JP6199725B2 - Piping connection method and piping connection structure - Google Patents

Description

  The present invention relates to a pipe connection method and a pipe connection structure.

In chemical plants, stainless steel small-diameter pipes such as SUS321 are installed and welded to boss portions (material is stainless steel such as SUS321) of large-diameter pipes of low alloy steel.
When an actual machine having such a pipe connection structure is used in a high-temperature and high-pressure gas environment (for example, a temperature of 520 ° C. and 25 kg / cm ² ), a small-diameter pipe welded to a large-diameter pipe will be around the heat-affected zone (HAZ). May cause cracking.

The following (1) and (2) are presumed as causes of the crack.
(1) “Sensitivity by welding heat input and operating temperature”, “Corrosion caused by corrosive substances (polythionic acid) generated during operation” and “Deterioration due to load stress including welding residual stress” Stress corrosion cracking (hereinafter also referred to as SCC) occurred due to superposition.
(2) Reheat cracking (hereinafter also referred to as SRC) is caused by the superposition of “deterioration due to load stress including welding residual stress” and “decrease in ductility due to precipitation of intra-grain fine carbides during high-temperature operation”. It was.

As countermeasures for the above (1) and (2), it is assumed that the material of the small diameter pipe is changed or the heat treatment of the welded portion.
However, a significant cost increase cannot be avoided by changing the material.

In addition, piping is welded at the time of installation on the actual product. For this reason, since the dimension of a large diameter pipe is large, a heat treatment furnace cannot be used.
On the other hand, local heat treatment using a muffler has the following problems.
(1) The large diameter pipe made of low alloy steel causes a decrease in strength (the required strength cannot be satisfied if it is affected by heat).
(2) The small-diameter pipe is long, and a region where sensitization occurs near the local heat treatment is formed. That is, the vicinity of the heat treatment part of the small-diameter tube is kept warm in the 450 ° C. to 850 ° C. region, thereby causing sensitization.

As described above, when welding a small-diameter pipe to a large-diameter pipe, there is a problem that SCC and SRC cannot be prevented or avoided at low cost.
A prior art such as Patent Document 1 (Japanese Patent Laid-Open No. 54-18438) is known. However, this is a technique that tries to prevent only SCC, and does not solve such a problem.

Japanese Patent Laid-Open No. 54-18438

  In view of the above circumstances, the present invention provides a pipe connection method and a pipe connection structure capable of preventing and avoiding SCC and SRC at low cost when welding a small diameter pipe to a large diameter pipe. With the goal.

  In order to solve the above-mentioned problem, the present invention is a pipe connection method, in which a short pipe material having SCC resistance and SRC resistance is welded in advance to a small diameter pipe connected to a large diameter pipe, and the small diameter pipe and The whole welded body with the short pipe material is heat-treated, and then the welded body is welded to a large-diameter pipe.

As the heat treatment, it is preferable to perform a solution treatment or a stabilization treatment in order to impart SCC resistance, and a stress relief heat treatment to impart SRC resistance.
In addition, by performing the stabilization treatment in a temperature range overlapping with the stress relief heat treatment, the heat treatment imparting SCC resistance and the heat treatment imparting SRC resistance can be performed simultaneously.

  Another aspect of the present invention is a pipe connection structure, in which a short pipe material having SCC resistance and SRC resistance is welded in advance, and the entire welded body is heat-treated, and then the end of the short pipe is enlarged. It is formed by welding to a diameter pipe.

  According to the present invention, there is provided a pipe connection method and a pipe connection structure capable of preventing and avoiding SCC and SRC at low cost when welding a small diameter pipe to a large diameter pipe.

FIG. 1 is a conceptual diagram illustrating a state in which a small-diameter pipe and a short pipe material are welded in the first embodiment of the pipe connection method according to the present invention. FIG. 2 is a conceptual diagram illustrating a state in which a welded body is heat-treated in the first embodiment of the pipe connection method according to the present invention. FIG. 3 is a conceptual diagram illustrating a state in which a small-diameter pipe is connected to a large-diameter pipe in the first embodiment of the pipe connection method according to the present invention. FIG. 4 is a perspective view for explaining a state where a small diameter pipe is welded to a large diameter pipe in the first embodiment of the pipe connection method according to the present invention.

  Hereinafter, embodiments of the pipe connection method and the pipe connection structure according to the present invention will be described in more detail.

First Embodiment First, a first embodiment of a pipe connection method according to the present invention will be described with reference to the accompanying drawings. The pipe connection method according to the first embodiment assumes a header structure in which ammonia is supplied from a large-diameter pipe and branched into a plurality of parallel small-diameter pipes to supply ammonia in small portions. However, the pipe connection method according to the present invention is not limited to the one according to such a form.

