Castings are a product form that often receive radiographic inspection since many of the defects produced by the casting process are volumetric in nature, and are thus relatively easy to detect with this method. These discontinuities of course, are related to casting process deficiencies, which, if properly understood, can lead to accurate accept-reject decisions as well as to suitable corrective measures. Since different types and sizes of defects have different effects of the performance of the casting, it is important that the radiographer is able to identify the type and size of the defects. ASTM E, Standard for Radiographs of castings has been produced to help the radiographer make a better assessment of the defects found in components.
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Castings are a product form that often receive radiographic inspection since many of the defects produced by the casting process are volumetric in nature, and are thus relatively easy to detect with this method. These discontinuities of course, are related to casting process deficiencies, which, if properly understood, can lead to accurate accept-reject decisions as well as to suitable corrective measures.
Since different types and sizes of defects have different effects of the performance of the casting, it is important that the radiographer is able to identify the type and size of the defects. ASTM E, Standard for Radiographs of castings has been produced to help the radiographer make a better assessment of the defects found in components. The castings used to produce the standard radiographs have been destructively analyzed to confirm the size and type of discontinuities present.
The following is a brief description of the most common discontinuity types included in existing reference radiograph documents in graded types or as single illustrations. These discontinuities are usually smooth-walled rounded cavities of a spherical, elongated or flattened shape. If the sprue is not high enough to provide the necessary heat transfer needed to force the gas or air out of the mold, the gas or air will be trapped as the molten metal begins to solidify.
Blows can also be caused by sand that is too fine, too wet, or by sand that has a low permeability so that gas cannot escape. Too high a moisture content in the sand makes it difficult to carry the excessive volumes of water vapor away from the casting. Another cause of blows can be attributed to using green ladles, rusty or damp chills and chaplets. Sand inclusions and dross are nonmetallic oxides, which appear on the radiograph as irregular, dark blotches. Careful control of the melt, proper holding time in the ladle and skimming of the melt during pouring will minimize or obviate this source of trouble.
Shrinkage is a form of discontinuity that appears as dark spots on the radiograph. Shrinkage assumes various forms, but in all cases it occurs because molten metal shrinks as it solidifies, in all portions of the final casting. Shrinkage is avoided by making sure that the volume of the casting is adequately fed by risers which sacrificially retain the shrinkage.
Shrinkage in its various forms can be recognized by a number of characteristics on radiographs. There are at least four types of shrinkage: 1 cavity; 2 dendritic; 3 filamentary; and 4 sponge types. Some documents designate these types by numbers, without actual names, to avoid possible misunderstanding. Cavity shrinkage appears as areas with distinct jagged boundaries. It may be produced when metal solidifies between two original streams of melt coming from opposite directions to join a common front.
Cavity shrinkage usually occurs at a time when the melt has almost reached solidification temperature and there is no source of supplementary liquid to feed possible cavities. Dendritic shrinkage is a distribution of very fine lines or small elongated cavities that may vary in density and are usually unconnected. Filamentary shrinkage usually occurs as a continuous structure of connected lines or branches of variable length, width and density, or occasionally as a network.
Sponge shrinkage shows itself as areas of lacy texture with diffuse outlines, generally toward the mid-thickness of heavier casting sections. Sponge shrinkage may be dendritic or filamentary shrinkage.
Filamentary sponge shrinkage appears more blurred because it is projected through the relatively thick coating between the discontinuities and the film surface. Cracks are thin straight or jagged linearly disposed discontinuities that occur after the melt has solidified. They generally appear singly and originate at casting surfaces.
Cold shuts generally appear on or near a surface of cast metal as a result of two streams of liquid meeting and failing to unite. They may appear on a radiograph as cracks or seams with smooth or rounded edges. Inclusions are nonmetallic materials in an otherwise solid metallic matrix. They may be less or more dense than the matrix alloy and will appear on the radiograph, respectively, as darker or lighter indications.
The latter type is more common in light metal castings. Core shift shows itself as a variation in section thickness, usually on radiographic views representing diametrically opposite portions of cylindrical casting portions.
E155-15 Standard Reference Radiographs for Inspection of Aluminum and Magnesium Castings.pdf
A number in parentheses indicates the year of last reapproval. A superscript epsilon? This standard has been approved for use by agencies of the Department of Defense. Scope 1.
More E Scope 1. The reference radiograph films are an adjunct to this document and must be purchased separately from ASTM International if needed. If performing digital radiography of aluminum-alloy castings, refer to Digital Reference Image Standard E