JPS61186166A - Prevention of carburization of weld zone - Google Patents

Abstract

PURPOSE:To improve easily the carburization resistance of the weld zone of a high-temp. structure and to improve the life of a device by coating or sticking a material which forms mainly an SiO2 oxide film to the inside surface of said weld zone and near the same after root pass then welding the 2nd and subsequent layers. CONSTITUTION:The material 12 which forms the oxide film consisting essentially of SiO2 is coated or stuck to the inside surface of the root pass weld zone 7 of a high temp. reaction pipe 5 for petrochemical industry, etc. and near the same. The material 12 is the colloid of, for example, an org. silicon compd. contg. Si as skeleton or an inorg. silicon compd. contg. SiO2, Si, etc. or a self- fluxing alloy sprayed thermally with Ni-Cr contg. >=2-3% Si. The 2nd and subsequent layers 8 are thereafter welded and the oxide film 15 consisting essentially of SiO2 is formed by the weld heat thereof. The carburization resistant of the weld zone is easily improved, by which the high-temp. long-term operation of the device is made possible and the life thereof is improved.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for preventing carburization of welded parts, and in particular, a method for preventing carburization that occurs mainly in the inner welded parts of high-temperature reaction tubes used in the petrochemical industry, etc. It is related to.

(Conventional technology) High-temperature reforming furnaces, cracking furnaces, etc. in the petrochemical industry are 1.00
Since it is operated under extremely harsh conditions close to 0°C, high Cr-N is generally used as the material for the reaction tubes and bends.
I series heat resistant casting & II HK40 (ASTMA 567
Gr, HK40: 25Cr-2ONi-04C) and HP (ASTMA 297 Gr, HP: 25C
Cast materials such as r-35Ni-05C) are used.

Figure 6 shows an example of a horizontal pyrolysis reactor for producing ethylene etc. from raw material naphtha (or hydrocarbons). It consists of several tubes welded together horizontally, and both ends of which are connected by a bend tube 2 to form several stages of reaction tubes 1. In the figure, 4 is the hearth.

The reaction tube 1 in this case is usually heated to about 700 to 900°C with a burner to produce ethylene from naphtha, but sometimes it is operated at temperatures exceeding 1°000°C to increase the yield. However, in the high temperature section (outlet side) of the reactor, carbon (C), one of the thermal decomposition products of naphtha,
It is often experienced that carbon oxide (CO) causes carburization of the pipe IA and the welded joint 3, resulting in significant embrittlement. When this is combined with creep and thermal fatigue, cracks occur, significantly shortening the life of the pipe, so carburization has become one of the important material problems in this type of industry. Note that carburizing here refers to Cr in the high-temperature pipe material.
(25% Cr in the case of HK40) reacts with C and CO generated during thermal decomposition of the raw material, producing so-called Cr carbides (for example, Cr7C5), which spreads into the tube thickness over time. When such a reaction occurs, Cr in the material decreases and not only does it no longer exhibit its original high-temperature strength, but also because Cr carbide is extremely hard and brittle, it is prone to cracking due to thermal stress during operation. Become.

This carburization phenomenon is exacerbated by casting defects (porosity, etc.) on the inner surface of the tube, so conventionally, measures such as machining and finishing the inner surface of the tube have been taken to prevent it.
In addition, from a structural perspective, silicon (S) has the effect of preventing carburization.
t) may be added in the required amount.

The effect of adding St is thought to be due to the formation of an oxide film consisting of 5io2 on the surface of the material, which acts to prevent the infiltration of C. Therefore, a large amount of Si is more effective, but if it is too large, the formation of brittle δ phase is promoted, resulting in significant embrittlement of the material. On the other hand, if it is too small,
There are problems such as decreased castability. Therefore, the appropriate composition range considering carburization resistance is 1.6 to 3.0% Si (by weight, the same applies hereinafter), but as a general carburization resistant material, Si is around 2%. many.

On the other hand, when discussing the appropriate amount of Si from the viewpoint of welded joints, there is a problem that if a large amount of Si is added, hot cracking becomes extremely likely to occur. For this reason, ASTM
The regulations specify a maximum of 2% Si for HK40.
For materials for which carburization resistance is not particularly considered, the amount of Si is generally set at around 1%. The situation at the welded joint is described in more detail as follows.

FIG. 7 shows the groove shape of the butt welded part of the cast pipe, and FIG. 8 shows the cross-sectional shape after welding.

