JPS63192567A - How to reweld double layer pipe - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a welding method for reusing a double-layered pipe for thermal decomposition and reforming reactions of hydrocarbons.

[Conventional technology]

When tubes for thermal decomposition and reforming reactions of hydrocarbons (cracking tubes, reformer tubes) are used in actual equipment, solid carbon that precipitates in the reaction system inside the tube adheres to the inner surface of the tube (caulking), causing damage to the tube body. Carburization occurs. Carburization that occurs in the reaction tube causes a change in the material of the reaction tube (particularly a decrease in ductility), which causes cracks to occur in the reaction tube during high-temperature, high-pressure operation.

FIG. 4 shows the carburization situation in the reaction tube (P).

(d) in the figure is a carburized part. Carburization of the reaction tube does not always occur on the entire surface of the tube as shown in the figure, but occurs partially, so if the reaction tube has been used for a certain period of time, carburization (d) will occur.
Cut and remove the part (B) where carburization has occurred, collect the sound part (A) where carburization has not occurred, and join the recovered sound parts (A) together by welding, or add a new reaction tube to this. In addition, these days, when it is common to reuse the reaction tube, as shown in Figure 3, the inner layer (10
) and an outer layer (20) have been proposed (Japanese Unexamined Patent Publication No. 198587/1987).

The double-layer tube has an inner layer (10) made of low-Ni, high-Mn heat-resistant steel, which suppresses the precipitation of solid carbon that occurs in the reaction system inside the tube, and suppresses coking on the inner surface of the tube and carburization inside the tube body. On the other hand, high Cr, which has been used as reaction tube material,
By forming the outer layer (20) with high Ni heat-resistant steel, the high-temperature strength required for the reaction tube is maintained. This double-layer pipe has less solid carbon deposited on the inner surface of the pipe and has a low carburizing rate, so it has a long service life and greatly contributes to stabilizing and increasing the efficiency of hydrocarbon thermal decomposition and reforming reaction operations. .

[Problem that the invention seeks to solve]

Although the two-layer reaction tube has excellent carburization resistance, partial carburization as shown in Figure 4 occurs after long-term use, so if used for a certain period of time, The carburized part (B) is cut and removed, and the sound part (A) is
They are collected, welded together, and reused.

However, since the inner layer (10) of the double-layer pipe is made of low Ni, high Mn heat-resistant steel, the inner layer (10) of the double-layer pipe once used is
No. 0) has a σ phase precipitated in its metal structure, and has an extremely low elongation of 1% or less. For this reason, if the recovered two-layer pipe is welded with the inner layer remaining, significant cracks will occur in the welded portion.

Therefore, before welding, it is necessary to remove the inner layer (10) of the part of the double-layer pipe to be welded by machining, grinding, etc. FIG. 2 shows a cross section near the joint of a two-layer pipe welded in this manner. (30) is the welded metal, and (al) is the area from which the inner layer (10) has been removed in advance.

As mentioned above, it is not only troublesome to remove the inner layer (10) prior to welding, but also the welded part of the reaction tube regenerated by welding and its vicinity has a single-phase structure with the outer layer (20) exposed. Since the effect of the inner layer (10) (suppressing solid carbon precipitation and carburization) has been lost, if it is reused as a reaction tube, it will cause problems (such as narrowing of the tube diameter) due to coking of solid carbon in that part. This results in premature carburization, and in the end the significance of using a double-layer pipe is greatly diminished by the disadvantages of its local area.

The present invention provides a welding method for solving the above problems.

Means and operation for solving problem c] The welding method of the present invention heats the recovered double-layered pipe to a temperature of 1000°C or higher, thereby causing the σ phase in the metal structure of the inner layer to disappear as a solid solution. The feature is that the welding is performed after the welding is done.

According to the present invention, in the recovered double-layer pipe, the σ layer of the metal structure of the inner layer has disappeared through heat treatment prior to welding, and good ductility has been restored, so it is not necessary to scrape off the inner layer. By welding as is, the first
As shown in the figure, it can be regenerated as a two-layer tube whose entire inner surface is coated with an inner layer (10).

Typical steel compositions of the inner and outer layers of the two-layer reaction pipe to which the welding method of the present invention is applied are as follows: First, the inner layer material, low Ni, high Mn heat-resistant steel, has C: 0.3 to 1. .5
%, Si: 3% or less, Mn: 6-15%, Cr: 20-
30%, Nb: 3% or less, N: 0.15% or less, balance F
Heat-resistant steel that is e, or C: 0.3 to 1.5%, Si:
3% or less, Mn: 6-15%, Cr: 20-30%, N
Examples include heat-resistant steel with i: 10% or less, Nb: 3% or less, N: 0.15% or less, and the balance being Fe.On the other hand, as a high Cr high Ni heat-resistant mesh as the outer layer material, C:
0.01-0.6%, Si: 2.5% or less, Mn: 2%
Below, Cr: 20-30%, Ni: 18-40%,
N: 0.15% or less, balance Fe or part of Fe is 5
Examples include austenitic heat-resistant steels substituted with one or more elements selected from MOIW and Nb within a range of % or less.

