JPS6033886B2 - Local solution treatment method for austenitic stainless steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for eliminating the sensitized structure caused by heating and temperature rise when a steel pipe made of austenitic stainless steel (hereinafter referred to as a stainless steel pipe) is welded, etc. The present invention relates to a solution treatment method.

Austenitic stainless steel, such as 18-8 stainless steel, has excellent corrosion resistance and mechanical properties, so it is used to form steel pipes and construct them in chemical plants, nuclear reactors, etc. It is suitable for each device to be used as a component or the like.

However, ordinary austenitic stainless steel pipes are
When heated to a temperature of 0.0 to 80,000, a so-called sensitization phenomenon occurs in which carbides precipitate at the grain boundaries, and the corrosion resistance and mechanical properties described above deteriorate. It has been subjected to solution treatment in advance.
However, when the stainless steel pipe is specifically formed into a part or part of equipment for constructing a chemical plant, a nuclear reactor, etc., the chemical plant must weld it or process it at the construction site of the nuclear reactor, etc. For example, when heated for welding, stainless steel pipes that have been previously subjected to solution treatment not only become sensitized due to the heating during welding, but also have a It has been found that tensile residual stress is generated due to thermal expansion during welding and contraction during the cooling process immediately thereafter, which is extremely inconvenient from a practical and safety standpoint.

When constructing or assembling a chemical plant, for example, stainless steel pipes are formed by welding in a factory or on-site as part of the piping equipment, and this is assembled into the plant body, and inside the piping, for example, C and - This was discovered as a result of investigating the cause of stress corrosion cracking that often occurs in welded parts of piping when high-temperature, high-pressure water is passed through an aqueous solution containing halogens such as ions.

In other words, as mentioned earlier, the structure of the welded part has become sharp and carbides have precipitated at the grain boundaries, so intergranular corrosion occurs due to the flowing substances, and this is accompanied by tensile residual stress and other tensile stress. Stress acts and easily progresses to stress corrosion cracking,
This is because the welded portion will eventually be destroyed.

Therefore, in order to eliminate or improve the above-mentioned inconveniences, it is obvious that in accordance with the above example, the welded parts of the pipes incorporated in the plant body should be subjected to solution treatment. With conventional solution treatment equipment, it is not possible to perform localized heat treatment on only the welded parts of piping, and even if it is treated, it is difficult to treat the parts that have already undergone stabilization treatment and have not been sensitized. Since the melting process is carried out up to the point, it cannot be put to practical use at all.

As a workaround, it may be possible to individually liquefy each part at the stage of subassembling the piping, but in reality, in order to connect the treated parts to the main body and complete the piping device, it is necessary to perform the final welding process. Since some parts remain without solution treatment,
In any case, this is not a satisfactory solution to the drawbacks of the prior art, since the above-mentioned stress corrosion cracking occurs in this part.

As mentioned above, when constructing a chemical plant or nuclear reactor, etc., the present invention is assembled as some devices or parts constituting the main body by welding in a factory or on-site, and is incorporated into the main body. It is possible to easily and quickly liquefy only the locally sensitized structure that occurs in the welded portion of the stainless steel pipe without newly sensitizing the healthy structure adjacent to the outside. This method was developed with the aim of providing a local solution treatment method that generates and retains compressive stress on the inner surface of an austenitic stainless steel pipe.The structure is to heat an austenitic stainless steel pipe and cool it by passing water through the inner surface. In the heat treatment, the entire circumference of the steel pipe in the vicinity of the circumferential joint weld, and the entire circumference of the steel pipe in the wall thickness direction, is arranged on the outer surface of the steel pipe along the weld line of the weld. Using a ring-shaped lightning electrode, the temperature is rapidly raised to the solution temperature at a heating rate and holding time that do not sensitize the structure at the boundary with the non-heated part, and then the heated part is immediately heated to the solution temperature. By forcing water to come into contact with the inner surface of the steel pipe and rapidly cooling it, the structure of the boundary part on the inner surface of the steel pipe, which had a healthy structure before the heating, is sensitized by welding without becoming sensitized. The present invention is characterized in that, at the same time, the residual stress during welding is dissolved, and compressive residual stress is newly generated in the vicinity of the welded portion on the inner surface of the steel pipe.

