JP2000291914A - Radiant tube superior in high-temperature oxidation resistance and internal structure member, and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide improved service life by a method, wherein high-temperature oxidation resistance of the radiant tube and the air nozzle for combustion of a steel product heat treatment furnace is reinforced. SOLUTION: A thermal spraying film (a lower layer film) of an Al or an Al-Si alloy (0.5-15% of Si) and a thermal spraying film (an upper layer film) of a heat resisting alloy containing two or more kinds of elements selected from the group of Ni, Co, Cr, Al, Y, Ta, and Si are formed in a laminated state on the inner surface of a radiant tube. An air nozzle for combustion has its outer surface covered with and protected by the same thermal spraying film as described of an Al or an Al-Si alloy (a thermal spraying film of a heat resisting alloy having a composition similar to one described in the upper layer film, as required). Thermal spraying film thicknesses are respectively approximately 30-500 μm. The films have firm adhesion by diffusion and penetration to the base material and fusion of the upper and lower films and an Al2O3 film superior in heat resistance and oxidation resistance is formed on the film surface, and stable operation of a tube and a nozzle is ensured 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 a radiant tube provided in a heat treatment furnace and an improvement in high-temperature oxidation resistance of a combustion air nozzle mounted on the radiant tube.

[0002]

2. Description of the Related Art A radiant tube provided in a heat treatment furnace for steel materials such as a steel plate and a steel pipe has a burner mounted at an open end of a pipe line, and a flaming gas and a combustion gas of the burner make the radiant tube approximately 90 mm.
The steel material is heated to 0 to 1100 ° C. to be in a red heat state, and emits radiant heat to heat the steel material to be heat-treated in the furnace to a predetermined temperature. Conventionally, as the radiant tube and the combustion air nozzle, Cr-Ni heat-resistant cast steel (SC of JIS G5122 regulation)
H 12, SCH 15, SCH 22, SCH 24, etc.).

[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.

On the other hand, the combustion air nozzle mounted on the open end of the radiant tube is also consumed by the radiant heat of the high-temperature combustion flame and the oxidizing effect of the combustion gas, and is locally concentrated by the radiant heat. Often deformed due to localization. This deformation of the nozzle promotes a bias in the temperature distribution of the radiant tube, further accelerating the local heating and oxidative consumption phenomenon. For this reason, the conventional tube has a short service life, and not only a great deal of labor and cost burden associated with frequent replacement work, but also a reduction in productivity due to stopping the furnace operation each time.

[0005]

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 of improving the oxidation resistance by applying a film 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). No.) or Co or N
There has been proposed a method of forming a sprayed film of i or an alloy containing Co and Ni as main components (Japanese Patent Laid-Open No. 6-281119). The effect of improving the heat resistance and oxidation resistance can be obtained by the film treatment, but the cost of the film treatment 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 and a combustion air nozzle that further enhance the high-temperature oxidation resistance and have low processing costs. The present invention has been made for the purpose of meeting the above demand.

[0007]

A radiant tube and an internal structural member according to the present invention are characterized in that a high-temperature exposed portion on the inner surface of the tube includes Al or Al containing 0.5 to 15% by weight of Si.
-An undercoat, which is a thermal spray coating of a Si alloy, and Ni,
An outer surface of a combustion air nozzle which is formed by laminating a top coat which is a thermal sprayed coating of a heat-resistant alloy comprising two or more elements selected from Co, Cr, Al, Y, Ta and Si, and which is attached to an open end of a tube. Al or 0.5
It has a coating protective film structure in which a thermal spray coating of an Al-Si alloy containing up to 15% by weight of Si is formed.

[0008] The high-temperature exposed portion of the outer surface of the combustion air nozzle may be made of Ni, Co, Cr, Al,
A coating protective film structure in which a top coat, which is a thermal sprayed coating of a heat-resistant alloy composed of two or more elements selected from Y, Ta, and Si, is formed by lamination is provided.

