It is often said that "one picture is worth a thousand words." This is especially true when trying to describe the appearance of certain types of discontinuities which are so often seen in industrial radiography.

Discontinuities in castings such as sponge or dendritic shrinkage are difficult, if not impossible, to describe in words. Gas porosity in weldments or in aluminum and magnesium castings is another example of a discontinuity which does not lend itself well to description by the written word. These discontinuities can readily by evaluated by comparing them to a reference radiograph of the same discontinuity type.

Reference radiograph sets, such as those distributed by the American Society for Testing and Materials (ASTM), are collections of radiographic images showing graded levels of discontinuities which can occur in castings and weldments. For each discontinuity type there can be four to eight graded levels, with level one being the best. Some of the discontinuity types illustrated are gas holes, shrinkage, foreign material inclusions, incomplete fusion, and penetration and porosity.

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One picture is worth a thousand words.

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Reference radiographs are available for steel and aluminum weldments, aluminum, magnesium, copperbase, tin bronze, titanium, and steel castings. The reference sets for weldments and heavy wall steel castings (25 mm [1 in.]) thickness and greater) are made at various energy levels. The set selected for use should be of the same energy level as that used on the items being evaluated. An example of this is ASTM E446 - "Standard Reference Radiographs for Steel Castings up to 51 mm (2 in.) in Thickness." This set consists of images of 34 steel specimens radiographed using 250 KVP X-ray units for Volume I, Ir-192 for Volume II, and Co-60 for Volume III.

Some reference radiograph sets can cover multiple thickness ranges by having illustrations of discontinuities in several thicknesses. An example of this is ASTM E192 - "Standard Reference Radiographs for Investment Steel Castings for Aerospace Applications." Table 1 shows the actual illustration plate thicknesses and their applicable range of use.

Text supplied with each reference set includes definitions of discontinuity types, a description of the illustrations and a basis for their use. These reference radiographs can be:

• a source from which manufacturers and purchasers may select particular radiographs to serve as standards representing minimum product acceptability.

• a guide and training aid in the recognition of discontinuities common to the material being examined

• a guide showing the differences in discontinuity appearance obtained by using different radiation producing sources and energy levels.

References are available showing the changes in appearance of radiographic images when certain parameters such as film type or radiation level are changed. Two examples of this are ASTM E242 - "Standard Reference Radiographs for Appearances of Radiographic Images as Certain Parameters are Changed" and ASTM E592 - "Standard Guide to Obtaining ASTM Equivalent Penetrameter Sensitivity for Radiography of Steel Plates 6-51 mm (0.25 - 2 in.) Thick with X-rays and 25-152 mm (1-6 in.) Thick with Co-60."

ASTM reference radiograph sets are recognized throughout the world and many material specifications, both military and commercial, specify their use for the determination of product acceptability. They may be used in totus, or, specific illustrations and grade levels may be selected based on the end use of the product being evaluated.

Using reference radiographs enables the film interpreter to make fast and accurate determinations of discontinuity type and acceptability. The interpreter is given a visual image for comparison rather than a table or graph and does not need to resort to elaborate measurements of size and spacing.

Reference radiographs allow product designers, product manufacturers, and product inspectors to "speak the same language" when it comes to "what it should be," "what I have to make," and "what really is."

Table 1  Range of illustration
Illustrations			Illustration Plate Thickness mm (in.)	Applicable Casting Thickness mm (in.)
Graded:
	Gas holes		3.2 (0.13)	6.4 (0.25) and under
	Gas holes		9.5 (0.38)	Over 6.4-12.7 (0.25-0.5), incluisive
	Gas holes		19.0 (0.75)	Over 12.7-25.4 (0.5-1), incluisive
	Shrinkage cavity		19.0 (0.75)	All thicknesses
	Shrinkage, sponge		3.2 (0.13)	6.4 (0.25) and under
	Shrinkage, sponge		9.5 (0.38)	Over 6.4-12.7 (0.25-0.5), incluisive
	Shrinkage, sponge		19.0 (0.75)	Over 12.7-25.4 (0.5-1), incluisive
	Shrinkage, dendritic		3.2 (0.13)	6.4 (0.25) and under
	Shrinkage, dendritic		9.5 (0.38)	Over 6.4-12.7 (0.25-0.5), incluisive
	Shrinkage, dendritic		19.0 (0.75)	Over 12.7-25.4 (0.5-1), incluisive
	Shrinkage, filamentary		19.0 (0.75)	All thicknesses
	Foreign material, less dense		3.2 (0.13)	6.4 (0.25) and under
	Foreign material, less dense		9.5 (0.38)	Over 6.4-12.7 (0.25-0.5), incluisive
	Foreign material, less dense		19.0 (0.75)	Over 12.7-25.4 (0.5-1), incluisive
Ungraded:
	Discrete Discontinuities:
		Foreign material, more dense	9.5 (0.38)
		Hot tear	9.5 (0.38)
		Cold crack	9.5 (0.38)
		Cold shut	9.5 (0.38)
		Misrun	9.5 (0.38)
		Core shift	9.5 (0.38)
	Defective Mold:
		Mold buckle, positive	9.5 (0.38)
		Mold buckle, negative	9.5 (0.38)
		Mold ridge	9.5 (0.38)
		Excess metal in cracked core	9.5 (0.38)
	Diffraction pattern:
		Columnar	9.5 (0.38)
		Mottled	9.5 (0.38)

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