JPH11351518A - Radiant tube having high high-temperature oxidation resistance and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the service life of a radiant tube laid in a heat-treating furnace for steel by increasing the high-temperature oxidation resistance of the tube. SOLUTION: A melt-sprayed film composed of Al or an Al-Si alloy containing 0.5-15 wt.% Si and having a thickness of 30-500 μm is formed in a desired area (particularly, the area which receives high heat from a burner during combustion) on the internal surface of a radiant tube. An Al2 O3 coating film formed on the surface of the melt-sprayed film is highly compact and stable and effectively suppresses or prevents the oxidation loss of the tube. In addition, the melt-sprayed film is firmly adhered to and coupled with the base material of the tube due to an Al-Si alloy layer formed at the boundary between the base material and the film and does not readily comes off the base material even when a thermal impact is repeatedly applied. Therefore, the melt-sprayed film can stably maintain its function as a protective film over a long period.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a high-temperature oxidation resistance of a radiant tube provided in a heat treatment furnace.

[0002]

2. Description of the Related Art A heat treatment furnace for a steel material such as a steel plate or a steel pipe has a radiant tube disposed in the furnace. The radiant tube is provided with a burner at the end of the pipe, and is heated to about 900 to 1100 ° C. by a combustion flame and a combustion gas of the burner to be in a red-hot state, thereby emitting radiant heat, thereby performing heat treatment in the furnace. The steel is heated to a predetermined temperature. Conventionally, as a radiant tube, Cr-Ni heat-resistant cast steel (JIS
A cast steel product consisting of SCH 12, SCH 15, SCH 22, SCH 24 of G5122) is used.

[0003] Since the radiant tube is strongly heated by a burner, deformation due to thermal stress is inevitably generated. Furthermore, due to the high temperature steam, carbon dioxide, etc. contained in the combustion gas,
Oxide scale is generated, and the peeling and generation of the scale are repeated, so that the tube thickness decreases over time (oxidative thinning)
I will do it. Tube deformation and oxidation thinning are likely to occur in areas directly heated by the burner combustion flame. In addition, local separation and deposition of the oxide scale increases the temperature deviation of the tube and the resulting thermal stress, thereby accelerating the deformation of the tube. For this reason, the conventional tube has a short service life and requires not only a great deal of labor and cost burden due to frequent replacement work, but also a decrease in productivity due to stopping the furnace operation each time.

[0004]

As a measure for improving the service life of a radiant tube, the component itself is devised by adding a large amount of Co, W, Nb, or the like to improve the high-temperature oxidation resistance and high-temperature strength of the tube itself. It is conceivable to increase. However, since a large amount of expensive elements are used, the material cost is significantly increased, and further, it is difficult to expect an improvement effect commensurate with the high cost. On the other hand, as a method for improving the oxidation resistance by applying a coating treatment to the inner surface of the tube, a treatment liquid containing chromic acid or the like is applied and baked to form a Cr ₂ O ₃ film (Japanese Patent Laid-Open No. 6-280043). Alternatively, there has been proposed a method of forming a sprayed film of Co or Ni, or an alloy containing Co and Ni as main components (Japanese Patent Laid-Open No. 6-281119). These show improved heat resistance and oxidation resistance as a coating treatment effect. However, there is a drawback that the coating treatment cost is high, and there is room for further improvement in the oxidation resistance. In order to further improve the service life, there is a demand for the development of a radiant tube having better high-temperature oxidation resistance and low processing cost. The present invention has been made for the purpose of meeting the above demand.

[0005]

According to the present invention, there is provided a radiant tube comprising an Al tube formed on a desired area on an inner surface of the tube.
Or Al-Si containing 0.5 to 15% by weight of Si
It has a sprayed film made of an alloy and having a thickness of 30 to 500 μm. The radiant tube on which the Al or Al-Si alloy film has been sprayed is heated, so that the Al component in the sprayed film diffuses and penetrates into the tube base material, and the interface between the sprayed film and the tube base material has Al. -Fe alloy layer (FeAl3 _, F
e ₂ Al _5, FeAl ₂ etc.).

