페이지 이미지
PDF
ePub

MODERN STEEL AS A STRUCTURAL MATERIAL.

By Mr. W. Worby Beaumont, Associate Member Inst. C. E.

From Papers of the Society of Engineers.

The object of this paper is not to tion of work when almost completed. bring forward any new experiments on Even twenty years ago, before the modthe properties of steel as employed in ern cheap steels had been offered, steel constructive work, but to invite discus- made by the crucible process had been sion on a few points which may occur to successfully employed on a small scale the engineer, to whom it seems desirable for boiler work. Very careful attention to consider the relative advantages of to the behavior of steel under treatment mild steels and iron in some mechanical had no doubt been observed, as it might or constructive work. For several years easily be on a small scale; but when, the substitution of steel for iron in vari- some years after, attempts were made to ous structural work has occupied the use Bessemer steel for boiler and bridge attention of engineers and metallurgists, work difficulties arose, some of which but the progress which has been made, were attributed to the nature of the macompared with the hopes that were terial and some to the want of thorough entertained when steel made by the care on the part of the workmen. Even Bessemer process became cheap enough when plates of this material cost more to make its cost, strength for strength, than iron for boiler construction, and comparable with that of iron, has not very much more than iron for bridge been great. There are classes of struct- and girder work, attempts were made to ures in which a reduction in weight use it for the latter on account of the would be a material advantage, even several advantages which would arise when attended with an increased total from a decrease in the permanent load cost. Ships, boilers, and girders may of bridge structures. Failures, howbe instanced as examples. In this direc- ever, of parts of these structures both tion, therefore, engineers turned their before and after completion, due to the attention, with a view to the application want of the observance of the special of steel. The difficulties, however, which treatment which the material demanded, attended the manipulation and connec- discouraged its employment. The ention of steel in structures largely com- deavors of steel makers during the past posed of plates checked its application. few years have, therefore, been directed Many failures attended the attempts to to the production of steel possessing use it, chiefly resulting from apparently properties so far similar to wrought anomalous behavior of the metal under iron that no great departure from the what were considered similar conditions, methods of working this material need but more, perhaps, owing to the special be made. Apart from the frequent want treatment which it required as compared of uniformity in the mechanical properwith iron. Those who had been long ties of the Bessemer steel plates of a few accustomed to the manipulation of iron years ago, it generally exhibited an were unable to depart from past practice absence of toughness, which not only sufficiently to operate upon the new made it difficult of manipulation in the material, with all the careful attention to hands of the plate smith, but caused its its different properties which these de- destruction after being built into a manded. Thus, although the difference structure. Its uncompromising rigidity in its behavior in the hands of the smith rendered it incapable of accommodating and plate worker, as compared with iron, itself to, and gradually eliminating from, was considered in several of the details itself, the residual strains originating in of manipulation, certain other peculiari- unequal heating and cooling, and also to ties were lost sight of in the assemblage the unequal strains visited upon it by of parts, which resulted in the destruc- other parts of a structure, of which it

was made to form a part, either by rivet- extension, or in proportion to the relaing or other modes of connection. It tion in which it combines toughness was not that the steel did not appear to with strength. Thus a metal which possess the qualities when tested in the possesses high ultimate strength and testing machine, but its behavior was high elastic limit, with small ranges of apparently different when employed in extension, will not have great structural full size pieces or plates, from that value, as it will not be capable of withexhibited when tested in small pieces in standing impact strains. Again, a metal the machine. This difference indicated possessing high ultimate strength with that there were differential internal low elastic limit will have small value for molecular strains resident in the whole structural purposes, as, though high plate which were eliminated when the ultimate strength is an essential propsame plate was cut into strips. This erty, the limit of elasticity determines in elimination, it was considered, could not almost all cases the sectional area, and, take place in the whole plate on account therefore, weight of metal required for a of the harshness of its material. Anneal- given duty. A high elastic limit, with ing was resorted to, but only with par- considerable elastic extension, must be tial success, the complex nature of the obtained, combined with great toughstrains in a plate of the material, even ness beyond that limit, or in other when cooled slowly, being sufficient, words, a high elastic limit must be when aggravated by extraneous strains attended by a considerable range of to cause its rupture. It is perhaps not extension both within and beyond that necessary here to consider the nature of limit, combined with high" ultimate the strains in a plate of iron or steel strength. resulting merely from slightly differen The tendency, however, in the productial contraction in cooling, but it may be tion of the modern mild steels and ingot observed that though small, these strains irons, which will eventually no doubt may have a material influence on the supplant wrought iron made by the pudstrength of a plate as a whole, and that aling process, is to make a metal which they would be greater in the thicker is far from combining these properties plates. This may be referred to here in such relations that it is best suited after.

