WILD HEATS.
After steel is melted, whether in a crucible, an open hearth, or a Bessemer vessel, it boils with more or less violence. This boiling is caused by ebullition of gases, and if steel be poured into moulds while it is boiling the resulting ingot will be found to be honeycombed to an extent that is governed by the degree of the boiling.
If a heat boils violently and persistently, it is said to be “wild,” and if a wild heat be teemed the ingots will be honeycombed completely; such ingots cannot be worked into thoroughly sound steel, and no melter who has any regard for his work will teem a wild heat if he knows it.
To stop the boiling is called “dead-melting,” “killing” the steel, so that it shall be quiet in the furnace and in the moulds.
A crucible-steel maker who knows his business can, and he will, always dead-melt his steel. It only requires a few minutes of application of a heat a little above melting temperature, and this can be applied by a skilled melter without burning his crucible or cutting down his furnace; this is indeed about all of the art there is in crucible-melting, the remaining operations being easy and simple.
Dead-melting in the Bessemer vessel is not possible by increase of time; wild heats are managed differently, probably by adding manganese or silicon, or both, but exactly how is not within the author’s experience.
Dead-melting in the open hearth would appear at first sight to be always possible, but there are more difficulties in the way than in the case of crucible-melting.
The heat may be wild when the right carbon is reached, and then the melter must use a little ferro-silicon, or silico-spiegel, or highly silicious pig, or aluminum, and he must use good judgment so as not to have his steel overdosed with any of these. From half an ounce to an ounce of aluminum to a ton of steel is usually sufficient, and although any considerable content of aluminum is injurious to steel there is little danger of its being added, because of its cost, and because a little too much aluminum will cause the ingots to pipe from top to bottom.
Silicon seems to be the most kindly element to use, and it is claimed that a content of silicon as high as 20 is not injurious; some people claim that it is beneficial. That it does help materially in the production of sound steel there can be no doubt, and if such steel meets all of the requirements of the engineer and of practice it would seem to be wise not to place the upper limit for silicon so low as to prevent its sufficient use in securing soundness. But the author cannot concede that as much as 20 silicon is necessary. In crucible practice high silicon is not necessary; in “melting-iron,” or iron to be melted, it means so much dirt, indicating careless workmanship; but there will always be a little silicon present which the steel has absorbed from the walls of the crucible during the operation of melting. In high tool-steel silicon should be at the lowest minimum that is attainable.
This discussion of wild heats may appear to be outside of the scope of this work, and to belong exclusively to the art of manufacturing steel, of which this book does not pretend to treat. This is true so far that it is not recommended that the engineer shall meddle in any way with the manufacturer in the management of his work; on the other hand, it is vital to the engineer that he should know about it, because wild steel may hammer or roll perfectly well, it may appear to be sound, but the author cannot believe that it is ever sound and reliable.
Again, it has a scientific interest; that wildness is due to too much gas, and probably to carbon-gas, may be shown by an illustration.
It has its parallel in the rising of the iron in a puddling-furnace at the close of the boil, a phenomenon with which every one is familiar who has watched a heat being boiled or puddled. That all of the iron does not run out of the puddling-furnace at this stage is owing to the fact that there is not heat enough in the puddling-furnace to keep the iron liquid after it has been decarbonized.
During the running of a basic open-hearth furnace an apparently dead heat was tapped; before the steel reached the ladle there was a sort of explosion; the steel was blown all over the shop, the men had to run for their lives, and not one tenth of the steel reached the ladle. The manager was rated roundly for carelessness in not having dried his spout, and the incident closed. A few days later another quiet heat was tapped and it ran into the ladle; about the time the ladle was full the steel rose rapidly, like a beaten egg or whipped cream, and ran out on to the floor, cutting the sides of the ladle, the ladle-chains, and the crane-beams as it flowed. The men ran, and there was no injury to the person.
Again the manager was blamed, this time for having a damp ladle, and he was notified of an impending dismissal if such a thing occurred again. He protested that he knew the ladle and the stopper were red-hot, that he had examined them personally and carefully, and knew he stated the truth.
There were several reasons for looking into the matter farther: first, the man in charge was known to be truthful and careful, so that there was no reason for doubting his word; second, if the vessel and rod were red-hot, there could be no aqueous moisture there; and, finally, such an ebullition from dampness was contrary to experience, as a small quantity of water under a mass of molten iron, or slag, results almost invariably in a violent explosion, like that of gunpowder or dynamite.
Upon inquiry it was found that prior to both ebullitions there had been a large hole in the furnace-bottom, requiring about a peck of material to fill it in each case. Magnesite was used; the magnesite was bought raw, and burned in the place. It is well known that it takes a long time and high heat to drive carbonic acid out of magnesite, and it was surmised that insufficient roasting might have caused the trouble. Samples of burned and of raw magnesite were sent to the laboratory, and the burned was found to contain about as much carbonic acid as the raw magnesite. Then the case seemed clear: This heavily charged magnesite was packed into the hole; the heat was charged and melted. The magnesite held the carbonic acid until near the close of the operation; then the intense heat of the steel forced the release of the gas, which was at once absorbed by the steel. Owing to the superincumbent weight of the steel the gas was absorbed quietly, and when the weight was removed the gas escaped, exactly as it does at the close of puddling or in the frothing of yeast.
Whether the carbonic acid remained such, or whether it took up an equivalent of carbon and became carbonic oxide, and then again took up oxygen from the bath, and so kept on increasing in volume, is not known.
The facts seem clear, and the collateral proof is that thorough burning of the magnesite, and of any dolomite that was used, prevented a recurrence of any such accidents.
Such ebullitions have occurred and caused the burning to death of pitmen, and the statement of the above case may be of use to melters in the future who have not met such an experience.