Notes on Iron and "Mild Steel" Castings for Motor Construction.
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The most useful metal which the engineer has at his command is that which is produced, in a blast furnace, from natural ores, and this metal is known as cast or pig iron. It is never in a pure state, but always contains carbon, phosphorus, sulphur,
silicon, manganese and other elements, each of which affects the ulti mate results in a different manner.
For instance: if the carbon be combined with the iron and form an alloy,
a hard and brittle " white " iron is
produced, but, if it be present only as graphite and distributed through out the entire mass, the iron will be soft. and " grey," but not so strong as the " white " iron. The amount of phosphorus is not harmful if less than one per cent., but, in general, it weakens the castings. Sulphur leads to hard and unsound castings, and also interferes with the free " flow " of the metal ; the amount should never exceed one-half per cent. A large quantity of silicon leads to " stiff " and weak castings, but the amount of silicon, and the rate at which the castings are allowed to cool, determines the ratio of " graphitic. " to " combined " carbon in the casting. Manganese increases hardness and makes the iron capable of holding more carbon.
Useful Properties of Cast Iron.
Cast iron, when freed from excessive quantities of the above-mentioned elements, has many useful properties, and one of the most important of these is that it expands slightly during the process of solidification, thus taking a faithful copy of the impression left in the mould by the pattern. After the process of solidification is completed, the metal commences to shrink; it continues to do so until the casting is quite cold, the amount of contraction in all cases being greater than the expansion during the solidification stage.. The tensile strength of cast iron is too low to be of much use for motor vehicles.
Taking the pig iron as the principal raw material, metallurgists and steel makers have, on the other hand, devised means for the almost-complete purification of the metal and, by the subsequent addition of such elements as nickel, vanadium, molybdenum, tungsten, aluminium, etc., the conversion of the refined iron into highgrade steels suitable for the making of forgings and stampings for a variety of purposes in machine construction. Some details of machinery, however, are necessarily of such intricate formation that it is almost impossible to make forgings or stampings with shapes anything approaching those of the finished articles; these parts must, therefore, at great cost, he machined from solid " blanks." Under such circumstances, many an engineer has been tempted to employ steel or malleable-iron castings, which latter material is really a very crude form of wrought iron, or mild steel, obtained he the decarbonisation of
ordinary " " iron castings. The castings are made in the ordinary way, and they are then embedded in oxide of iron, or peroxide of manganese, and heated te a re:I heat until most of the carbon is removed from the surface of the castings. This operation may take anything from three days to three weeks for completion. Since all the strength of this material lies in the 'skin," as little machining as is absolutely necessary should be clone.
"Crucible" Castings of Mild Steel.
During the past few years excellent results have been obtained from castings whieh have been produced by means of the fusion of wrought-iron scrap in plumbago crucibles. This process has for its object the production of a metal which has all the properties of mild steel, or wrought iron, but which metal, unlike steel or wrought
Specimens cast by the Stassano process.
iron, can he run off from the crucible into moulds for the making of castings (in sand) that have a carbon percentage of 0.05 to (1.07. About 65 to 75 per cent, of the metal at each " melt " is marketable, and even the remainder, made up of " gates " and " runners," can be re-melted and again used. The system is. accordingly, very economic-al. Provided that the metal be run off from the crucible with as little delay as possible after the melting process is completed, and that the sand of the mould be mixed with a suitable refractory material and provided with ganisters, etc., in order to prevent the sand's " burning on " the faces, the castings are clean and smooth, free from blow-holes, and perfectly homogeneous. The temperature of the " melt "-3,000 degrees Fahrenheit—nnist be maintained until the moment of pouring into the moulds. Unlike malleable cast iron, castings by the crucible process have no hard skin they are uniform throughout the metal, and, therefore, do not require annealing. Consequently, there is practically no warping or distortion of the castings. This, of course, offers a very great advantage, and may often lead to a considerable saving in time and cost for finishing. The amount of tooling required on castings made by this process, as compared with forgings, is very slight, as the absence of hard skin permits of a comparatively small allowance for machining.
