JPH02258190A - Method for reforming welded part of austenitic stainless steel - Google Patents

Abstract

PURPOSE:To obtain a stainless steel welded joint having long service life by executing fusing treatment at one side face of thermally affected range in the welding joint of austenitic stainless steel while cooling from the other side face and changing the structure of the range into crystal structure containing delta ferrite. CONSTITUTION:A stainless steel pipe 5a is penetrated through a carbon steel- made thick plate 5b and fixed with fillet weld 6. The steel pipe 5a is made of the austenitic stainless steel. In the thermally affected range 7 of the fillet welding 6 part in the welding joint, the fusing treatment is executed with non- filler TIG weld to one face side while cooling from the other face side. Then, the structure of the part is changed into the crystal structure containing the delta ferrite. By this method, the stainless steel welded joint having high reliability can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for modifying welds of austenite-based stainless steels, and particularly relates to solution heat treatment (stress corrosion cracking in welds of stainless steels).
ion cracking))
The present invention relates to a method for modifying an existing welded joint portion of an apparatus structure, in which melting treatment in place of solution heat treatment can be performed only from one side of the member. Equipment parts used in high-temperature, high-pressure water containing dissolved oxygen, especially B
Equipment components for WR plants are required to have high corrosion resistance in order to ensure reliability and extend the life of the components. The present invention is most suitable for technical fields that address this problem.

[Conventional technology]

boiling-water reactor
intergranular stress corrosion cracking (hereinafter referred to as SCC) occurs in the weld heat-affected zone of the welded joint of the primary cooling water piping (made of JI 5SUS304 material) in the (BWR) plant. There is a direction. As shown in FIG. 3, SCC has a tensile stress of 1. It occurs in a region 4 where a chromium-deficient layer 2 (sensitized region) that occurs along grain boundaries due to the influence of welding heat and a corrosive environment 3 such as dissolved oxygen overlap.

JISSUS3 with conventional natural cooling
When welding 04 pipes, as shown in Fig. 2, welding (see welding part 6) causes a loss of several tens of kg/n on the inner and outer surfaces of the pipe.
A high tensile residual LE, : force (see characteristic 8 in FIG. 2) of up to 2 is generated. The TS side in FIG. 2 is a tensile stress region, and the CS side is a compressive residual stress region. straight line 9
indicates 10 kg/+1wn 2 lehels. A chromium-11-deficient layer is formed in the heat-affected zone 7 near the welded portion of the base metal 5a. In this way, when a corrosive fluid comes into contact with both the inner and outer surfaces of the base material, where high tensile residual stress and chromium-deficient layers have occurred,
The risk of SCC occurring in the heat-affected zone of the base material increases.

As one of the known SCC countermeasures, the Special Publication No. 59217]1
The direction θ described in the publication is correct. The method described in this publication involves applying a delta to the surface in contact with corrosive fluid near the weld joint of multiple stainless steel members to be welded together.
(δ) A corrosion-resistant material containing Ferray 1 to 1 is built up, and then heat is input 5 to the surface in contact with the corrosive fluid at the toe of the internal buildup.
Melting treatment is applied to K J / c1n or less, and then 1
) How to weld the welded joints of stainless steel members θ (The purpose of this melting treatment is to eliminate the chromium deficiency J? that forms in the heat-affected zone of the base by overlaying the corrosion-resistant material. At the same time, a structure with excellent corrosion resistance containing delta (δ) ferrite h k is generated.
The method described in Japanese Patent Publication No. 3-561.34 is similar to the force a described in Japanese Patent Publication No. 59-21711 in that the method described in Japanese Patent Publication No. 59-21711 is performed in that melting treatment is applied to the vicinity of the welded joint of stainless steel materials prior to welding. are doing. Sunao] also has this force θ,
Prior to welding of austenitic stainless steel members, the surface layer at certain points affected by the heat of welding is
This is a method of melting by applying thermal energy (by arc, plasma, etc.).

