ROLLING MILLS Hot Working.—A spongy lump of wrought iron, hot from the furnace, requires forging in order to squeeze out much of the trapped slag, and to consolidate the metal. Even the best ingot of steel is improved by hot work, such as forging or pressing; its fracture is changed from a coarse crystalline cleavage like sugar to a mossy or fibrous appearance associated with greatly increased toughness ; it also becomes more amenable to heat treatment. Formerly all this was done on the blacksmith's anvil. Massive gun tubes and armour-plates are forged slowly in hydraulic presses; heavy engine parts, axles and cranks are beaten out by steam hammers; repetition tools, levers and machine parts are forged under drop-hammers. In every instance the aim is to convert a steel casting (ingot) into a special shape and with improved physical properties.
Modern machines and structures—even foods and personal con veniences—require large quantities of steel parts of uniform cross section, such as plates, bars, beams, rails, pipes, sheets, etc. These are made in rolling-mills where the work is performed by passing the red hot metal through one pair of heavy rollers after another, so grooved and spaced that each succeeding pass is smaller and more nearly the required final shape. While the products of a rolling mill vary widely in weight and shape, a common charac teristic is that each piece is of uniform cross section from end to end, and is of a length limited only by the size of the original ingot, or by the size of the runout floor (see ROLLING-MILL).
The word mill may mean the entire steel making plant, just one department, or one stand of rolls in the department. Complete departments have various names, such as rail mills, structural mills, plate mills, merchant bar mills, depending upon the product they most efficiently produce. Each such mill contains a number of roll-stands of decreasing size and power, from the blooming or cogging mills which do the heaviest work, down through the inter mediate stands to the last pass, finishing mill or finishing stand, which impresses the exact final cross section.
There still exist many rolling-mills, some producing the highest type of tool steel, which start with a small piece of metal (a 150 lb. ingot or a chunky rolled bar called a billet) and produce rods, bars, hoops or flats a fraction of an inch thick. Metal is passed back and forth by tongs in the hands of skilled labourers. Such roll stands are usually arranged side by side and the hot bars are looped from stand to stand, entering from alternate sides, until the final shape is produced. Sometimes curved guides are installed to do the work of alternate workmen ; as the size of the bars in creases hand labour cannot cope with the situation. Most rolled shapes start from white hot ingots, perhaps six tons in weight, which are taken from soaking pits by a travelling crane, placed on the first of a series of "tables"—power driven roller conveyors— which transport the metal wherever a distant electric switch opera tor directs (see Plate II., fig. 3).
Blooming mills are usually arranged so the ingot is passed back and forth several times through the same roll stand. This re quires a screw-down device and indicating dial on top of the housing for lowering and reversing the rolls progressively as the piece becomes smaller. The spindles connecting the rolls to the driving engine or motor are of most rugged construction, for up wards of 20,000 h.p. is transmitted at times.
a long rectangular hearth, slightly inclined, as wide as the billet is long, and a low roof. Water-cooled skids carry the weight of the steel and prevent scraping on the brick furnace-bottom. At the cool end billets are pushed in sideways by a plunger, one by one, moving all ahead closer to the hot end. Every time a cold piece enters the furnace, a hot one is pushed out the other end on a roller table leading to a roll stand ; other continuous furnaces have water-cooled rollers or other devices built into the bottoms for moving lighter pieces such as wire coils through them at a slow steady rate.
Plate manufacturing departments are less complicated than a rail mill, although the machinery may be more rugged. First the ingot-6 to I o tons or even heavier—is rolled to a flat slab in a blooming mill, sheared into pieces, and the slabs reheated. Fur ther work is done by cylindrical (ungrooved) rolls. Sometimes a single reversing 2-high roll stand does all the work; at other times two three-high mills with elevating tables may be observed. On account of the enormous bending stresses carried by the rolls when rolling wide plates, many two-high reversing stands have been built as four-high mills, having two rather small working rolls, each backed up with a larger one for stiffness. Plates of more uniform thickness, edge to centre, may be made in this way. It should be observed that the action of a rolling-mill does not widen the piece very much—most of the change in volume results in increasing its length. Consequently a heavy slab is cut to a length which equals the width of the required plate, and it enters the plate rolls sideways. Cross rolling in this way results in a better plate; metal worked in only one direction acquires trans verse weakness. This means that the tensile strength of test bars cut with their axes parallel with the direction of extension during rolling will be higher than when cut from the plate with axis in any other direction. Other interesting mechanical devices are in cluded in a plate mill which cannot be described here in detail. Among them are plate straighteners, cooling beds, cross cut and edge shears, turnover devices, castor beds and magnetic grips. Side shearing on narrow plates may be avoided by using a univer sal mill, which is a roll stand to which is added a pair of stout vertical rolls. The latter are adjustable, up to perhaps 48 in. apart, and are set a proper distance to keep the hot plastic plate the exact width during its formation.