JPS5854175B2 - Local solution treatment method for rust-free steel pipes - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is designed to protect steel pipes, strips, and other materials made of austenitic stainless steel (hereinafter simply referred to as stainless steel pipes, etc.) from thermal effects caused by heating and temperature rise during welding. The sensitized tissue that occurs locally during treatment disappears without creating new sensitized tissue in areas that were healthy tissue before treatment, and at the same time generates new compressive stress on the inner surface to eliminate the remaining sensitized tissue. The present invention relates to a solution treatment method for the purpose of

It is well known that steel pipes made of JIS standard SUS 304 are often used as corrosion-resistant pipes in nuclear reactor equipment, chemical equipment, and other fields because rust-free steel pipes have excellent corrosion resistance and mechanical properties. be.

However, when the above-mentioned steel pipes are formed into equipment or incorporated into equipment as parts, due to the heat effect during welding, chromium carbide precipitates near the weld and the structure becomes sharp, causing chromium to form at grain boundaries. For example, when a liquid containing dissolved oxygen or the like comes into contact with the chromium-depleted layer, the chromium-depleted layer combines with the dissolved oxygen and corrodes electrochemically, and furthermore, the tensile stress remaining in this corroded part during welding. acts to cause intergranular cracking, which eventually causes the phenomenon of stress corrosion cracking in the weld, which is difficult to use in practice.
It has been pointed out that this is extremely inconvenient in terms of safety.

Such inconveniences occur not only in the above-mentioned welded parts, but also in parts where the structure of the welded part is locally affected by heat due to other processing and becomes sensitive.

In order to prevent stress corrosion cracking caused by the sensitization of the structure as in the welded parts of steel pipes, etc., the inventor of the present invention has developed We first proposed a local solution treatment method that eliminates the sensitized structure caused by chromium carbide precipitated near the weld, and does not affect other parts due to the heat (Patent No. 1).
No. 129247 (Special Publication No. 57-19169)).

Therefore, the method proposed earlier heats only the vicinity of the circumferential weld of a stainless steel pipe, including the welded part, by induction heating to the solution temperature at a rapid temperature increase rate, and the inner surface of the pipe is heated to the solution temperature. The idea is to bring water into contact with the inner surface as soon as it reaches the temperature to rapidly cool it down, and although it is somewhat effective,
This method uses a rapid temperature increase rate only in the vicinity of the weld, including the weld, in order to avoid sensitizing the parts that are not affected by heat during welding and have a healthy structure due to the heating of the solution treatment using this method. A large-capacity power source that can raise the temperature to the target temperature is essential, which poses difficulties in terms of equipment and construction.

Incidentally, if the original invention method is carried out using a power supply with a small capacity, the rate of temperature rise will be slow, and parts of healthy tissue will be kept in the sensitized temperature range for a relatively long time, causing the parts sensitized by welding to Even if solution treatment is possible, a new sensitized structure is likely to develop in the surrounding area.3 In addition, the rapid heating using a large-capacity power source, which is practiced between nuclear power plants, is difficult because stainless steel pipes are assembled as piping in plants, etc. Because it is applied after welding, heating can only be done from the outer periphery of the welded part, so
In order to raise the temperature of the inner surface of the tube, whose sensitized tissue must be improved, to the solution temperature (approximately 1050 to 11500 C), the outer surface of the protective tube should be heated to a temperature higher than that of the inner surface, and the thickness of the G step should be 10 mm. If the temperature exceeds 1,300°C, it will be necessary to raise the temperature to around 1,300°C. However, such heating is not only undesirable because it causes coarsening of the crystal grains in that area, but also causes the outer surface to melt. There are some drawbacks, such as the possibility of damage.

Therefore, when induction heating is used for heating in the above method, the difficulty in heating can be improved to some extent by selecting the frequency, but even with induction heating with a selected frequency, heat generation does not occur due to the heating target ( Here, the temperature rise on the inner surface is concentrated on the outer surface of the tube) and is largely dependent on heat conduction, so the above-mentioned region of coarse grains on the outer surface still tends to be eroded.

