3.2 Assurance of Integrity of Casting
1) Control of molten metal using of degassing method
The primary cause of occurrence of minute defects (blow holes) on the surface or inside a casting lies with the remaining gases. To eliminate these blow holes, a method is effective to degas from the molten metal. In this method, inert gas, i.e., N2 gas which is not dissolved in the molten metal is blown into the molten metal to absorb harmful H2 gas by the N2 gas, and the gas combined is discharged together into the atmosphere. Also, because blow holes tend to occur on the upper surface of the casting, a plan to float the gas to the rear surface side (compressive stress) by facing down the pressure surface (tensile stress in use) of the propeller blade is adopted for molding. Fig.2 shows the effect of our degassing method. The number of the defects shown in the figure indicates the number of echoes detected by the ultrasonic inspection. The figure does not show that all defects are blow holes.
When the hydrogen content is 1.0 or below, the number of the detects is reduced remarkably.
2) Application of padding to boss
Relatively large defects (shrinkage cavities) which tend to occur in a propeller, particularly in the connection part between the root of the blade and the boss, or linear defects (hot tearing) occurring on the surface of or near the fillet of the blade root section occur due to temperature distribution and temperature gradient at the time of solidification of the molten metal. As a result, when a propeller having a novel geometric form is designed, for example, when the balance between the propeller blade root thickness and boss thickness is different remarkably from usual propeller , a simulation of solidification is performed beforehand. If required, an excess thickness called the padding is provided at the corn part or others on the internal surface of the boss to control the inside temperature of the propeller casting at the time of solidification. In a recent propeller, the temperature control at the time of solidification particularly by this padding or hot top device (heater for continuous heating located on the upper surface of a riser) exerts its effect, and consequently linear cavities are prevented from the surface of the blade. Fig.3 and fig.4 shows the solidification simulation and hot top device, respectively.
3) Prevention of entry of foreign substance into casting.
In addition to shrinkage cavities, minute defects occurring on the surface of a propeller casting include inclusion of foundry sand or oxidized slag. Inclusion of sand tends to occur when the flow-in speed of the molten metal is high at the time of pouring. The sand on the surface of the mold is separated and caught into the molten metal. Some types of oxidized slag solidify in the state of being caught in or near the surface of the molten metal. When the pouring is performed at a permissible and steady speed paying attention to a reduction in temperature of the molten metal during pouring, an inclusion of sand is prevented.
Fig. 2 The effect of degassing Method
Fig. 3 Simulation of solidification
Fig. 4 Hot Top device
