Magnetic Powder Inspection of Castings

Although radiographic examination was required by the Navy Department as early as 1930 in the inspection of seams in welded boiler drums, it was not until 1936, at which time gamma-ray technique was developed by the Naval Research Laboratory, that radiographic inspection was required on cast materials. In the beginning this method of inspection was specified only on those castings produced by Navy Yard foundries. Somewhat later, because of the rather general discovery of defects in hull castings, radiographic inspection was specified in contracts for important and highly stressed hull castings. The necessity for such inspection to determine internal defects was made doubly important with the advent of lighter sections to provide decreased weight. Combined with permission to repair defects by welding, radiographic examination has provided one method to insure soundness in critical areas of castings which enter into the construction of combatant vessels. In 1939, with the advent of the increased program of Naval construction, specifications required that all castings subjected to superheated steam, all castings in the main steam system, and all hull castings directly concerned with maneuvering the vessel be subjected to radiographic inspection. Although radiographic inspection has not been completely effective in preventing service failures, such failures have fallen to a very small percentage of those experienced prior to specifying this method of nondestructive testing. An order specifying radiographic inspection does not necessarily require that all parts of any casting or even that all castings be radiographed. The fact that any casting may be subjected to such inspection has usually provided fairly definite insurance that the necessary effort has been exerted by the foundry to produce castings free from harmful defects. In cases where defects have been discovered, it has usually been possible to make any necessary repairs. while magnetic powder inspection has been used for some time in testing forgings, and to a certain extent in inspecting ordnance castings regularly subject to high shock in service, this method of inspection has not generally been specified for ship castings. Because of lack of willingness by the steel foundry industry in general to use radiography when first specified, it was not desired at that time to add the burden of an additional inspection method. It was also believed that radiography would indicate all harmful defects with the possible exception of certain surface cracks which would be found by visual inspection. It was noticed, however, that occasional valves and fittings which had successfully passed all radiographic inspection and hydrostatic tests at twice the service pressures, developed leaks through the bolt holes of flanges and although radiographed after such leakage was discovered, no definite defect could be found on the negative. Upon subjecting the casting to magnetic inspection, however, a discontinuity was noted having the form of a hot tear or a crack running circumferentially around the bore immediately under the flange. Further magnetic inspection by Naval activities and by commercial foundries showed that approximately 30 per cent of all flanges produced contained this hot tear defect which in most cases was invisible to the eye, and which could not be indicated by radiographic inspection even when the exact size and direction of the defect was known, and a source and film location was used which would provide the maximum rendition of the defect.