JPS629761A - Method for preventing casting defect to be generated near advancing face of propeller made of ni-al bronze for large-sized ship - Google Patents

Abstract

PURPOSE:To prevent pipy defects and to suppress the generation of the fatique crack of a propeller by disposing chillers to a drag on the side to directly contact with a molten metal over approximately the entire surface thereof and specifying the solidification time of the molten metal thereby reducing the time until the end of the solification. CONSTITUTION:The molten metal consisting of the Ni-Al bronze is poured from a gate part 15 into a cavity 12 to cast the propeller A. The chillers 18 spaced from each other in the diametral direction are disposed on the side of the drag 11 on the side to directly contact with the molten metal over approximately the entire surface thereof. The solidification time ratio alpha(=T0/T1) of the molten metal is made >=1.8 by the chillers 18. T0 is the solidification end time of the largest wall thickness part in the respective r/R sections if both the drag 11 and cope 10 are sand molds, T1 is the solidification end time of the largest wall thickness part in the respective r/R sections when the cooling capacity of the drag 11 is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for preventing pipe-shaped casting defects that occur near the advancing surface of a blade when casting a propeller for a large ship made of Ni-Al bronze.

(Prior Art) In casting a propeller for a ship, as shown in FIG. 11, an upper mold 1 made of a sand mold and a lower mold 2 made of a sand mold are made to fit together to form a propeller casting cavity 3.
A core 4 having a molded boss inner surface is inserted into the core 4, and the molten metal is poured in an upward motion.

In the propeller manufactured in this way, the 12th
As shown in the figure, pipe-shaped defects are likely to occur in a region 6 surrounded by <0.25 to 0.5 S R in the r/R direction and Tll1ax in the wing span direction to approximately A wing span from the water separation 5.

This casting defect is made clear from the relationship between the wall thickness of the propeller and the time until solidification is completed, as shown in FIG.

That is, in FIG. 10, when the maximum wall thickness of the propeller blade becomes approximately 120 nm or more, the time required to complete solidification increases rapidly. This is because when the thermal conductivity of the mold is low, such as a sand mold, the mold temperature is sufficiently low compared to the molten metal temperature immediately after casting, so the amount of heat removed from the mold is large, but as time passes after casting, the mold temperature decreases. This is because the amount of heat removed gradually decreases.

Solidification of thin-walled parts of the casting is completed in areas where the heat removal rate is high, but in thick-walled parts, the heat removal rate is low except in the early stages of solidification, so the completion of solidification is delayed.

For these reasons, casting defects are likely to occur in region 6 shown in FIG. 12.

Moreover, the pipe-shaped defect 7 in the wall thickness direction is likely to occur on each of the forward moving surface 8 and the backward moving surface 9, as shown in FIG.

This is because solidification in this region in the thickness direction is delayed compared to other regions, and this is because Ni-Aj! This is due to the unique solidification characteristics of bronze, which solidifies slowly.

(Problem to be Solved by the Invention) If a casting defect occurs on the forward facing side (lower die side) of the blade B of the propeller A, the forward facing side 8 will appear as shown in FIG. 14 during the propeller operation. Thus, a tensile force F1 is generated, and a compressive force F2 is generated on the backward traveling surface side 9. In particular, on the forward facing side where tensile force is generated, fatigue cracks may occur starting from this type of defect, the cracks may propagate, and even blade breakage may occur. In Ni-A 1 bronze propellers, where the maximum wall thickness of the propeller blade root exceeds approximately 120 mm, pipe-shaped defects are observed at the starting points of blade breakage or fatigue cracks due to fatigue, and these are serious defects. This will cause an accident.

In particular, as ships become larger and faster, propellers tend to be used under harsher conditions, and propeller materials are required to have better properties (corrosion fatigue strength, crushing corrosion resistance, static strength, etc.). Since Ni-Al bronze has these properties larger than high-strength brass, it is becoming the mainstream propeller material.
It is necessary to eliminate this occurrence as something unique to Aozuna.

