Steel

by William Metcalf

Steel in cooling from the liquid passes through the granular and the plastic conditions to the cold state.

The granular form is of special interest to the steel-maker for the reason that in this condition the steel has more of adhesion than cohesion; it will stick to anything it touches, and so cannot be made to flow. This is the cause of “bears,” “stickers,” and many of the troubles of the melter. Therefore steel must be put into the moulds while it is still molten, and moulds should be well smoked or lime-washed to prevent stickers. This condition is of great interest to engineers, because the failure to roll or shape molten steel by pouring it directly between the rolls is doubtless due to this adhesive, non-cohesive condition.

To produce sheets, bars, and all sorts of shapes from molten steel direct, without the expense of making, handling, and re-heating ingots, is an enticing idea which has occupied the minds and efforts of many able mechanics and engineers.

If steel passed directly from the liquid to the plastic condition as glass does, hammers and rolls would soon be replaced by dies at a great saving of cost and labor. It is no wonder that such a desirable end has led to many persistent and costly efforts, but until some way can be devised to eliminate this granular form in cooling it would seem that all such efforts must end in failure.

As steel cools down through the plastic condition the cooling is not continuous; there are two or three points where it is arrested for a time, and at one notable point the cooling is not only arrested, but after a few moments of stop the operation is reversed, the steel becomes visibly hotter, and then the cooling goes on regularly; there may be other slight pauses, but they are of little importance compared to this one, which is known as the point of recalescence. There are many theories of the cause of this recalescence; the ablest scientists are still working at it; and until some definite conclusion is reached it is not worth while to write pages of discussion which may be found fully stated and illustrated over and over again in the various technical journals, and transactions of different engineering societies.

There are some properties of steel of great interest which seem to cluster around this recalescence-point; they will be noted as they are reached.

We have seen that there is a marked, definite structure of the grain of ingots due to every quantity of carbon, and also that there is a fixed limit of malleability for every quantity of carbon. It is known also that the recalescence-point shifts slightly with a change of carbon, and that it is much more marked and brighter in high-carbon steel than in low.

There are no other sure indications of the quantity of carbon present. As soon as an ingot is heated up to orange color, or the recalescent-point, it loses its distinctive structure and its fracture no longer furnishes a sure guide.

If three ingots of say, 20, 80, and 120 carbon respectively be heated to orange and then cooled slowly, their fractures will be so different as to enable an expert to place them properly in their order of carbon, and to classify them as mild, hard, and harder; beyond that he could not go; if he attempted to give them their temper numbers, he would be likely to miss by four or five numbers either way, and a correct mark would be only a lucky guess.

Hammering and rolling heated steel affect the grain or structure profoundly; a high steel may be worked so that the grain will look mild, and a mild steel may be so worked that the grain will look hard. It is common to see a bar of steel with a fine grain at one end and a coarse grain at the other, and this state of things often frightens a consumer, who imagines that he has received a very irregular, uneven article, and he is as often astonished when it is shown to him that at the same proper heat the two ends will refine and harden equally well, and be exactly alike. In such a bar one end has been finished a little hotter than the other, and the grain is due to the heat in each case. This uneven heating may have been incidental or careless; with skilful workers it is rare.