In the present invention, eliminating the σ phase by heat treatment does not necessarily mean completely eliminating all of the σ phase, but only allowing up to 1% of the σ phase to remain in the metal structure. It shall be. This is because if the remaining amount is at that level, it will not interfere with subsequent welding and a successful welded joint can be achieved.

The reason why the heat treatment was performed in a temperature range of 1000° C. or higher is that at lower temperatures, even if a relatively long heating time is required, it is not possible to eliminate the σ phase to the required residual amount. The higher the processing temperature, the faster the specified processing can be completed, but if the processing temperature is too high, surface scale will occur due to oxidation and become a source of wall thickness, so 1200°C is the upper limit. good. The heating holding time is appropriately set depending on the processing temperature, but for example, in the case of processing at 1000° C., the purpose can be sufficiently achieved with a holding time of 20 minutes or more. Cooling after the heat treatment may be performed by air cooling.

There are no particular restrictions on the welding conditions after the heat treatment, and the welding may be carried out in accordance with conventional general welding conditions used for reusing used reaction tubes.

〔Example〕

A centrifugally cast two-layer tube (pipe diameter: 160φ, inner layer thickness: 2t, outer layer thickness: 10t, m), whose inner layer is low Ni, high Mn heat-resistant steel and outer layer is high Cr, high Ni heat-resistant mesh, is made into a cracking tube ( Operating temperature/pressure crab 800~900℃/1.1~
1.3 kg/cm") and was used in actual equipment for two years, it was taken out from the reactor, the carburized part was cut and removed, and the recovered healthy part was heat treated, and the sound parts were bonded together without removing the inner layer. Butt welding was carried out on the pipe end faces of.
σ of the inner layer of the recovered double-layer pipe specimen before heat treatment
The phase had an area percentage of about 5-6%.

The steel compositions of the inner and outer layers of the double-layered pipe were as follows.

Inner layer material C: 0.6%, Si: 1.8%, Mn: 9.5%, Cr
: 25%, Nb: 0.3%, N: 0.06%, balance Fe.

Outer layer material C: 0.45%, Si: 1.7%, Mn: 0.3%,
Cr: 25%, Ni: 35%, Nb: 1.2%, N: 0
．． 08%, balance Fe.

Table 1 shows the heat treatment conditions, the amount of σ phase remaining in the inner layer after heat treatment (area ratio 2%), and the crack inspection results by color check of the welded part after welding.

In the table, columns 1 to 4 are invention examples, and columns 1) and 12 are comparative examples. Among the comparative examples, Arashi 1) is an example in which heat treatment was omitted, and Arashi 12 is an example in which heat treatment was performed, but the σ phase disappearing effect is insufficient.

As shown in Table 1, in the invention examples (cases 1 to 4) treated at a heating temperature of 1000°C, the residual amount of σ phase decreased to 1% or less, and the ductility of the inner layer was recovered accordingly. According to
There were no cracks in the welded parts. On the other hand, in Na1), in which heat treatment was omitted, a large amount of σ phase remained in the inner layer, and in Na12, in which the σ phase disappearing effect was insufficient due to the low heat treatment temperature, cracks occurred in the welded parts.

〔Effect of the invention〕

According to the method of the present invention, in welding work for reusing a reaction tube having an inner layer made of a low-Ni, high-Mn heat-resistant net, there is no need to scrape off the inner layer, and the inner layer can be removed by simply performing a simple heat treatment. It can be re-welded with the remaining parts intact. Therefore, the regenerated reaction tube has a two-phase structure coated with an inner layer over its entire circumference, including the welded joint area, and has excellent coking resistance on the inner surface of the tube, just as before reuse. It has carburization resistance, guarantees stable operation over a long period of time, and is effective in improving operational efficiency.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view in the tube axis direction showing a welded joint of a reaction tube formed according to the present invention, FIG. 2 is a cross-sectional view in the tube axis direction of a welded joint of a reaction tube formed by a conventional welding method, and FIG. FIG. 4 is a partially sectional front view of the two-layer reaction tube, and is a sectional view in the tube axis direction schematically showing the carburization state of the reaction tube. 10: inner layer, 20: outer layer, 30: weld metal.

Claims (1)

[Claims] (1) Rewelding after use of a double-layered pipe for hydrocarbon pyrolysis/reforming reactions that has a laminated structure of an inner layer made of low Ni, high Mn heat-resistant steel and an outer layer made of high Cr, high Ni heat-resistant steel. A method for rewelding a two-layered pipe, characterized in that the two-layered pipe is heated to 1000° C. or higher to cause the σ phase in the metallographic structure of the inner layer to disappear as a solid solution, and then welding is performed.