Inclusions

T.O. 33B-1-1 2-85 2.7.6.2.2.2 Inclusions. Inclusions are particles of foreign material, usually slag, oxides, sulfides or silicates trapped in the metal during solidification. If the material is mechanically worked into plate, sheet or bar, the inclusions will be elongated by the forming operations. They are not usually at the part surface but may become exposed by subsequent machining. Since inclusions are solid foreign matter, they will not form penetrant indications unless the foreign material is porous. Inclusions are considered defects only when they are open to the surface, have a measurable length, and are located in a critical area. 2.7.6.2.2.3 Seams. Seams occur in rolled bar stock or parts machined from bar stock. They are inclusions, porosity, or more commonly, metal folds that have been elongated by the rolling process during fabrication. They are long, straight discontinuities that run parallel to the direction of mechanical working. If the seams contain foreign material, they may produce no indications, or very faint indications. They may be classified as defects depending on size and location. 2.7.6.2.2.4 Forging Laps. Forging laps are formed when a portion of the metal is creased and folded over during the forging operation. They produce a wavy, irregular, linear indication. The indication may be faint or intermittent, since the lap breaks the surface at an angle and the edges may be partially welded. They may or may not be considered a defect, depending on size and location. 2.7.6.2.2.5 Flash-Line Cracking. Forging flash is the line of excess metal extruded into the space at the junction between the top and bottom dies. Cracking can occur when this excess metal is removed causing the linear type of indications. The cracking always occurs along and within the trimming marks. 2.7.6.2.2.6 Extrusion Tears. Extrusion involves forcing a metal through a die to produce a desired shape. The process is similar to squeezing tooth paste out of a tube. If the die lip has a nick, burr or lump of oxide, the die can produce tears in the extruded part. Extrusion tears are usually short linear defects perpendicular to the extrusion direction. 2.7.6.2.2.7 Thermal Cracks. When metals are subjected to a high temperature, localized stresses can occur due to unequal heating or cooling; restricted movement within the part; or unequal cross-section. Cracking will occur when the stresses exceed the tensile strength of the material. There are several types of thermal cracking depending upon the heating process. 2.7.6.2.2.8 Grinding Cracks. Grinding of hardened surfaces frequently introduces surface cracks. These thermal cracks are caused by localized over- heating due to insufficient or poor coolant; improper grinding wheel; too rapid feed or too heavy a cut. The cracks are shallow and sharp at the root; generally occur at right angles to the direction of grinding; and usually but not always, occur in multiples. Grinding cracks are considered defects since they reduce the fatigue strength. 2.7.6.2.2.9 Heat Treat Cracks. Heat treat or quench cracks form as a result of unequal heating or cooling within a part. The cracks are deep, usually forked, and seldom form a pattern. These cracks are considered defects. 2.7.6.2.2.10 Weld Cracks. Welds can contain a number of discontinuities detectable by penetrant. They may be due to lack of penetration, lack of fusion, heating or quenching cracks in the weld bead and heat affected zone, and grinding cracks occurring during removal of the weld crown. Crack-like discontinuities are considered defects. Two typical examples are, weld grinding cracks; and, shrinkage or quench crack. 2.7.6.2.3 Service Induced Discontinuities. The most frequently encountered service discontinuities detected by penetrant inspection are fatigue cracks. Stress corrosion and overload cracking is also common. Overload fractures occur when the stress exceeds the tensile strength