When steel had been cast into an ingot it is in a suitable form for fabrication into steel rails, plate, bars or any form desired. Massive rolling mills produce plate and rails, and powerful hydraulic presses squeeze forgings into shape
WITHDRAWING AN INGOT from the soaking pit. Ingots when taken from the moulds in which they are cast are hotter in the centre than on the outside and cannot be rolled or forged in this state. They are therefore stored in heated soaking pits for some time until the temperature becomes uniform throughout their mass.
THE clink of hammer on anvil and the cheery glow of the blacksmith’s hearth were for long centuries the only signs of the fabrication of iron into shapes of use and ornament. Less widely distributed, but just as picturesque, was the shaping of steel, the craft of the armourer and cutler being so much more localized that the names of Toledo, Damascus and Sheffield still stand for sword and knife.
With the coming of the machine the sound of the hand hammer was drowned by the heavier blows of the water-driven tilt hammer. Then, about 150 years ago, came the rumbling of the rolling mill of that hardly-used genius Henry Cort and, nearly a century ago, the thudding of James Nasmyth’ssteam hammer began to be heard. But still iron was the material handled, and the first steel rail used was rolled some seventy-five years ago — only, that is, after Bessemer had invented his converter.
The chapter “Steel - From Ore to Ingot” describes how steel, molten from the Bessemer converter or the open-hearth furnace, is poured into moulds, not to form a particular part as in the iron foundry, but to provide a piece of steel of convenient shape and weight for further manipulation. These pieces are known as ingots. Although the processes to which ingots are subjected and the machines in which they’ are carried out are, broadly speaking, the same in all steelworks, it is not possible to find two in which every detail is precisely the same. This account, therefore, is general rather than particular.
The ingot moulds are not of foundry sand, but are of iron. For normal purposes they are generally square, with rounded corners, in cross section. They are tapered slightly throughout their length. They are, in most instances, stood upon trolleys with the large end downwards and resting on a cast-iron plate which closes in the bottom. The trolley of filled moulds is drawn away to make room for empty moulds and is removed to a convenient place, where the moulds are allowed to stand for perhaps forty minutes. At the expiration of this time an overhead crane comes along and draws off the mould upwards in much the same way as a child lifts off his inverted bucket from the sand at the seaside. The stripped ingots are nearly always still molten in the interior, though the outside may have cooled down to a dull red. If they were allowed to cool in the open air, cavities would develop in the inside, and in any event they would not be in a fit condition to have any work, either forging or rolling, done upon them.
The ingots, therefore, as soon as possible after having been stripped, are transferred to the soaking pits — large, covered-in pits lined with firebrick and, in the most modern works, electrically heated. In the pits the ingot reaches an even temperature throughout its mass, the internal heat spreading outwards, while the outside also is heated. Modern practice allows for reheating in the soaking pits to a temperature of about 2,150 degrees Fahrenheit. In this state the white-hot ingot, now solid throughout, is gripped by a huge pair of tongs suspended from an overhead crane and borne off to the rolling mills. Hot metal is much more readily worked than cold metal: hence the reheating process.
The heated ingot is so soft that when it goes into the mills its tensile strength may be as low as 1 ton per square inch and. rarely exceeds 3 tons, though the strength of the same steel in a cold, finished state may be anything from 25 tons to 100 tons per square inch.
The glowing ingot is laid on its side on what are known as live rollers — that is, on a pathway of closely spaced rotating rollers with their tops just projecting above the mill floor. These carry the ingot into the rolling mill, which may be likened, especially when a plate mill is concerned, to a gigantic mangle or clothes wringer. The function of the mill is to reduce the ingot in cross section and at the same time to lengthen it into a flat sheet, rail, bar, or any other shape desired. Once the live rollers have pushed the ingot into contact with the roils these draw it through them, for they are driven by a powerful motor or engine.
The change from ingot to plate or rail, however, is not made in a single rolling mill. There are generally several mills in line, through which the steel is passed in succession; but it does not always go straight on without a pause. The type of rolling mill first used was exactly the same in principle as the domestic mangle. There were two rolls constantly revolving so as to draw the material through in one direction only. As one pass did not give sufficient reduction of thickness, the material was returned above the top roll and passed through again and again, the top roll being screwed down a little further at each pass. This type of mill still survives, but its use is limited.
American steelmakers added a third roll above the other two so that the material could be worked upon during its return journey, and this mill is known as the three-high mill. Then came a big advance, due to a locomotive engineer. John Ramsbottom, at the Crewe works of the former London and North Western Railway (now L.M.S.), fitted an old locomotive to a two-high rolling mill and demonstrated successfully that the rolls could be reversed so as to draw the material through, in the second pass, in the opposite direction to the first pass. This arrangement obviated the necessity for lifting the material to a higher level, as had to be done with the non-reversing two-high or three-high mill. The two-high reversing mill is probably that most often found in steelworks to-day.
