While one might assume defects or abnormities occur during a part’s manufacturing process, there are some discontinuities which occur before a part’s creation even starts. These types of discontinuities are known as “inherent” and they are present in metal as the result of its initial solidification from the molten state, before any of the processes to forge or roll it into useful sizes and shapes has begun. Below are common inherent discontinuities you might encounter.
Pipe
As the molten steel which has been poured into the ingot mold cools, it solidifies first at the bottom and walls of the mold. Solidification progresses gradually upward and inward. The solidified metal occupies a somewhat smaller volume than the liquid, so that there is a progressive shrinkage of volume as solidification goes on. The last metal to solidify is at the top of the mold, but due to shrinkage there is not enough metal to fill the mold completely, and a depression or cavity is formed. This may extend quite deeply into the ingot. After the early breakdown of the ingot into a bloom, this shrink cavity is cut away or cropped. If this is not done completely before final rolling or forging into shape, the unsound metal will show up as voids called "pipe" in the finished product. Such internal discontinuities, or pipe, are obviously undesirable for most uses and constitute a true defect. Special devices ("hot tops") and special handling of the ingot during pouring and solidifying can, to a large degree, control the formation of these shrink cavities.
Blowholes
As the molten metal in the ingot mold solidifies, there is an evolution of various gases. These gas bubbles rise through the liquid and many escape. Many, however, are trapped as the metal freezes. Some, usually small, will appear near the surface of the ingot; and some, often large, will be deeper in the metal, especially near the top of the ingot. Many of these blowholes are clean on the interior and are welded shut again into sound metal during the first rolling or forging of the ingot; but some, near the surface, may have become oxidized and do not weld. These may appear as seams in the rolled product. Those deeper in the interior, if not welded shut in the rolling. may appear as laminations.
Segregation
Another action that takes place during the solidification of the molten metal is the tendency for certain elements in the metal to concentrate in the last-to-solidify liquid resulting in an uneven distribution of some of the chemical constituents of the steel as between the outside and center of the ingot. Various means have been developed to minimize this tendency, but if for any reason sever segregation does occur, the difference in permeability of the segregated areas may produce magnetic particle indications. Unless severe, such segregation is generally not deleterious.
Non-metallic Inclusions
All steel contains more or less matter of a non-metallic nature. The origin of such matter is chiefly the de-oxidizing materials added to the molten steel in the furnace, the ladle or the ingot mold. These additions are easily oxidizable metals such as aluminum, silicone, manganese and others. The oxides and sulfides of these additions constitute the bulk of the non-metallic inclusions. When finely divided and uniformly distributed, such non-metallic matter does not usually injure the steel. However, it sometimes gathers into large clumps which, when rolled out, become long “stringers.” These stringers in some cases are objectionable. When such a string occurs at the surface or just under the surface of a highly stressed part or bearing surface, it may lead to fatigue cracking. In general, non-metallic inclusions in steel seldom constitute a real defect, through they are often indicated with magnetic particles. Non-metallic inclusions are sometimes added to steel intentionally. The addition of lead of sulfur to steels for the purpose of improving machinability is common practice. Such steel will show excessive amounts of non-metallic inclusions, which serve to break up the chips when the metal is turned or otherwise machined. Machine time is reduced, and tool life is lengthened. Such steels, if tested with magnetic particles, may show a alarming looking patterns, which have no significance as defects. The magnetic particle must be familiar with this type of steel. Though serving a useful purpose in their proper field, these steels should never be used for critical or highly stressed parts or parts subject to fatigue in service.
Internal Fissures
Because of the stresses set up in the ingot as the result of shrinkage during cooling, internal ruptures may occur which may be quite large. Since no air normally reaches the surfaces of these internal bursts, they may be completely welded shut during rolling or other working and leave no discontinuity. If there is an opening from the fissure to the surface, however, air will enter and oxidize the surfaces. In such a case welding does not occur, and they will remain in the finished product as discontinuities.
Scabs
When liquid steel is first poured into the ingot mold there is considerable splashing or spattering up and against the cool walls of the mold. These splashes solidify at once and become oxidized. As the molten steel rises and the mold becomes filled, these splashes will be reabsorbed to a large extent into the metal. But in some cases, they will remain as scabs of oxidized metal adhering to the surface of the ingot. These may remain and appear on the surface of the rolled product. lf they do not go deeply into the surface, they may not constitute a defect, since they may be removed on machining.
Ingot Cracks
Surface cracking of ingots occurs due to surface stresses generated during cooling of the ingot. They may be either longitudinal or transverse, or both types may occur together. As the ingot is worked into billets by rolling, these cracks form long seams. Inspection of quality-product billets for seams of this type with magnetic particles is now common practice in modern mills. This permits removal of the seams by flame scarfing, chipping or grinding without waste of good metal. If not removed before further rolling these deeper seams appear, greatly elongated, on finished bars and shapes, often making them unsuitable for many purposes.
Want to learn more about discontinuities and defects? Read our detailed blog about the difference between the two.
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