We have learned in considering the general properties of metals that they always crystalize upon solidification. This is eminently so in the case of the molten steel solidifying in the large ingots of commercial operations. These masses of steel, often weighing many tons, aggravate the segregation of impurities by the necessarily long time which elapses before the interior of such an ingot becomes entirely solid.
Because of this characteristic solidification phenomenon - the growth of the crystals from the exterior to the heart of the ingot during the solidification - it is found that ordinary ingots exhibit wide variations in their chemical compositions. Carbon may vary many per cent of its total amount, as between the outer shell and the core. Phosphorus often is worse in this respect, and the same is the case with sulphur. Elements which form solid solutions with ferrite in the cold show little tendency to segregate. It is those elements which are thrown out of solution in ferrite - and in so doing form low fusing alloys and eutectics - which not only segregate most but are injurious because of this segregation in the finished material. This is known as segregation in steel, and is one of the great features to contend with in modern practice.
Another phenomenon exhibited by steel in solidifying is the contraction of the metal during the solidification and cooling of the solid; the effect is that the metal solidifying in its exterior portion has a solid shell formed, against which the molten interior gradually is contracting and solidifying. Obviously, a hole will be left in the very heart of the ingot when all of the metal has become solid. This cavity is called the pipe. It is a very serious defect in any ingot, because, if it is not removed, it will leave a flaw in the finished steel.
Another defect in many ingots is caused by gases, which were perfectly soluble in the molten material, extruding into little blowholes as the metal assumes the solid state. This condition possibly is more peculiar to converter practice than to any of the other methods, and it needs the most careful technique to overcome it fully.
As shown in Fig. 26, Ingot No. 1 plainly is damaged seriously by the presence of the large blowholes throughout the entire metal; as the steel solidified, this gas generation even forced the metal up in the mold. Ingot No. 2 has a few blowholes about the outside of the ingot and a core of conspicuously segregated metal, the pipe is small. Ingot No. 3 has a most aggravating pipe but is otherwise sound; to cut out the piped part as discard would mean losing half the ingot. Ingot No. 4 is sound with a big cavity in the feeder head; when this head is cut away, 95 per cent of the ingot may be sent to the mill in perfect condition. The first three ingots show what may happen in ordinary practice; the fourth shows what scientific study will accomplish.
The presence of small particles of solid foreign materials - like slag and oxides - in the ingot, as well as o/ defects like cracks and checks, is a matter of faulty operation and does not require the serious study to overcome as in the case of segregation, pipes, and blowholes.
To overcome all these defects is one of the serious efforts of current study. We know that small ingots, as a rule, are not affected as seriously as the large ingots, but it is seldom practical to resort to this. Bottom casting of the ingots is not infrequently used and gives a much better metal than the ordinary teeming.
A pouring basin on top of the mold may be a step in the right direction but is only a partial remedy. Casting ingots with the big end uppermost, and with the mold very thick at the base, much reduces the defect due to the pipe. The intrusion of a can of thermit into the very bottom of the still molten metal at the proper moment has proved efficacious and worth using. Squeezing the ingot while the core is still partly molten of course will close up any cavity and give a solid ingot. Heating the top of the ingot and keeping it molten as long as possible will concentrate the cavity in the very top of the ingot and prevent much metal being wasted when the ingot is cropped preparatory to rolling.
Particular chemical composition to a certain extent can regulate the amount and position of any blowholes which will be formed. Finally, the technique of deoxidizing and recarbonizing will have an extreme effect on the solidity and uniformity of the solid ingot. Gases must be removed as completely as possible while the metal still is molten; slag particles and particles of oxide must be floated to the top of the bath while the metal is held fluid. This chemical and physical purification is accomplished by using the proper deoxi-dizers in exactly the right amount. The most common deoxidizers are aluminum, ferromanganese, ferrosilicon, ferrotitanium, and ferrovanadium.