THE FINISHING MILLS of the steel rolling plant at the Homestead Works of the Carnegie-Illinois Steel Corporation, in the United States. There are four of these mills, each with four rolls. The hot steel plate passes between the inside pair, which are driven by a motor of 5,000 horse-power. The outside rolls serve as supports to the inside ones. The lowest outside, or backing roll, is not visible. The. plates pass successively through all four mills and thence to the roller levelling machine.
The top and the bottom rolls of the two-high reversing mill are driven from a pair of gears. The bottom pinion is coupled to the reversing motor supplying the power or, in older works, to the
reversing steam engine, and the top pinion is driven by the bottom one. The drive to the pinions may be either direct or through speed-reducing gears.
The pinions are coupled to the rolls by shafts called driving spindles, and the top spindle has to be provided with a special form of joint to permit the top roll to be moved up or down as required. This movement is made, according to the thickness of the work required, by heavy screws. The screws are nowadays generally driven by an electric motor, though hand-operated screws are still fairly common, especially in the smaller mills.
The exact treatment of the ingot depends upon the final product. Thus, when plates are to be produced the ingot is flattened out into a slab about 6 in. thick in a slabbing mill having plain cylindrical rolls. When bars or rails are required the ingot is drawn out into blooms of about 6 in. square in cross section, the rolls of the bloom mill being formed with grooves of different sizes so that, as the work is passed backwards and forwards, it is reduced in stages. Both slabs and blooms are further reduced in other mills. In some mills both slabs and blooms are produced with flat rolls, the ingots being turned over between the passes by a manipulator, often operated hydraulically, which handles the glowing mass with uncanny precision.
Again, when slabs are being rolled the manipulator is nearly always necessary, as the slab has to be rolled on its edges also. When a slab about 4 feet wide is turned on its edge and passed through the rolls the ingenuity of the manipulator is fully appreciated. Some mills, however, have vertical rolls as well as horizontal ones, so that all four surfaces of the slab are rolled without any turning over of the work. Blooms are, as a rule, further reduced in another mill to long bars of square cross section. These bars are then cut into shorter uniform lengths by a saw, and are known as billets.
Oval and Square Cross Sections
By this time the metal is too cold for further rolling, so the billets are removed to a reheating furnace and brought back to a working temperature. The billets are then rolled in a mill having a series of grooves in the rollers, and are passed backwards and forwards from larger grooves to smaller ones until the final reduction is obtained. The number of passes depends on the size of the billet and of the finished bar. Thus, if a bar of ½ in. diameter is made from a billet 4 in. square, about eleven passes are necessary.
The alternate cross sections are oval and square, of gradually diminishing area in cross section, the cylindrical shape being imposed in the final pass only. Great skill is needed in designing rolls for such reductions and, until recently, this work was one of the most jealously guarded secrets of the steelworks. Sometimes, instead of one reversing finishing mill with many passes, the billets are passed through a series of non-reversing mills in line, each mill taking down the work in one step. This type of mill is called the continuous mill and is used generally with small section bars only, so that too many passes are not involved.
Complicated sections require a large number of passes. For instance, channels and joists, such as are used in structural work, and rails, the earliest type of un-symmetrical section rolled, may require as many as eighteen passes. The practice in rail rolling has always varied, but practical considerations prevented rails being produced in lengths exceeding 90 feet until early in 1937, when a British firm, the Skinningrove Iron Co., Ltd., Of Saltburn, Yorkshire, perfected a method of rolling rails 200 feet long from an ingot weighing 3 tons 6 cwt. This long rail, after the ends have been cropped, is cut up into one 60-feet rail and one 120-feet rail, the latter weighing just over 1¾ tons. The installation of sixty of these rails on the L.N.E.R. main line south of Peterborough, Northants, may well initiate an important change in railway practice. The fewer joints there are in track the smoother running for the train and the fewer the fishplates and bolts.
GIGANTIC HYDRAULIC PRESS in a steelworks at Dortmund, Germany. The press, which is shaping a huge steel forging, can exert a pressure of 15,000 tons. This method of forging has largely superseded the steam hammer for heavy work. The hydraulic press can handle ingots weighing more than 100 tons.
