JPH02137690A - High-alloy pipe and circumferential welding method thereof - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance of the weld heat affected zone of the high-alloy pipe by providing a specific amt. of a coating layer formed by applying and drying of a chromating liquid onto the inner peripheral surface of the heat affected zone at the time of circumferential welding. CONSTITUTION:The chromating liquid is applied on the inner peripheral surface of the heat affected zone at the time of the circumferential welding of the high-alloy pipe. The coating layer is formed by drying the coating after the application and the amt. thereof is specified to >=10mg/m<2>. The high-alloy pipes formed with the coating layer are joined to each other and thereafter, the ends of these pipes are circumferentially welded. The corrosion resistance of the weld heat affected zone of the high-alloy pipe is improved in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a high alloy tube and a method for circumferential welding of high alloy tubes. More specifically, the present invention provides a high Cr Fe-based alloy pipe or a Ni-based alloy pipe, and a high alloy pipe and a welding method thereof that can provide welded parts with excellent corrosion resistance in piping installation using these. Regarding.

(Prior Art) For example, high alloy pipes for line pipes or chemical industry piping are circumferentially welded at the construction site. At this time, oxides, that is, weld scale, are generated on the surface of the weld heat affected zone by heating. This welding scale generation part is
- It is known that corrosion resistance deteriorates during boat fishing.

On the other hand, high alloy pipes for line pipes or chemical industry piping are often used for the purpose of transporting corrosive liquids containing halide ions.

Therefore, the aforementioned deterioration of corrosion resistance in the weld scale generation area causes pitting corrosion and crevice corrosion in this area, posing a major problem in terms of equipment and operation.

Various methods have been proposed in the past as means for solving such problems. (ii) A method of coating the end of the tube with metal by thermal spraying, plating, brushing, etc. prior to welding (2) A method of shielding the inside of the pipe to be welded with inert gas to prevent oxidation during welding. Japanese Patent Application No. 62-71200, No. 63-28315, etc.).

(Problems to be Solved by the Invention) However, these known methods cannot prevent the formation of scale in the weld heat affected zone of high alloy tubes.

That is, with the method shown in (i), it is practically impossible to completely eliminate oxygen from inside the pipe using an inert gas as long as it is carried out on-site, and therefore it is not possible to prevent the generation of scale.

In addition, the method shown in (ii) requires special equipment to perform thermal spraying or spraying on-site; and, with the method of applying metal powder, it is difficult to uniformly coat the entire surface of the target area. Corrosion resistance is difficult to achieve, and the applied metal powder is simply physically attached to the surface of the high-alloy tube, requiring a large amount of coating to obtain sufficient oxidation-preventing ability. This is not a desirable method from the point of view of preventing deterioration.

Although it is possible to remove weld scale and prevent deterioration of corrosion resistance by polishing or pickling the weld scale after it has occurred, it is also possible to remove the weld scale and prevent deterioration of corrosion resistance by polishing or pickling the inner circumferential surface of the pipe that has been circumferentially welded at the local construction site. It is virtually impossible to do so.

None of the above methods could prevent the formation of scale in the weld heat affected zone of high alloy tubes.

SUMMARY OF THE INVENTION An object of the present invention is to provide a high alloy tube and a circumferential welding method thereof, which make it possible to obtain a circumferential welded portion having high corrosion resistance when welding the high alloy tube.

(Means for Solving the Problems) In order to solve the above problems, the present inventors first investigated the cause of the occurrence of weld scale. In other words, there have been various opinions regarding the causes of welding scale, and the representative ones are: 1) It is effective in ensuring corrosion resistance in the base material directly under the scale, and ROM (hereinafter referred to as rcrJ) is reduced;
A so-called Cr-depleted layer is generated, and halide ions enter through the cracks in the scale and attack the Cr-depleted layer, which has poor corrosion resistance ■Crevice corrosion progresses because gaps are formed between the scale and the base material■ The scale itself acts as a kind of anti-corrosion film, but since the Cr concentration in the scale is lower than the Cr concentration in the passive film on the scale-removed surface or the non-scale-generated surface, the corrosion resistance in the scale-generated area deteriorates. Are known.

Therefore, the present inventors conducted the following experiment in order to identify the main cause of the occurrence of weld scale.

That is, in order to reproduce the welding scale in the laboratory, a plate of SUS 316L, a typical high-alloy material, was heated in the air to a temperature of 500 to 1100°C for 1 second, and a corrosion test piece was cut from the heated plate. , CQ-concentration is 1103
The pitting potential was measured by a constant potential step method for 24 hours under atmospheric conditions of pp and temperature of 30°C. In addition, surface SEM observation of the same test piece before and after the corrosion test and surface elemental analysis using a secondary ion mass spectrometer (IHMA) were performed.

