Figure 3-49. Cross-Section of Ingot Showing Shrink Cavity

Figure 3-48. Magnetic Particle Indications of Segregations

Figure 3-50. Magnetic Particle Indication of a Sub-Surface Stringer of Non-Metallic Inclusions

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 3-96 3.7.4.1.1 Inherent Discontinuities. This group of discontinuities is present as the result of its initial solidification of metal from the molten state, before any of the operations to forge or roll it into useful sizes and shapes have begun. The names of these inherent discontinuities are given and their sources described below. a. 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 (See Figure 3-49.) After early breakdown of the ingot into a bloom (see Glossary), 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 solidification can control the formation of these shrink cavities. Figure 3-49. Cross-Section of Ingot Showing Shrink Cavity b. 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 small percentage escape. The remainder is trapped as the metal freezes. Most of these, 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 fused shut into sound metal during the first rolling or forging of the ingot; but some, near the surface, may have become oxidized and do not fuse. These may appear as seams in the rolled product. Those deeper in the interior, if not fused in the rolling, may appear as laminations. c. Segregation. Another action that takes place during the solidification 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 in the ingot. Various means have been developed to minimize this tendency, but, if for any reason, severe segregation does occur, the difference in permeability of the segregated areas may produce magnetic particle indications. Segregation can adversely affect physical properties as well as contribute to the formation of defects later in the processing cycle. d. Nonmetallic Inclusions. Nonmetallic inclusions are usually oxides, sulfides or silicates. They can be introduced by the use of dirty raw materials, crucibles or rods. Other contributing factors can be faulty linings and poor pouring practices. The inclusions can form stringers during subsequent rolling operations. These stringers can affect the physical properties of the materials and are usually considered defects. See Figure 3-50 for an example of an indication of nonmetallic inclusions.

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