for structural purposes, or so as to offer The necessity of imparting to steel those advantages which were at one time plates the toughness characteristic of supposed to be attainable by the use of good wrought iron has thus induced steel. steel manufacturers to devote their The best index to the comparative attention to the production of steel pos- structural value of different materials is sessing great ductility. It is on this probably that obtained by the applicapoint that much might be said, because tion of Poncelet's formulæ as simplified in the search for toughness the advant- and made applicable for practical purages which the employment of steel in poses by Robert Mallet. This formulæ some structures seemed at one time to

Te and Tr and promise, are being almost lost sight of. expresses by coefficients Te and TT Not only are these advantages likely to the work done by an extending or combe lost, but the great ductility being pressing force upon an elastic prismatic imparted to soft steels and ingot iron, body, at the point where its elasticity in search for this necessary toughness, becomes permanently impaired, and its seems likely to give us a material inferior form distorted, and at the further point in some essential qualities to much of the where rupture occurs, or, in other words, wrought iron hitherto in use. Tough- it expresses the balance in any material ness in plates has not been obtained, but between strength and toughness, and instead thereof great ductility attended was designed to give a more exact index with low elastic strength.

to the structural value of materials than It may be premised that the structural was given by the early experimenters value of a metal will be proportionate to down to the days of Telford, when the the degree in which it combines high force necessary to produce rupture was ultimate strength with high elastic limit alone considered. The formulæ are and wide range of elastic and ductile probably known to most of the members

e

=22.31,

=

[ocr errors]

of the society, having been given by Mallet in a paper “On the Coefficients of

PX 2240 x12 Elasticity and of Rupture in Wrought

Ter

2 Iron," published in vol. XVIII of the “Proceedings" of the Institution of or

0.02 Civil Engineers, and subsequently em

Ter

12 X 2240 x 12 ployed by Dr. Pole in his book on “Iron

2 as & Material of Construction," but they and

1.0 may be repeated here.

Tr=
22 x 2240 by 12

=2052.5, Let L = the length of the prism;

2 1 = the unit of length;

while with another material having an A -- the transverse sectional area elastic limit of, say, 14.22 tons, elastic of the prism;

extension 0.024 inch; ultimate strength, i = the extension, or compres- say, 23 tons, and total extension of 1.6

sion which is proportionate inch in a foot, the coefficient Te=31.8

1
to

and Tr 3501.
L
i

In order to determine the relative
P= the force of extension or structural values of several materials, it

compression which is pro- is thus necessary to take Te and Tr sim

portionate to A; and ultaneously into consideration, though e = the elastic resistance which Te is the coefficient of the greater im

balances P, or the modulus portance in most structural work.
of elasticity.

About a dozen years ago Bessemer The force P is, of course, variable be- metal was offered for bridge and ship tween 0 and i,

construction, which in the testing PL eA

machine showed an ultimate tensile i= and P i. eA

strength of from 34 to 40 tons per If x be any small extension, or compres- 23 tons, and a range of ductile extension

square inch; an elastic limit from 20 to sion less than the nth part of i, P corre- of from 10 to 18 per cent., while the

eA sponding to x= The work done in tests of plates considered suitable for

the shells or barrels of boilers showed extending, or compressing through the figures not much lower than these. The infinitely small additional range A'x failures which occasionally attend the

eA (assumed uniform)=

application of this steel, however, dis2 A'x, and the L

couraged the extension of its applicawhole work done when i=the compres- tion by engineers, who hoped that sion or extension at which the elasticity greater uniformity in the mechanical of the material is permanently changed, properties of the metal would gradually or the elastic limit reached, is