"Adjusting the Melt."
" Mild-steel " castings may be made of almost any tensile strength, as it is an easy matter, while the metal is still in the molten state, by the addition of a small quantity of one or more of the elements which impart the properties of high-grade steel, to adjust the mixture to suit any specification. The facility with which the character of the " melt " can be altered to suit urgent requirements is, indeed, a great convenience, and one which may be obtained for but little extra cost. Apart from the cost of raw materials (the bulk of which may be scrap boiler-plates, rivets, and wrought-iron and steel plates containing from 0.03 to 0.2 per cent, of carbon), the cost of production, including the renewal of plumbago crucibles, the wages of the furnace attendant, the elements used for " adjusting the melt," fuel, and general expenses is about £11 to C12 per ton of the material as it leaves the crucible. This works out at about lld. per lb., apart from the cost of moulding and fettling, and the overhead charges of the establishment. Small castings (weighing under 21b.) are sold at about is. per lb., whereas castings over, say, 301b., cost only half that rate.
Although the foregoing charges are higher than those for malleable cast iron or steel, they are warranted by the more reliable nature of the products. The following results have been obtained by tests of materials cast by this process. High-speed Melting.
Another company which has—for a considerable period — been experimenting with this system of casting is the Bar Metal Company, Limited, of Broad Sanctuary Chambers, Westminster, S. W., and which concern has its experimental foundry at Bermondsey. A series of tests was recently made, on specimens selected from three different ' melts " of the furnace at Bermondsey, and it gave the following average results:— Cltimate tensile strength, 25.6 tons per square inch; elongation, 11.1 per cent, on a length of five inches, or 13 per cent, on two inches; and 16.3 per cent, contraction of sectional area. The tests were undertaken by David Kirkahly. The chief claim made by the Bar Metal Company is in respect of the rapid reduction of the metal to a molten state, ready for pouring into the moulds. The company undertakes to melt 3001b. of scrap wrought iron and steel, in four 751b. crucibles heated, at the same time, in one of its special furnaces, in the short space of 21 hours, from all cold, whilst subsequent charges may be melted in an hour and a half. These times are not equalled by any other method of melting—except in the electric furnace.
The company is not yet fully equipped for the production of castings on a large scale, but arrangements are in progress, and will shortly be completed, whereby an ample supply of high-grade castings, of great purity, will be assured, and the average price is expected to work out at about 4d. per lb.
Electric Smelting Furnaces.
The whole idea of the crucible system is to melt quickly, and to prevent the taking up of any impurities by the steel, as is the ease when the metal is fused in contact with coal, coke, or gas. Even better results can be obtained if the metal is melted in an electric furnace, al ihough there are certain technical diilieulties connected with the construction and maintenance of the baths of such furnaces.
The idea of melting the charge by means of electricity, so far as one is able to judge at the present time, is exceedingly good, and steel so produced is exceptionally pure. We have never had brought to our notice any case of weakness of the material which could be traced to this method of smelting. The electric furnace for the melting of steel is receiving a large amount of attention at the present time, although, from the commercial point of view, it is still in its infancy ; in fact, it 18 not much more than five years old. One of the oldest and most successful of these furnaces is the " Stassano," three views of which we reproduce herewith. Messrs. Arthur Reiner and Company, of 63, Queen Victoria Street, E.C., holds the British patents for this make of furnace, anc is not only prepared to supply them to steel makers and founders, but is. about to equip a foundry in which " Stassano '' furnaces will exclusively be employed. The locality in which the foundry will be situated is not yet settled, as the question of an ample supply of electrical energy at a low rate per unit must be one of the principal factors in enabling the patentees to arrive at a decision. Messrs. Reiner and Company are even now accepting large orders for its " ' steel castings, but these orders are, of course, being executed abroad, as will all orders booked for delivery before the completion of the arrangements for castings in this country.
The Stassano System.