[0009] The thermal spray coating made of Al or Al-Si alloy formed on the required surface of the radiant tube and the combustion air nozzle is heated to a high temperature in the operating environment of the heat treatment furnace, and has a temperature higher than the melting point of Al (about 660 ° C). In the temperature range,
The Al component diffuses and penetrates into the substrate, and an Al—Fe alloy layer (Fe—Al ₃ , Fe ₂ —Al ₅ , F
e-Al ₆ ). This Al-Fe alloy layer is
By having a high melting point (about 1160 ° C. or higher) and being metallurgically bonded, it enhances the adhesion of the sprayed film and provides stability that is not easily peeled off by heating, cooling, deformation, or the like. In addition, the film is made of Al
Produce a ₂ O ₃ film. Since the Al ₂ O ₃ film is dense and has excellent thermal stability, it serves as a barrier layer that blocks oxygen intrusion, thereby suppressing and preventing oxidation damage of the substrate.

[0010] In the case of a thermal spray coating made of an Al-Si alloy, in addition to the above effects (high adhesion by the Al-Fe alloy layer, high oxidation resistance by Al ₂ O ₃ ), the effect of Al The effect of appropriately suppressing the formation reaction of the Fe alloy is obtained. For example, in the case of an Al-5% Si alloy, 70%
The thickness of the formed Al—Fe alloy layer under the heating condition of 0 ° C. is suppressed to about 75% of that of the Al single phase. Therefore, by appropriately adjusting the Si content in accordance with the actual usage conditions of the radiant tube, it is possible to suppress excessive generation of the Al—Fe alloy layer and easily generate Al ₂ O ₃ sufficiently. The Si generates SiO2 having excellent heat resistance in a high temperature environment, the _Al 2 _{O 3,} which contribute to the suppression of oxidation wear of the substrate.

The thermal spray coating (Al or Al-Si alloy)
Is an undercoat, and Ni, Co, Cr, A
By forming a thermal spray coating made of a heat-resistant alloy composed of two or more elements selected from l, Y, Ta, and Si as a top coat, the protective film function for the base material is strengthened. That is, when this alloy film is heated to a high temperature in the operating environment of the radiant tube, it is metallurgically bonded to the lower film (Al or Al-Si alloy sprayed film) and Cr ₂ O ₃ , Al ₂ O _3. exerts generate protective oxide film such as SiO ₂ excellent oxidation resistance. Moreover,
The melting point of the alloy layer is in a high temperature range of 1000 to 1500 ° C., and at the same time, fine carbon particles and the like contained in the combustion gas to generate Ni-Al-based and Co-Al-based hard intermetallic compounds. Also shows good resistance to erosion due to

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The formation of a thermal spray coating on the radiant tube and the combustion air nozzle of the present invention is carried out by subjecting a surface area of a base material to degreasing and roughening (such as blasting) in accordance with a conventional method. It is performed by thermal spraying. As the material of the base material, various heat-resistant cast steels (JIS G5122) or various stainless steels (JIS G4303) such as SUS 321 can be used as appropriate, and there is no particular limitation on the selection of the material type.

When the thermal spray coating for coating the substrate surface is formed of Al, the thermal spray coating having a purity of 99% by weight or more (JI
SH 4040 (corresponding to the specified alloy number 1070, 1050, 1100, 1200, etc.) is preferred. If the purity is lower than the above range, the performance of the thermal sprayed coating is deteriorated due to the mixed impurities. When an Al-Si alloy is used instead of Al, the Si content is preferably 0.5 to 15% by weight.
If the Si content is less than this, the Si-containing effect (the effect of suppressing the reaction of forming an Al-Fe alloy at the thermal sprayed film-substrate interface) is insufficient, and if it exceeds this range, the Si-containing effect is almost saturated. is there. The impurities accompanying the Al-Si alloy are desirably 1% by weight or less as in the case of Al.

The thermal sprayed coating of Al or Al-Si alloy is used as an undercoat, and a heat-resistant alloy (Ni, C
o, Cr, an alloy consisting of two or more elements selected from Al, Y, Ta, and Si) as the top coat, the heat-resistant alloy may be Cr: 5% by weight.
-25%, Al: 1-29%, and if desired, Y:
0 to 5%, Si: 0 to 14%, containing one or two or more elements, and the balance from one or two elements of Ni and Co (however, Ni ≦ 75%, Co ≦ 70%) An alloy having the following composition is preferably used.

Thermal spray coating (Al or Al-Si alloy coating,
The thickness of each of the heat-resistant alloy films is preferably 30 to 500 μm. If the thickness is smaller than this, the effect of improving the high-temperature oxidation resistance and the like becomes insufficient. On the other hand, even if the thickness is increased beyond this, the increase in the effect is small and economic efficiency is impaired.

