Modern Machine-Shop Practice Part 99

The bands for this purpose are usually of wrought iron, and require in the case of irregular surfaces to be driven on by hammer blows, so that the fit may be correct. As the band is forced on a heavy hammer is held against it, to prevent its moving back and off the work as the other parts are forced on.

[Ill.u.s.tration: Fig. 1428.]

Very slight bands may be forced on by levers: thus, wagon makers use a lever or jack, such as in Fig. 1428, for forcing the tires on their wheels. The wheel is laid horizontally on a table as shown, and the tire A forced out by the vertical lever, the arm B affording a fulcrum for the lever, and itself resting against the hub C of the wheel.

The following extracts are from a paper read by Thomas Wrightson, before the Iron and Steel Inst.i.tute of Great Britain.

"The large amount of attention bestowed upon the chemical properties of metals, and the scientific methods adopted for their investigation, have led to the most brilliant results in the history of iron and steel industries. It must not, however, be overlooked that iron and steel have highly important properties other than those which can be examined by chemical methods. The cause for so little having been done in accurate observation of the physical properties of iron is twofold: 1. The molecular changes of the metals are so slow, when at ordinary temperatures and when under ordinary conditions of strain, that reliable observations, necessarily extending over long periods, are difficult to obtain: 2. When the temperatures are high--at which times the greatest and most rapid molecular changes are occurring--the difficulties of observation are multiplied to such an extent that the results have not the scientific accuracy which characterizes the knowledge we have of the chemical properties of metals.

"The object of the present paper is to draw attention to some phenomena connected with the physical properties of iron and steel, and to record some experiments showing the behavior of these metals under certain conditions.

"In experimenting the author has endeavored to adopt methods which would, as far as possible, eliminate the two great difficulties mentioned.

"It is obvious that the possible conditions under which experiments may be made are so numerous that all which any one experimenter can do is to record faithfully and accurately his observations, carefully specifying the exact conditions of each observation, and this must eventually lead to a more complete knowledge of the physical properties of the metals.

"The author's observations have been led in the following directions:--

"1. The changes in wrought and cast iron when subjected to repeated heatings and coolings.

"2. The effect upon bars and rings when different parts are cooled at different rates.

"3. These changes occurring in molten iron when pa.s.sing from the solid to the liquid state, and _vice versa_.

PART I.

"To ill.u.s.trate the practical importance of knowing the effects of reiterated heating and cooling on iron plates, one of the most obvious examples is the action of heat upon the plates of boilers which are alternately heated and cooled, as in use or otherwise. When in use, the plates above the fire are subjected to the fierce flame of the furnace on one side, and on the other side to a temperature approximating to that of the steam and water in the boiler. Where the conducting surfaces of the metal are thickened at the riveted seams, a source of danger is frequently revealed in the appearance of what are known as 'seam-rips.'

"The long egg-ended boilers, much used in the North of England, are very subject to this breaking away of the seams. From some tests made by the writer on iron cut from the plates of two different boilers which had ripped at the seams, and one of which seam-rips had led to an explosion resulting in the destruction of much property, though happily of no lives, it was found that the heat acting on the bottom of the boiler had, through time, so affected the iron at the seam as to make it brittle, apparently crystalline in fracture, and of small tensile strength. Farther from the seam the iron appeared in both cases less injuriously affected. But although the alternate heating and cooling of the plates over a long period had produced this change in the molecular condition of the iron, a method of restoration presents itself in the process of annealing. In subjecting the pieces cut from the seam-rips to a dull red heat, and then allowing them to cool slowly in sawdust, the writer found that the fibrous character of the iron appeared again, and renewed testing showed that the ductility and tensile strength were restored.

"The same process of annealing is equally effectual in restoring the tenacity of iron in chains rendered brittle, and apparently crystalline, by long use, and is periodically applied where safety depends upon material in this form. Thus the heating and cooling of iron may be looked upon as the bane or the antidote according to the conditions under which the process is carried out. This affords an example of the importance of the physical effects produced by repeated changes of temperature. The change effected by one heating and cooling is so small that a c.u.mulative method of experiment is the only one by which an observable result can be obtained, and this is the method adopted by the writer in the investigation now to be described.

