Quench Cool Casting Of Copper Propellers
|In the past, nickel-aluminum-bronze marine propeller casting is a casting method in which a molten copper alloy of a propeller is poured into a sand mold, and the casting is cooled to normal temperature before being taken out of the mold.|
Due to the low thermal conductivity of the molding sand, the large propellers weighing tens of tons can be cooled for several days. The cooling rate of the machining castings is slow, and the mechanical properties and corrosion fatigue strength that constitute the quality effect are significantly reduced.
The mechanical properties and corrosion fatigue strength of special quality effects are significantly reduced. Especially around 0.2 ~ 0.3R of the airfoil with the largest working stress, the thickness of the airfoil (except the hub) is thicker than other parts, so the corrosion fatigue strength is repeated 2×107, 18kgf / mm2, and the diameter of more than 6000mm weighs twenty tons. For large-scale ship propeller castings, the corrosion fatigue strength at the root of the wing drops to 2×107; 10 ~ 20 kgf / mm2. The reason is that the casting speed is between about 1000 ° C and 700 ° C. For large propellers, the cooling rate is about 0.1 ° C / min.
Therefore, the metal compounds of Al, Ni, and Fe (such as FeAl, NiAl, etc.) that dominate the mechanical properties of nickel-aluminum bronze alloys have agglomeration In addition, nickel-aluminum bronze castings are sensitive to hydrogen absorption during the melting process, and slow-cooling large castings are more likely to produce hydrogen defects.
Accordingly, the new casting method is characterized by:
- 1. Regarding the weight percentage of the chemical composition in the material: aluminum 8.5 ~ 10.5%, nickel 4 ~ 6%, iron 4 ~ 6%, manganese 4% or less, copper and common impurities
- 2. During casting, after casting the molten copper alloy into a mold that can control the cooling rate, the cooling rate is controlled between 1000 ~ 700 ℃, 5 ℃ / min; below 700 ℃, 1 ℃ / min.
This casting method is different from the sand mold and air cooling with low thermal conductivity in the past. Instead, it uses metal particles with a thermal conductivity 20 to 30 times higher than that of sand, and embeds through-cooling water. In addition, prior to pouring, a method of forcibly cooling a molten copper alloy having a predetermined chemical composition while cooling water in the mold is poured and forced cooling is performed.
The cooling rate is significantly higher than that of the conventional casting method, which effectively prevents the deterioration of the corrosion fatigue strength and mechanical properties that constitute the quality effect.
This casting method stipulates that the cooling rate between 1000 and 700 ° C is 5 ° C / min or more, because the cooling rate has no relationship with the precipitation and growth of the precipitates above 1000 ° C, and the precipitates (FeAl, NiAl between 1000 and 700 ° C) Etc.)
Precipitation and growth temperature; below 700 ° C, crystal grains almost no longer grow, and the cooling rate potential precipitates under this temperature condition must have a limit value to grow to an appropriate size. The upper limit of the cooling rate is not specified because the shape of the casting varies, and it is ideal that 5 ° C / min or more is sufficient to control the growth of precipitates. The cooling rate below 700 ° C is specified to be below 1 ° C / min, in order to transform the β phase produced by rapid cooling above 1000 ° C to 5 ° C / min between 1000 ° C and 700 ° C into an α phase, to ensure the toughness of the material, and to allow the alloying elements to diffuse sufficiently to achieve material homogeneity Into.
There is no lower limit for the cooling rate below 700 ° C. Naturally, the cooling rate varies with the shape of the casting. If the cooling rate is below 1 ° C / min, the α-phase transformation can be more sufficient and high corrosion fatigue strength can be obtained. With this casting method, the casting does not need to be heat-treated again at high temperature, avoids the problems of oxidation and deformation caused by heat treatment, and saves energy.
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