In this method, delta (δ) ferrite is produced when the molten part from the melting process solidifies. Furthermore, even if the structure obtained by this method is affected by heat during welding, it does not become sensitized due to carbide precipitation at grain boundaries, as in the case of overlaying.

Another measure against SCC is Japanese Patent Publication No. 60-453033.
This is the method described in ncd Publ, 1 catjon). The method described in the publication includes, prior to welding,
A corrosion-resistant material containing delta (δ) ferrite is deposited on the surface of the welding part that comes into contact with the corrosive fluid, and then, without cooling the deposited side, the deposit is deposited on the opposite side of the deposited toe. This is a method of applying

The purpose of overlaying while cooling is to improve the residual stress in the chromium-deficient layer region created in the heat-affected zone of the base metal by the initial (or primary) overlay of the corrosion-resistant material.

[Problem to be solved by the invention]

In the known method, only the surface on which SCC countermeasures have been applied is subject to SCC.
It is effective against the other side, and no consideration is given to the opposite side. Therefore, when both the front and back sides of a welded joint come into contact with corrosive fluids, SCC is applied to both sides of the joint.
Measures must be taken. This indicates that the conventional technology cannot effectively deal with cases where both the front and back sides of a welded joint come into contact with corrosive fluids, and where melting treatment can only be applied to one side of an existing welded joint. It means.

On the other hand, in order to prevent SCC occurring in the heat-affected zone of a welded joint of Austety 1-stainless steel, there is a method in which thermal energy is applied to the heat-affected areas of the base material to melt them prior to welding, as disclosed in Japanese Patent Application Laid-Open No. 53-1981. It is disclosed in Japanese Patent Application Laid-open No. 56134 (already mentioned) and Japanese Patent Application Laid-Open No. 177972/1983 (the latter is described as a method for cold-worked materials in general). These methods are not intended for existing welded joints.

The present invention was created against this technical background.

The main purpose of the present invention is to eliminate the chromium-depleted layer and high upper tensile residual stress that occur in the weld heat affected zone of austenitic stainless steel, and to eliminate the chromium-deficient layer on one side of the welded joint, as well as to eliminate the delta (δ) ferrite. The purpose of the present invention is to obtain a welded joint of austenitic stainless steel with excellent corrosion resistance by changing the structure to a structure containing excellent corrosion resistance and reducing the residual stress in the chromium-deficient layer region on the other side of the welded joint. This objective also includes providing a method d that can suppress the occurrence of SCC when maintaining or repairing the austenitic stainless steel piping of the existing plan 1-(equipment).

[Means to solve the problem]

The first means to achieve the above goal is to weld a structure in which a fluid comes into contact with corrosion on both the front and back surfaces of the heat-affected zone after Austety 1~ series stainless steel is affected by welding heat. Part: The heat-affected area of the hand is cooled from one side of the joint, while the other side is melt-treated, and the melt-treated part is made of a highly corrosion-resistant material containing delta (δ) ferrite.
+, '+) This is a method for modifying a welded part of austenitic stainless steel characterized by changing the weld part to a second stage, and the second means is to modify the welded part of an austenitic stainless steel by changing it to Another method is to modify the welded part of austenitic stainless steel by applying high-density energy.

[Effect]

According to the first method, after the austenitic stainless steel is affected by welding heat, the heat affected zone of the welded joint is changed to one side of the joint for a structure in which a corrosive fluid comes into contact with both the front and back surfaces of the heat affected zone. While cooling from the side, melting treatment is applied to the other side, and due to the melting and solidification treatment method, the chromium carbide that had formed in the welding heat affected zone becomes ferrite and Austenai J.
A structure divided into two parts, namely delta (δ)
- By aging, it becomes a structure with excellent corrosion resistance.

On the other hand, on the one side, the high tensile stress (7
(residual thermal stress due to 8-junction) is improved to below +10 kg/view, and as a result, the occurrence of SCC is prevented. What is the reason for the improvement in residual thermal stress? 8. Cooling during the melting and solidification process creates a large temperature difference on both sides of the joint, similar to water-cooled welding, and improves residual thermal stress on the cooling side.