The inventor of the present invention investigated the cause of the above-mentioned difficulties caused by the implementation of the nuclear power plant gap method, and as a result, learned that the cause lies in the heating mode in the nuclear power plant gap.

In other words, between nuclear power plants, rust-free steel pipes, etc. (if the carbon content is 0.05%) are used.

The fact that chromium carbide precipitates in the structure of (the same applies hereinafter) is related to the temperature range in which the structure is held or passed during heating or cooling after heating, and the time the structure is kept in that temperature range. Specifically, the temperature range is 400 to 800°C, although it varies slightly depending on the carbon content, and the relationship between the temperature range and the time for which it is held or passed is, for example, 60°C.
It has been previously reported that intergranular corrosion occurs in about 100 to 200 seconds at 0°C, and the structure becomes sensitized in about 20 seconds in the temperature range of 700 to 800°C. (See the area surrounded by solid lines in the diagram in Figure 1) and an experiment conducted by the inventor that showed that if the temperature range is between 400 and 800 degrees Celsius, the tissue becomes sensitized even if it is kept in that temperature range for about 10 seconds. Based on the results (see the area surrounded by the dashed line in the diagram in Figure 1), during local solution treatment, other healthy tissues adjacent to the treated area and which have not yet been sensitized. In order to solutionize only the above-mentioned local treatment target area while preventing the above-mentioned areas from becoming sensitized, the above-mentioned other healthy tissue parts should be heated to a temperature of approximately 400 to 800°C during the heating for local solutionization. This was based on the conclusion that it is necessary to minimize the time during which the solution is retained or passed through the area. The above-mentioned difficulties arose because the attempt was made to prevent this part from becoming newly sensitive by increasing the heating rate.

Therefore, regarding the precipitation of chromium carbides, the inventors of the present invention investigated the relationship between the temperature range and the time for which the structure is maintained or passed through the temperature range, and found that it is generally said that carbides precipitate in the structure at We learned that even in the 8000C sensitization temperature range, chromium carbide may not precipitate depending on the time the structure is exposed to that temperature range (see the shaded area in the diagram in Figure 1), and we took advantage of this phenomenon. If heating is applied, local solutionization can be achieved without causing the temperature difference that occurs during heating with respect to the thickness of the object to be heated, and without precipitating chromium carbide in areas with a healthy structure near the area to be treated. The present invention was completed based on the conclusion that the treatment could be applied to the structure, and the structure is similar to the circumferential weld of a stainless steel pipe and the structure in the vicinity including the circumferential weld. The entire circumference of the affected and locally sensitized part may be raised or lowered by an annular inductor depending on the carbon content of the steel pipe base material.
Immediately after preheating the sensitized part so that the inside and outside of the sensitized part is approximately the same temperature in the thickness direction, in a temperature range of around 400°C and for a time during which carbides do not precipitate in parts with a healthy structure, the preheating is performed. The part is rapidly heated to the solution temperature, and as soon as the heated part reaches the solution temperature, the heated part is forcibly cooled by bringing water into contact with the inner surface of the heated part to form a healthy structure before solution treatment. This method is characterized in that only the locally sensitized tissue is made into a solution without sensitizing that part, and compressive residual stress is generated on the inner surface side of this solutionized part.

Next, examples of the method of the present invention will be described.

JIS 5US-304゜5ch8 with nominal diameter of 4 inches
0 and wall thickness 7.6 as shown in the second figure.
When we prepared butt welded pieces as
The structure of '' had changed to a sensitized structure 3, 3' in which chromium carbide was precipitated at the grain boundaries due to heating and cooling during welding.

Note that 4 and 4' are stable structures that are not affected by heat due to welding.

Therefore, a large number of samples having such a structure were divided into two groups, and by induction heating using the annular inductor 5, they were rapidly heated and cooled using the heating mode of the original invention and the heating mode of the present invention. The state of temperature increase on the outer surface A and inner surface B of the tube in the two parts was observed and was as follows.