"It is well known that if a wrought-iron bar be heated to redness, a certain expansion takes place, which is most distinctly observed in the direction of its length. It is also known, although not generally so, that if a bar be thus heated and then suddenly cooled in water, a contraction in length takes place, the amount of this contraction exceeding that of the previous expansion, insomuch that the bar when cooled is permanently shorter than it originally was. If this process of heating and cooling be repeated, a further amount of contraction is found to follow for many successive operations.

"Experiments Nos. 1 and 2 were made to verify this, and to show the increment of contraction after each operation.

"EXPERIMENTS ON WROUGHT-IRON BARS 1-1/8 IN. SQUARE BY 30.05 IN. LONG, HEATED TO A DULL RED, THEN COOLED SUDDENLY IN WATER.

------------------+------------------------+------------------------ | EXPERIMENT NO. 1. | EXPERIMENT NO. 2.

| Common Iron. | Best Iron.

+------------+-----------+------------+----------- | |Percentage | |Percentage |Contraction.|on original|Contraction.|on original | | length. | | length.

------------------+------------+-----------+------------+----------- | Inches. | | Inches. | After 1st cooling | .04 | .13 | .04 | .13 " 2nd " | .10 | .33 | .10 | .33 " 3rd " | .16 | .53 | .14 | .46 " 4th " | .17 | .56 | .16 | .53 " 5th " | .23 | .76 | .20 | .66 " 6th " | .28 | .93 | .24 | .80 " 7th " | .31 | 1.03 | .27 | .89 " 8th " | .33 | 1.10 | .30 | 1.00 " 9th " | .40 | 1.33 | .33 | 1.10 " 10th " | .47 | 1.56 | .39 | 1.30 " 11th " | .52 | 1.73 | .42 | 1.40 " 12th " | .54 | 1.80 | .47 | 1.56 " 13th " | .58 | 1.93 | .51 | 1.70 " 14th " | .62 | 2.06 | .54 | 1.80 " 15th " | .68 | 2.26 | .56 | 1.86 ------------------+------------+-----------+------------+-----------

"The Table of Experiment No. 5 shows that at the twenty-fifth cooling a contraction of 3.05 per cent. had taken place, or an average of .122 per cent. after each cooling. This is almost identically the same average result as shown in Experiment No. 1 with straight bars.

"The above experiments only having reference to the permanent contraction of the iron in the direction of its length, the author made the following experiments to ascertain the effect in the other dimensions, and to see whether the specific gravity of the iron was affected in the reduction of dimensions.

[Ill.u.s.tration: Fig. 1429.]

"_Experiment No. 6._--Wrought-iron plate, .74 inch thick, planed on both surfaces and all edges to a form nearly rectangular, and of the dimensions given in Fig. 1429.

"_Specific Gravity._--Two small samples were cut out of different parts of the same piece of plate from which the experimental piece was planed, and the specific gravity determined as follows:--

No. 1 piece 7.629 } No. 2 piece 7.651 } Mean, 7.64.

"_Quality._--Subjecting a piece to tensile strain in the direction of the grain, it broke at 21.2 tons per square inch of section, the ductility being such that an elongation of 8.3 per cent. occurred before fracture, with a reduction of 9.6 per cent. of the area of fracture.

This may be looked upon as representing a fairly good quality of iron.

"A bar of wrought iron, 1-1/8 inches square and 30.00 inches long, was heated to redness, and then allowed to cool gradually in air.

Measurements after each of five coolings showed no perceptible change of length.

"_Experiment No. 4._--Wrought-iron bar, 1-1/8 inches square by 30 inches long, heated to a white heat and cooling gradually in air.

------------------+------------+----------------+---------------- |Contraction.| Percentage on | Remarks.

| |original length.| ------------------+------------+----------------+---------------- | Inches. | | After 1st cooling | No change. | | ---- " 2nd " | " | | ---- " 3rd " | .02 | .07 | ---- " 4th " | .05 | .17 | ---- " 5th " | .05 | .17 | ---- ------------------+------------+----------------+----------------

"It may be remarked, that if the bars be heated to white heat a slight contraction does occur, as shown by Experiment No. 4, where a bar of the same dimensions as No. 3 contracted .17 per cent. after the fifth cooling. As, however, the further remarks on this subject have only reference to bars heated to redness and then cooled, the writer would summarize the results of Experiments Nos. 1, 2, and 3, by stating that wrought-iron bars heated to redness permanently contract in their length along the fibre when cooled in water of ordinary temperature; but when cooled in air, they remain unchanged in length.