The plate mill may be continuous in the same way as that used for rolling rails, bars and sections, though it takes up a great deal of space. Even a semi-continuous mill is an enormous piece of plant. The mill at the Homestead Works of the United States Steel Corporation, near Pittsburgh, Pennsylvania, is perhaps the best example of modern plate rolling plant. This mill is designed to convert steel slabs, weighing 2 tons each, and 4 ft. 6 in. wide and 6 in. thick, into plates up to 8 ft. 4 in. wide and up to ½ in. thick. The slabs, which have been formed from ingots in a slabbing mill, are reheated in a series of furnaces stretching for a distance of 80 feet. They are charged at one side of the furnace by machines whose motion is so regulated that the slabs are pushed through the furnace and come out at the other side at the rate of one slab every three-quarters of a minute. Emerging at a temperature of about 2,400° Fahr., the slab slides down an incline on to a pathway of live rollers which hurry it off to the mill. Here the slab is first passed through a two-high non-reversing mill known as the scale breaker.
After having been passed through a reheating furnace, steel has a coat of grey shale-like iron oxide which, if not removed, might be rolled into the steel to its great detriment. This scale is cracked in the mill and is washed off the glowing slab as it comes out of the rolls by jets of water discharged at the enormous pressure of 4,000 lb. per sq. in. This operation generates great quantities of steam.
The cleaned slab is not immediately deposited on to live rollers to continue its journey. Instead, it is delivered to a device which immediately twirls it round in the horizontal plane through a right angle. The slab is then deposited on the rollers on which it is conveyed to the next mill “sideways on”.
Pressure of Great Jaws
The second mill is a four-high non-reversing mill, which means that there are four rolls piled, as it were, one above the other. Only the two inner rolls, however, are power-driven, the outer rolls being driven by friction from them. This appears to be an unnecessary arrangement as the material is not passed between the outer and inner rolls but only between the inner rolls. The tremendous pressure on these, however, might cause them to bend under the strain, and it is to prevent this bending that the inner rolls are backed by an outer pair. This mill flattens out the slab to perhaps double its original width. When the slab leaves the mill, the edges may be irregular; so the slab is passed through what is known as the squeezer, which trues up the edges by the pressure of great jaws. After this squeezing the slab, now much thinner and longer, is hurried on by live rollers to the roughing mill. This mill is the unit that makes the plant semi-continuous instead of wholly continuous, as it is of the four-high reversing type, that is, the slab is passed forwards and backwards through it until it is rolled out into a plate.
AN IMMENSE SHEARING MACHINE at the Homestead Works cuts the long strip of finished plates into the required lengths. The moving blade is curved and cuts with a rocking motion, an arrangement that prevents distortion of the plate which is held up against a long stop by the jaws on the right.
The roughing mill is provided with reversible vertical rolls in addition to the flattening rolls. These vertical rolls true the edges of the slab or plate and another high-pressure water scale-re-moving device is fitted, as fresh scale tends to form as long as the steel remains fairly hot. The plate must go through the mill an odd number of times so that after its last pass it is travelling forward. Another set of live rollers takes it to the finishing mills, of which there are four, arranged one behind the other, and all of the four-high non-reversing type.
The function of the finishing mills is to give the plates a smoothly finished surface. Having left them, the plates are sprayed with jets of water at high pressure. The jets in this instance are provided not to remove scale but to control the cooling of the plates and to assist in the formation of a protective coating of hard oxide. After this the plates pass on through a roller levelling machine. This adjunct is not found in rolling mills producing plates such as are used for ships and boilers, but only in those mills which produce plates of a gauge suitable for pressing into motor car bodies and for light constructional work. In the Homestead plant, however, the roller leveller is used. After having left it, the long strip of plate, by now nearly cold, goes on to other machines for cutting up.
First of all, the strip is led through a machine fitted with two rotating knives, one at either side. These trim the edges as the plate passes under them, making these edges parallel, straight and the required distance apart. An ingenious arrangement of electro-magnets keeps the strip from getting out of line and feeds it under the knives. A short further journey is taken and the strip reaches the shears straddling across the live roller path. These shears are of a new type.
The more commonly used shears have straight blades, the upper and moving blade being set at an angle. Thus, as each part of the blade cuts through the plate it travels on below it and drags it downwards so that, when the plate is thin, distortion may occur. The curved blade now used is given a motion similar to that of the runner of a rocking chair. The part in action at any particular moment leaves the plate at its top side immediately it has cut through it, and so no bending can occur. The plates, when cut to different lengths to fill different orders, are run along by live rollers which can be stopped at will when a particular plate has to be added to the pile to which it has been assigned. The plate is then tilted off.
Rolling mills for armour plates, thick boiler shells and so forth are much more massive than these mills. At the other end of the scale are rolling mills for thin rods such as are commonly used nowadays for reinforcing concrete. These mills often have the successive units arranged side by side instead of one behind the other.
