Modern Machine-Shop Practice Part 100

Fig. 1430.]

The plate was subjected to fifty heatings to redness and subsequent coolings in water of ordinary temperature. At every tenth cooling accurate measurements were taken of the contraction in superficial dimensions, and Fig. 1430 shows the final form after fifty coolings. The intermediate measurements at every tenth cooling showed a uniform and gradual decrease in the superficial dimensions, but the thicknesses were only measured after the fifty coolings had been completed. The thickness appears to have varied considerably; in some places, notably towards the centre and outside edges, being much reduced. Between the centre and outside edges the thickness appears to have increased, and in some few places the plate has been split open. The average dimensions in inches before and after the experiment were as follows (dimensions of cracks being allowed for):--

----------------------+---------+----------+----------+----------- | | | | Cubic | Average | Average | Average | inches | length. | breadth. |thickness.|capacity.

+---------+----------+----------+---------- | Inches. | Inches. | Inches. | Original | 11.995 | 5.98 | .74 | 53.08 After 50 coolings | 11.25 | 5.59 | .774 | 48.72 Per cent. variation { |Decrease |Decrease | Increase | Decrease from original { | of | of | of | of { |6.2 p. c.|6.52 p. c.| 4.6 p. c.| 8.2 p. c.

"Three triangular pieces of iron were then cut out of the plate from positions indicated on the diagram; No. 1A from the part most reduced in thickness, No. 3A from the part most increased in thickness, and No. 2A from a part where the thickness was a mean between the thickest and thinnest part. The specific gravities were accurately determined as follows:--

No. 1A 7.552 thinnest part.

No. 2A 7.574 average thickness.

No. 3A 7.560 thickest part.

"The average of these specific gravities is 7.562.

"The average before experiment was 7.64. Hence the average loss in specific gravity has been 1.02 per cent.

"The small triangular piece No. 1A, specific gravity 7.552 (already subjected to fifty heatings when forming part of the solid plate), was next heated and cooled fifty times more. The specific gravity at the end of the one hundred total coolings was 7.52, being .43 per cent. lower than after fifty heatings in plate, and 1.57 per cent. lower than 7.64, the original mean specific gravity of the plate.

"The same piece, 1A, was then heated twenty-five times more, making 125 in all. On taking the specific gravity it was found to be 7.526, or practically the same as after 100 total heatings and coolings.

"It thus appears that there is an undoubted decrease in specific gravity on repeated heating and cooling as described up to one hundred coolings, the specific gravity decreasing as much as 1.57 per cent.; that this percentage appears to be less when the pieces of iron operated upon are very small; that while there is a decrease of specific gravity there is also a decrease of total volume.

[Ill.u.s.tration: Fig. 1431.]

"From the above it was evident that the volume was affected by several causes:--

"1. By the permanent contraction of the outer skin, either the volume would be lessened, or relief by bulging out the sides must occur.

"2. By the decrease of specific gravity an increase of volume must occur, which could also find relief in bulging.

"3. A diminution of the whole ma.s.s must occur through scaling of the surface.

"Having determined the change in specific gravity by Experiment 6, we only now want to determine the loss of volume due to surface scaling, and we can then infer the actual contraction of the outer skin.

"_Experiment No. 7._--To ascertain the amount of scaling which took place in heating and cooling under same conditions as Experiment No. 6, a wrought-iron plate was cut from the same piece as No. 6, thickness .74 in., planed on both surfaces and all edges to a form nearly rectangular, and to the dimensions given in Fig. 1431.

"The only difference (except the very small difference in the dimensions) between this and 1430, was that the princ.i.p.al grain of the iron was in 1431 in the direction of the arrow, whereas in the other it was lengthwise of the plate.

"This piece was subjected to fifty heatings to redness and sudden coolings in water of ordinary temperature, as in the case of No. 6. The change in form was exactly the same in general character, but the contraction was not quite so great either in length or breadth; the increase in thickness, however, was proportionately greater, the volume (measured by displacement of water) after fifty heatings being 48.6 cubic inches, which is nearly the same as in No. 6 after the same number of heatings. The weight of the piece:--

Avoirdupois.

lbs. oz. dr.

Before heating 14 10 15 After fifty heatings 13 5 10 ------------ Difference 1 1 5

"This represents a loss of 9.07 per cent. of the original weight by scaling, and upon the whole original surface (sides and edges) represents a thickness of .0284 of an inch for the fifty immersions, or .00057 of an inch for the thickness of the film lost at each immersion over the whole surface.

"Calculating the weight of No. 6 before and after experiment from the volumes and specific gravities, we find the following:--

Mean Weight of specific cubic inch Volume. gravity. water. Pounds.

Weight before heating should be 53.08 7.64 .036 = 14.599 " after " " 48.72 7.562 .036 = 13.262 ------ Difference in weight 1.337

the ascertained difference in the case of No. 7 being 1.332, thus sufficiently accounting for the discrepancy between specific gravity and change of volume by the scaling.

"By Experiment 7 it has been shown that the loss of thickness due to scaling after fifty immersions was .0284 inch over the whole surface (sides and edges.) Therefore, a.s.suming this scaling as uniform over the surface, the girth, whether measured lengthwise or breadthwise, should be eight times .0284, or .23 inch less after immersion than before. Now the gross loss of girth is:--

--------------------------------------+-----------+------------ |Lengthwise.|Breadthwise.

--------------------------------------+-----------+------------ | Inches. | Inches.

