JPS5940892B2 - Heat treatment method for welds in stainless steel pipes - Google Patents

Description

[Detailed Description of the Invention] The present invention is useful for welding steel pipes, strips, and other materials (hereinafter referred to as stainless steel pipes, etc.) made of austenitic stainless steel, etc., and for heat-treating the core after welding. This relates to a heat treatment method for eliminating or improving sensitized structures and other structures that cause corrosion cracking caused by heating or raising the temperature.

Stainless steel pipes and the like are frequently used as structural members for piping and other parts of nuclear power equipment, chemical plants, etc. because of their excellent corrosion resistance and mechanical properties, but they also have drawbacks.

That is, when the stainless steel pipes and the like are used, for example, as piping for a nuclear reactor, piping parts of an appropriate shape and length are assembled into a piping system by welding, etc., but the grain boundaries of the base material are This causes a sensitization phenomenon in which carbides precipitate in the weld, which sharpens the structure, deteriorating its characteristic corrosion resistance and mechanical properties, and heat treatment of the part after welding further promotes the sensitization of the structure of the base metal. This causes practical inconvenience.

This inconvenient situation is caused not only by the base metal near the weld being heated to 400 to 800°C during welding, but also by the heat treatment of the weld, which causes its structure to become more sensitive. Furthermore, tensile residual stress is also generated on the inner surface of the pipe due to thermal expansion during welding or heat treatment of the welded part and contraction due to cooling immediately thereafter, so there may be It is known that when a hot aqueous solution containing NOx is passed through, stress corrosion cracking occurs in the vicinity of the weld.

Therefore, in order to eliminate or improve the above-mentioned disadvantages, the sensitized parts can be treated with a solution, but a general solution treatment in which the object to be treated is heated in a heating furnace and then cooled down is used. In the chemical treatment method, unnecessary parts are heat treated, which is not only inconvenient but also uneconomical.
However, solution treatment of piping, etc. built into the main body is impossible.

The present invention is designed to solve the problem that the base metal in the vicinity of the welded part of stainless steel pipes and the like becomes sensitized due to the influence of heat during welding, and to improve the structure of the welded part. The structure of the welded part was improved in view of various points such as the need to perform post-heat treatment, and the fact that the base metal in the vicinity becomes even more sensitive due to the post-heat treatment, so the structure of this area must be further improved. At the same time, not only does the sensitized structure generated during welding disappear, but the heat treatment for this purpose also prevents the formation of sensitized structures in other parts, and the piping is made of stainless steel pipes assembled to the main body. This method was developed with the aim of providing a heat treatment method that can be applied to materials such as stainless steel pipes, and the method is to treat the entire circumference of only the circumferential joint welded part of stainless steel pipe and its vicinity, and the thickness of the steel pipe is The whole area of the weld is heated to a solution temperature by induction heating using an annular inductor placed along the weld, held for an appropriate period of time, and then rapidly cooled to improve the structure of the weld and dissolve the structure sensitized by the welding. After the heat treatment, only the thermal boundary area due to this heat treatment or the area including the welded area and the thermal boundary area is rapidly heated to the solution temperature by induction heating of an annular inductor, and water is immediately poured onto the inner surface of the area. It is characterized in that it is brought into contact and rapidly cooled, and the portions sensitized by the heat treatment are made into a solution, and at the same time compressive stress remains on the inner surface thereof.

Next, embodiments of the method of the present invention will be explained with reference to the drawings.

Figure 1 A is a cross-section of the welded part as welded, Figure 1 opening is a cross-section of the welded part and the part that has become sensitized due to the heat effect of the welding, after the first stage of heat treatment is applied, and Figure 1 C is the cross-section of the welded part as it is welded. This is a cross-sectional view of the part that has been sensitized by the first stage treatment and has been solutionized by the second stage heat treatment, where 1 and 1' are the base metal made of welded stainless steel pipes, 2 is the welded part, and 3 is the welded part. ,3'
is a sensitized structure in which carbides precipitate at the grain boundaries due to the thermal influence during welding of the base metal 1.1'.