In the case of butt welding, the first layer weld 7 in FIG. 8 is formed by tanning the root face 6 (FIG. 7) using the TiC method, which is a method of welding the main tubes 5 together without using a core wire. . At this time, if the amount of Si in the main pipe 5 is high, welding cracks are likely to occur, so from the viewpoint of weldability, Si
The smaller the amount, the better. In addition, assuming that the welding of the base metal (approximately 2% St) in consideration of carburization resistance in the first layer welding part 7 was performed well, the present invention would not work because the welding is between high-Si materials. Although it is thought that such an operation of adding Si is not necessary, in this case, Si in the base metal becomes Si and slag due to the action of acid and is fixed in the weld metal or exposed on the outer surface side. The amount of Si for film formation on the inner surface side is reduced. Further, in order to prevent initial layer cracking, an insert ring with a low Si content is sometimes welded to the root face 6 portion, but since this ring is made of a low Si material, carburization resistance cannot be expected.

Next, in Fig. 8, when welding the second layer 8 onwards, so-called co-metal welding is performed, in which a core wire with the same composition as that of the main tube 5 is usually used, but it is extremely important that both the main tube 5 and the core wire are made of high-Si material. Since weld cracking is likely to occur, a core wire with a low Si composition (approximately 1%) is generally used. Furthermore, δ ferrite crystallizes heterogeneously in the weld metal 9 and secondary carbide 11 precipitates in the heat affected zone 10, so that the carburization resistance of the weld metal is significantly deteriorated. In fact, in plants using high St materials, cases have often been experienced in which the material does not undergo any carburization, but carburization occurs only in the weld joint 3. That is, no matter how much the carburization resistance of the main tube 5 is improved, as long as the problem remains in the welded part, the carburization problem will not be solved.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for improving the carburization resistance of welded parts such as reaction tubes, where carburization resistance is difficult to improve, without changing the material composition. There is a particular thing.

(Means for Solving the Problems) In short, the present invention applies or adheres a Si-containing substance to the inner surface after the first layer is welded, and uses the welding heat (for example, around 1.100°C) for the second and subsequent layers. This is reacted or melted to form a dense oxide film mainly composed of Sio2.

That is, the present invention is directed to welding of high-temperature reaction tubes, bent tubes, etc. used in the petrochemical industry, etc., such as 25 Cr-20N i cast steel, into long lengths or welded structures. Applying or adhering to the welded part after the initial layer welding a substance that forms an oxide film mainly composed of SiO□ on the inner surface of the welded part and the vicinity thereof,
The method is characterized in that the second and subsequent layers are welded after that.

The substances used in the present invention that generate an oxide film mainly composed of SiO2 include colloids such as organic silicon compounds having Si as a skeleton, SiO□, Si and inorganic silicon compounds containing Si, and at least 2 to 3% of colloids such as inorganic silicon compounds containing Si. N containing Si
Examples include i-Cr-based thermally sprayed self-fluxing alloys.

Hereinafter, the present invention will be explained in more detail with reference to the drawings.

FIGS. 1 and 2 are diagrams showing an embodiment in which the present invention is applied to butt welding of an HK40 main pipe 5. FIG. In FIG. 1, after a first layer weld 7 is applied to the main pipe 5 by the TIC method, a Si-containing substance 12, that is, an organic or inorganic silicon compound having a Si skeleton, or 2 to 3% Si is added to the inner surface of the main pipe 5. A Ni-Cr self-melting alloy is thermally sprayed. Then the second
As shown in the figure, the welding heat of the two-layer weld 8 is used to cause a reaction. After welding two layers, white SiO□ is formed on the surface layer.
It was observed that a film 15 was formed. The results of measuring the heating temperature in this case are shown in FIG.

The measurement method is to install thermocouples at two locations on the inner surface of the first layer weld (A: center of the first layer, B: two points from the boundary between the first layer and the base material) as shown in Figure 3A. This is what I did. From Figure 3, it is 1.200℃ at point A and 1 at point B.
．． It can be seen that the temperature reached 100"cC.

Example 1 As shown in Fig. 4, after performing the initial N welding 7 on the HK40 main pipe 5, silicone grease 16 (generally available commercially available agent) was applied to a thickness of 0.5 to 1 m and a width of 10 to 15 mm. did. Thereafter, two-layer welding 8 was performed on this as shown in FIG. When we cut the sample after welding and observed its cross section, we found that
In the area where silicone grease 16 was applied, a white SiO□ film with a thickness of about 10 μm was formed in close contact with HK40.

In this example, silicone grease 16 was applied, but silicone oil also has the same effect, and generally S
It has been found that a similar effect can be obtained by applying an organic silicon compound having i as a skeleton. In the case of silicone oil, it is difficult to apply a thick layer in one go, so it is necessary to apply it several times.

Example 2 A colloid of silica sol (inorganic silicon compound containing about 20% Si) was added to the main tube 5 in the same manner as in Example 1.
Fine powder dispersion after 1μmyr1
~15mm was coated. When two layers of this are welded and the cross section is observed, it is found that SiO
□ Formation of oxide film was observed.