"To show that this is true as applied to circular hoops, Experiment No.

5 was made upon a wrought-iron bar of 1-1/8 inches square in section, welded into a circular hoop, 57.7 inches outside circ.u.mference.

"_Experiment No. 5._--Wrought-iron hoop, 1-1/8 inches square by 57.7 inches outside circ.u.mference, heated to a dull red, then cooled suddenly in water.

------------------+------------+--------------+----------------------- | |Percentage of | |Contraction.| original | Remarks.

| |circ.u.mference.| ------------------+------------+--------------+----------------------- | Inches. | | After 1st cooling | .06 | .10 | Red heat.

" 2nd " | .06 | .10 | This was nearly white, " 3rd " | .16 | .28 | but before cooling " 4th " | .26 | .45 | red hot.

" 5th " | .35 | .61 | " 6th " | .46 | .80 | " 7th " | .54 | .93 | " 8th " | .60 | 1.04 | " 9th " | .68 | 1.18 | " 10th " | .76 | 1.32 | " 11th " | .80 | 1.38 | " 12th " | .87 | 1.51 | " 13th " | .94 | 1.63 | " 14th " | 1.00 | 1.73 | " 15th " | 1.08 | 1.90 | " 20th " | 1.30 | 2.25 | On opposite edge 1.66; " 25th " | 1.76 | 3.05 | hoop splitting.

"This hoop was heated to redness and cooled in water twenty-five times, the circ.u.mference of the hoop being accurately measured after each cooling.[23]

[23] The lengths of circ.u.mference were taken, in this and other hoops, after each cooling, by encircling the periphery with a very fine piece of "crinoline" steel, the ends of which were made just to meet round the original hoop. By again encircling the hoop with the same piece of steel the expansion was shown by a gap between the ends, and a contraction by an overlap, either of which was measured with great accuracy by means of a finely divided scale.

"Two wrought-iron bars, 1-1/8 inches square and 30.05 inches long, were selected.[24] No. 1 was of common "Crown" quality; No. 2 of a superior quality known as "Tudhoe Crown." These bars were heated to redness in a furnace and then plunged into water of ordinary temperature, the length being accurately measured after each cooling. After fifteen heatings and coolings the permanent contraction on No. 1 bar was 2.26 per cent. of the original length, and that on No. 2 bar 1.86 per cent., or an average on the two bars of about .13 per cent. after each cooling, the increment of contraction being nearly equal after each successive operation. It is noticeable that after the first two coolings the better quality of iron did not contract quite so much as the common quality, and that in the latter the contraction was going on as vigorously at the fifteenth as at the first cooling.

[24] In some of these experiments the original sizes of the iron were only measured with an ordinary foot-rule, in which case the dimensions are given in the ordinary fraction used in expressing the mercantile sizes of iron. When accurate measurement was taken decimals are invariably used both in this paper and the Tables of Experiment.

"Similar bars of wrought iron, heated to redness and then allowed to cool in air at ordinary temperature, do not appear to suffer any permanent change in their length.

"Experiment No. 3 was made to verify this.

"_Experiment No. 3._--Wrought-iron bar, 1-1/8 inches square by 30 inches long heated to a dull red and cooled gradually in air.

------------------+------------+----------------+----------- |Contraction.| Percentage on | | |original length.| Remarks.

------------------+------------+----------------+----------- After 1st cooling | No change. | ---- | ---- " 2nd " | " | ---- | ---- " 3rd " | " | ---- | ---- " 4th " | " | ---- | ---- " 5th " | " | ---- | ---- ------------------+------------+----------------+-----------

[Ill.u.s.tration: _Wrought iron rectangular plate. 14" thick 11" 995 598 planed on both surface and edges. Heated to redness, and cooled in water 50 times. The dotted lines show original form, the black lines the form after the experiment._

(Two-ninths of full size.)