JPH02299760A - Casting method for reaction tubes for ethylene production - Google Patents

Abstract

PURPOSE:To produce the reaction tube for producing ethylene having prescribed carburization resistance by immersing the lower part of a cooled casting mold having a mold hole penetrating to the top and bottom into a specific molten metal, cooling and solidifying the molten metal intruded into the hole and continuously pulling up the solidified part, thereby forming a tubular body. CONSTITUTION:The molten metal 6 of an Fe-Cr-Ni refractory alloy is formed and a pull up rod 2 is lowered to stick the molten metal 6 to a shaft 21 at the bottom end. The molten metal near the shaft 21 is cooled along the mold hole 11 by the cooling water in a water cooling jacket 13 enclosing a die hole wall 12, by which solidification 61 is generated. The solidified part 61 of the molten metal is pulled up as the tubular body 62 when the pull up rod 2 is risen. The molten metal which freshly comes into contact with the mold hole wall solidifies. The tubular body can be continuously cast by rising the pull up rod 2 at a specified speed. The tubular body is used in an as-cast state without machining the inside surface and the inside surface of the reaction tube exhibits the prescribed carburization resistance possessed by the above- mentioned heat resisting alloy.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for manufacturing a reaction tube used in the thermal decomposition reaction of raw material naphtha in the ethylene production process, and more specifically, the present invention relates to a method for manufacturing a reaction tube used for the thermal decomposition reaction of raw material naphtha in the ethylene manufacturing process. The present invention relates to a method for manufacturing reaction tubes requiring the following at low cost.

(Prior art and problems) Reaction tubes used in the ethylene production process are made of ASTM standard HP material (25Cr-35Ni-Fe), H, K material (25Cr-35Ni-Fe),
It is used as a Fe-Cr-Ni heat-resistant alloy material such as Cr-2ONi-Fe) and manufactured by centrifugal casting.

When pipes are manufactured using the centrifugal casting method, non-metallic inclusions and impurities in the molten metal are separated by centrifugal force and diffused toward the rotation axis, and are then disposed of on the innermost surface of the pipe. solidify. When these impurities concentrate on the inner surface of the tube, there is a problem in that casting defects such as cracks occur due to the difference in solidification temperature.

It goes without saying that if such casting defects exist, the original characteristics of the material cannot be fully exhibited. For example, in the production of ethylene, raw material naphtha and steam are introduced into a reaction tube,
This is done by heating the tube from the outside and using the radiant heat to decompose the naphtha inside the tube. During the decomposition process, free carbon is generated and deposits on the inner surface of the tube.

However, if the above-mentioned casting defects exist on the inner surface of the tube, carbon adhering to the inner surface of the tube significantly accelerates carburization inside the tube.

For this reason, if a tube manufactured by centrifugal casting is used as a reaction tube for ethylene production, carburization of the inner surface of the tube will be promoted due to insufficient carburization resistance, resulting in material deterioration after short-term use. Ta.

Therefore, when manufacturing pipes using the centrifugal force casting method, after casting,
It was necessary to perform mechanical cutting on the inner surface of the tube to remove casting defects on the inner surface. However, machining the entire length of the reaction tube increases processing costs and reduces material yield, which is extremely disadvantageous economically.

(Problems to be Solved) An object of the present invention is to provide a method for manufacturing a reaction tube for ethylene production that has a predetermined carburization resistance in an as-cast state without performing machining after casting.

(Means for Solving the Problems) The method for manufacturing a reaction tube for ethylene production of the present invention includes Fe-Cr-
A molten metal of a Ni-based heat-resistant alloy is formed, the lower part of a cooling mold having a mold hole extending vertically is immersed in the molten metal, the molten metal is allowed to enter the mold hole from the lower opening of the mold hole, and the molten metal is poured from around the mold hole. A tube body is formed by cooling to form a solidified layer that follows the shape of the mold hole wall, and by continuously pulling up the solidified layer.

(Function) In the method of the present invention, a solidified layer is formed in the molten metal. That is, since the outer and inner surfaces of the tube are solidified under substantially the same conditions, impurities are not locally concentrated on the inner surface of the tube, and no casting defects due to the difference in solidification temperature occur in the solidified layer.

(Effects) Since no casting defects occur on the inner surface of the tube, even if it is used as a reaction tube in the ethylene production process in an as-cast state, the reaction tube can exhibit carburization resistance commensurate with the material used. Therefore, there is no need for machining the inner surface of the tube after casting, unlike when manufacturing tubes by centrifugal casting, and the economical effects are extremely large.

(Example) Fig. 1 shows a cooling mold (1) used for carrying out the present invention, and the cooling mold has a mold hole (1) penetrating vertically around the axis.
1), the mold hole (11) is surrounded by a water cooling jacket (13), and cooling water is circulated through the jacket (13).

