled six times; beyond that point, however, its strength declines, and is reduced to that of a puddled bar when piled twelve times.

The same authority has shown that with steel, the strength of a puddled bar being 1.0, it becomes 1.253 at the fourth piling, after which it declines and is reduced to 0.94 at the seventh piling.

(11.) “Welded Joints.—The strength of wrought-iron welded joints appears by the experiments of Kirkaldy to be very variable, the mean from eighteen experiments on bars from 11 to 1 inch diameter = .8066, the strength of a solid bar being 1.0; in extreme cases it is as low as •562, or little more than half, in others as high as • 974, the great difference being due no doubt to imperfect workmanship.

With steel the loss of strength by welding is still more considerable; the same authority shows that the strength of welded steel joints varies from •55 to • 404 of that of a solid bar. The strength of steel is also affected considerably by hardening, tempering, annealing, &c., as is shown by Table 1: when heated and quenched in oil, Mr. Kirkaldy obtained the extraordinary strength of 96.1 tons per square inch, which is exceptional and anomalous. The same steel made as hard as possible by being highly heated and quenched in water, gave 40.2 tons only: the mean for ordinary rolled or tilted bars being 47.84 tons per square inch: see cols. 3, 9 of Table 2.

(12.) “Screwed Bolts.”—There are two ways of measuring screwed bars, namely by the diameter at the top of the thread, or that of the plain bar before screwing, and by the diameter at the bottom of the thread: the former is the most convenient, and will be followed here. Mr. Kirkaldy obtained some curious results; he found that when the thread was chased in the lathe, or cut by new dies, the strength was nearly proportional to the diameter at the bottom of the thread as might be expected, and varied with different sizes, between 67 and 82 per cent of that of a plain bar, the mean being 72.5 per cent. But when old dies were used, the metal seemed to be compressed rather than cut, and the strength was much greater than with new dies, varying from 77 to 89 per cent. of that due to a plain bar, the mean being about 85. It will be the safest course to reckon the strength as due to new dies, or 72.5 per cent. of plain bar; see cols. 5, 6 of Table 2.

(13.) “Plate-iron and Steel.—Rolled plates of iron and steel are rather weaker than the same materials in the form of bars, as shown by Table 1; the ratio happens to be nearly the same for both: thus taking Kirkaldy's results, with wrought iron we

?

have 21.6 - 25.7 = 84, or 84 per cent., and with steel 38.4 : 47.8 = .80, or 80 per

cent. Effect of the Grain.”—Experiments have shown that the tensile strength of both wrought iron and steel, lengthways of the grain, is greater than that crossways, as shown by Table 1 : thus with wrought iron we have 22:6 = 20.6 = 1.097, or 9.7

per cent.; and with steel 40.1 = 36.6 = 1.096, or 9.6 per cent., being practically the same for both.

In arranging the plates for girders, &c., those subjected to tension should be cut so that the strain is in the direction of the grain, and in boilers, where the circumferential strain is double the longitudinal (71), the direction of the grain should be arranged accordingly.

(14.) Effect of Annealing.-It has been found by experiment that the effect of annealing wrought iron in the bar, plate, and chain form is to reduce the tensile strength : this is the more remarkable, being just the reverse of the effect on steel plate, which is to increase the strength as much as 55 per cent. (38). ,

By hammering cold the strength of wrought iron is much reduced, but by annealing it is partially restored : experiments at Woolwich show the effect of both processes on bars of different sizes : thus bars

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per square inch. The mean tensile strength of ordinary bar
iron is 25.7 tons per square inch by Table 1, hence the loss by
cold-hammering is
32

13
per cent.: the mean being 27 per cent. After annealing, the
strength became

36

[blocks in formation]

The tensile strength of plate iron also is reduced by annealing as shown by Mr. Kirkaldy's experiments on six kinds of Yorkshire iron , ], and inch thick, which gave the loss

5.6 per cent. lengthways, and 5.2 per cent. crossways of the grain ; Lowmoor giving 4.8 and 1.8; Bowling, 8.0 and 9.1 per cent. respectively.

The effect of annealing chain is shown by Nos. 19, 20 in Table 21, to be 16.34 • 17:54 = • 93, or 7 per cent. loss of strength (109)

CHAPTER II.

ON RIVETED JOINTS.

(15.) Riveting two plates of metal together may appear to be a very simple matter, but the fact is that there is more philosophy involved in it than is commonly supposed. The extreme importance of riveted joints, not only as applied to steam-boilers, but also to girders, railway bridges, and other structures will justify the most careful attention to the principles by which the strength is governed, and the proper proportions are fixed.

The strength of a riveted joint is dependent, first, on the tensile strength of the plate, measured at its weakest place, namely, through the line of the rivet holes ; second, on the shearing strength of the rivets; and third, on the friction of the plates against one another due to the pressure of the rivets.

(16.) Strength of Punched Plates.”—It has been found by experiment that when wrought-iron plates are punched cold in the usual way, the strength of the plate is reduced not only by the removal of the metal punched out, but also by the damaging of the fibres of the metal that remains between the rivet-holes. Direct evidence of this is given by the experiments on Yorkshire plates by Mr. Kirkaldy in Table 4, which shows that the mean loss was 13 per cent. with the grain, and 17.26 per cent. across the grain, this being the result of eighteen experiments on six kinds of iron by different makers, with plates , 1, and sinch thick. The plates were in all cases 8 inches wide, with four rivet-holes

TABLE 4.-Of the Loss of STRENGTH by PUNCHING RIVET-HOLES

in YORKSHIRE BOILER-PLATES.

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in a line, •85 inch diameter; hence the ratio of the solid part of the plate to the metal between rivet-holes was 8

{8 – (-85
(4

4} • 8 = 575 to 1.0. This proportion is about the same as that adopted in ordinary riveting, a fact which is important, for obviously, the damage to the fibres will be the greatest close to the rivet-holes, and will diminish with the distance: now when the holes are pretty close together as in ordinary riveting, we may suppose that the whole of the metal between them will be affected, but where the distance is very great, the metal at middistance may be wholly unaffected, and in that case the mean strength would be much greater than in others where the pitch of the rivets is small. Mr. Kirkaldy's experiments are the more conclusive because the strengths of the punched plates were compared with those of unpunched ones cut out of the same plate. Moreover, in order to avoid any possible loss of strength by

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