As shown in FIG. 1, in the pipe connection method according to the first embodiment, a short pipe material (buttering material) 2 is provided between a small diameter pipe 1 and a boss portion (not shown) of a large diameter pipe. Intervene.
The material constituting the small-diameter pipe 1 and the large-diameter pipe is generally stainless steel, and a typical example is SUS321.

  As a material constituting the short tube material 2, a material having SCC resistance and SRC resistance is suitable. That is, a material having low SCC sensitivity and SRC sensitivity depending on the welding and use environment is selected. As a material constituting such a short tube material 2, a material containing a Cr content of 25% by weight or more (up to a maximum of about 52% by weight industrially) is preferable. In the case of austenitic stainless steel, materials that do not contain components that cause intragranular hardening (Nb and Ti, and high Ni-containing materials such as Al and Ti) are also suitable. Specific examples include materials such as FM82 and ENiCr-4 (AWS standard).

  The length in the axial direction of the short pipe material 2 is designed to be longer than the width of the HAZ (heat affected zone) generated during actual machine installation welding. If the length of HAZ is a general welding method, it will be 5 mm or less. The axial length of the short tube material 2 satisfies HAZ, and is preferably up to about twice that of HAZ. Therefore, 10 mm or less is common while satisfying the length of HAZ.

  Next, the entire welded body 3 formed by welding the short pipe 2 to the small diameter pipe 1 is heat treated in the heat treatment furnace 4.

As described above, (i) stress corrosion cracking (SCC) and (ii) SRC (reheat cracking) are considered as causes of damage to the pipe connection structure.
In contrast, in the first embodiment, the welded body 3 is heat-treated to eliminate the damage factors (i) and (ii).

SCC measures (processing to give SCC resistance)
In the first embodiment, as a countermeasure against SCC, a solution treatment (solid solution) process or a stabilization process is performed on the welded body 3.

  When performing the solution treatment, the welded body 3 is heated to a temperature of 1000 ° C. or higher and lower than 1200 ° C., and then rapidly cooled to room temperature (400 ° C. or lower) to be in a supersaturated solid solution state.

Thus, in the solution treatment (solid solution treatment), heating is performed at 1000 ° C. or higher to reset the sensitized tissue.
However, at a high temperature (1200 ° C. or higher), the crystal grains become coarse and the scale is generated on the surface, so that the mechanical properties and the corrosion resistance are lowered. On the other hand, even at low temperatures (under 980 to 1000 ° C.), the disappearance of carbides and σ phases is not sufficient, and mechanical properties and corrosion resistance are reduced. For these reasons, heating to a temperature of 1000 ° C. or more and less than 1200 ° C.

  In the case where the stabilization process is carried out and the SCC countermeasure is taken, the welded body 3 is held at 850 ° C. or higher and 930 ° C. or lower, and Cr in the material is sufficiently diffused to cause a decrease in the corrosion resistance of the sensitized structure. The deficient layer disappears and the corrosion resistance is restored. At the same time, in this temperature range, it is possible to positively cause a bond between the carbon-forming element such as Ti and Nb and C rather than the bond between Cr and C, which is the cause of the Cr-depleted layer. If it is the solid solution temperature (less than 1000 ° C.) of the stable carbide formed by combining these, the subsequent Cr deficient layer generation can be suppressed.

  The holding time in this stabilization process is at least 2 hours when the weld thickness is 25 mm or less, and when it exceeds 25 mm, the holding time is added at a rate of +1 hour / 25 mm for every 25 mm exceeding. That is, the addition is performed such that at least 26 hours from 50 to 50 mm, at least 4 hours from 51 mm to 75 mm, and at least 5 hours from 76 mm to 100 mm.

Measures against SRC (processing to give SRC resistance)
For SRC (residual stress), stress relief heat treatment is performed.
The stress relief heat treatment prevents residual stress and coarsens the stable carbide (precipitate) to prevent intragranular hardening. The temperature range where SRC is likely to occur is assumed to be about 500 ° C to 850 ° C.

  From the viewpoint of removing residual stress, it is necessary to maintain the welded body 3 at 850 ° C. or higher and 930 ° C. or lower, and from the viewpoint of coarsening of precipitates, it is necessary to maintain the welded body 3 at 900 ° C. or higher and 930 ° C. or lower. . Therefore, as a countermeasure against SRC, it is preferable to maintain the welded body 3 at 900 ° C. or higher and 930 ° C. or lower when performing this stress relief heat treatment.