According to a second means, the melting treatment in the first means comprises:
It is carried out at every f118i by arc discharge (e.g., injection (or irradiation!) of high-density energy of Z% '4) to the treatment area
ij, it is possible to increase the temperature drop between the injection site and the surrounding area. By doing so, the cooling after the injection is done quickly due to the influence of the surrounding low-humidity canal, and the effect that chromium carbides are less likely to be generated can be obtained.

〔Example〕

In the example, a chromium-depleted layer produced by welding austenitic stainless steel members was irradiated (or irradiated) with high-density energy such as arc or laser beam 11.
It is characterized as a treatment method in which chromium carbide in the weld heat-affected zone is decomposed and dissolved into ferrite 1 and austenite by melting and solidifying it. This treatment produces delta (δ) ferrite in the treated surface layer, resulting in a structure with excellent corrosion resistance. If the opposite side is forcibly cooled with water or the like during the melting process, the heat input required for the melting process is 1 to 30 KJ/cl11, and the opposite side can be cooled naturally without being forcedly cooled. When relying on
As stated in the publication, it is 5 KJ/cm or less. If the heat input is within this range, the formation of a chromium-depleted layer at the toe of the melting process due to thermal effects will be suppressed.

From another point of view, the embodiment reduces the high tensile stress in the chromium-depleted zone region caused by welding austenitic stainless steel members by cooling one side of the weld joint and melting the other side in the heat input range. It is characterized as a method that improves the cooling side to +10 kg/c +& or less by applying this.

By the way, austenitic stainless steel is 18Cr
It is well known that -8Ni steel is a typical example. Austenite 1 stainless steel generally has better corrosion resistance than Cr stainless steel, and also has good high temperature strength and low temperature toughness, as well as excellent weldability, so it has an extremely wide range of uses.

Examples of the present invention will be described in more detail below.

FIG. 2 shows an outline of the test specimen of this example. Thickness 100
No carbon steel thick plate 5b, outer diameter 50n+n+, inner diameter 38
A structure is shown in which a stainless steel tube 5a of mn is penetrated and fixed by fillet welding 6.

Table 1 shows the chemical composition of the stainless steel pipe used in this example.

Table 1 Fillet welding conditions are shown in Table 2.

The depleted layer region 7 of the second table is welded by non-filler TIG welding.

The non-filler TIG welding performed by melting treatment was carried out under conditions of 1 to 30 KJ/an in the water-cooled region 11 on the outer surface of the tube, and 1 to 5 KJ/an in the region 12 relying on natural cooling. An example of construction conditions is shown in Table 3.

Table 3 This welding produces a chromium-deficient layer 7 and high tensile residual stress on the inner and outer surfaces near the weld. An example of residual stress on the outer surface of the tube is shown in the second example.
As shown in the figure. In this way, high tensile stress occurs in the chromium-depleted layer region, and if a corrosive fluid comes into contact with this region, SCC
There is a possibility of occurrence.

After fillet welding, as shown in Fig. 1, while cooling the outer surface of the tube with water (see water cooling area 10), weld the inner surface of the tube with chromium non-filler TI.
The region including the toe that has been welded by G welding has a structure with excellent corrosion resistance in which the chromium-depleted layer has disappeared and δ ferrite 1 has been formed, and exhibits excellent corrosion resistance.

Figure 1 shows the state of residual stress on the outer surface of the tube. As shown in this figure, the residual tower force on the outer surface 13 of the tube in contact with the corrosive fluid has been improved to the compressive stress side, and the stress is 10 kg/1, which does not cause SCC.
It becomes less than nII+2.

The above can be summarized as shown in Table 4.

In this example, if we consider 5b to be the lower head plate of the reactor pressure vessel of a BWR type nuclear reactor and 5a to be the in-core monitor (usually ICM) housing part of the BWR type nuclear reactor, the present invention will prevent deterioration due to the heat effect of fillet welding. It is possible to repair the ICM housing with good corrosion resistance without replacing the stainless steel ICM housing.

