The straightening frame, or English tempering system, is as follows: The plates of steel are made of a size that will cut into four saws. The furnace front is provided with a tramway extending to the floor of the furnace, and on this runs the stretcher-frame. The plates are stretched in the frame, which is run into the furnace so that the plate is heated under a tension, which operates to straighten them. As the temper lowers, the screws of the stretcher are turned, increasing the tension; when the tempering is done, the screws are made to stretch the plates very tight just previous to taking it from the fire, and the plates are allowed partly to cool off while kept in the frame. In this process the indications of the temper are determined as in the first process. In the third process, the saws are placed between a stationary and a movable die provided in the body of the furnace, the movable one descending and pressing the saw to the other die; thus the tempering is accompanied by a flattening process (the dies being operated by pressure). The degree of temper is regulated by the temperature to which the saws are heated, which is ascertained by a pyrometer. The furnace is kept at a constant temperature, and the length of time the saw remains between the dies is varied to suit the thickness of the saw. The gain due to this system is, that less straightening is required and a determinate temperature is secured. Some makers claim that in this system the vapour of oil that exudes from the saw has no means of escape, and that a chemical effect injurious to the steel ensues; and furthermore that the temperature of the dies will be greatest at or near their circumference, and hence the teeth and back and the ends of the saw will be softer than the middle of the width and length of the saw, and that if two saws, one above the other, be placed on the dies at once, the contacting surfaces of the saws will be the hardest, and those surfaces will be black by reason of the oil burning into the steel, instead of exuding, as in the open furnace process.
The floor of the tempering furnace should be flat and even; for if any part of the saw-plate lies suspended, it will sag when heated, greatly increasing the amount of straightening required. The furnace must be so constructed as to heat evenly all over, otherwise the temper of the saw will not be even. The air must be carefully excluded to prevent the steel from decarbonizing, which being thin, it is very apt to do. Thin saws warp proportionally as they are heated more, and if they are allowed to remain longer in the furnace and not heated too quickly, existing buckles or bends will partly straighten themselves in the furnace. Care must be taken to keep the tongs clear of the teeth, and in taking the saws from the furnace the length of the saw must stand at a right angle to the operator (two pairs of tongs being used), so that the saw’s own weight shall not cause it to bend. The saw must be transferred from the furnace to the bath very quickly, to prevent, as much as possible, its cooling in the air; for such cooling would take place unequally, causing the saw to warp, as well as impairing the temper. It should be dipped with the length horizontal, the teeth downward and the side faces vertical, and plunged quickly into the bath. On being dipped in the hardening liquid, they warp again, but the dipping may be manipulated to partly regulate the warping. From the moment the cold air strikes the plate a warping process sets in, hence quickness in transferring from the furnace to the bath is a great point. When the saw is hot enough to temper, the scale will begin to rise upon its surface, and if the furnace is unequally heated, the scale will arise first at the hottest part, instantly notifying the operator of the defect.
From the appearance of the surface of the saw after it comes from the hardening bath, the operator can see if it is properly hardened. If so, the scale will be what is termed “struck,” that is, it has come off, leaving the surface from a grey to a white color; while if the scale remains in dark patches, the saw is too soft in those parts.
After the saws are tempered they are allowed to cool in the open air, and then require to be straightened by the hammer, and in this process the tempering has been interfered with, inasmuch as that the elasticity due to the tempering has been counterbalanced to some extent by the local condensation of the metal induced by the immediate effects of the hammer blows. The condensation of the metal has impaired the natural grain or fibre of the metal, and stiffens it so that if the saw be bent these stiffened hammer marks will cause it to remain set instead of springing back straight, as it should do. To remove this defect the saws are what is termed stiffened, that is, they are heated until the surface assumes a yellow color, when they are removed and allowed to cool. This causes the metal condensed by the hammer to assume its natural structural condition, and permits the tempering to spring the saw back straight, even though it be bent until the two ends touch, and the bend carried half way along the blade by carrying one end forward along the blade surface. The yellow color is subsequently removed by an application of a solution of muriatic acid.
The method employed by the Tomlinson Carriage Spring Company for carriage springs is as follows:—
The spring plates are heated to bend them to the former, which is a plate serving as a gauge whereby to bend the plate to its proper curve, which operation is performed quickly enough to leave the steel sufficiently hot for the hardening; hence the plates after bending are dipped edgeways and level into a tank of linseed oil which sets in a tank of circulating water, the latter serving to keep the oil at about a temperature of 70° when in constant use. About 3 inches from the bottom of the oil tank is a screw to prevent the plates from falling to the bottom among the refuse.
To draw the temper the hardened springs are placed in the furnace, which has the air-blast turned off, and when the scale begins to rise, showing that the adhering oil is about to take fire, they are turned end for end in the furnace so as to heat them equally all over. When the oil blazes and is freely blazed off, the springs are removed and allowed to cool in the open air, but if the heat of a plate, when dipped in the oil to harden is rather low, it is cooled, after blazing, in water. The cooling after blazing thus being employed to equalize any slight difference in the heat of the spring when hardened.
The furnace is about 10 inches wide and about 4 inches longer than the longest spring. The grate bars are arranged across the furnace with a distance of 3⁄8 inch between them. The coal used is egg anthracite. It is first placed at the back of the furnace, and raked forward as it becomes ignited and burns clearly. For shorter springs the coal is kept banked at the back of the furnace, so that the full length of the furnace is not operative, which, of course, saves fuel. By feeding the fire at the back end of the furnace, the gases formed before the coal burns clearly pass up the chimney without passing over the plates, which heat over a clear fire.
For commoner brands of steel, what is termed a water-chill temper is given. This process is not as good as oil-tempering, but serves excellently for the quality of steel on which it is employed. The process is as follows: The springs are heated and bent to shape on the former plate as before said; while at a clear red heat, and still held firmly to the former plate, water is poured from a dipper passed along the plate. The dipper is filled four or five times, according to the heat of the plate, which is cooled down to a low or very deep red. The cooling process on a plate 11⁄2 × 1⁄4 inches occupies about 6 seconds on an average, but longer if the steel was not at a clear red, and less if of a brighter red, when the cooling began. Some brands of steel of the Swede steel class will not temper by the water-chill process while yet other brands will not harden in oil, in which case water is used to dip the plates in for hardening, the tempering being blazing in oil as described. In all cases, however, steel that will not harden in oil will not temper by the water-chill process.
The Columbia Car Spring Company temper their springs as follows:—Using “Gregory crucible steel,” heating is performed in a furnace consuming gas coke, but the furnace has a number of return enclosed flues, and between these flues (one over the other), are ovens, the heat passing through the brick-work forming the flues into the ovens. To facilitate renewing the ovens (which of course also renews the flues), the floor of each oven (which forms the ceiling of the oven below), is built on iron supports, protected by the brick-work and suitable fire clay, the bricks all being made to pattern, thus involving very little labor in building. The furnace doors are at the ends, and are kept closed as much as possible. In this way the steel has no contact with the products of combustion of the fuel, and the air is excluded as far as practicable (two valuable features). The furnaces are long and narrow, and not being connected with the flue there is but little disposition for the cold air to rush in when the furnace doors are opened.