The experimental results are summarized in the first column that expresses the relationship between heating temperature and pitting potential
Figure 2 shows the relationship between the heating temperature and the maximum Cr concentration in the scale. From FIG. 1, it can be seen that the pitting corrosion potential decreases significantly when heated to a temperature of 600 to 800°C, that is, the corrosion resistance deteriorates significantly. Also, from FIG. 2, according to IHMA analysis, when heated to a temperature of 600 to 800'C, where corrosion resistance is significantly deteriorated, Crff1. It can be seen that the Crff degree is significantly lower than the Crff of 4 degrees on the surface of the material from which scale has been removed by polishing. Incidentally, neither the cracks in the scale nor the Cr-depleted layer immediately below the scale were detected.

In other words, from the above experimental results, the cause of the deterioration of corrosion resistance is
The present inventors have found that this is mainly a decrease in C4 degrees on the scale (the above-mentioned ■).

Therefore, in order to prevent or compensate for this decrease in Cr concentration in the scale, it is necessary to enrich the weld surface with Cr before welding, in other words, weld a chromate treatment on the end of the tube that is heated during welding. The present invention was completed based on the knowledge that it is possible to obtain a circumferential weld with high corrosion resistance by welding the weld beforehand and then welding.

The gist of the present invention is to provide a high-alloy tube with a coating layer of 10 μm/M by applying and drying a chromate treatment liquid on at least the inner circumferential surface of the tube in the heat-affected zone during circumferential welding.
This is a high alloy tube characterized by having the above.

Another aspect of the present invention is a circumferential welding method for high alloy tubes, which is characterized in that the above-mentioned high alloy tubes are joined together and then the ends of these tubes are welded circumferentially.

Here, high alloy refers to a material that contains alloying elements such as Ni, Cr, and Mo in a larger amount than normal materials, and specifically, it is a Fe-based alloy containing 12% by weight or more of Cr or a Cr.
An example of a Ni-based alloy containing 12% by weight or more of

The heat-affected zone during circumferential welding refers to the area where weld scale occurs due to circumferential welding, and although it varies depending on the pipe size, for boat fishing, for example, from the end of the high alloy pipe to be welded,
The range is within oCIII.

Furthermore, in addition to chromate treatment liquid, Cr2O3
・It may be one containing an oxide such as 5iO7 or an acid such as nitric acid or sulfuric acid, and a commercially available chromium-based surface treatment agent may be used.

(effect) The present invention will be described in detail below along with its effects.

First, the chromate treatment solution adjusted appropriately prior to welding is converted to non-volatile content and is 10mg/rr! The above is applied to the inner peripheral surface of the heat-affected zone of the pipe to be welded.

The method of applying the chromate treatment liquid is not limited to a specific method.

In addition to Cr acid, the chromate treatment solution also contains Cr2O3, S
The surface treatment agent may contain an oxide such as ing, or an acid such as nitric acid or sulfuric acid, and a commercially available Cr-based surface treatment agent may be used.

For example, 6.25% of Cr2O3 is added as a non-volatile component in the stock solution.
wt%, Cr03: 6.25 wt%, 5t (h: 12.
A Cr-based surface treatment agent containing 5% by weight may be used. Furthermore, since Cr exists as trivalent or hexavalent Cr ions in the processing solution, ■ Cr is not dispersed as a powder, but is ionized, so uniform coating can be achieved even with a smaller amount. ■Unlike organic solvents, it does not contain carbon that degrades the performance of welded parts. ■It forms a Cr-rich film through chemical reaction with the high alloy surface, so no defects are formed in the film and it has good adhesion. There are benefits.

The amount of chromate treatment liquid applied is 10mg/non-volatile content.
If it is less than 10 mg/r, the Cr enrichment amount will be insufficient, the Crlfi degree in the scale generated in the heated part during welding will be low, and deterioration of corrosion resistance cannot be prevented. Therefore, 10 mg/r
It is effective to apply more than W. The amount of chromate treatment liquid to be applied is 10a+g/n in terms of non-volatile content. However, no improvement in corrosion resistance can be expected, and on the contrary, it will lead to higher costs, so the coating amount is 1000mg/
n (preferably less than or equal to n).

It goes without saying that the chromate treatment liquid may be applied to the entire inner circumferential surface of the pipe, but it is sufficient to apply it to the area where oxide scale is generated after welding, that is, at least to the heat-affected zone during circumferential welding.

The range in which oxidized scale occurs varies depending on the pipe diameter, wall thickness, welding conditions, etc., but the range in which welding scale occurs is generally the same regardless of the range in which the pipe diameter, wall thickness, welding conditions, etc. change. Since it is within lQcm from the end of the tube, it is desirable to apply the chromate treatment liquid to this area.

The chromate-treated high alloy tube obtained in this way is dried, and a coating layer consisting of a non-volatile chromate treatment liquid is applied to at least 100% of the inner circumferential surface of the tube in the heat-affected zone during circumferential welding.
The high-alloy tubes having a carbon content of mg/m or more are joined together, and then the ends of the tubes are welded circumferentially.

This circumferential welding is usually performed using GT8 (gas tungsten welding method), but it is not limited to this method, and other welding methods may be used, such as using a mechanically or electrically focused plasma column. Plasma welding using a plasma arc, friction welding in which materials are brought into contact with each other under pressure, causing relative motion on the contact surfaces, and welding using the generated frictional heat.Friction welding also uses the thermal diffusion of elements between the materials to be joined. A diffusion bonding method may also be used.