In addition, the work can be done without draining the reactor water from the reactor pressure vessel, which saves the trouble of draining the reactor water.The radiation shielding effect of the reactor water reduces the amount of radiation that workers are exposed to, reducing exposure and increasing the Work becomes possible.

Table 4 According to this example, it is possible to improve the corrosion resistance of both the front and back surfaces of a stainless steel welded joint in high-temperature, high-pressure water containing dissolved oxygen. Even in cases where melt processing can only be performed from one side of the joint, a highly reliable and long-lasting stainless steel welded joint is obtained.

In particular, the present invention is effective in maintaining and repairing stainless steel piping in existing plants (in cases where high temperature, high pressure water containing dissolved oxygen comes into contact with both the front and back of welded joints).
It can be used for atomic couplant equipment and its component parts used in SCC generating environments.

As is clear from the above explanation, in cases where the corrosion resistance of both the front and back surfaces of a stainless steel welded joint is achieved, both the front and back surfaces of the stainless steel welded joint are in contact with high temperature, high pressure water containing dissolved oxygen, and the melting treatment is applied to the joint. Even under working conditions, stainless steel welded joints are obtained that are highly reliable and have a long service life.

〔Effect of the invention〕

According to the invention of claim 1, the corrosion resistance of both the front and back surfaces of a stainless steel welded joint can be improved, and when both the front and back surfaces of the stainless steel welded joint are in contact with high temperature, high pressure water containing dissolved oxygen, and the melting treatment is performed on the joint. If it can only be done from one side, the effect is that a stainless steel welded joint can be obtained that is highly reliable and has a long service life even under working conditions.

According to the invention of claim 2, in addition to the effect of the invention of claim 1, the heat input to the improvement section is less likely to be dispersed to the surrounding area, so that partial temperature raising work can be performed efficiently. The subsequent cooling process is also carried out more efficiently than when the temperature of the surrounding area is still rising, and the effect is that a stainless steel welded joint with a long service life can be obtained efficiently.

[Brief explanation of drawings]

FIG. 1 shows a cross-sectional structure of a welded pipe joint that has been subjected to the welded joint modification treatment according to the present invention, and a graph of an improved residual stress characteristic curve of the outer surface of the pipe shown corresponding to the welded joint. FIG. 2 shows a cross-sectional structure of a pipe welded joint that has not been subjected to the welded joint modification treatment according to the present invention after welding, and also shows a diagram corresponding to the welded joint. Figure 3 is a conceptual representation of the cause of SCC occurrence;
FIG. 4 is a diagram showing the cross-sectional structure of a welded joint by ordinary natural cooling welding in relation to a graph of a residual stress characteristic curve on the outer surface of the pipe shown corresponding to the welded joint. 1... High tensile stress, 2... Chromium-deficient layer, 3 Corrosive environment, 4-S CC occurrence area, 5a Stainless steel pipe, 5
B ・Carbon steel, 6 Welding, 7 Heat affected zone, 8 ・Residual stress, 9-+ 10 kg/Inm2.10 ・Water cooling area, 11...Residual stress improvement area by water cooler melting treatment, 12
・Structural improvement area by melting treatment, 13・Water cooler 8 Figure 2 Figure Figure

Claims (1)

[Claims] 1. After austenitic stainless steel is affected by welding heat, the heat affected area of a welded joint in a structure where corrosive fluid comes into contact with both the front and back sides of the heat affected zone is defined as one side of the joint. A method for modifying a welded part of austenitic stainless steel, which is characterized by performing melting treatment on the other side while cooling from one side, and changing the melted part to a crystal structure with excellent corrosion resistance that includes delta (δ) ferrite. 2. Claim 1, wherein the heat of fusion for performing the melting process is obtained by inputting high-density energy.
A method for modifying a welded part of austenitic stainless steel described in .