JPH0117042B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a double steel tube for boilers that has excellent high temperature corrosion resistance and high temperature strength. Steel pipes for boilers are used in environments where they come into contact with steam at temperatures around 570°C, and 18Cr-8Ni austenitic stainless steel is known as a material that can be used at such high pressures and temperatures. 18Cr, which is currently used, is expected to be used in harsher environments than the current one, such as the use of fuel with high content of Na, K, V, and S
Materials such as −8Ni-based austenitic stainless steel are insufficient in terms of corrosion resistance, and materials with excellent high-temperature strength and high-temperature corrosion resistance are required. Conventionally, it is well known that high Cr and high Cr-high Ni alloys are excellent measures against high-temperature corrosion of boiler steel pipes, but high Cr-high Ni alloys with a content of 30% Cr or more are difficult to hot work. It is often used as a cast product because of its poor hardness and cold workability, and it is difficult to manufacture thin-walled, small-diameter, and long tubes like boiler tubes. In addition, overlay welding for boiler tubes is difficult to manufacture and expensive, making it difficult to apply, and thermal spraying has disadvantages such as poor adhesion to the base material and poor workability. Therefore, 50Cr−50Ni and NCF2TB (20Cr−
Attempts have been made to apply double tubes combining 32Ni steel) to boiler tubes, but 50Cr-
50Ni does not have a single austenite phase, but also has some ferrite phase, resulting in poor hot workability and poor high-temperature corrosion resistance.Also, since it is a double tube with high Ni NCF2TB, it is expensive and disadvantageous when considering application to boiler tubes. There is a point. The present invention aims to eliminate the above-mentioned drawbacks and provide a steel tube for boilers that has both strength and corrosion resistance at high temperatures. One of the exposed tubes is an austenitic stainless steel tube that contains 30 to 45% Cr, which has good corrosion resistance, and satisfies the formula Cr-0.5Ni19.0%, and the other tube has 25% or less Cr, which has excellent high-temperature strength. Contains austenitic stainless steel pipes, and if necessary, prevents the diffusion of Fe or Cr during high-temperature use.
A nickel layer was provided between the inner tube and the outer tube to prevent precipitation of the σ phase. Here, the above conditional expression Cr-0.5Ni19.0% is a mathematical expression showing the relationship between the contents of Cr and Ni, which was derived based on the experimental results shown in FIG. The reason why the pipe has a double structure is that the pipe that is exposed to the corrosive environment contains 30 to 45% Cr, which has good corrosion resistance.
We use austenitic stainless steel pipes that satisfy the formula Cr-0.5Ni19.0% to prevent corrosion, and other pipes are made of austenitic stainless steel pipes that contain 25% or less of Cr, which has excellent high-temperature strength, to prevent corrosion at high temperatures. This is to provide strength. By forming a double-structured pipe in this way, a pipe with excellent corrosion resistance and high-temperature strength can be obtained. Therefore, it is better to reduce the wall thickness of the tube used for corrosion resistance. One tube contains 30-45% Cr and has the formula Cr-
The austenitic stainless steel tube that satisfies 0.5Ni19.0% has a markedly improved corrosion resistance when the Cr content exceeds 30%, as shown by the experimental results described later, and can be used as a boiler steel tube without any problems.
This is because if it exceeds the austenite single phase region, it becomes impossible to secure the austenite single phase region, which impairs hot workability and also deteriorates corrosion resistance.Also, by satisfying the above formula, hot workability becomes good, and other As a component, to create a single-phase austenite structure.
The main component is Ni, the amount of Cr is 30-45%, and the amount of Ni is 36-45%.
The above formula is satisfied at about 60%. Also, within this range, precipitation of σ phase will not occur during use of the boiler. Cr to ensure corrosion resistance
However, if the Cr content exceeds 30%, the workability will be slightly inferior, so if necessary, Ti,
Nb and Zr may be contained alone or in combination at a concentration of 1% or less, or Mg, Ca, La
Processability can be improved by containing Ce and Y in a total amount of 0.1% or less, singly or in combination. That is, by adding these elements to fix impurities such as S and O that degrade hot workability, hot workability is improved. Ti, Nb, and Zr are ferrite-forming elements, and if they are increased, it is necessary to increase the amount of Ni in order to stabilize austenite, which increases cost, so if added, it may be 1% or less. Also Mg, Ca, La−Ce,
If the amount of Y increases, the hot workability will deteriorate, so if Y is added, it should be 0.1% or less. On the other hand, the reason why the other tube is an austenitic steel tube containing 25% or less Cr is that the Cr content is 25% or less.
% or less, the corrosion resistance is inferior, but the high temperature strength and workability are excellent, and the Cr content is 15 to 15%.
Austenitic stainless steel with a Ni content of about 25% and a Ni content of about 8 to 23% is suitable as the base material for the double pipe. Next, providing a nickel layer between the inner and outer tubes of the double tube prevents the diffusion of Fe and Cr in the steel.
This is to prevent phase precipitation. If Cr-containing steel is exposed to high temperatures for a long time, Fe and Cr will diffuse, and at high temperatures below about 900°C, a σ phase will form, making the alloy brittle and causing the outer tube to peel. To solve this problem, a nickel layer is provided.
The nickel layer is produced by electroplating the outer circumferential surface of the billet, which is the material for the double pipe, or by wrapping nickel foil around the outer circumferential surface of the billet, which becomes the inner tube, and manufacturing the pipe using normal pipe manufacturing methods. The thickness of the nickel layer may be about 5 to 40 μm. Methods such as cladding, thermal spraying, and overlay welding can be applied to manufacture the double-walled pipe of the present invention, but composite steel ingots are manufactured, and this is extruded, rolled, and drawn based on the manufacturing method of seamless steel pipes. It is preferable to manufacture it by, for example, The above was obtained as a result of various experimental studies, and the contents of the experiments will be explained below. In order to study the effect of the amount of Cr in high-Cr-high-Ni alloy steel on high-temperature corrosion, the amount of Cr was varied as shown in Table 1. A 30Kg steel ingot with the ingredients shown in Table 1 below was melted in the air, forged, hot-rolled and cold-rolled to form a 14mm thick plate.The plate was then subjected to a high-temperature corrosion test of 15 x 15 x 3 mm. A piece was cut out and tested under the following conditions.