That is, according to the method of the original invention, the inductor 5 is operated with an input power of 600 KW and an output frequency of 2000 Hz, and the base materials 1 and 1 are
Although the temperature of the inner surface B was raised to a solution temperature of approximately 1050°C in about 4 seconds when the weld bead 2 and the nearby structure 33' were directly heated to the solution temperature. On the other hand, the temperature of the outer surface A has been raised to approximately 1330°C (the third
(Refer to the diagram in the figure), not only did the crystal grains in this area tend to become coarser, but some samples also showed some melting loss.

On the other hand, when the method of the present invention was used to preheat at a temperature of approximately 500° C. for approximately 10 seconds using the same heat source as described above, and then the output was increased, the inner surface B was approximately 10
The temperature was raised to a solution temperature of 50°C, and the outer surface A was heated to approximately 1170°C at this time (see the diagram in Fig. 4).
Of course, no melt damage was observed in any of the samples, and no coarse crystal grains were observed.

Based on the above, in order to locally solution heat a thick object, if you preheat the area to be treated before rapidly heating it, you can not only raise the temperature to the solution temperature in a shorter time. , since the side (outer surface A in this case) near the heat source (inductor 5) is not overheated and no unfavorable changes occur in the tissue, local solution treatment can be applied more effectively, and the heat source It can be seen that rapid heating is possible even with a smaller capacity than in the case of the original invention.

Therefore, the above preheating is not just preheating, but an appropriate preheating temperature and the same time are set based on the relationship between the temperature range for precipitation of chromium carbide and the time for a healthy part of the structure to be in that temperature range. , it can be easily carried out without requiring special intuition or skill level.

Next, the two types of samples heated in different manners as described above were quenched by injecting water into the tube immediately after raising the temperature, but in the end, the samples heated in the manner of the original invention were Since the temperature of the outer surface A was raised to 1330° C., it took a longer time to cool down compared to the sample heated by the method of the present invention (see the diagrams in FIGS. 3 and 4).

The fact that cooling takes such a long time means that it takes a long time to pass through the sensitizing temperature range of 400 to 800° C. when the temperature drops, which is not preferable because there is a risk that chromium carbide will precipitate in the structure.

For this reason, in local solution treatment, it is preferable to rapidly raise the temperature between the inner and outer surfaces of the part to be treated to the solution temperature with little temperature difference, and the method of the present invention is also better than the original invention in this respect. It can be said that it is excellent.

Next, the state of the structure of a sample subjected to local solution treatment by the method of the present invention as described above will be explained.

@) 7 Boundary area between weld bead 2 and base metals 1 and 1' (hereinafter referred to as "
Although it was clearly observed that chromium carbides were precipitated at the grain boundaries in the welded portions (referred to as "boundary portions"), these carbides completely disappeared after treatment by the method of the present invention.

(b) Sensitized portions 3, 3' outside the boundary portion (
In the sensitized part (hereinafter referred to as the "sensitized part"), precipitation of chromium carbides was observed at grain boundaries under the influence of heat during welding, but after treatment by the method of the present invention, these carbides were completely removed. It had disappeared.

(c) Although the structure was stable and healthy before solution treatment,
After treatment by the method of the present invention, precipitation of chromium carbide was observed in the area 4゜4' that is affected by heat during solution treatment (hereinafter referred to as the "treatment-affected area"), but the amount thereof was less than that described above. The amount of carbide was extremely small compared to the amount of carbide observed in the boundary and sensitized areas, and no sensitization of the structure in this area was observed as a whole.

This is thought to be because the heating and cooling times of the parts to be treated by the method of the present invention were extremely short and there was no sufficient thermal influence to change the structure of these parts 4, 4'.

Incidentally, when another sample was rapidly heated to the solution temperature using the heating mode of the method of the present invention and then allowed to cool, a large amount of chromium carbide was precipitated in the structure of these portions 4 and 4', and the chromium carbide was almost completely precipitated. It was becoming more sensitive.