  For SRC countermeasures, the holding time when performing the stress relief heat treatment is at least 1.5 hours when the weld thickness is 25 mm or less, and when it exceeds 25 mm, the holding time is set at a rate of +1 hour / 25 mm for every 25 mm exceeding. to add. That is, at 26 to 50 mm, at least 3.5 hours, 51 mm to 75 mm, at least 4.5 hours, and 76 mm to 100 mm, at least 5.5 hours.

Finally, the pipe end 5 of the buttering part constituted by the short pipe material 2 of the welded body 3 and the boss part 7 of the large-diameter pipe 6 are welded at the installation site of the small-diameter pipe 1, and the actual machine installation is completed.
Thus, one embodiment of the pipe connection structure according to the present invention is completed in a state where welding is completed.
FIG. 4 shows this form as a perspective view. FIG. 4 also shows a welded portion 8 between the small diameter pipe 1 and the short pipe material 2 and a welded portion 9 between the boss part 7 of the large diameter pipe 6 and the short pipe material 2.

  As described above, according to the first embodiment, the strength of the small-diameter pipe is not reduced. In addition, it is possible to improve the sensitized structure of the entire small-diameter pipe, which was impossible with local heat treatment. That is, by using a short pipe material (battering material) having low SCC sensitivity and SRC sensitivity, a pipe connection structure (actual product) that can prevent the occurrence of SCC and SRC can be supplied.

  The pipe connection method according to the first embodiment is not a change over the entire small-diameter pipe 1, but a material change only in the vicinity of the welded portion 8 of the boss 6 of the large-diameter pipe 7, and the SCC, Measures against SRC are possible.

  Further, when installing the small-diameter pipe 1 in the actual machine with the large-diameter pipe 7, it is possible to prevent SCC / SRC without any on-site heat treatment.

Second Embodiment Next, a second embodiment of the pipe connection method according to the present invention will be described.
In the first embodiment, in the heat treatment, the HAZ sensitization structure of the small-diameter pipe is reset as an SCC countermeasure for the damage factor (i). Furthermore, it is necessary to perform stress removal and softening by coarsening of precipitates as an SRC countermeasure for the damage factor (ii). The heat treatment temperature for eliminating SCC and SRC is as described above.

  In the second embodiment, when the heat treatment is performed on the welded body 3 in order to eliminate the damage factors (i) and (ii), the stabilization process of the first embodiment is performed as an SCC countermeasure. . However, the stabilization treatment is performed at 900 ° C. or higher and 930 ° C. or lower, not 850 ° C. or higher. Within this temperature range, the holding time is carried out as follows.

  When the weld thickness is 25 mm or less and exceeds 25 mm for at least 2 hours, the holding time is added at a rate of +1 hour / 25 mm for every 25 mm exceeding. That is, the addition is performed such that at least 26 hours from 50 to 50 mm, at least 4 hours from 51 mm to 75 mm, and at least 5 hours from 76 mm to 100 mm.

If it is carried out with this holding time, the stress removal heat treatment carried out as the SRC countermeasure in the first embodiment is also carried out at the same time. And it can implement at low temperature rather than solution treatment.
That is, according to the second embodiment, it is possible to expect cost reduction by lowering the heat treatment temperature and shortening the heat treatment time.

Claims (6)

  A short pipe material having SCC resistance and SRC resistance is pre-welded to a small diameter pipe connected to a large diameter pipe, the entire welded body of the small diameter pipe and the short pipe material is heat-treated, and then the welded body A pipe connection method characterized by welding a pipe to a large-diameter pipe.   2. The pipe connection method according to claim 1, wherein as the heat treatment, a solution treatment or a stabilization treatment is performed to impart SCC resistance, and a stress relief heat treatment is performed to impart SRC resistance.   The pipe connection method according to claim 1, wherein the stabilization process for imparting the SCC resistance is performed in a temperature range overlapping with the stress relief heat treatment for imparting the SRC resistance.   A pipe formed by previously welding a short pipe material having SCC resistance and SRC resistance, heat treating the entire welded body, and then welding the end of the short pipe to a large-diameter pipe. Connection structure.   2. The pipe connection structure according to claim 1, wherein as the heat treatment, a solution treatment or a stabilization treatment is performed to impart SCC resistance, and a stress relief heat treatment is performed to impart SRC resistance. .   The pipe connection method according to claim 1, wherein the stabilization treatment for imparting the SCC resistance is performed in a temperature range overlapping with the stress relief heat treatment for imparting the SRC resistance.

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