In No. 6 | 1.38 | .86 In No. 7 | 1.2 | .52 +-----------+------------ Or for both experiments a mean of | 1.29 | .69 Deducting from them the loss of | | girth due to scaling | .23 | .23 +-----------+------------ Net contraction after fifty immersions| 1.06 | .46 Or in percentage of original girths, | | which were | 25.46 | 13.43 | per cent. | per cent.

We have a percentage of | 4.16 | 3.42 Or for each immersion an average of | .083 | .07 --------------------------------------+-----------+------------

"Comparing these results with those of Experiments Nos. 1, 2, and 5, we find that the contraction of the skin of the plate is less for each immersion than that of a bar or hoop, in the proportion of .125 to .083.

This is what might be expected, as the contraction of the plate is resisted by the volume of heated matter inside, which is eventually displaced by bulging, while the bar finds relief endwise without having to displace the interior.

"We have now before us the following facts, substantiated by the experiments described:--

"1. That in heating to redness, and then cooling suddenly in water at ordinary temperatures, bars and plates of wrought iron, a reduction of specific gravity takes place, the amount being about 1 per cent. after fifty immersions, and 1.57 per cent. after one hundred immersions, further heatings and coolings not appearing to produce further change.

"2. That a reduction of the surface takes place after each heating and cooling, this being due to two causes:--

"_a._ The scaling of the surface, which is shown to amount to a film over the (sides and edges) entire area of .00057 inch in thickness for each immersion, or 0.284 inch for fifty immersions (Experiment 7).

"_b._ A persistent contraction, which takes place after each immersion.

This varies according to the form of the iron, being in plates from .07 per cent. to 0.83 per cent. (Experiment 6), while in long bars it varies from .122 to .15 per cent. (Experiments 1, 2, and 5). This contraction continues vigorously up to fifty immersions, and probably much farther.

"3. That in the case of plates a bulging takes place on the largest surfaces, increasing the thickness towards the centres, although the edges diminish in thickness.

"4. That wrought-iron bars heated to redness, and allowed to cool slowly in air, do not show any change in dimensions (Experiment 3).

"The reduction of specific gravity, and the bulging out of the sides, have been explained as follows by the learned Secretary of the Royal Society, Professor Stokes, who has taken considerable interest in these experiments, and who has kindly allowed the author to publish the explanation:

"'When the heated iron is plunged into water, the skin tends everywhere to contract. It cannot, however, do so to any significant extent by a contraction which would leave it similar to itself, because that would imply a squeezing in of the interior metal, which is still expanded by heat, and is almost incompressible. The endeavor, then, of the skin to contract is best satisfied, consistently with the retention of volume of the interior, by a contraction of the skin in the two longish lateral directions, combined with a bulging out in the short direction. The still plastic state of the interior permits of this change.

"'Conceive an india-rubber skin of the form of the plate in its first state, the skin being free from tension, and having its interior filled with water, treacle, or pitch. I make abstraction of gravity. It would retain its shape. But suppose, now, the india-rubber to be endowed with a tension the same everywhere similar to that of india-rubber that has been pulled out, what would take place? Why, the flat faces of considerable area, being comparatively weak to resist the interior pressure, would be bulged out, and the vessel would contract considerably in the long directions, increasing in thickness. This is just what takes place with the iron in the first instance. But when the cooling has made further progress, and the solidified skin has become comparatively thick and strong, the further cooling of the interior tends to make it contract. But this it cannot well do, being encased in a strong hide, and accordingly the interior tends to be left in a porous condition.'

"The reduction by scaling does not require any explanation. The only fact which appears unaccounted for is this persistent contraction of the cooled iron skin, which does not appear to be explicable on any mechanical grounds; and we are, therefore, obliged to look upon it as the result of a change in the distance of the molecules of the iron, caused by the sudden change of temperature in the successive coolings.

"Our next subject is the curious effect of cooling bars or rings by partial immersion in water. Bearing in mind the results at which we have arrived, viz., that wrought iron contracts when immersed in water after heating, and that when allowed to cool in air it remains of the same dimensions, let us ask what would be the behavior of a bar or circular hoop of iron cooled half in water and half in air, the surface of the water being parallel to the fibre and at right angles to the axis of the hoop?

"Arguing from the results of Experiments 1, 2, and 5, it might be expected that the lower portion cooled in water would suffer permanent contraction; and, arguing from Experiment 3, that the upper or air-cooled edge would not alter. This apparently legitimate conclusion is completely disproved by experiments. This will be seen by a reference to Experiments 8, 9, and 10.

[Ill.u.s.tration: Fig. 1432.--Experiments with a circular hoop of wrought iron. Appearance of the hoop at the beginning.]

"In No. 8 a circular hoop of wrought iron was forged out of a 3-1/2-inch by 1/2-inch bar, the external diameter being about 18 inches, the breadth, 1/2 inch, being parallel to the axis of the hoop. This hoop, Fig. 1432, was heated to redness, then plunged into cold water half its depth, the upper half cooling in air. The changes in the external circ.u.mference of the hoop were accurately measured after each of twenty successive coolings, at the end of which the external circ.u.mference of the water-cooled edge had increased 1.24 inches, or 2.14 per cent. of its original length, and the air-cooled edge had contracted 7.9 inches, or 13.65 per cent.

"_Experiment No. 8._--Wrought-iron hoop, 3-1/2 inches by 1/2 inch by about 18 inches in diameter, or exactly 57.85 inches in circ.u.mference at top, and 57.95 inches at bottom edge.

-------------+--------------------+--------------------+-------------- | Top Edge. | Bottom Edge. | +--------+-----------+--------+-----------+ |Contrac-|Percentage | Expan- |Percentage | | tion. |of original| sion. |of original| | | circ.u.m- | | circ.u.m- | Remarks.