JPS586762A - Centrifugal casting method - Google Patents

Abstract

PURPOSE:To produce a two-layered pipe wherein respective layers have prescribed uniform layer thicknesses and the adhesiveness between the layers is high in the manufacture in the two-layered centrifugally cast pipe by charging thermit simultaneously with the casting of the 2nd layer or after said casting. CONSTITUTION:In a centrifugal casting device, a required amt. of molten metal is charged to cast the 1st layer having design wall thickness. After the 1st layer solidifies down to the inner side thereof, the molten metal for the 2nd layer is charged to cast the 2nd layer of design wall thickness. In the stage of casting the 2nd layer, a proper amt. of thermit is charged thereto simultaneously with the charging of the molten metal or after said charging to remelt the thin-solidified layer (chill layer) of the 2nd layer produced in contact with the 1st layer by the quantity of heat evolved in accordance with thermit reaction. Despite the presence of the flux captured in the chill layer part (part of the flux charged after the casting of the 1st layer), it is floated and separated on the top surface of the molten metal for the 2nd layer by the remaining of said chill layer, and the 1st layer is melt-stuck to the 2nd layer, whereby the adhesiveness between both layers is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a centrifugal casting method, and more particularly to a method for manufacturing a two-layer centrifugally cast tube in which each layer has a predetermined and uniform layer thickness and has excellent interlayer adhesion.

Double-layer centrifugally cast pipes are made by casting two different metals concentrically to form a two-layer structure, making use of the characteristics of each layer of metal to withstand harsh usage conditions. It is used as a reaction tube for thermal decomposition and reforming of hydrocarbons, a so-called racking tube. In order to make this double-layer centrifugally cast pipe exhibit the desired performance and characteristics,
In the casting process, it is necessary to prevent the metals in both layers from mixing together, to form each layer to a predetermined uniform thickness, and to bring the two layers into metallurgical contact at the interface to create a strong bond. be.

Two-layer centrifugal casting tube has an outer layer (first layer) inside the centrifugal casting mold.
It is produced by injecting molten metal as a layer, then applying flux to the inner surface of the molten metal, and then injecting molten metal to form the inner layer (second layer). If this is done at a relatively early point in time, Figure 1C
As shown in I), the surface layer (1.1) of the first layer metal (1) in the mold (M) is in an unsolidified state, so the first layer and second layer (2) are easily fused. However, as a result of mutual melting of the molten metals of the first and second layers, the finally formed first layer (1') is thicker than the intended layer thickness, as shown by the mark in the figure. The -1 second layer (2) becomes excessively thick due to the large amount of the first layer metal mixed in, and has a chemical composition different from the original one.

In order to avoid the above-mentioned disadvantages, it is necessary to completely solidify the first layer to the inner surface and cool it to a temperature at which it will not re-melt even when it receives the heat of the second layer molten metal, and then pour the second layer molten metal. However, if this is done, a problem will arise in the adhesion between the two layers. That is, as shown in Fig. 2, flux (F) is administered to the inner surface of the injected first layer molten metal (1), and the first layer (1) is solidified in that state. [I])
, when the second layer molten metal (2) is injected (same figure [■]),
The second layer has a thin solidified shell (chill layer) (
2.1) is formed (same figure [■]). In that case, most of the flux (F) floats to the surface with the injection of the second layer molten metal, but because the solidified shell (2.1) is formed early, some of the 7 fluxes cannot float completely. 1st as it is
Solidification of each layer is completed in a state where it remains between layer (1) and second layer (2) (Figure [■]). For this reason, the bond between the two layers becomes extremely incomplete, and the flux remaining between the two layers becomes a fatal defect in the tube material.

Such a tendency becomes more pronounced especially when the melting point of the second layer metal is higher than that of the first layer metal.

As a method to prevent this, the formation of the solidified shell (2.1) is delayed by raising the casting temperature of the second layer molten metal or increasing the amount of casting to increase the amount of heat retained in the molten metal, or its regeneration. It is also possible to try to melt it. However, the former method cannot raise the temperature of the molten metal so high, so its effectiveness is limited, while the latter method requires an increase in the thickness of the second layer.
It cannot be applied when layers are required.

The present invention has been made in view of the above, and includes a method for casting the second layer after casting the first layer, or at the same time or after the casting. Provides a centrifugal casting method for obtaining a two-layer tube having an arbitrary layer thickness with excellent adhesion between the two layers by administering a powder mixture of iron (FexOy) and metal aluminum in a thermite reaction ratio. do.

The method of the present invention will be explained below.

According to the method of the present invention, in a centrifugal casting apparatus, a required amount of molten metal is first injected in a conventional manner to cast a first layer having a designed thickness. A flux commonly used for the purpose of preventing oxidation on the inner surface may be applied to the inside of the first layer according to a conventional method. Then, after the first layer has solidified to its inner surface, a second layer molten metal is injected, and a second layer having a designed thickness is cast. Since the second layer is cast after solidifying up to the inner surface of the first layer, the metals between the two layers will not mix.