be obtained by the steel makers. A steel

of somewhat lower tenacity and greater

1 A
Ter
* A'x=

ductility, attended by great uniformity 2 L

in composition and behavior, was then Terlei", or simply=*Pi;

produced, and this indicated that steel

makers and engineers must look to steel for the unit of length and section. The of milder character for the removal of value of the coefficient Tr is arrived at in the difficulties which had attended the the same way, by substituting the corre structural application of cheap steels, sponding values for P and i due to the that is, steels not produced by the moment of crushing or of rupture. Thus crucible. The result of this was that for a material having under tensile strain engineers specifying steel for, say, an elastic limit P of 12 tons per square bridge-work, stipulated that it should inch, and elastic extension e of 0.02 inch not possess more than a certain maxiin a foot of length, and having an ulti- mum tenacity, a reversal of the stipulamate of 22 tons per square inch and a tion that had always and does obtain total extension of, say, i inch, the coeffi- with respect to iron. As a further recient

sult of this, and to ensure that the

[ocr errors]
[ocr errors]

Sra's

[ocr errors]
[ocr errors]

2

L

,

[ocr errors]

or

harder steels of comparatively high tenac- With respect to the latter, Messrs. ity, but less uncertain character, should Greig and Max Eyth observe that “the not be used in the construction of tensile and shearing strength of the bridges, the Board of Trade regulations material supplied for these tests by some upon the subject limited the tensile of the most experienced makers of steel, strain on any part of a structure to 7 and by them, no doubt, considered the tons per square inch. This has led to best for the purpose, has in the experithe endeavor on the part of all steel ments proved to be not more than 10 makers to produce the very mild soft per cent. above that of the iron supplied steels now largely used, some of which at the same time, and its want of hardafford the engineer no help towards pro- ness, as distinct from tensile strength, ducing the lighter structures which a' has proved to be a very serious disaddozen years ago it was promised that vantage in boiler work. What the trade steel would give them. Boiler shells now requires is a return to a harder must be made nearly or quite as thick material of increased tensile strength as if they were constructed of iron. without losing the homegeneity which is

As an instance in illustration reference as present obtained at the expense of may be made to the results of a series of hardness.” Again, in speaking of the experiments made on iron and mild steel, results of the test, under hydraulic in order to determine their respective pressure, of the complete boilers made values for high-pressure boiler construc-, of this steel, they say, " the compression tion, and described by Mr. David Greig of the rivet and elongation of the rivet and Mr. Max Eyth in a paper read hole resulted invariably in leakage which before the Institution of Mechanical prevented the necessary pressure from Engineers, in June, 1879. Amongst being obtained. Each rivet became its other experiments, the mechanical prop- own safety valve, and the strain put on erties of mild steel and Yorkshire plates the weakest part never reached more under tensile strain was determined. than 70 per cent. of the breaking strain. The plates—both iron and steel—were This is the point where additional hardobtained from Messrs. John Brown & ness of the material would be most useCo. and Messrs. Cammell & Co. In the ful, as it would prevent the opening of paper referred to the

tensile the rivet holes, which now makes a strength, elastic limit and extension are boiler useless long before the breaking given as follows:

strain is reached." This evidence is, Ultimate strength. Elastic limit. Extension therefore, to the effect that no structural

Tons per square inch. per cent. advantage is obtained by the use of Iron plates.....22.27

16.06

36.2 these steels in place of iron. Steel plates....25.80 16.74

28.3

If we turn now to the result of tests, The extension given is in a length of made by Mr. Kirkaldy, of Siemens' mild 6 inches. If all reference to the second steel, we find that the mean results for ary elastic limit, which may be induced plates of from 0.37 to 0.70 in thickness, by repeated application of strain up to annealed and unannealed are as follows: and slightly exceeding the elastic limit

Ultimate strength. Elastic limit. Extension. of each preceding test, be omitted, it

