Fig. 5 Relation between a casting speed and number of detected echoes
In addition, the amount of molten metal sufficient for the weight of a casting must be prepared so that oxidized slag is not mixed into the molten metal when the molten metal is discharged from the melting furnace, and the oxidized slag formed continuously on the surface of the molten metal must be completely removed as far as practicable. Fig.5 shows the relationship between a casting speed (casting time divided by weight of casting) and the number of detected echoes. From Fig.5, it can be found that the number of the detects by echoes is reduced at a casting time of 0.03 sec/kg or higher.
3.3 Defects for which Propeller Must Be Inspected by Nondestructive Inspection
As described above, even today's propeller casting technology, it is unfortunately difficult using to completely eliminate the internal defects of a casting.
From the view point of the assurance of reliability, it becomes important to reinforce the nondestructive inspection . As already described, gas defects, shrinkage and entry of foreign matter are cited as the internal defects found in the casting of a propeller. And, a shrinkage occurring on or directly under the surface of the pressure surface of the blade and the entry of foreign matter are tend to cause fatigue cracks during the operation. This is because the fatigue cracks are produced from linear defects which increase stress sensitivity as initial notches. This can be found in Fig.6 showing an example of actual breakage of propeller blade which is contained in the document of the Nippon Kaiji Kyokai [1]. Namely, the shape of the defect at its initiation indicates a shrinkage or entry of foreign matter directly under the surface of the blade.
Observing in detail a series of shapes of the defects, it can be found that those defects excepting the defect (b) shown in Fig.6 result in crack propagation from the surface to the inside of the blade. In other words, many such defects in a propeller that become the imitation of fatigue cracking appear partly on the surface of the blade. Therefore, these defects can be detected if a proper appearance inspection and dye penetrant inspection are performed.
The reason why the classification society has an obligation to perform the dye penetrant inspection is considered due to this.
Judging from its shape, the exceptional defect (b) is considered to be blow holes produced directly under the surface of the blade at positions rather deep from the surface of the blade. It has already be described that this type of defect can be prevented by an appropriate degassing method.
4. Application of Nondestructive Inspection to Propeller Casting
The inspections used generally for the nondestructive inspection of a propeller casting include the dye penetrant inspection specified in the rules of the classification society and the ultrasonic inspection which is not considered to have been established technically. Here the inspection procedure is explained briefly and the present technical level is confirmed.
4.1 Dye Penetrant Inspection Procedure
The dye penetrant inspection is performed in accordance with the Manufacturing Inspection Standard for Large-Size Marine Propellers (SMA276) of Japanese Marine Equipment Association [2].