Cold Rolling of Razor Blades
A rod can be passed through these mills without a great deal of handling, although it has to travel through all the units. A man is stationed behind and between a pair of the units. When a new billet is started a twisting, thrusting spear of glowing steel comes through, to be seized unerringly with a pair of tongs and bent round a bollard. The rod is all this time increasing in length, and presently it is long enough for the man to insert the free end into the second unit, which is running in the opposite direction.
There is now a huge hairpin of glowing rod with the man in the loop, and on the other side of the units is another man waiting for the rod to come through the second unit and to bend it back to a third one. There may be as many as four loops altogether, the rod, as long as the billet lasts, travelling in a form resembling the outline of that homely firework, the cracker. The rod comes out from the final unit quite straight, and is received, on a cooling rack which not only prevents it from becoming distorted when cooling, but also moves it automatically out of the way to make room for the next bar from the next billet.
SCALE-BREAKING AND SPREADING MILLS form the first two units of the steel rolling plant in the Homestead Works. The glowing slab from the reheating furnace is fed into the mill in the foreground, and in it is elongated so that the scale flakes off. The slab is then turned through a right angle and enters the second mill, from which it emerges with perhaps double the width that it had originally.
So far the rolling of hot metal only has been described, but steel can also be tiled in a cold state. This is a practice which has extended greatly of late liars. The rolling is not, however, of the same nature as with hot metal. For instance, the reduction is not made from a billet. The steel rolled cold has been previously rolled hot and the object of cold rolling is to harden it and to effect some other physical changes. The steel from which safety razor blades are made is, for instance, cold rolled. The material in any instance rarely exceeds 9 in. in width, and is extremely thin, a typical example being steel of 0·15 in. thick reduced by cold rolling to 0·025 in. thick in three passes at a speed of 600 feet a minute.
These mills are invariably of the four-high reversing type, the backing, or outer rolls being of much larger diameter than the inner working rolls, to provide the necessary stiffness. Steel tubes are rolled hot, but the procedure is not quite the same as with bars or rails. In one kind of mill the rolls are not parallel, but are set at an ingle to one another.
The steel ingot does not always find its way to the rolling mill. It sometimes goes to the forge to be shaped under powerful hydraulic presses into shafts, boiler drums and other parts working under great loads or heavy pressures.
The hydraulic press can handle ingots of a size unsuitable for treatment by a steam hammer. Such ingots may weigh over 100 tons and are often octagonal in cross section, as this approximates more closely to the final circular shape required for such parts as boiler drums. The action of a hydraulic press is more akin to that of rolling than is the action of a steam hammer. Its deforming and consolidating effect is more uniform throughout the mass of hot metal and the pressure may be as much as 14 tons per sq. in. The steam hammer, on the other hand, gives an instantaneous local blow. The hydraulic press is an important tool in the modern steelworks.
The steam hammer, with some other types of mechanical hammer, is, however, extensively used in what is known as “drop forging”. Many of the parts of the modern motor-car are drop-forged; for example, crankshafts, connecting rods, axles and gear blanks. Two blocks of steel with smooth, level faces are carefully hollowed out so that half of the shape of the desired part is formed in each. One of these blocks is attached to the anvil of the hammer and the other to the hammer head, the two being alined to give a complete mould of the part when the faces are in contact.
A billet of steel of a size proportioned to the volume of the mould, and heated to a suitable temperature in a reheating furnace, is laid over the bottom block, or die, as it is called, and the top die is then let fall with a smashing blow on to it. On the hammer being again lifted, a forging of the exact shape is left and is immediately removed to make room for another billet. The process takes but a small fraction of the time required for forging by hand, especially when a complex part, such as a crankshaft, is to be made. The drop forging is generally heat-treated in some way to remove internal stresses, and may be machined all over, as with a crankshaft, or at the ends only, as with a connecting rod.
It has not been possible to describe the variations of the several processes referred to above. The rolling of armour plate, for instance, requires special machinery; so does its opposite extreme, the tinplate for which South Wales is famous. Then there are forging machines — a kind of compromise between an hydraulic press and a drop forging hammer. Forging machines are used for making bolts and the like. The steel works, in short, is full of illustrations of the mastery man has gained over metal.
Not all steel produced is made into ingots. A large amount is cast into machine parts in sand moulds, steel castings being much stronger than those of iron or bronze.
ROUGHING MILL at the Homestead Works. This mill converts the slabs into plates,the slabs being passed forwards and backwards until the desired thickness has been reached. The rotation of the rolls is reversed at each pass. The edges of the plate are looked after by the two vertical rolls in the foreground. They are driven by a 600-horse-power motor, the horizontal rolls needing one of 7,000-horse-power.