Tons per square inch.
14.50

23.4 will be seen that the so-called steels, to Unannealed....31.02

Annealed.. .28.84

12.84

24 6 which the above figures relate, present scarcely any structural advantage over From these figures it will be seen that iron. The elastic limit is nearly the though this material (like that used at same, and though its ultimate strength Messrs. Fowler's Works) may be very is greater than that of iron, its ductility, safe, it presents little structural advantas shown by its range of extension, age over iron plates, the limit of elasrenders its greater ultimate strength of ticity being low, permanent set taking little value; for very long before it can place at as low as 12.84 tons per square be strained to that limit, in any struc- inch when the plates are annealed. It ture, it stretches so much that the is, however, observable that though the accumulation of strain is prevented, and extension of this metal reaches a total this may be shown to take place in of 24.6 per cent. it is less than 11 per bridge as well as boiler structures. cent. annealed, and under 7 per cent.

mean

per cent.

unannealed at a strain of 26.78 tons, that even with some of the so-called thus indicating very considerable tough- mild steels a little misgiving is pardonness and probably greater value as a able on the part of the boilermaker, who structural material than that tested by wonders what he may find in a boiler Messrs. Greig and Max Eyth.

shop to-morrow where he has left a Material described still more recently nearly or quite finished boiler to-night. gives the engineer even less of hope that Not only, moreover, is this mild steel the modern steel for structural purposes way of getting over the difficulties atwill enable him to produce lighter struc-tending the use of the strong steels tures than he can do with iron. It likely to rob engineers of much that the would even appear that greater hope of latter material promised; but by the still obtaining a material of high structual existing custom of denominating what is value is to be found in iron; for it was really iron by the term steel, we are like recently stated at a meeting of the Iron to lose the advantages gained by using and Steel Institute by a member that he the harder steels where that has been had been experimenting with puddled possible, for already a good deal of soiron, the elastic limit of which ranged called steel, in which there is not much from 19 to 20 tons, while its ultimate more carbon than there is in a Lowmoor strength was 29 tons and its elongation bar, is being worked up into steel rails between these strains was 23 per cent. and steel tires. It is true that rails of in a length of 8 inches. The elongation very hard steel do not wear quite so well within the elastic limit was not men-, as those of a tougher quality, but it tioned. Unless a higher elastic limit can remains to be seen whether some of the be obtained no advantage in a structural very mild metal being rolled into rolls sense is secured. High ultimate strength to day may not soon acquire battered is a comparatively useless quality if the heads under the heavy traffic of modern elastic limit is low, and great range of locomotives. Such rails will not laminextension is also of no service. In ate, but it remains to be seen whether bridge work a very high range of ex- splitting will not result from the deformtension is useless, because the members ation by battering. under tension would elongate if strained It will have been observed that in this beyond the elastic limit, to a greater short paper the attention of the writer extent than the members under compres- has been almost wholly confined to ref. sion would compress, and thus the struc- erences to plates. This has been done ture would fail by the destruction of the because chiefly in plates improvement balance in the disposition of the strains has to be looked for from the steel makon the different parts.

Even if the , ers. With bar steel, the difficulties in material showed a range of compression manipulation and connection in strucequal to its range of extension this tures have not been so great as with would remain equally true, because in plates, and failures, whether due to imorder to secure the necessary resistance posed or internal differential molecular to bending or buckling, members under strains, have not been so frequent; seccompression must be of greater sectional tions are less in area and forms less caarea than is indicated by the static pable of initiating and transmitting strain. Thus a higher elastic limit is destructive strains. Consequently, mild the first essential in steel for structural bar steel need not be so ductile as mild purposes. This it appears can only be plate steel, and hence the structural obtained by making the steel harder, value of mild bar steel is much higher which, by the Bessemer process at least, than that of plates, and engineers can if not by other largely used processes, it use it to advantage. In the course of a seems cannot be secured without other very interesting lecture delivered by Dr. attendant qualities or properties w ich Siemens at the Royal United Service Inmake the material difficult to work, and stitution in March, 1879, he gave a table more or less uncertain in its behavior showing the results of a serious of tests under mechanical treatment. Whether of mild and hard steel bars up to, and this is really now so, or must continue slightly beyond their elastic limit. This so, it remains for the steelmakers to table is of great value as affording evishow, but it does at least seem plain, I dence of the structural value of these

« 이전계속 »