JPH1161275A - Weld zone of austenitic stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a weld zone of austenitic stainless steel, where the fatigue strength of the weld zone which is inferior to a base material in fatigue characteristic in a high temp. fluid including high temp. water is improved. SOLUTION: The surface part of a weld metal on the side to be brought into contact with high temp. fluid is heated up to the recrystallization temp. or above and then cooled at a cooling rate not lower than that of natural air cooling at ordinary temp., by which the amount of ferrite in the vicinity of the surface of the weld zone is reduced to improve fatigue strength. At this time, in order to lower the heating temp., a worked layer is previously formed at the surface part of the weld metal on the side to be brought into contact with high temp. fluid.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welded portion of austenitic stainless steel, and more particularly to a welded portion of austenitic stainless steel used for a portion in contact with a high-temperature fluid.

[0002]

2. Description of the Related Art Conventionally, austenitic stainless steels have been widely used from the viewpoint of corrosion resistance in portions that come into contact with high-temperature water or high-temperature fluid such as boilers and pipes of nuclear power generation facilities and thermal power generation facilities. Many of these members are joined by welding. Most of the metal structure of this weld is austenite, but several percent of a ferrite phase is generated during the solidification process during welding, resulting in a mixed structure of austenite and ferrite.

[0003]

Since the welded portion having the mixed structure has a shape discontinuity such as a weld toe, a boiler or a pipe made of austenitic stainless steel is used. When a repetitive stress is applied due to a rapid change in temperature of the fluid, a change in pressure, etc., stress is concentrated in the welded portion, which is the shape discontinuity, and fatigue cracks occur. Fatigue strength is a problem.

Therefore, in order to examine the fatigue characteristics of the weld metal in high-temperature water in accordance with the amount of ferrite contained in the mixed structure, SUS316 stainless steel (1) SUS316 base metal (2) SUS316 Weld metal (3) 316 series cast members were subjected to a high temperature underwater fatigue test. Incidentally, the amount of ferrite in the steel is SUS316 of (1).
0% for base metal, 8 for SUS316 weld metal of (2)
%, And 20% in the 316 cast material of (3). The fatigue test is a complete oscillating stress under axial strain control, with a strain amplitude of 0.6% and a strain rate of 0.0001% / sec.
c, performed in degassed water at 325 ° C.

FIG. 2 shows the obtained results. The horizontal axis in FIG. 2 is the amount of ferrite in the steel, and the vertical axis is the fatigue life ratio when the fatigue life of the SUS316 base material (1) is set to 1. As is clear from FIG. 2, the fatigue life increases with increasing ferrite content.
There is a tendency to decrease, and the fatigue strength of the welded portion having the weld metal is lower than that of the base metal, indicating that a fatigue crack tends to occur in the welded portion due to the material.

[0006] In order to solve the above technical problems, the present invention provides a welded portion of austenitic stainless steel having improved fatigue strength of a welded portion having poorer fatigue properties than a base material in a high temperature fluid containing high temperature water. It is in the thing.

[0007]

Means for Solving the Problems In order to examine how fatigue damage of stainless steel occurs in high-temperature water, a SUS304 base material was used as a test material, and a shaft was formed in degassed high-temperature water at a temperature of 250 ° C. A strain control fatigue test was performed while measuring cracks generated on the surface. The result is shown in FIG.
The horizontal axis in FIG. 3 is the number of repetitions, and the vertical axis is the total length of the largest crack observed on the surface. From Fig.3, in high temperature water, small cracks occur early in the repetition, and the cracks
It has been found that the crack propagation life up to growth to m accounts for most of the total fatigue life. That is, the difference in fatigue life due to the difference in the amount of ferrite shown in FIG.
m is determined to be due to the difference in crack propagation velocity during growth to m.

Therefore, as a method of improving the fatigue strength of the welded portion, it is effective to significantly reduce the amount of ferrite near the surface of the weld metal where cracks occur due to corrosion. As the depth at which the amount of ferrite is reduced, the life during growth to a surface length of 0.5 mm occupies most of the results in FIG. 3, and the ratio of crack depth to length in the above test is about 0.5 mm. Therefore, it is considered that even a small amount of about 0.25 mm is sufficiently effective. It has also been confirmed that when austenitic stainless steel is heated to about 1000 ° C. or higher, recrystallization occurs. At this time, when the cooling rate is high, ferrite is hardly generated.

Accordingly, the present invention provides a method of heating a surface of a weld metal in contact with a high-temperature fluid to a temperature higher than a recrystallizable temperature and then cooling at a cooling rate higher than natural cooling at room temperature. A first feature is that the amount of ferrite near the surface of the weld is reduced to improve the fatigue strength. In this case, the cooling rate higher than the natural cooling at room temperature is preferably a cooling condition in which forced air cooling is performed by air injection at room temperature or forced water cooling is performed by injection of cooling water.

