« ForrigeFortsett »
PRINTED BY PEWTRESS & Co.,
Steam Printing Works, 15, GREAT QCEAN STREET, INCOLN'S INN FIELDS, W.C THE
VOLUME XLVIII.-NO. I.
STEEL FOR SHIPBUILDING.
(Continued from page 968, Vol. 47.)
HE paper read at the meeting of the “ Iron and Steel
Institute," by Mr. Daniel Adamson, on the mechanical and other properties of iron and mild steel is fully
as interesting as that by Professor Akerman, to which we have already referred. Mr. Adamson has conducted a large series of experiments on iron of various qualities, on mild steels, and on steel more highly carbonised, and his experiments afford much information as to the behaviour of the several metals when broken by the ordinary tensile test, when exposed to concussive strains, and when subjected to corrosion in a bath of diluted sulphuric acid, the experiments in each case being of especial value from the fact that a statement of the exact chemical composition of each plate is given. He has also experimented upon the mechanical properties of various kinds of the metal when subjected to a temperature of about 600° Fahr.
In the first series of experiments a cast iron anvil block was used having a hole gouged out of its upper surface in the form of a segment of a sphere ten inches in diameter, and four inches deep. The plate to be experimented upon was laid upon the anvil, and
then gun-cotton was exploded at a height of ten inches above it, the result of course being that the plate was bent or broken so as to be forced down into the hole in the anvil. Various kinds of metal were experimented upon in this way, and the specimens were exhibited by Mr. Adamson as illustrating the capability of the respective metals for enduring a great concussive force. The metals compared were four specimens of iron, two being of best best boiler plate, one of special class Yorkshire iron, and one of special class Lancashire iron. The steels were two specimens of Bessemer mild steel, one of Martin-Siemens mild steel, and two plates of crucible steel having, however, a low percentage of carbon. One of the crucible steel plates was not annealed, all the other steel plates were.
The result of these experiments was that the four iron plates were cracked and broken. Two of the annealed mild steel plates stood the effect of the concussion without a crack, the metal being bent and stretched into a hollow half the depth of the cup in the anvil. These plates were then turned upside down, and a second charge of gun-cotton exploded over them, with the result of producing a second smaller hollow opposite to the first, and still without any cracks. The annealed crucible steel plate gave results nearly as favourable. The unannealed mild steel plate, however, was cracked and broken as much as the iron plates, and one of the annealed Bessemer mild steel plates was also as much cracked and broken as the Lancashire iron plate. Mr. Adamson finds an explanation of the failure of the last-named Bessemer plate in the fact that the analysis shows that it had three times as much sulphur and phosphorus in it as any of the others; it contained •126 per cent. of sulphur and .154 per cent. of phosphorus. It must be remarked that the iron plates used were a sixteenth inch thicker than the mild steel, which last were three-eights of an inch thick. Taking this into account, we can hardly see that the experiments were as favourable to the use of mild steel as would appear at first sight. They prove that it is possible to produce a metal, some specimens of which shall show an extraordinary power of enduring sudden and violent strains ; but one plate, which had slipped in with the others, failed as much in the test as the
ordinary iron plates, the reason only appearing after a minute and careful chemical analysis. The steel plate, which was not annealed, though of as good material as the other, stood the test po better than the iron plates.
Pieces of metal of circular shapes were cut out of each of the plates, and a hole being drilled in each, of the size rivet that would be employed in plates of that thickness, the hole was enlarged by a tapering punch being driven into it till the ring burst. The mild steel when annealed showed itself under the test to be much superior to the iron, but here again the purest plates, that is, those containing least phosphorus and sulphur, came off best.
The results of these tests, as might be expected, brought up the question of annealing in the subsequent discussion, and it was stated that the Admiralty have their mild steel annealed after punching in the case of work done in the Royal dockyards, but do not insist upon it in the case of work done for them by private shipbuilders, believing that unless their own officers can so carefully supervise annealing as to make sure that it is properly performed, it had better not be attempted.
We certainly are of opinion that in an ordinary shipbuilding yard it would be impracticable to make use of metal which required such tender treatment.
Mr. Adamson's tensile tests of steel, &c., are, as we have said, valuable, because an exact chemical analysis was made of each, the metal in each plate tested. Various kinds of iron of different degress of purity, mild steels in which the proportion of carbon varied from .03 to .22 per cent., and hard steel containing as much as •4 per cent. of carbon, were tested with results decidedly in favour of the mild steels, more especially the purest and mildest of them. The hard steel stood a maximum strain of 53 tons per square inch, and did not take permanent set till it was subjected to 27 tons per square inch, but its percentage of elongation before breaking was only 14.5, whereas the elongation of the mild steel specimens varied from 21 to 31 per cent., the length in each case being ten inches. The tests prescribed by the Admiralty and Lloyd's for mild steel require a minimum elongation, before fracture, of 20 per cent. in a length of eight inches.
With reference to his experiments upon bending iron when heated to a temperature of about 600° Fahr., Mr. Adamson remarks :
“ It is desirable that the worker of metal should have some experience of the best working heat of any iron or steel that may be under manipulation. A knowledge of the mechanical endurance at variable temperatures is also important to every one, as life and property depend as much upon the efficiency of metallic structures. Few or no malleable metals, such as wrought iron or mild steel, can be found in the open market that possess a range of endurance at all varying temperatures, say from cold up to red heat, but nearly all ordinary bar or boiler iron and mild steels will endure considerable percussive force when cold, and up to 450° Fahr., after which, as the heat is increased, probably to near 700 degrees, they are all more or less treacherous and liable to break up suddenly by percussive action. The poorer class of metals at this temperature, which may be called a colour heat, varying from a light straw to a purple and dark blue, are simply rotten. The colour heat test ought to be impressed upon all workmen to prevent the hammering of metals when half cold, or the heating of iron by red-hot iron for some final adjustment; where hammering is required it would be a better and wiser policy to only heat the iron with boiling water, or by applying steam against the surface a short time. Finishing forgings or smith's work by hammering at a black heat at all times proves highly injurious unless great care is afterwards used in annealing, and it is questionable then whether the full measure of strength of the metal in many cases is ultimately restored."
The experiments as to the corrosion of iron and mild steel referred to by Mr. Adamson, were carried out by the Weardale Iron Company, Spennymoor, Durham. The pieces of metal were placed in a water bath containing one per cent. of sulphuric acid, for seventeen days, the amount of corrosion being noted each day. The specimens tested were one of common iron, containing as much as 5 per cent. of phosphorus, two specimens of superior irons, one of hard steel, one of mild steel, and one of a very pure iron, containing as much as 99.9 per cent. of pure metallic iron.