The present invention aims to provide a method that can prevent pipe-shaped defects by increasing the cooling capacity (heat removal amount) of the lower die (advancing surface side) and shortening the time until the completion of solidification. It is.

(Means for solving the problem) The technical means taken by the present invention to achieve the above object are characterized by the upper mold 10 and the lower mold 1 being matched with each other.
1 constitutes a propeller casting cavity 12, a core 13 having a boss inner molded shaft portion 14 is inserted into the cavity 12, and molten metal is poured into the cavity 12 to form Ni-A.
1. In an apparatus for casting a bronze propeller for a large ship, chillers 18 are arranged over substantially the entire surface of the lower mold 11 on the side that comes into direct contact with the molten metal, with intervals in the radial direction. The goal is to make the solidification time ratio α of the molten metal 1.8 or more.

However, α=To/Ts To: Solidification completion time of the maximum thickness part in each r/R cross section when both the lower mold 11 and the upper mold 10 are sand molds.

T1: Each r/when the cooling capacity of the lower mold 11 is increased
Solidification completion time of the maximum thickness part in the R section.

(Example) Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a casting mold, and 10 is an upper mold made of a sand mold, and is used to mold the backward moving surface 10A of the blade part B of the propeller A.

Reference numeral 11 denotes a lower mold made of a sand mold, which molds the advancing surface 11A of the blade portion B of the propeller A.

The upper mold 10 and the lower mold 11 are fitted together to form a propeller casting cavity 12.

A core 13 has a shaft portion 14 forming the inner peripheral surface of the propeller boss C and a spout portion 15, and is inserted into the cavity 12.

16 is a runner, in which molten metal made of Ni-Ajl bronze is poured into the cavity 12 from the molten metal weir part 15 by a push-up method in this example, and the propeller AIJ is cast; numeral 17 is a feeder part.

Reference numeral 18 denotes a chiller, which is disposed over substantially the entire surface of the lower mold 11 on the side that directly contacts the molten metal with a radial interval therebetween.

That is, it increases the cooling capacity (heat removal amount) of the lower mold 11 and shortens the time until the end of solidification.In this case, the condition for increasing the cooling capacity of the mold is the solidification time ratio α shown in the following equation.
is set to 1.8 or higher.

α= To / T I To = If both the lower mold 11 and the upper mold 10 are sand molds, each r/R
This is the solidification completion time of the maximum thickness part in the cross section, and its value can be approximately determined from FIG.

T1: Each r/when the cooling capacity of the lower mold 11 is increased
Solidification completion time of the maximum thickness part in the R section.

In other words, the mold cooling capacity of the upper mold 10 remains the same as before, and the mold cooling capacity of the lower mold 11 remains the same.
It was found that when the cooling capacity of the steel sheet was increased to shorten the solidification time, there were no local areas where solidification was delayed in the wall thickness direction, and pipe-shaped defects near the forward and backward surfaces did not occur. be.

Furthermore, even if the cooling capacity of the upper mold 10 is increased while the lower mold 11 remains the same, pipe-shaped defects can be similarly prevented;
This is not preferable because other defects, such as a hole-like defect, may occur near the lower die side (advancing surface side).

The range in which the mold cooling capacity of the lower mold 11 is increased is, in principle, the entire region of the blade tip of <0.2 SR, as shown in FIGS. 2 and 7.

For example, as shown in Figure 8, 0°25R where defects are likely to occur.
If the chiller 18 is arranged only in the ~0.55R section as shown in FIG. 9, it is not preferable because there is a possibility that a drag cavity 19 will occur on the wing tip side of the chiller 18.

It should be noted that in the area of fins L-20-0 and 25R, it is not particularly necessary to arrange the chiller 18 because molten metal replenishment (boiler) from the boss C is effective, but it may be arranged as necessary. The width and thickness of the chilled metal 18 are selected appropriately, and the chilled metal 18 is formed with sand 21 interposed as shown in FIG.
(so-called indirect chill method), but instead, as shown in FIG. 4, the chiller 18 is disposed so that its surface is in direct contact with the molten metal (so-called direct chill method).