Next, examples of the method of the present invention will be described. As shown in the figure, when base metals made of stainless steel pipes 1 and 1' are butted and welded, the surrounding area of the weld bead 2 changes into a sensitized structure 3 and 3' due to temperature rise and cooling during welding. As mentioned above, there is a residual tensile stress in this part.

Therefore, we prepared a large number of samples of JISSUS-304 with a nominal diameter of 2 inches and butt-welded tubes with a wall thickness of 6 as the base material, and after heating these samples to the solution temperature under various conditions and modes, After trying cooling in various ways, we found that 50Hz AC 50Kw was applied directly to the area on the 100 side in the wisteria direction centering on the weld for about 30 seconds, and the current-carrying part was cooled to 100cm.
The best results were obtained by raising the temperature to 90 oo and then immediately cooling it by passing water through the tube.

That is, when the sample was cut in the mar direction, the welded part and the surrounding structure that appeared on the cut surface were observed, and it was confirmed that the sensitized structure of the welded part had disappeared, and that the heated part Almost no sensitized structure was observed at the boundary between the non-heated part and the inner surface of the tube, and compressive residual stress was observed to occur on the inner surface of the tube, whereas under other conditions, No such results were obtained for those treated in this manner.
From the above results, the following was clarified.

i. If the sample is locally heated to 109,000, there will be a gap of 400 to 800q between the heated part and the non-heated part.
Although there is always a portion that is maintained in the sensitization temperature range of 0,
As mentioned above, the reason why almost no sensitized structure was observed at the boundary is because the time to 109,000 was relatively short, 30 seconds.
0~8000C) becomes even shorter,
Therefore, the tissue in that area did not become more sensitive. Incidentally, a clear tendency toward sensitization was observed in another sample in which the heating time to raise the temperature to 109,000 ℃ was investigated for a relatively long time. (b) Even when the sample is heated to 1090 qo, held, and cooled, the heated part passes through the sensitized temperature castle when the temperature drops, similar to when the temperature is raised, but no sensitized structure was found in the heated part of the sample. The reason for this is that if the material was rapidly cooled, the time required to pass through the sensitization temperature gate would be extremely short, and for the same reason as in case A, the tissue in that area would not become sensitized.

Incidentally, when we air-cooled another sample that had been heated under the same conditions, it took about 6 seconds for the heated part to pass through the sensitized temperature range of 400 to 800o○, and the whole heated part of the sample had changed into a sensitized tissue. C. It was recognized that there was a tensile residual stress on the inner wall of the welded part of the sample before the treatment, but compressive residual stress was found to have occurred on the inner wall of the welded part after the treatment. This is because the distribution state of residual stress after treatment is influenced by the mode of cooling.

Incidentally, when another sample heated under the same conditions as above was cooled with water from the outside of the tube, a tensile stress of 30 kg/ground was generated on the inner wall of the tube, and another sample was heated from the inner and outer surfaces of the tube. When water-cooled at the same time, no such stress was observed. Therefore, in order to prevent the generation of tensile residual stress that causes stress corrosion cracking, direct quenching of the wall surface is effective. Therefore, it is preferable to carry out the method of the present invention in the manner described below based on the above A to C.

In other words, (i) In order to solutionize only the locally sensitized structure of a material in which a stable structure and a locally sensitized structure due to welding, etc. coexist, when heating the material to the solution temperature, Localized heating is possible, and the time that stable parts of the structure are exposed to the sensitized temperature range is minimized, and even if there are differences in the thickness of the heated part, rapid heating can be done almost uniformly throughout the thickness direction. Exploring current heating means or induction heating means capable of localized rapid heating.2) Cooling of heated parts should be done by sufficiently shortening the time it takes the heated parts to pass through the sensitized temperature range when the temperature drops. In order to shorten the heating time on the inner surface of the tube, a rapid cooling method is adopted in which the water passing through that surface is brought into contact with the heated portion. Even if there is residual tensile stress, this stress will disappear while the part is heated and held at the solution temperature, and if the tube is rapidly cooled from the inside after being heated to the solution temperature, it will disappear. In view of the fact that compressive residual stress is generated in the process, in order to prevent the generation of tensile residual stress in the treated area, it is sometimes necessary to directly cool the treated area.