Referring to the accompanying illustrations, the novel construction of the furnace may at once be seen. The bath consists of a comparativelysmall chamber, which is built up of slabs of magnesite, and around this, and separated by a narrow air space, is a second chamber of magnesite bricks; this second chamber is, in turn, enclosed by a sheet-metal casing, and the intervening space between the casing and the magnesite bricks is filled with circulating water. The "arc " is formed across the ends of three large carbon electrodes, but these are above the level of the " melt " and are fed towards the centre, as required, by hydraulic pressure. Three-phase alternating current is preferable, but the furnace will turn out good material with direct current at about 250 volts.
The whole is mounted on conical rollers, which run on a circular track that is inclined to the horizontal. The furnace is kept rotating at a speed of about one revolution every two minutes, about the centre of the circular track, by means of a small electromotor and suitable gearing. Every part of the " melt " is thus brought under the influence of the arc, and is uniformly mixed and heated, which advantages, coupled with the fact that the material is melted in an inert atmosphere, ensures the production of high-quality castings of great purity and free from all blowholes. Phosphorus and sulphur are practically eliminated during the process of melting. Such castings may be made of any required degree of hardness and tensile strength. and having any desired percentage of elongation. One of our illustrations shows a few castings that have been bent when cold and which, after bending, showed no signs of surface cracks. " Electro-Flex " castings, like those made from metal fused in a closed crucible, are without any hard skin, and do not need annealiug ; consequently, there•is practically no warping of the material after it leaves the sand.
The prices for small castings made by this process are about the same as for those made from metal melted in ,crucibles, but for larger pieces " Electro-Flex " castings are the cheaper, and, when the company gets to work with a foundry in this country, considerable reductions may be .expected; the prices will then more nearly approach—and in some cases rival—those for malleable cast iron and steel.
The Italian Government, in 1904, installed one of these furnaces, and two years later, on the 12th April, 1906, the " Colonel Director " of the Artillery Construction Department at Torino, wrote to Cav. Ernesto Stassano as follows :— " We are pleased to attest with the approbation of the Minister of War that : " 1.—The revolving furnace (model Stassano), constructed under your supervision at these works, fully answers the conditions of the contract made between you and the Military Administration, namely, to produce a good quality steel for projectiles by utilising scrap and turnings, iron and steel, obtainable from the ordinary workings in these works.
"2—After two years of working, we are able to attest to the reliability of the system, and of the material used in construction, from a mechanical as well as an electrical point of view.
"The Ministry for War has decided, based on the reports and the steel produced, to utilise the furnace for the regular production of steel for projectiles. The furnaces erected at these works produce 2i tons of steel per 24 hours in four smeltings, consuming 850 kilowatt-hours for each smelting."
The employment of the class of castings produced by the processes described is to be recommended for such. parts as crankcases, gearbox carcases, clutch cases, frame brackets of all descriptions, hand and foot levers, steering wheels, back-axle casings, wheel hubs, and numerous other parts of commercial vehicles. Such castings are free from the serious objections which have been raised against the use of malleable cast iron and steel, which materials, as may be gathered from our remarks in the centre column of page 129, are only suitable for such parts as have not to be machined. Castings made by one or other of the methods described, on account of the entire absence of hard skin, may be welded like ordinary wrought iron and mild steel. Many of the harder grades may not only be forged, but also hardened and tempered like tool steel, and such grades are, therefore, particularly suitable for the casting of blanks for many kinds of cutting tools, including large milling cutters, punching and bending dies, and a variety of others which may be cast accurately to pattern, thereby avoiding the heavy expense for forging to shape before machining.
Just as we go to press, the " Flexilis " brand of steel castings is brought to our notice. This is a steel for which Ludw. Loewe and Co., Ltd., of 30-32, Farringdon Road, E.G., has been appointed agent. It is made in three grades, the softest having a tensile strength of 22-25 tons per square inch, and an elongation of 25-30 per cent., whilst the " tool-steel " grade will stand up to a tensile strength of 50-57 tons per square inch with about a six per cent. elongation.