The mold cavity wall (12) forming the mold cavity (11) is made of a material with good thermal conductivity and a high melting point, and the jacket (13)
is made of steel plate.

The cooling mold (1) is placed in a vessel (5) containing the molten metal, with the upper part of the mold (1) protruding from the scene and the lower part submerged in the molten metal (6). Note that the container (5) has a built-in heating device (not shown) to maintain the molten metal (6) at a constant temperature.

The cooling mold (1) is bolted to a tubular bracket (41) fixed to a support (4) above the mold, and suspended coaxially with the bracket. Tubular bracket (
The hole diameter of 41) is the same as or slightly larger than the hole (11) of the mold (1).

A molten metal pulling rod (2) is disposed on the tubular bracket (41) so as to be movable up and down. A plurality of protruding shafts (21) are provided on the outer periphery of the lower end of the molten metal pulling rod (2) and protrude downward at approximately equal intervals, and the upper end of the pulling rod (2) is provided with a hanging cable (3) that is movable up and down. It is connected to.

As shown in FIG. 1, the pulling rod (2) is lowered to deposit the molten metal on the shaft (21) at the lower end of the rod.

The molten metal near the shaft (21) is cooled in the mold cavity (11) by cooling water in the water cooling jacket (13) surrounding the mold cavity wall (12).
is cooled and solidified (61) (see Figure 2).

Next, as shown in Fig. 2, when the pulling rod (2) is raised, the solidified portion (61) of the molten metal is pulled up as a tube (62), and the molten metal newly contacts the mold hole wall (12). solidifies. In this way, by raising the pull rod (2) at a constant speed, the tubular body can be continuously cast.

For example, when manufacturing a tube with an outer diameter of 60 mm and a wall thickness of 8 mm from a heat-resistant alloy consisting of 0.4% C-25% Cr-20% Ni-Fe, the tube is poured into a molten metal at 1450°C at a speed of 1000 mm/min. By pulling up the body, the desired tube body could be manufactured.

Next, a carburization test was conducted on the tubes manufactured by the centrifugal casting method and the tubes manufactured by the method of the present invention to examine carburization resistance.

In Table 1, the comparative example was made by centrifugal casting, and had an outer diameter of 60 n+m, an inner diameter of 44 mto, and a length of 2500 mm. The invention example is a pipe manufactured by the method of the present invention, and has an outer diameter of 60 mm, an inner diameter of 44 mm, and a length of 50 mm.
00mm. As shown in Table 1, the alloy components of test tubes Nα1 and 2 are considered to be substantially the same.

Table 1 (Remaining Fe and unavoidable impurities) 0.42 1.
06 1.01 0.020 0.011 24.75
21.22 Invention Example Next, from these tube bodies, a test tube (outer diameter 60 mm, inner diameter 44
mm, length 100 mm), and a carburization test was conducted on these test tubes. In the carburizing test, the inner surface of each test tube was filled with a solid carburizing agent (Dufrit K G 30).
After holding at a temperature of 1050° C. for 300 hours, the amount of carburization from the inner wall surface of the test tube was measured. To measure the amount of carburization, material was scraped off from the inner surface of the test tube at a pitch of 0.5 mm, so-called chips were collected, and the amount of C in each chip was analyzed. The value obtained by subtracting the initial C amount shown in Table 1 (0.42% for both the comparative example and the invention example) from the analytical value is shown in FIG. 3 as the C increase amount.

As is clear from FIG. 3, the inventive example showed a smaller increase in C than the comparative example, indicating superior carburization resistance. That is, even when a Fe-Cr-Ni heat-resistant alloy tube is formed by the method of the present invention and used in an as-cast reaction tube for ethylene production, the carburization resistance corresponding to the material properties of the heat-resistant alloy can be maintained. Be equipped.

Unlike the conventional centrifugal casting method, there is no need to machine the inner surface of the tube to remove casting defects.

[Brief explanation of drawings]

FIGS. 1 and 2 are explanatory views of the casting method of the present invention, and FIG. 3 is a graph showing the amount of carbon increase in a carburizing test. (1), Mold (2), Pulling rod (3
), sling (4), support (5), ,
, container (6) , , , molten metal (11)
,,, mold hole

Claims (1)

[Claims] (1) Step of forming molten metal of Fe-Cr-Ni based heat-resistant alloy, immersing the lower part of a cooling mold with mold holes penetrating vertically into the molten metal, and allowing the molten metal to enter the mold hole from the lower opening of the mold hole. The process consists of a step of cooling the invading molten metal from around the mold hole wall to form a solidified layer that follows the shape of the mold hole, and a step of forming a tube by continuously pulling up the solidified layer. The ethylene is used as a reaction tube for the ethylene manufacturing process in an as-cast state without machining the inner surface, and the inner surface of the reaction tube exhibits the predetermined carburization resistance possessed by the heat-resistant alloy. Casting method for manufacturing reaction tubes.