The Al—Fe alloy layer formed by the diffusion and infiltration of Al into the base material has a high melting point (about 1160 ° C. or higher) and is metallurgically bonded to enhance the adhesion of the sprayed film. It provides stability that does not easily peel off due to heating, cooling, deformation and the like. In addition, the sprayed film forms an Al ₂ O ₃ film by contacting with a high-temperature oxidizing atmosphere.
Since Al ₂ O ₃ is dense and has excellent thermal stability, it effectively suppresses and prevents oxidative abrasion of the substrate as a barrier layer that blocks oxygen intrusion.

[0007] In a sprayed film formed of an Al-Si alloy,
The above effect (high adhesion by Al-Fe alloy layer, Al ₂ O
_{In addition to (3} ) high oxidation resistance, the effect of appropriately suppressing the formation reaction of the Al-Fe alloy can be obtained. For example, Al-5
% Si alloy, the Al-
The formed layer thickness of the Fe alloy layer is about 75% of that of the Al sprayed film.
To a degree. Thus according to the actual conditions of use of the radiant tubes, by adjusting the Si content of the sprayed film appropriately, to suppress excessive generation of Al-Si alloy layer due to the extension of the heating time of the tube, of the sprayed coating surface Al ₂ O It becomes easy to generate ₃ sufficiently. Further, Si generates SiO ₂ having excellent heat resistance in a high temperature environment, and Al ₂ O
Together with ₃ , it contributes to the prevention and prevention of oxidative wear of the tube base material.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The grade of the radiant tube of the present invention is selected from various heat-resistant cast steels (JIS G512) conventionally used.
2) or various stainless steels such as SUS 321 (JIS G4
303) can be used as appropriate, and there is no particular restriction on the selection of the material type. At the time of thermal spraying, blasting is appropriately performed as a process for roughening the surface of the thermal sprayed film and removing foreign substances adhered to the surface and cleaning the surface according to a conventional method of thermal spraying technology.

[0009] Al, which is a thermal spray material, has a purity of 99% by weight or more (alloy number 1070, 105 specified in JIS H4040).
0, 1100, 1200 etc.). If the purity is lower than the above range, the performance of the sprayed coating is degraded due to the mixed impurities. The Al-Si alloy preferably has a Si content of 0.5 to 15% by weight. If the Si content is less than this, the effect of containing Si (the effect of suppressing the reaction of forming an Al—Fe alloy at the interface between the sprayed film and the base material) cannot be obtained, and if the content exceeds this range, the effect of containing Si is almost saturated. It is. The impurities accompanying the Al—Si alloy are the above Al
As in the case of the above, it is desirable that the content be 1% by weight or less.

[0010] The thermal spraying method is not particularly limited, but the thermal spraying material (Al, Al-Si alloy) is easy to process as a wire, has a low melting point, does not require a high-temperature heat source such as plasma, and has a narrow tube. The arc spraying method, the flame spraying method and the like are preferably used from the viewpoint of good workability at a place. The thickness of the sprayed film needs to be 30 to 500 μm. If the film thickness is smaller than this, the effect of improving the high-temperature oxidation resistance becomes insufficient. On the other hand, if the film thickness is increased beyond this, the increase in the effect is small and economic efficiency is impaired.

Since the sprayed film is relatively porous as it is, it is preferable to coat the surface of the sprayed film with an airtight coating after the spraying. Thereby, the subsequent heat treatment can be successfully achieved without concern about deterioration of the film quality due to intrusion of air into the pores. Since the sprayed film is densified by heat treatment, the coating film only needs to function as a temporary barrier layer until then,
It is not necessary to leave the coating film thereafter, and it is preferable that the coating film disappear from the sprayed film surface or be easily peeled off.

As the coating film, a heat-resistant resin coating such as a silicon resin coating or a bituminous coating is used. If a mixture of metal Al powder and an appropriate amount (0.5 to 10% by weight) is used, the protective effect of the coating film is enhanced, and the Al component in the coating film is melted in the heat treatment process to form a sprayed film and Fused,
Contributes to the reinforcement of the thermal spray coating. The paint is applied by spraying, dipping, or the like, and the thickness of the coating film may be about 5 to 150 μm.