Example 3 Similarly to Example 1.2, after the initial layer welding 7 was performed on the main pipe 5, a Ni-Cr self-fluxing alloy containing 2 to 3% Si (Si) was applied to the inner surface of the pipe.
JIS H8303, MS F'Ni 1-5) was thermally sprayed. The remelting temperature of this self-fluxing alloy is usually i, oso ~
The temperature is 1,100℃, and it is remelted by the subsequent two-layer welding to form a dense layer, and a white Si layer is formed on the surface.
Formation of an O2 film was observed. As a result of cross-sectional observation, the film thickness was about 5 μm, similar to Example 1.2.

It is assumed that the formation of the 5 in 2 film continues even during actual machine operation, further improving the carburization resistance.

In addition, in the treatment of the present invention, as shown in FIG. 5, if a weld heat affected zone 10 is generated in the joint, and a large number of secondary carbides 11 are precipitated and there is a risk of embrittlement, solid solution treatment after welding is required. The self-fluxing alloy 17 is densified and strengthened by heat treatment (for example, heating and quenching at 1,050 to 1.100°C for 2 hours) to dissolve the secondary carbide 11 in the heat affected zone 10 and strengthen it. It is possible to promote the formation of the SiO2 film 15 and increase its thickness, and the effects of 3M can be expected.

The present invention is not particularly limited to welded joints, but can also be applied to the entire main pipe or other structural parts, and can prevent carburization of the entire main pipe or other structural parts.

(Effect of the invention) According to the present invention, the following excellent effects can be obtained.

(1) The carburization resistance of welded parts such as pipe joints can be economically improved with simple operations without changing the base metal composition.

(2) The inner surface of the main pipe is finished to prevent Si.
The amount close to the lower limit of the carburization-resistant composition range (1.6 to 3%) can be used, making welding easier.

(3) Improved carburization resistance enables high-temperature operation, leading to increased yield and cost reduction.

(4) Since a long-life reaction tube can be obtained, it is economically advantageous.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing an example of the present invention in which a St-containing substance is applied to the inner surface of the tube after the first layer welding, and FIG. 3 is a cross-sectional view showing the state in which the SiO2 oxide film is formed, FIG. 3 is the thermal cycle temperature curve during the two-layer welding, FIG. 3A is an explanatory diagram showing the installation position of the thermocouple, and FIG. 4 is A cross-sectional view of silicone grease applied and two-layer welding to form a SiO2 film. Figure 5 is a cross-sectional view after all welding is completed. Figure 6 is a schematic diagram of a horizontal pyrolysis reaction tube. Figure 7 8 is a cross-sectional view showing the shape of the butt weld groove, and FIG. 8 is a view showing the cross-sectional shape after welding of FIG. 7. DESCRIPTION OF SYMBOLS 1... Reaction tube, 2... Bend pipe, 3... Weld joint part, 4... Hearth, 5... Main tube, 6... Root face, 7... First layer welding Part, 8... Two-layer welded part, 9.
... Weld metal, 10... Heat affected zone, 11... Secondary carbide, 12... 3i-containing substance, 15... SiO2 film, 16... Silicon grease, 17... Self-fluxing alloy . Agent Patent Attorney Takeshi Kawakita Cho 5: + Official 7: First 4 Raku Sep 8: 21 Raku Sep 12: Si [stock] Setsumono! 15: 5iO2i, Jll Fig. 3 Fig. 3A Time (sec) 4th factor Fig. 5 Fig. 6 Fig. 1: Anti-Kyu 2: Vent pipe 3: Transition #Itohiteku 4: Hearth Fig. 7 Figure 8

Claims (4)

[Claims] (1) On the welded parts of high-temperature structures and on the inner surface near them,
A method for preventing carburization of a welded part, characterized by applying or adhering a substance that forms an oxide film mainly consisting of SiO_2 to the welded part after first layer welding, and then welding the second and subsequent layers. (2) Mainly SiO_ in claim 1
1. A method for preventing carburization of a welded joint, characterized in that the substance that forms an oxide film consisting of 2 is an organic silicon compound having a Si skeleton. (3) Mainly SiO_ in claim 1
The substance that generates the oxide film consisting of 2 is SiO_2, S
1. A method for preventing carburization of a welded joint, characterized by using a colloid such as an inorganic silicon compound containing i and Si. (4) Mainly SiO_ in claim 1
The substance that generates an oxide film consisting of at least 2 to
A method for preventing carburization of a welded part, characterized in that the Ni-Cr-based self-fluxing alloy is thermally sprayed and contains 3% or more of Si.