The embodiments of each item as described above can be easily implemented using an apparatus as shown in FIG. 2, so this will be explained below.

Reference numerals 11 and 11' denote electrodes that are designed to hold the weld bead 2 in the pipe P formed of the stainless steel pipes 1 and 1' at appropriate intervals, and the inner surface of the electrodes can almost completely grasp the entire circumference of the pipe 1. It is formed in the shape of a clamp that has a substantially circular cross section and can be opened and closed.

12 is an electrode device to which the electrodes 11 and 11' are connected; 13
is a cooling device that passes cooling water through the two parts of the weld bead after heating, and allows the cooling water supplied from an appropriate supply source (not shown) to freely flow or shut off through a valve 14 or the like. It has become.

Since the above-mentioned items 11 to 14 are combined into one relatively small device, they can be formed into a portable or stationary type, and therefore can be used at construction sites or factories of chemical plants, nuclear reactors, etc. In such cases, stainless steel pipes are formed into equipment or parts that are intended to be part of the plant, etc., or only the parts that have become locally sensitized by welding etc. during the formation of stainless steel pipes are removed from healthy adjacent parts. It is useful when solution treatment is performed without causing new sensitization of a tissue portion, and at the same time compressive stress remains on the inner wall of the treated portion.

The method of the present invention is as described above. Conventionally, when constructing a nuclear reactor in a chemical plant, stainless steel pipes are used to form devices and parts constituting the plant by welding or the like on site or in a factory. Even if the welded parts of steel pipes have been stabilized in advance, they may become sensitized due to the effects of heat during welding, and residual stress may occur. Stress corrosion cracking is likely to occur, which poses a practical problem, and those involved have struggled to find countermeasures.However, by using the method of the present invention, stainless steel pipes can be easily formed into some equipment and parts at a factory. Of course,
Even after welding on site and then assembling it into the main body, only the welded part where the tissue has become sensitized can be treated with lacquer without causing new sensitization in the part with healthy tissue adjacent to that part. At the same time, it is possible to leave compressive stress on the inner wall of the processing section, so the above conventional problems can be solved at once, which is extremely useful industrially, and the apparatus for carrying out the method of the present invention It can be constructed very simply and is very convenient to implement.

[Brief explanation of the drawing]

The figures are for explaining the method of the present invention, and Figure 1 is a sectional view of a welded part, and Figure 2 is a schematic diagram of an apparatus for carrying out the present invention. 1,1'...Stainless steel pipe, 2...
Weld bead, 3, 3'... Sensitized tissue, 11,
11'...electrode, 12...power supply device, 13...
...Cooling device, 14.../Gunolev. Ticket” map 2

Claims (1)

[Claims] 1. In a heat treatment in which an austenitic stainless steel pipe is heated and cooled by passing water through the inner surface, the entire circumference of the steel pipe in the vicinity of the circumferential joint weld is treated as described above. An annular current-carrying electrode placed on the outer surface of the steel pipe along the weld line of the weld allows the steel pipe to reach the solution temperature at a heating rate and holding time that does not sensitize the structure at the boundary with the non-heated part. Rapidly raise the temperature to , and immediately after this,
By forcibly quenching the heated portion by contacting the inner surface with water, the structure at the boundary on the inner surface of the steel pipe, which had a healthy structure before the heating, is sensitized. The method is characterized in that the structure that has been sensitized by welding is made into a solution without being sensitized by welding, and at the same time, the residual stress at the time of welding is eliminated, and compressive residual stress is newly generated in the vicinity of the welded part on the inner surface of the steel pipe. Local solution treatment method for austenitic stainless steel pipes.