In addition, in the method of the present invention, cooling immediately after heating is performed by bringing cooling water into direct contact with the inner surface of the tube, so that the treated area is simultaneously treated with local solution treatment without creating new sensitized structures. Since compressive stress can remain on the inner surface side, stress corrosion cracking can be prevented.

In the above embodiments, the welded portion of the stainless steel pipe and the vicinity thereof are to be locally solutionized. Of course, this method can also be applied to structures where the structure in and around the area has been locally sensitized by heat treatment or the like due to the thermal influence at that time.

The method of the present invention is as described above, and rust-free steel pipes, etc. that are conventionally incorporated into nuclear power equipment, chemical equipment, etc. as corrosion-resistant pipes, etc., have a structure that is affected by heat during welding etc. during the installation. When sensitized, residual stress also acts on the sensitized portion, causing stress corrosion cracking in the sensitized portion during use, which has been a problem.However, according to the method of the present invention, the structure is not affected by the heat caused by welding, etc., as described above. Only the sensitized part, whether it is a part or after it has been assembled into the device,
Stress can be improved locally at the same time as solution treatment, which not only solves the conventional problems at once, but also improves the structure of the healthy part of the area adjacent to the area to be treated when heating for solution treatment. Since preheating is performed at a temperature and time that does not cause sensitization, the heating time can be shortened compared to the original invention, and overheating on the side closer to the heat source can be prevented. The capacity of the heat source can be made smaller than in the case of the original invention, resulting in economical effects.

An additional related original invention is that when rustless steel pipes are incorporated into nuclear power equipment, etc., the structure of the pipes becomes locally sensitized due to heat effects during processing or assembly, such as at welded parts. In order to locally solution treatment only the sensitized portion without thermally affecting other portions, the sensitized portion is rapidly heated to the solution temperature using a heating means capable of local heating; This is a method of rapidly cooling the inner surface by bringing water into contact with it, but this method requires a large-capacity heat source and is therefore uneconomical. If the temperature of the inner surface is raised to the solution temperature, the temperature of the outer surface near the heat source may be raised to a temperature at which the crystal grains become coarse or melted, causing new problems. Therefore, in order to eliminate these difficulties, the present invention preheats the part to be solution-treated at a temperature and time that does not sensitize parts of healthy tissue adjacent to the part to be solution-treated, and then lowers the temperature to the solution-treated part. This method rapidly raises the temperature to , and then immediately brings water into contact with the inner surface to rapidly cool it down, thereby reducing the temperature difference between the inner and outer surfaces of the part to be treated and reducing the capacity of the heat source. be.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between temperature and time at which the structure of austenitic stainless steel becomes sharp, Figure 2 is a vertical cross-sectional view of a sample used to carry out the method of the present invention, and Figure 3 is a diagram showing the relationship between the temperature and time at which the structure of austenitic stainless steel becomes sharp. FIG. 4 is a chart showing temperature changes on the inner and outer surfaces of a sample when the sample is solution-treated by the method of the present invention. 1.1'...Base metal, 2...Weld bead, 3,3'...Sensitized structure, 4,4'...
・・Stable tissue, 5・・・・Inductor, A・・・・
Outer surface, B...Inner surface.

Claims (1)

[Claims] 1. The entire circumference of a portion where the structure has been locally sensitized due to local thermal effects, such as the circumferential joint weld of a stainless steel pipe and the surrounding structure including the circumferential joint weld, is removed using an annular inductor. At a temperature range of around 600 to 400 degrees Celsius, which may be raised or lowered depending on the carbon content of the material, and at a time during which carbides do not precipitate in areas with a healthy structure.
Immediately after preheating the sensitized portion so that it is almost isothermal inside in the thickness direction, the preheated portion is rapidly heated to the solution temperature, and as soon as the heated portion reaches the solution temperature, the heating is performed. The part is forcibly cooled by bringing water into contact with the inside of the part, and only the locally sensitized tissue is solutionized without sensitizing the part that is healthy tissue before solution treatment. A method for local solution treatment of rustless steel pipes, which is characterized by generating compressive residual stress on the inner surface side of a rust-free steel pipe.