According to this invention, only the surface of the welded portion in contact with the high-temperature water is locally heated and recrystallized to reduce the amount of ferrite only in the vicinity of the surface. As shown in FIG. Improvement can be measured.

[0011] The invention according to claim 2 is that, after forming a work layer on the surface of the weld metal in contact with the high-temperature fluid, the surface is heated to a temperature at which recrystallization can be performed, and after the heating, at room temperature. It is characterized by cooling at a cooling rate higher than the natural cooling in the above, reducing the amount of ferrite near the surface of the welded part, and improving the fatigue strength. According to this invention, the surface of the welded portion is subjected to a strong working such as a roll working, or a surface worked layer such as a shot peening to form a surface worked layer, and then heated to lower the recrystallization temperature, thereby reducing the ferrite disappearance. Becomes larger. The formation of the processed layer is preferably performed at room temperature before heating the surface layer. The lower recrystallization temperature depends on the state of formation of the processed layer. For example, when the processed layer is formed by shot peening, a heating temperature of about 850 ° C. or more is sufficient.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention unless otherwise specified, and are merely illustrative examples. Only. FIG. 1 shows an example of a heat treatment of a butt weld according to an embodiment of the present invention. Reference numeral 1 denotes a V-shaped butt-welded portion in which the lower layer has a wetted surface on the first layer side. The lower layer of the first layer was heated at a heating temperature of about 1000 ° C. or more using a gas burner 2 for about 10 minutes. Then, it is cooled to room temperature by forced air cooling with room temperature air injection or the like. Next, in order to confirm this effect, SUS316 pipe (outer diameter about 100 mm, plate thickness about 13
mm) Using the welded joint as a test material, a test piece having the dimensions and shapes shown in FIG. 4 was taken therefrom, and subjected to a fatigue test with deaerated water at 325 ° C. under load control oscillating stress and nominal stress at the base material. Amplitude 2
The test was performed at 50 MPa and a frequency of 0.001 Hz.

FIG. 5 shows the test results. FIG. 5 shows that the fatigue crack initiation life when the treatment shown in FIG.
It shows the fatigue crack initiation life ratio when there is a treatment. By the heating / cooling treatment, the fatigue life was about twice as long, and the effect was recognized.

Next, as another embodiment of the present invention, after the shot peening process is performed on the first layer side of the lower surface of the welded portion, the flame of the gas burner 2 is squeezed to reduce the flame of the gas burner 2 to about 850. After heating at a heating temperature of not less than 10 ° C. for about 10 minutes, the same test was performed by forcibly air-cooling with room temperature air injection or the like and cooling to room temperature, and the fatigue life was reduced by the forced processing / heating / cooling treatment. The service life was about twice as long, and the same effect was observed.

[0015]

As described above, according to the present invention, the amount of ferrite in the surface layer is reduced by locally heating / cooling the surface layer of the weld to a recrystallization temperature of about 1000 ° C. or higher. As a result, the resistance to the occurrence of fatigue in high-temperature water increases, and as a result, the fatigue life of the welded portion is improved.
Further, the heating temperature can be reduced by performing shot peening or forcible processing on the surface layer in advance as described in claim 2, and safer and more effective treatment can be performed.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a surface heating method according to an embodiment of the present invention.

FIG. 2 is a graph showing the effect of the amount of ferrite on fatigue life.

FIG. 3 is a graph showing the number of stress repetitions and the crack length.

FIG. 4 is a diagram showing dimensions of a test piece shape used for confirming effects.

FIG. 5 is a graph showing the results of confirming the effect of the fatigue crack initiation life of a conventional technique without heating / cooling treatment and the present invention.

[Explanation of symbols]

 1 butt weld 2 gas burner

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/00 302 C22C 38/00 302Z // B23K 103: 04

Claims (2)

[Claims] 1. A welding metal surface portion on a side in contact with a high-temperature fluid is heated to a temperature at which recrystallization is possible or more, and then cooled at a cooling rate equal to or higher than natural cooling at room temperature, and the vicinity of the surface of the welding portion is cooled. Austenitic stainless steel welds characterized by reducing the amount of ferrite and improving fatigue strength. 2. After forming a working layer on the surface of the weld metal in contact with the high-temperature fluid, the surface is heated to a temperature at which recrystallization is possible, and after the heating, natural cooling at room temperature or higher is performed. Austenitic stainless steel welds characterized by cooling at a cooling rate of less, reducing the amount of ferrite near the weld surface and improving fatigue strength.