The radiant tube on which the sprayed film (and a coating film which coats the surface if desired) is formed, is heated and maintained at a temperature higher than the melting point of Al (660 ° C.) for a suitable time (about 1 to 5 hr). The Al component of the sprayed film diffuses and penetrates into the inside of the base material to form an Fe-Al alloy layer. Heating temperature is 8
The upper limit is preferably 00 ° C. If the amount exceeds the above range, the sprayed coating may be excessively melted and the form of the coating may be impaired. This heat treatment can also be performed by incorporating the radiant tube into a heat treatment furnace. At the start of the operation of the heat treatment furnace incorporating the radiant tube, the heating effect of the heating and heating process of the radiant tube by the burner combustion can be utilized to sufficiently achieve the Al-Fe alloy layer formation reaction.

As an effect of forming the Al—Fe alloy layer by the heat treatment, a strong bond is formed between the sprayed film and the tube base material, and the sprayed film is densified with melting of Al. Furthermore, the surface of the sprayed film is dense and has excellent thermal and chemical stability due to contact with a high-temperature oxidizing atmosphere.
l ₂ O ₃ (Al ₂ O ₃ and SiO _{2 in} the case of an Al-Si alloy sprayed film) is generated, and high-temperature oxidation resistance for effectively protecting the tube base material from the heat of the burner combustion flame is secured. Is done.

The above-mentioned sprayed film is formed at least in a region where the burner is directly subjected to an intense heat. In the W-shaped radiant tube shown in FIG. 1, four straight pipe portions (1
1) (12) (13) (14) and three curved pipes (21) (22) (23) having a pipe connected by welding, and a straight pipe (11) at one end thereof
A burner (B) is inserted and installed in the tube, and a U-shaped radiant tube shown in FIG. 2 is provided at one end of a pipe line in which two straight pipes (11) and (12) and one curved pipe (21) are connected. Straight pipe (1
The burner (B) is inserted and installed in 1).

A sprayed pipe is used as a straight pipe at the end of a pipe in which a radiant tube burner (B) having a W-shaped or U-shaped pipe is installed. In addition,
Burners are installed at both ends of the radiant tube, and 2
In the case of the alternating combustion system in which the combustion of two burners is performed alternately, a structure in which a sprayed film is formed on each straight pipe at both ends of the pipeline is applied. The tubes other than the straight pipes where the burners are installed need not necessarily have a sprayed film, and may be spray-coated according to the actual use conditions of the actual machine.

[0017]

Example [A] Evaluation of high-temperature oxidation resistance (1) Preparation of test material After blasting (abrasive: alumina particles) on both front and back surfaces of the base material, thermal spraying was applied to both surfaces, and resin was applied to the surface of the sprayed film. A test material is obtained by forming a coating film and performing a heat treatment. (1) Base material: SUS 321 stainless steel (0.05C-10.5Ni-18.1Cr-Fe) (2) Thermal spraying method: arc spraying Thermal spraying material: Al, Al-Si alloy Film thickness: 150 μm (3) Resin coating : Silicon resin film containing Al powder (Al content 3 wt%) Film thickness: 150 μm (4) Heat treatment: 700 ℃ × 2 hr (in an electric furnace)

(2) Oxidation test and test results The above test material and a separately prepared comparative material (substrate of the same material type without thermal spraying) were subjected to an oxidation test (atmosphere: air,
Temperature: 1050 ° C). After the test, the change in weight (oxidative weight loss) of the test material was measured, and the results shown in Table 1 were obtained.

[0019]

[Table 1]

As shown in Table 1, in the comparative example (without thermal spraying), remarkable oxidative weight loss was caused by heating for only 20 hours, and the generation and peeling of oxidized scale was remarkable, whereas that of the inventive example. Of course, heating for 20 hours, 600
Even after hours, the oxidation thinning (weight loss) is still small and has outstanding high temperature oxidation resistance. In the test materials of the invention examples, the surface of the sprayed film after the test has changed from the initial metallic luster to gray white or black white, but this color change does not impair the film performance at all.

