Steel working and tool dressing

The Melting Furnace

The melting furnace consists of a row of oval melting pots, one in front of the other, and deep enough to allow of sufficient coke to cover the lids from each melting hole. A flue leads in old-fashioned furnaces into a flat stack, each flue having a separate flue in the stack. But many furnaces are now made with short flues for each hole leading in a main flue with ends in a single square chimney.

The application of gas to the melting of steel has been successful, but for the highest qualities coke is principally used, as the control which the melter has over the temperature of each pot, which sometimes requires keeping back and sometimes hurrying on, is supposed to be more absolute, although in Pittsburgh gas is now used almost entirely for melting steel.

The pots in which the steel is melted are generally made in a room adjoining the melting furnace. They are composed of a mixture of different clays which are best adapted to stand a great heat. A small quantity of ground coke, as well as of ground old pots, is also added. Great care is taken that the clay is absolutely disintegrated and perfectly mixed together. This is accomplished by treating it in a trough. The pots are moulded in an iron flask by means of a wooden plug and are slowly dried at the back of the stack, and the night before they are used gradually heated to a dull red, a process called annealing.

Pot making is a very important part of the manufacture of cast steel. It is absolutely impossible to make good cast steel if the pots are defective. Each pot lasts a single day and is used three times. In it are about fifty, forty-four or thirty-eight pounds of steel successively. The object of lessening the weight of each successive charge is to bring the surface of the molten metal to a different place in the pot, because the flux or scum which accumulates on the surface has a chemical action on the silica of the pot, which is consequently decomposed for some depth just at that point and the pot is reduced in thickness. The bar steel is first carefully selected of the exact temper required. All flushed or aired bars are rejected. After it has been broken up into small pieces and carefully weighed, it is conveyed to the pot, which has already been placed in the melting hole through an iron funnel called a charger. The lid is carefully adjusted and the melting hole filled with coke.

The degree of heat to which the furnace is allowed to go IS controlled by means of the two flues into either of which can be inserted a fire brick if required. A brick in the melting hole flue lessens the heat by lessening the draft. The head melter periodically inspects the pots and gives the final instructions to the puller out and decides the precise moment when the steel is dead melted and the holes sufficiently burnt down to allow of its being turned or poured into the mould with a fair chance of producing a sound ingot. When the puller-out has put on his sacking wraps which envelop the arms and legs and which are soaked with water to protect him from the heat, he raises the pot with a pair of pulling-out tongs and lifts it from the hole to the floor of the furnace. The lid is instantly taken off with a pair of lid tongs and the scum or flux is removed with a skimmer from the surface of the molten steel which is then poured into a cast iron mould formed of two halves tightly wedged together. The interior of the mould has been previously covered with a coat of coal-tar soot to prevent the ingot from adhering to it.

The melting of the higher qualities of steel is a process requiring the greatest skill ; and one of the principal reasons why the trade has become to such a remarkable extent localized in Sheffield is the importance to this branch of the trade of being able to select from a large class of more or less experienced workmen the few exceptional men in whom sound judgment, technical skill and steady habits are combined.

The chances of accident in the melting of steel are many and various. Not only badly made pots, but badly annealed or badly worked pots, are sure to run and be practically of no value. Should a piece of coke accidentally find its way into the pot, the ingot will show bright, sparkling fracture. Under the hammer it will prove hot short and crumble to pieces. If the steel is not in the fire long enough, it will turn fiery and produce a honeycombed ingot, and the same result will follow if it be too hot when it is poured. If it remains too long in the fire, it will turn dead. The fracture of the ingot will look scorched, and though exceptionally sound, it will be brittle if hard, and wanting in tensile strength if mild.

If the molten steel be chilled before it is poured into the mould, which may be detected by the stream skimming over, as it is termed, the fracture of the ingot will appear dull in color and full of small holes or honey combed.

All ingots having a proportion of one per cent, or more, if properly melted, will pipe; that is to say, the steel in the center of the ingot will settle down as it cools, leaving a hollow space in the middle of the top of the ingot to the depth of from three to five inches. When the ingot has become cold, the hollow part must be broken off until the ingot is sound, and before this fracture has had time to rust, the ingots must be carefully examined. The ingots which are not properly melted must be rejected, and the exact percentage of carbon which each ingot contains must be marked upon it. An experienced eye can judge of the percentage of carbon contained in an ingot to a wonderful nicety by the appearance of the fracture. Every tenth per cent is marked and an experienced hand will detect a difference between, for example, 1.3 and 1.35 per cent. In order to reduce the ingot of cast steel to the size and shape required by the consumer, it must be reheated, and when hot enough, hammered or rolled to the dimensions ordered.

Great care must be exercised in this process not to harm or overheat the steel, and to prevent this the bar must be entirely turned around in the fire, and ground fire clay or sand and borax sprinkled upon it. In many cases it is necessary to give the surface of the bar, after it has been once drawn down under the hammer, a welding or wash heat to close the small honeycombs which are scattered here and there on the surface of the ingot.


Forging and Rolling.

The forging and rolling of cast steel requires experienced workmen and a considerable outlay of expensive machinery. It is seldom that a workman attains exceptional skill in many departments, and loss is sustained by the often changing faces or rolls, so that these processes cannot be satisfactorily or cheaply carried on upon a small scale, and this is one of the chief reasons why the crucible cast steel trade has to such a large extent become localized in a single town.

It might be supposed that when the best quality of iron had been selected and the greatest care used in the process of manufacture the result must of necessity be good steel and the troubles of the manufacturer would be over. But this is not the case by any means. The greatest difficulty has yet to be faced. The results may be of good steel, but good steel only for certain purposes. There was a time in the Golden Age of steel manufacturing when steel was steel, and if it did not answer the purpose for which it was required, it was taken for granted that the fault lay with the workman. In some cases the manufacturer altered the percentage of carbon, but the "temper" of the steel was kept a profound secret from the consumer in most cases, no doubt, because the manufacturer had very vague ideas on the subject himself. Chemical analysis was unknown in the trade; the despotic sway of the "rule of thumb" reigned supreme. Now it is customary for the manufacturer to take the consumer into his confidence and not only to inform him of the percentage of carbon which the steel contains, but also to give him the benefit of his opinion as to the purposes for which it is or is not suitable. Formerly if the consumer discovered this, it was entirely due to his own wit.

There can be no doubt that for many purposes a considerable latitude may be permitted if the steel has the good fortune to fall into the hands of a clever workman who understands how to humor it; but next to quality by which is meant percentage of phosphorous, sulphur, etc., combined with some other obscure points of crystallization - the most important thing is temper or percentage of carbon. For some purposes, indeed, temper is of more importance than quality. Nothing is more common than for steel to be rejected as bad in quality, because it has been used for a purpose for which the temper was unsuitable. The size and shape generally furnished come close to the purpose for which steel is likely to be used.