Castings

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 Change 3 6-99