Primary Processing Discontinuities

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

Figure 3-53. Magnetic Particle Indication of a Seam on a Bar

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 3-98 rolling these seams appear, greatly elongated, on finished bars and shapes, often making them unsuitable for many purposes. 3.7.4.1.2 Primary Processing Discontinuities When steel ingots are worked down into usable sizes and shapes, such as billets and forging blanks, some of the above described inherent defects may appear. But the rolling and forging processes may themselves introduce discontinuities that also may constitute defects. Primary processes are those, which work the metal down, by either hot or cold deformation, into useful forms such as, bars, rod and wire, and forged shapes. Casting is another process usually included in this group. Even though it starts with molten metal it results in a semi-finished product. Welding is included for similar reasons. A description of the discontinuities that can be introduced by these primary processes follows: a. Seams. Seams in rolled bars or drawn wire are usually highly objectionable. As previously described, seams may originate from ingot cracks. Conditioning of the billet surfaces by scarfing, grinding or chipping can eliminate the cracks before final rolling is performed. But seams can be introduced by the rolling or drawing processes themselves. Laps can occur in the rolling of the ingot into billets as the result of over-filling of the rolls. This produces projecting fins, which on subsequent passes are rolled into the surface of the billet or bar. In similar fashion, under-fills in the rolling process may on subsequent passes be squeezed to form a seam, which often runs the full length of the bar. Seams derived from laps will usually emerge to the surface of the bar at an acute angle. Seams derived from the folds produced by an under-filled pass are likely to be more nearly normal to the surface of the bar. Seams or die marks may also be introduced in the drawing process due to defective dies. Such seams may or may not make the product defective. For some purposes, such as springs or bars for heavy upsetting, the most minute surface imperfections (or discontinuities) are cause for rejection. For others, where machining operations are expected to remove the outer layers of metal, shallow seams will be machined off. (See Figure 3-52 and Figure 3-53.) Figure 3-52. How Laps and Seams Are Produced from Over-Fills and Under-Fills

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