The above-mentioned thermal spray coating is formed on the inner surface of the tube which is directly subjected to the ignition by the burner and on the outer surface of the combustion air nozzle. In the W-shaped radiant tube shown in FIG. 1, four straight pipe portions (11) (12) (13) (1)
4) and three curved pipes (21), (22) and (23) are connected by welding, and a burner (B) is inserted and installed in a straight pipe (11) at one end thereof. In the U-shaped radiant tube shown in (2), two straight pipes (11) and (12) and one curved pipe (21)
The burner (B) is inserted and installed in the straight pipe (11) at one end of the pipe line to which is connected. The burner (B) has a combustion air nozzle (A). The figure shows an example in which the burner (B) has a double pipe structure, in which fuel is injected from the inner pipe, and the outer pipe (A) forms an air nozzle for supplying combustion air. In addition, when a configuration is used in which a thin tube is arranged as a combustion air nozzle in a direction parallel to the burner (B) at a plurality of positions in the circumferential direction near the outer surface of the burner (B). There is also.

As a straight tube at the open end of the tube where the burner (B) is installed, a sprayed coating (undercoat) made of Al or an Al-Si alloy and a sprayed coating (topcoat) made of a heat-resistant alloy were laminated and formed. A combustion air nozzle is used in which a sprayed coating of Al or an Al-Si alloy is formed on the outer surface thereof or a sprayed coating of a heat-resistant alloy is formed on the sprayed coating. In addition, in the case of the alternating combustion system in which burners are attached to both ends of the pipe of the tube and the burner is alternately burned, the sprayed coating is formed on each straight pipe and each combustion air nozzle at both ends of the pipe. Things apply. The area other than the pipe end to which the burner is attached does not necessarily require the application of a thermal spray coating, and the necessity is appropriately set according to the actual use conditions of the actual machine.

The radiant tube and the combustion air nozzle which have been subjected to the thermal spraying are subjected to a heat treatment, if desired, prior to the actual use. This heat treatment is carried out by heating and holding at a temperature range of about 660 ° C. or more for an appropriate time (about 1 to 5 hr). This heat treatment causes a diffusion reaction of the Al component (diffusion and infiltration into the inside of the base material and diffusion and penetration between the sprayed coatings), thereby strengthening the metallurgical bond to the base material and the bond between the coatings.

Since the thermal spray coating is relatively porous, it is preferable to coat the thermal spray coating with an air-tight resin coating when performing the heat treatment. Thereby, the heat treatment can be successfully achieved without fear of deterioration of the film quality due to intrusion of air into the pores. The resin coating film only needs to function as a barrier layer that temporarily prevents air from entering, and subsequent coating film remains unnecessary.
As the resin coating film, a silicon resin paint, a bituminous paint, or the like is used.

[0020]

[Example 1] (Evaluation of high-temperature oxidation resistance) (1) Preparation of test material Both sides of the base material were blasted (abrasive material: alumina particles) to form a thermal spray coating, and the coating was formed. After forming a resin coating on the surface, heat treatment is performed to obtain a test material. Base material: SUS 321 stainless steel (0.05% C-10.5% Ni-18.1% Cr
-Remaining Fe) Thermal spraying, coating composition, etc .: Refer to Table 1. Resin coating: Silicone-based resin is applied twice with a brush. Heat treatment: 700 ° C x 2 hr (in an electric furnace)

(2) Oxidation test and test results Each test material was subjected to an oxidation test (atmosphere: air, temperature: 1050
° C), the surface is observed for oxidative wear and the weight change is measured. As a comparative example, one in which the formation of the thermal spray coating was omitted (the base material was the same as above) was prepared, and an oxidation test was performed under the same conditions, and the results shown in Table 1 were obtained. In the table, the invention example No. in the column of “weight change (mg / cm ² )”. 11-13
The numerical value of “No.” indicates an increase in weight due to oxidation. 1
The numerical value of 4 indicates a decrease in weight due to peeling and falling off of the oxide scale.

As shown in Table 1, in Comparative Example No. 14 (without a thermal spray coating), the generation and detachment of oxide scale became remarkable by heating for only 20 hours, resulting in a significant weight loss. On the other hand, those of the invention examples have not only heating for 20 hours, but also a small change in weight (increase in weight due to oxidation) even after 600 hours, and have excellent 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.