When casting the second layer, an appropriate amount of thermite is administered at the same time as or after the injection of the molten metal, and a thin solidified shell (chill layer) of the second layer that is formed in contact with the first layer is formed as shown in the formula below. It is remelted by the heat generated by the thermite reaction.

FezOa+2A/-AlzOs+2Fe+20
0Kca/By remelting this chill layer, even if the flux (part of the flux injected after casting the first layer) that was trapped in the chill layer is present, it floats to the upper surface of the second layer molten metal and separates. The first layer and the second layer are fused and their adhesion becomes good.

That is, the present invention allows the thermite administered during second layer casting to function as a heat source for remelting the chill layer. Therefore, the dose of thermite must be sufficient to cover the amount of heat sufficient to remelt the chilled layer. The dosage varies depending on the casting amount of the second layer, the specific heat/melting point of the metal, etc., but it is usually about 0.2 kg or more per 1 kg of the second layer molten metal.

After casting the second layer and dosing the required amount of thermite, solidification is completed, and the alumina layer inside the second layer (produced by the thermite reaction) is peeled off and the desired layer thickness and chemical composition are achieved. A two-layer tube with good adhesion and a good composition can be obtained.

The thermite may be administered through holes provided in an end plate attached to the end of the mold, similar to the injection of molten metal. The introduction into the mold can be carried out using, for example, air as a carrier gas. It is desirable that the thermite to be administered is uniformly dispersed in the second layer molten metal, and for this purpose it is preferably administered as a mixture with a flux. The flux fluidizes the thermite and promotes its dispersion. This flux may be a commonly used flux for preventing oxidation on the surface of the molten metal.

In addition, since the iron produced by the thermite reaction fuses with the second layer molten metal, the thickness of the second layer increases by that amount.
This may be removed in the subsequent tube inner surface cutting step, or the amount of the second layer molten metal injected may be adjusted to account for the increase. Furthermore, fluctuations in the chemical composition due to the fusion of iron can be easily prevented by appropriately adjusting the composition of the second layer molten metal in advance.

In addition, in the method of the present invention, there are no particular restrictions on other casting conditions, and for example, the casting temperature of the molten metal of each layer may be the same as usual.

Next, examples of the present invention will be described.

In the example centrifugal casting apparatus, 0.4%C-25%Cr-2
Inject 25kg of 0% Ni-Fe molten metal to create a designed wall thickness of 15m.
The first layer of M was cast, and a flux was applied to its inner surface to prevent oxidation. Then, after solidifying to the inner surface of the first layer, 5 glazes of molten 18<Cr steel were injected to form a layer with a thickness of 4 mm.
In addition to casting the second layer of thermite (iron oxide (F
ezes) 75% by weight, metal AI! By administering 25% by weight) 1 and solidifying it as it is, the outer diameter i 8
A two-layer tube with a thickness of 5 mm, a first layer thickness of 15 mm, and a second layer thickness of 4.5 mm was obtained. It was confirmed that the two-layer pipe had no metals mixed between the layers, had a predetermined composition and designed wall thickness, and had perfect adhesion between both layers.

As described above, according to the method of the present invention, the molten metal for the second layer is injected after the first layer has solidified to the inner surface. No change in component composition occurs. In addition, since the chilled layer that forms in contact with the first layer is remelted, even if flux is trapped in that area, it is floated and separated, and the adhesion between both layers is made metallographically perfect. A strong bond can be created. In this way, a two-layer centrifugally cast tube with good adhesion, having a predetermined chemical composition and an arbitrary layer thickness can be obtained.

Furthermore, in the past, when the molten metal for the second layer was injected after solidifying the inner surface of the first layer as in the present invention in order to prevent mixing of the first layer and the second layer, the adhesion between the two layers was incomplete. As mentioned above, this tendency becomes more pronounced when a metal with a higher melting point than the first layer metal is used as the second layer, and therefore there are strong restrictions on the selection of materials for each layer metal. According to the method of the present invention, such restrictions are not imposed, so it is possible to manufacture a double-layered pipe using any combination of materials.
It can also meet various required characteristics for various uses.

[Brief explanation of the drawing]

Figure 1 [I], [■] and Figure 2 [I], [■],
[m”x [■] is a cross-sectional explanatory diagram showing the solidification status of each layer metal in the rotary mold for centrifugal casting. 1: First layer, 2: Second layer, M: Mold, F: Flux. Figure 1

[1] [arrived] Figure 2

Claims (2)

[Claims] (1) In the production of a two-layer centrifugally cast tube consisting of an outer layer and an inner layer, the outer layer is cast and solidified to the inner surface, and then thermite is administered at the same time as or after the casting of the inner layer. Centrifugal casting method. (2) The centrifugal casting method according to item (1) above, characterized in that thermite is mixed with flux and administered.