Further, the welding conditions are the same as those in the conventional circumferential welding method for high-alloy pipes, and may be appropriately set as necessary.

As described above, the present invention makes it possible to improve the corrosion resistance of the weld heat affected zone in circumferential welding of high alloy tubes.

Further, the present invention will be explained in detail using Examples, but these are merely illustrative of the present invention and the present invention is not unduly limited thereby.

Example 1 Two high alloy tubes made of alloy A having the composition shown in Table 1 were
The book was circumferentially welded at each end using methods (1) and (2) shown below. The circumferential welding conditions are shown in Table 2.

The tubes were left standing for one hour to dry the moisture, and then these tubes were joined and circumferential welding was performed. The coating amount was determined from the weight measurement results after drying.

Table 3 - Welded using the conventional method. That is, two high alloy tubes were immediately joined and circumferentially welded without performing chromate treatment.

3 ml 1 liter beyond the weld heat affected zone from the fusion boundary of the high alloy pipe circumferentially welded by method ① or method ②.
(Thickness) x 10mm (Width) x 50mm (Length) A test piece was cut out from the inner peripheral surface of the tube and used as sample 11ll or sample size 7, and was heated at 30°C, 3.5% NaC12 aqueous solution, and exposed to the atmosphere at 50mV/24h. The potential for pitting corrosion was measured using the constant potential step method. The pitting potential of the base material is 0.
．． 35 (V vs 5CE). The results are shown in Table 4.

Table 1 Table 2 ■Welding was carried out by the method according to the present invention. That is, before welding, a commercially available chromium-based surface treatment agent having the composition shown in Table 3 was diluted with deionized water to six levels from 2 times to 500 times on the inner circumferential surface of the pipe in the Loan range from the end of the pipe. 2 (Note) * is outside the scope of the present invention As is clear from Table 4, the samples according to the present invention (sample No. 1 to sample N15) have a high pitting corrosion potential and cannot be welded. It can be seen that even after the test, it has the same corrosion resistance as the base material.

On the other hand, sample no. 6 and sample no. 7 are comparative samples, but the chromate treatment at the end of the tube was not performed (sample no. 6) or was insufficient (sample no. 7).
), it can be seen that the pitting corrosion potential is lower than that of the base material.

Example 2 Two high-alloy tubes made of Alloy F or Alloy F having the composition shown in Table 1 were used for each alloy, and a 10-fold diluted chromate treatment solution was applied to the ends of the tubes, and the tubes were heated at room temperature. After leaving to dry for 1 hour, these high alloy tubes were joined and circumferential welding was performed in the same manner as in Example 1. Samples of the same size were cut out from the heat affected zone on the inner surface of the tubes in the same manner as in Example 1.
Samples NfiIO to Sample No. 15 were obtained. Further, circumferential welding was performed without chromate treatment on the end of the tube, and sample floors 16 and 27 of the same dimensions as in Example 1 were obtained from the heat affected zone of the tube. However, for samples 11h22 to 27, the scale was removed by polishing the base material after circumferential welding. Thereafter, the pitting potential of these samples was measured in exactly the same manner as described above. The results are shown in Table 5.

Table 5 (note) * is outside the scope of the present invention As is clear from Table 5, the method according to the present invention covers (i) high Cr Fe-based alloys Alloy A to Alloy C and Alloy F) ( 11) It is clear that all high Cr Ni-based alloys (alloy and alloy E) have the effect of improving the corrosion resistance of the weld heat affected zone.

On the other hand, Sample Corner 16 to Sample N [121] are samples cut from high-alloy tubes welded without chromate treatment on the inner circumferential surface of the tube in the welded heat affected zone, but they exhibit remarkable corrosion resistance compared to the base material. It can be seen that it has decreased.

Further, sample holes 22 to 27 are samples cut out after polishing and removing scale from the welded heat affected zone of a high alloy tube that was welded without chromate treatment on the inner peripheral surface of the tube in the welded heat affected zone. However, it can be seen that the pitting potential is high and the corrosion resistance is excellent. However, as mentioned above, this method cannot be used in actual implementation sites.

(Effects of the Invention) The method according to the present invention, which has been described in detail above, has a structure in which chromate treatment is applied to the end of the high-alloy tube before welding the high-alloy tube. The corrosion resistance of the parts can be ensured without the need for various treatments (polishing, etc.) after welding.

The significance of the present invention, which can improve the corrosion resistance of the weld heat affected zone of high alloy pipes circumferentially welded on site, is extremely significant.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between heating temperature and pitting corrosion potential; and FIG. 2 is a graph showing the relationship between heating temperature and maximum Crf1 degree on the scale.

Claims (2)

[Claims] (1) A high-alloy tube, characterized by having a coating layer of 10 mg/m^2 or more formed by coating and drying a chromate treatment liquid on at least the inner peripheral surface of the tube in the heat-affected zone during circumferential welding. High alloy tube. (2) A method for circumferential welding of high alloy tubes, characterized in that the high alloy tubes according to claim (1) are joined together and then the ends of these tubes are welded circumferentially.