Further, the chill area ratio is set to 60 to 70%.

Here, the chill application area ratio refers to the total cooling area on the blade surface, for example, SC1 + Sc2 + Sc3 in Figure 2.
+5c-8 divided by the surface area SO of the wing.

Next, an example of the present invention, a comparative example (indirect chill method), and a conventional example will be compared and explained.

Here, propeller blade root (0,3R,T+max 20
The coagulation time ratio of 0fi) is as follows.

(Next page) In addition, the relationship between the solidification time ratio and defect length for the 0.3R section and the vicinity of the advancing surface between Tmax and GL is shown in Figure 5, and the effect of the solidification time ratio on the defect length and defect appearance frequency is shown in Figure 5. Figure 6 shows the influence of

(Effects of the Invention) In summary, according to the present invention, pipe-shaped defects can be prevented by increasing the cooling capacity of the lower mold and shortening the time until the end of solidification.
This is advantageous because the frequency of blade breakage or fatigue cracks in large bronze propellers can be suppressed.

In other words, in recent years Ni-Al blue w4! It is believed that the frequency of blade breakage or fatigue cracking in II large propellers has increased for the following reasons.

1) The fluctuating stress applied to the propeller due to the increase in speed of the ship is
There is a tendency to increase. Furthermore, during stormy weather, it is thought that they are used under severe conditions that exceed the design stress.

2) In order to improve the propulsion efficiency of propellers, the uni-stress type, in which the radial blade thickness distribution was changed, became popular after the 1970s. As a result, the area of large fluctuating stress expanded.

■It is estimated that defects are less likely to occur near the blade root because the wall thickness gradient is smaller.

Even with this tendency, it is possible to provide a highly safe marine propeller in which casting defects are suppressed.

[Brief explanation of drawings]

Fig. 1 is a sectional view of the casting state in the present invention, Fig. 2 is a plan view of the entire cooling arrangement - an example, Figs. 3 and 4 are sectional views of the entire cooling arrangement, and Fig. 3 is a comparative example. , FIG. 4 is an example of the present invention,
Figure 5 is a graph showing the relationship between solidification time ratio and defect length, Figure 6 is a graph showing the effect of solidification time ratio on defect length and defect appearance frequency, and Figures 7 and 8 are graphs showing the relationship between solidification time ratio and defect length. Figure 9 is a cross-sectional view of Figure 8, Figure 10 is a graph showing the relationship between solidification completion time and maximum propeller wall thickness, Figure 11 is a cross-sectional view of the conventional example, and Figure 12 shows the same defect. FIG. 13 is an explanatory diagram of the occurrence area, FIG. 13 is an explanatory diagram of defect occurrence in the r/R cross section, and FIG. 14 is a sectional view of a part of the propeller. 10... Upper mold, 11... Lower mold, 12... Cavity, 13... Core, 18... Cold metal. Patent applicant: Kobe Steel, Ltd. (d) Figure 3 Figure 4.

Claims (1)

[Scope of Claims] 1. A propeller casting cavity 12 is constituted by an upper mold 10 and a lower mold 11 that are matched with each other, and a core 13 having a boss inner molding shaft portion 14 is inserted into the cavity 12, and 12, in which a Ni-Al bronze propeller for a large ship is cast by pouring molten metal into the lower mold 11, which is in direct contact with the molten metal, a cooling metal 18 is placed over almost the entire surface of the lower mold 11 with intervals in the radial direction. A method for preventing casting defects occurring in the vicinity of the advancing surface of a propeller for a large ship made of Ni-Al bronze, characterized in that the solidification time ratio α of the molten metal by the cooling metal 18 is set to 1.8 or more. However, α=To/T_1 To: Each r when both the lower mold 11 and the upper mold 10 are sand molds.
/ Time for completion of solidification of the thickest part in the R cross section. T_1: Each r/R when increasing the cooling capacity of the lower mold 11
Solidification completion time of the thickest part in the cross section.