Reference numeral 4 denotes a high-frequency inductor serving as a heating source for post-heat treatment to improve the structure of the welded part 2 and for solution treatment of the sensitized structures 3 and 3'. and is formed into a band-shaped ring having a width approximately equal to the width of the sensitized tissue 3, 3' portion (hereinafter referred to as the treatment target area), and is arranged so as to locally heat the treatment target area. has been done.

Reference numeral 5 denotes a cooling liquid injection nozzle for rapidly cooling the part to be treated that has been rapidly heated by the inductor 4. Here, it is formed in an annular shape similar to the inductor 4 and is placed adjacent to it.
It does not necessarily have to be located adjacent to the inductor 4 due to wiring, piping to the nozzle 5, or the next process.

Then, the inductor 4 is used to control the part to be treated, that is, the welded part 2.
The sensitized structure 3, 3' is heated to the solution temperature, and after maintaining that temperature for several tens of seconds, the heated part is cooled by a jet of coolant from the nozzle 5, which achieves the original purpose of this part. Although the structure improvement and solution treatment were completed, it was found that a sensitized structure had actually appeared at the boundary between the target area and the non-target area (hereinafter referred to as thermal boundary BH).

This is considered to be because the thermal boundary portion BH was exposed to the sensitization temperature range during heat treatment of the target portion, and carbides were precipitated at the grain boundaries of that portion BH, resulting in the sensitization of the structure.

Therefore, in the present invention, in order to eliminate the sensitized structure generated in the thermal boundary part BH by the heat treatment, after the heat treatment in the aspect shown in FIG. Inductor 6 and coolant injection nozzle 7 formed wider than 4
are arranged as shown in FIG. Immediately thereafter, a solution treatment (hereinafter referred to as post-treatment) was performed in which the sample was rapidly cooled.

As a result, the structure of the welded part 2 is improved and the sensitized structure 3.3'
The sensitized structure that occurred at the boundary between the target and non-target areas, that is, the thermal boundary area BH during the pre-treatment aimed at eliminating the heat, has completely disappeared by this post-treatment, and this area BH has been dissolved in solution. On the other hand, there was no effect of the post-processing on the thermal boundary between the heated part and the non-heated part during this post-processing.

This is thought to be because the post-treatment was performed under conditions of rapid heating and rapid cooling immediately after heating, so the thermal boundary area of the post-treatment was not affected by the heat treatment and the structure did not change. It will be done.

In the above embodiment, the pretreatment was performed by raising the temperature to the solution temperature, holding it for an appropriate period of time, and then rapidly cooling it. The purpose is to solutionize the parts 3, 3' whose structures have become sensitized due to the thermal influence, and even if it is possible to solutionize the sensitized structures 3, 3' unless the solution temperature is maintained. This is because the structure of the welded portion 2 cannot be sufficiently improved.

Therefore, this pretreatment makes the thermal boundary region BH more sensitive, but this is because the post-treatment, which includes rapid heating and rapid cooling immediately after temperature rise, can sufficiently make it into a solution, and the treatment time is extremely short. There was almost no adverse effect on the heat boundary during post-treatment, and an extremely good heat treatment effect was obtained overall.

Therefore, the method of the present invention carried out in the manner described above,
Further, even if the present invention is implemented in the manner described below, the same effects as those of the above embodiments can be obtained.

That is, the inductor 4 and the nozzle 5 are arranged so as to surround the part to be treated to form a device for pretreatment, which is similar to the above example, but here, there are approximately Inductors 8, 8' and nozzles 9, 9' of similar shape are installed and a device for post-processing is installed, and first, as shown in Figure 2A, only the inductor 4 is energized to remove the area to be treated. is locally heated to a solution temperature, maintained at that temperature for an appropriate period of time, and then rapidly cooled by jetting a cooling liquid from the nozzle 5 to complete the pretreatment of this portion.