[0023]

[Table 1]

Example 2 (Evaluation of Thermal Shock Characteristics) (1) Preparation of Test Material One side of a base material (50 mm × 50 mm × 5 mm thick plate-like material) was blasted (abrasive material: alumina) ), And then thermal spraying is performed, and heat treatment is performed or omitted to obtain a test material. Base material: Heat resistant steel (0.3% C-23.0% Cr-13.0% Ni-0.8%
Nb-Residual Fe) Thermal spraying, coating composition, etc .: See Table 2 Heat treatment: 700 ° C x 30 min (in an electric furnace)

(2) Thermal shock test and test results Each of the above test materials was heated in an electric furnace (950 ° C. × 15 min).
), The operation of heating / cooling into water (25 ° C.) was repeatedly performed, and the presence or absence of peeling damage of the sprayed film and the degree of peeling were visually observed, and the test results shown in Table 2 were obtained.

As shown in Table 2, Comparative Examples No. 29 and No. 30 (sprayed film: 80% Ni-20% Cr heat resistant alloy), No. 31
And No.32 (sprayed coating: 9% Ni-18% Cr-Fe heat-resistant alloy steel)
Has caused peeling damage of the film due to repetition of heating / cooling 2 to 6 times. On the other hand, in the example of the present invention, there is no peeling at all even after 20 repetitions, 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 diffusion and infiltration of Al into the inside of the base material (the formation of an Al—Fe alloy layer)
As a result, it was confirmed that the sprayed film and the base material were firmly bonded. No such diffusion-penetration phenomenon was observed at all in the sprayed film cross section of the comparative example. In addition, 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, but this color change does not impair the coating performance at all.

[0028]

[Table 2]

[0029]

The radiant tube of the present invention and the combustion air nozzle mounted on the end of the conduit opening have excellent oxidation resistance against a high-temperature oxidizing atmosphere as a protective effect of the sprayed film. 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, the improvement of productivity, and the like. Is what you do.

[Brief description of the drawings]

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

[Explanation of symbols]

 11-14: Straight pipe 21-23: Curved pipe A: Air nozzle for combustion B: Burner

Continuation of 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- 13-4 Fukae Kita-cho, Higashinada-ku, Kobe City, Hyogo Prefecture F-term in Tokaro Corporation (reference)

Claims (5)

[Claims] An undercoat, which is a thermal sprayed coating of Al or an Al—Si alloy containing 0.5 to 15% by weight of Si, is formed on a high-temperature exposed portion of an inner surface of a tube, and Ni, Co, Cr,
A high-temperature exposed portion on the outer surface of a combustion air nozzle which is formed by laminating a top coat which is a thermal sprayed coating of a heat-resistant alloy comprising two or more elements selected from Al, Y, Ta and Si Al or 0.5 to 15% by weight
A radiant tube and an internal structural member which are excellent in high-temperature oxidation resistance and have a thermal spray coating of an Al-Si alloy containing Si. 2. An Al or Al containing 0.5 to 15% by weight of Si is provided on a high-temperature exposed portion of an outer surface of a combustion air nozzle.
-Si alloy spray coating undercoat, Ni, C
2. The radiant having excellent high-temperature oxidation resistance according to claim 1, wherein a top coat which is a thermal sprayed coating of a heat-resistant alloy comprising at least two elements selected from the group consisting of o, Cr, Al, Y, Ta, and Si is formed. Tubes and internal structural members. 3. The thermal spray coating of the top coat, in weight%,
Cr: 5 to 25%, Al: 1 to 29%, Y: 0 to 5%,
Si: 0 to 14%, balance is Ni 75% or less, Co 70%
The radiant tube and the internal structural member having excellent resistance to high-temperature oxidation according to claim 1 or 2, comprising one or two of the following. 4. The thermal spray coating has a thickness of 50 to 5
The radiant tube and the internal structural member according to any one of claims 1 to 3, which are excellent in high-temperature oxidation resistance. 5. A high-temperature exposed portion on an inner surface of a tube or / and an outer surface of a combustion air nozzle installed at an open end thereof.
The method for producing a radiant tube and an internal structural member according to any one of claims 1 to 4, comprising forming a thermal sprayed coating and, if desired, heating and maintaining the coating at a temperature of 600 ° C or higher.