Casting Defects.

T.O. 33B-1-1 and kind of material under consideration with a technique giving inherent wide latitude with adequate sensitivity. In some instances even when radiographing light alloys castings, lead filter screens may be employed. 6.8.14.4.1 Correct radiographic procedure requires the selection of the lowest voltage that will do the job in a reasonable exposure time. Where many castings are examined, a convenient technique is to establish a reasonable exposure time and select the voltage required for the thickness of the particular section being radiographed. Good practice normally requires that exposures be longer than 1 minute. When castings with great difference in thickness must be radiographed in one exposure, an increase in voltage will provide wider latitude, as well as shorter exposure time. However, contrast is reduced. If other factors remain constant, the most desirable combinations of voltage and exposure time for a specific part being examined may be governed largely by the acceptable radiographic sensitivity. 6.8.15 Casting Defects. 6.8.15.1 Microshrinkage. Microshrinkage appears as a dark feathery streak or dark irregular patches, corresponding to a grain boundary shrinkage condition. This condition may be suspected in a magnesium base alloy. 6.8.15.2 Shrinkage Porosity. Shrinkage porosity or sponge appears as a local honeycombed or mottled pattern; it may be the result of pouring temperature or alloy composition. 6.8.15.3 Gas Porosity. Gas porosity appears as round or elongated smooth, dark spots occurring individually or in clusters, or distributed throughout the casting. This condition can be caused by gas forming during solidification, by the evaporation of moisture or volatile material from the mold surface. Insufficient core baking, venting and entrapment of air in the cope surface of the casting before complete solidification are other likely causes. 6.8.15.4 Inclusions. Inclusions of tramped material in the molten f low may be poured into the mold. This appears as light areas in a radiograph since the inclusions are denser than the alloyed casting. Sand inclusions appear as gray or light spots of uneven granular texture within distinct outline and tend to concentrate near the drag (bottom) side of the casting. Inclusions which are lighter than the casting appear as isolated, irregular or elongated variations of film blackening not corresponding to variations in thickness of material or to cavities. They may be due to sand slag or oxides. 6.8.15.5 Hot Cracks. Hot cracks appear as ragged dark lines of variable width and numerous branches with no definite line of continuity. They may exist in groups and start at the surface or they may be internal. This condition is usually a result of the normal contraction of the casting, being restricted by the mold and/or core during or immediately after solidification. 6.8.15.6 Cold Cracks. Cold cracks appear generally as a single, straight, sharp dark line usually continuous throughout its length. Cracks of this nature may be produced by cooling from elevated temperature during f lame burning, grinding or quenching operations and internal stresses that are set up by thermal gradients. 6-100 Change 3