Next, as shown in Figure 2, current is applied to the inductor 8°8' to rapidly heat both sides of the previously pretreated portion to the solution temperature, and immediately after the temperature rises, the nozzle 9, The whole body is heat-treated by rapidly cooling it by jetting a cooling liquid from 9'.

When heat treatment is carried out in this manner, the thermal boundary part BH, whose structure has become sensitive due to the thermal influence during pre-treatment, becomes a solution in the post-treatment, and the area near the post-treated part The structure was not affected by any heat due to the post-treatment, and the same heat treatment effect as in the first example described above was obtained.

In the second embodiment, the inductor 4 and nozzle 5 used in the pre-treatment may be sequentially moved to the thermal boundary areas BH, BH to be subjected to the post-treatment for heat treatment.

In this case, the inductors 8, 8' and nozzles 9, 9
' is not necessary, and the equipment for carrying out the method of the present invention can be simply configured.

In the above-mentioned embodiments of the method of the present invention, cooling after heating is performed from the outside of the tube, but if the tube is cooled from the inside, compressive residual stress will be generated in that part, so as mentioned at the beginning, It becomes possible to eliminate tensile residual stress.

The present invention is as described above, and for example, in a welding process in the process of forming a welded part in piping made of stainless steel pipe or in the process of assembling it into a main body, it is possible to improve the structure of the welded part. There is no local heat treatment method for solutionizing the structure of the base metal, which has become sensitized due to the heat effects during welding. Stress corrosion cracking occurs in welded areas and areas with sensitized structures as described above, which poses a practical and safety problem.However, with the method of the present invention, stress corrosion cracking occurs locally in such welded areas and during welding. Since the structure of the sensitized area can be improved and solution treated at the same time, the conventional problems can be solved all at once. This is a pretreatment process in which only the parts to be treated are heated to the solution temperature, held at that temperature for an appropriate period of time, and then rapidly cooled. Since this is a solution treatment method that involves rapid cooling immediately after heating, the pretreatment can sufficiently improve the structure of the weld zone and completely eliminate the sensitized structure that has formed in the base material to be welded. In addition, the sensitized structure generated at the thermal boundary due to the pre-treatment can be completely eliminated by the post-treatment, which is not only perfect as a heat treatment, but also in order to carry out the method of the present invention. induction heating means capable of rapidly heating the local area according to the form of the local area to be solution-treated;
For example, if the target to be treated is a welded section of piping, local heat treatment can be performed at any time, whether during assembly of the piping component or after assembly of the main body is complete. It has the effect of being.

[Brief explanation of the drawing]

FIGS. 1A to 1C are longitudinal cross-sectional views showing essential parts of an embodiment of the solution treatment method according to the present invention, and FIG. The openings are the same (this is a longitudinal sectional view showing the main parts of an embodiment of another solution treatment method. 1.1'...Base metal, 2...Welded part, 3
, 3'... Sensitized tissue, 4 , 6 , 8 , 8
'...Inductor, 5,7゜9...Cooling liquid injection nozzle.

Claims (1)

[Claims] 1. Heat the entire circumference of a circumferential joint welded part of a stainless steel pipe and its vicinity, and the entire circumference in the thickness direction of the steel pipe, to the solution temperature by induction heating with an annular inductor placed along the welded part, and heat it for an appropriate period of time. After holding and quenching to improve the structure of the welded part and solution treatment of the structure sensitized by the welding, only the thermal boundary part due to this heat treatment or the area including the welded part and the thermal boundary part, The annular inductor is rapidly heated to its solution temperature by induction heating, and its inner surface is immediately brought into contact with water to rapidly cool it, and the portions that have been sensitized by the heat treatment are made into a solution while at the same time compressive stress remains on the inner surface. A method for heat treatment of welds in stainless steel pipes, characterized by: