The X-Ray Testing of Metals at Watertown Arsenal 270037
TESTS of metals with X-rays, as made at Watertown Arsenal, are of two classes: (a) radiographic tests in which photographic images of internal details of the gross structure are obtained and (b) diffraction tests in which images are obtained that may be interpreted to give information regarding details of micro-structure of the constituents in the metal. The present paper deals with tests in the first class. Diffraction tests will eventually result in steels that have better physical properties required for special applications in industry, but such improvement must be accompanied by elimination of defects in the gross structure of forgings and castings before the greatest utility of better steels can be realized.
Radiographic testing gives pictures of defects whereby the nature of the defects can be determined, but their causes must be sought by logical deductions from other information. Cast-steel ingots are too large for study by X-ray methods, and forgings are not suited to these tests because the rolling reduces the defects in the ingot to too small a size for detection. It is more important to understand the nature of defects in structural units which are familiar as steel castings, hence the tests are of fundamental importance in studies tending to improve the gross structures, and are of importance also in certain kinds of inspection testing.
The test method involves merely taking X-ray pictures and interpreting them. Blow-holes, cracks and sand inclusions in the metal are revealed as dark spots on the negative. The apparatus used and the method of making the photographs are illustrated and described. Cavities revealed by the negatives result from various causes, as by a gas or by shrinking of the metal while cooling and insufficient size or improper placing of heads or risers intended to supply additional metal in the casting to offset shrinkage.
Cracks are revealed by X-rays as a series, some of which may not come to the surface, and are therefore very difficult to remove completely by welding. It is often necessary as a means of preventing repetition of such defects in castings from a given pattern, to correct the casting design.
As the cost of making radiographs is from $1 to $5 per picture and often from 40 to 50 pictures are needed for study of a single large casting, the method is slow and expensive; but in some cases it is justified as a means of inspecting an individual casting to assure against a serious accident. It may be used as a routine method of inspection of many small types of castings or of larger castings of simple design. Its great value lies, however, in developing better technique in the making of castings through the study of a few pilot castings. The expense of the test is returned to the foundry, often many times, in the reduction of rejections.
This method of X-ray testing tells nothing of the physical properties of the metal except as these are affected by the presence of defects in the gross structure, but the elimination of these defects through study may result in the realization in the castings of the intrinsic strength of the metal. The true strength of a steel casting is governed to a greater extent by the presence and location of gross structural defects than by the intrinsic strength of the metal, which cannot be realized in the strength of the casting until gross defects of structure can be eliminated. Alloy-steel castings may be little better than plain carbon-steel castings, as their strength is governed by the presence of gross defects. It seems to be poor economy to put expensive alloy-steels into poorly made castings. It is more important to seek to improve manufacturing methods than to seek to improve the intrinsic properties of the metal of the casting.