[B] Evaluation of Thermal Shock Characteristics (1) Preparation of Test Material After blasting (abrasive: alumina particles) one side of a base material (a 50 mm × 50 mm × 5 mm thick plate-like material) After performing thermal spraying and forming a resin coating film on the thermal sprayed film surface, heat treatment is performed or omitted to obtain a test material. (1) Base material: heat-resistant steel (C 0.3, Cr 23.0, Ni 13.0, Nb 0.8, Fe Bal) (2) Thermal spraying method: arc spraying Thermal spraying material: Al Al-Si alloy Film thickness: 200 μm (3) Resin coating Film: Silicon resin film containing Al powder (Al content 1.5 wt%) Film thickness: 50 μm (4) Heat treatment: 700 ℃ × 30 min (in an electric furnace)

(2) Thermal shock test and test results The above test material and a comparative material having a sprayed film prepared separately were heated in an electric furnace at 950 ° C. for 15 minutes, and then put into water (25 ° C.). Repeat the cooling operation,
The presence or absence of damage to the sprayed film and the degree of peeling are visually observed. Table 2 shows the test results. Comparative Example (Sample Nos. 25 to 2
The sprayed film thickness of 8), the conditions for forming the coating on the sprayed film surface and the heat treatment conditions are the same as those of the invention.

[0023]

[Table 2]

As shown in Table 2, Comparative Examples No. 25 and No. 26
(Sprayed film: 80% Ni-20% Cr heat-resistant alloy), No.27, No.28 (sprayed film: 9% Ni-18% Cr-Fe heat-resistant alloy steel) The repetition causes peeling damage of the film. On the other hand, in the example of the invention, even if it is repeated 20 times, there is no peeling at all and a sound film state is maintained. According to the optical microscope observation of the cross-section of the sprayed film of each test material after the test, all of the invention examples show that the diffused infiltration of Al into the inside of the base material (the formation of the Al-Fe alloy layer) It was confirmed that the material was strongly bonded. No such diffusion-penetration phenomenon was observed at all in the sprayed film cross section of the comparative example. Although the surface of the sprayed film of the invention example after the test changes from the initial metallic luster to a grayish white or blackish white, this color change does not impair the film performance at all.

[0025]

The radiant tube of the present invention has outstanding oxidation resistance to a high-temperature oxidizing atmosphere as a protective effect of the sprayed film coating the inner surface. The sprayed film is firmly and tightly bonded to the tube base material, and does not easily peel off even when subjected to a thermal shock or the like. Due to the protective effect of the thermal spray coating, the radiant tube of the present invention can be used stably for a long period of time, which contributes to the reduction of maintenance, the improvement of the efficiency of the heat treatment furnace and the improvement of productivity. Is what you do.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a radiant tube.

[Explanation of symbols]

 11,12,13,14: Straight pipe 21,22,23: Curved pipe B: Burner

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yoshio Harada 4-13-4 Fukae Kitamachi, Higashinada-ku, Kobe City, Hyogo Prefecture Inside Tokaro Corporation (72) Inventor Yukihiko Hirooka 4-13 Fukae Kitamachi, Higashinada-ku, Kobe City, Hyogo Prefecture No. 4 Inside Tokaro Co., Ltd.

Claims (3)

[Claims] 1. A high-temperature oxidation-resistant film in which a sprayed film of 30 to 500 μm thick made of Al or an Al—Si alloy containing 0.5 to 15% by weight of Si is formed in a desired region on the inner surface of a tube. Radiant tube with excellent properties. 2. A sprayed film having a thickness of 30 to 500 μm made of Al or an Al—Si alloy containing 0.5 to 15% by weight of Si is formed in a required region on the inner surface of the tube, and then heated to form a sprayed film. A method for producing a radiant tube excellent in high-temperature oxidation resistance, comprising diffusing and infiltrating Al into a base material to form an Al—Fe alloy layer at an interface between the sprayed film and the base material. 3. The method for producing a radiant tube having excellent heat resistance and oxidation resistance according to claim 2, wherein the surface of the thermal sprayed film is coated with a heat resistant resin film containing metal Al powder and subjected to heat treatment. .