[Table] Test conditions Atmosphere gas composition: -1%SO ₂ -5%O ₂ -15%CO ₂
−Remaining _N2 corrosion ash composition: −1.5MK ₂ SO ₄ −1.5M Na ₂ SO ₄ −
1M Fe ₂ O ₃ corrosion test conditions: - Crucible immersion method (aluminum crucible) test piece embedded in 5 g of corrosive ash Immersion temperature, time: -650℃ x 100 hr The amount of corrosion was evaluated by descaling after the heating test and weight This was done to find the amount of decrease. Results first
As shown in the figure. As is clear from the figure, it can be seen that when the Cr content is increased to 30% or more, the corrosion resistance is extremely improved. Next, high-temperature torsion tests were conducted to examine the hot workability of high Cr-high Ni alloy steel. Second
A steel ingot with a single-phase austenite phase and a two-phase structure of austenite + ferrite with the composition shown in the table was melted in the air, and a torsion test piece with a parallel part diameter of 8 mm and a length of 50 mm was prepared from this steel ingot and heated at 1250℃. Tests were conducted to evaluate hot workability. The twist value indicates the number of twists until breakage, and is the average value of each two pieces.

[Table] The results are shown in Fig. 2. In the austenite single-phase region, that is, in the shaded γ region in the figure, workability at high temperatures is improved, and in the α+γ two-phase region, workability decreases. ing. Next, examples of the present invention will be shown. Alloy steel with the components shown in Table 3 was melted to produce double pipes and ordinary pipes (single pipes) for comparison.
A JIS12B arc-shaped tensile test piece was prepared and a high-temperature tensile test was conducted at 650°C. The double tube was manufactured by the following method. A double billet was made using steel number 17 in Table 3 for the inner tube and steel number 8 in Table 2 for the outer tube, and a double billet with a nickel layer between the outer tube and the inner tube was used. For pipe use, the outer circumferential surface of the inner billet is plated with nickel. A blank pipe is manufactured from such a billet by Eugene extrusion, and a double pipe with an outer diameter of 51 mm and an inner diameter of 34 mm is created through cold drawing, softening annealing, and cold drawing processes, and is finished annealed at 1220°C and then sold. It was used as a sample material.

【table】

【expressed
**Aging treatment: -650℃×1000hr Heating in air The results are shown in Table 4. The double pipe of the present invention has a high-temperature strength equivalent to conventional boiler steel and has a Ni layer. The elongation of the double tube is slightly better than that of the double tube without the Ni layer. This is considered to be because precipitation of the σ phase was prevented. As is clear from the examples above, the double boiler tube of the present invention exhibits excellent corrosion resistance even in a more severe corrosive environment than conventional tubes, and its strength at high temperatures is equivalent to that of conventional steel. This is an excellent double tube for boilers that can withstand use in harsh high-temperature corrosive environments.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the high temperature corrosion test results of stainless steels with different Cr contents, and FIG. 2 is a diagram showing the high temperature torsion test results of austenite single phase and austenite + ferrite dual phase stainless steels.

Claims (1)

[Claims] 1. Either one of the double pipes has Cr30 to 45% by weight.
% and satisfies the formula Cr-0.5Ni19.0%, the other is Cr25
A double tube for boilers with excellent high-temperature corrosion resistance, characterized by being made of austenitic stainless steel tube containing less than %. 2 Either one of the double pipes is Cr30-45 in weight%
% and satisfies the formula Cr-0.5Ni19.0%, the other is Cr25
A double tube for boilers with excellent high-temperature corrosion resistance, which is made of an austenitic stainless steel tube containing less than 100% austenite, and has a nickel layer between the inner tube and the outer tube.