T.O. 33B-1-1
6.8.14.1
It is necessary to control the quality of the casting process to assure reliability of the castings. Radiographic
inspection is a satisfactory quality control since the conditions likely to make the casting unacceptable are
readily detected by this inspection. For the purpose of inspection, airframe castings may be divided into
classes based on their function and on their margins of safety for design loading conditions. These classes are
defined in specification MIL-C-6021 F ‘‘Castings, Classification and Inspection of’’ and are basically as
follows:
a.
Class 1. A casting, the single failure of which would cause significant danger to operating personnel
or would result in a significant operational penalty. In the case of missiles, aircraft, and other
vehicles, this includes loss of major components, loss of control, unintentional release or inability to
release armament stores, or failure of weapon installation components. Class 1 castings shall be
further classified under Class 1A and Class 1B below.
(1)
Class 1A. A Class 1 casting, the single failure of which would result in the loss of a missile,
aircraft, or other vehicle. These castings receive 100 percent radiographic inspection.
(2)
Class 1 B. Class 1 castings not included in Class 1A. Radiographic inspection in accordance with
sampling Table 1 of Spec. MIL-C-6021F.
b.
Class 2. All castings not classified, as Class 1. Class 2 castings shall be further classified under Class
2A and Class 2B below.
(1)
Class 2A. Castings have a margin of safety of 200 percent or less. Radiographic inspection in
accordance with Table 11 of Spec. MIL-C-6021F.
(2)
Class 2B. Castings have a margin of safety greater than 200 percent, or for which no stress
analysis is required. All target drone castings and aerospace ground support equipment fall in
this category, except for such critical parts, the failure of which would make the equipment
unsatisfactory and cause the vehicles which they are intended to support to be inoperable.
Radiographic inspection is not required.
6.8.14.2
Radiographic examination is ideally suited to the inspection of castings because the most common casting
discontinuities are three dimensional and are, therefore, almost independent of angle of inspection. Excep-
tions in some cases include fine cracks, cold shuts, unfused chills and chaplets. To reveal these, the radiation
must be at or near the same parallel plane as the discontinuity. Hairline surface cracks, such as those
produced by grinding are seldom, if ever, revealed by radiography.
6.8.14.2.1
It is possible in most cases to identify the radiographic images of the common types of discontinuities that are
inherent in the casting process. This information is valuable to the foundry in procedure development work
that may be necessary to meet a standard of quality. Although the discontinuities commonly encountered in
aluminum and magnesium castings are similar to those in ferrous metals, a group of irregularities called
‘‘dispersed defects’’ may frequently be present. These ‘‘defects,’’ prevalent in light alloy castings, consist of
tiny voids scattered throughout part or all of a casting. Gas porosity and shrinkage porosity in aluminum
alloys are examples of dispersed defects. On radiographs of sections more than one-half inch thick it is
difficult to distinguish images corresponding to the individual voids. Instead, dispersed defects may appear
on film deceptively as mottling, dark streaks or other irregularities.
6.8.14.3
Radiographic studies of new casting produced by the foundry reveal the type and location of internal
discontinuities. This aids the foundry to change the casting technique by altering the gating, relocating
chills, changing the pouring temperatures, repositioning, increasing or decreasing the risers or altering the
size, correcting a faulty sand condition, or increasing the venting in the mold. After developing an acceptable
casting procedure the casting can be duplicated with assurance of a quality part.
6.8.14.4
In general, castings are irregular in shape and may vary considerably in cross section thickness from area to
area. It is, therefore, important to utilize equipment of adequate capacity to penetrate the section thickness
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