JPS6055226B2 - How to weld austenitic stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for welding stainless steel piping for nuclear reactors and other austenitic stainless steels used in corrosive environments.

When austenitic stainless steel is welded, charcoal precipitates at the austenite grain boundaries in the range heated to 550 to 850° C. in the heat affected zone, making it susceptible to corrosion.

If this part is used in a corrosive environment, intergranular corrosion will occur, and if tensile residual stress is present, stress corrosion cracking will occur at the grain boundaries. In BWR nuclear reactors, stress corrosion cracking occurs most often in the heat affected zone of the primary stainless steel piping, and countermeasures are being taken. Measures to prevent stress corrosion cracking include limiting the welding heat input to 20000JIC771 or less, welding the heat-affected zone with a weld metal with excellent corrosion resistance in advance by overlaying the inner surface of the tube, or applying water cooling from the inner surface. The welding method is applied. However, both methods require significantly more work than normal welding, reducing work efficiency. When butt welding stainless steel pipes to reduce the sensitization of the stainless steel heat-affected zone, the first layer is welded to seal the groove root, and subsequent welding is performed on the inner surface of the welded pipe as described above. The method of preventing stress corrosion cracking by welding while cooling by flowing water has the following drawbacks. In other words, it requires a water storage tank, a water pump, a fountain nozzle, etc. to supply cooling water, and it takes time to prepare and clean up afterwards, and when the distance from the pipe end to the welding point is long, such as in on-site welding. Water cooling the inner surface of the welded part is quite difficult. Furthermore, since the first layer cannot be cooled with water to prevent direct contact between the weld metal and the cooling water, it is insufficient to prevent the structure of the heat affected zone from becoming sensitized. An object of the present invention is to provide a welding method for austenitic stainless steel that can reduce the sensitized region of the heat-affected zone without performing cooling, which requires a large number of man-hours.

The present invention sets the welding heat input of the heat of the part directly in contact with the base metal or its groove to be 8000 JICrfl or less,
The other heated parts are welded with the normal welding heat input.

An embodiment of the welding method for austenitic stainless steel according to the present invention will be explained below in comparison with a conventional method.
Applying the fact that the degree of carbide precipitation region (sensitized region) in the heat-affected zone is approximately proportional to the welding heat input for base metals of the same size and shape, welding heat input adjacent to the groove is This is a method of reducing the sensitized region of the base metal to the same level as water-cooled welding by keeping it to the minimum possible value.

As a specific example of the effect of limiting heat input, a 6-thickness SUS3
The results of the bead 2 (D3O8) placement test with the O4 base material 1 open are shown in Figure 1A (for the method of the present invention) and Figure 1 (for the conventional method). A is a dwarf heat bead 2 with a small diameter welding rod of 5000J1d, and the mouth is 14000JIcr! l
This is the case of the normal human heat bead 4. That is, SUS3O4
In covered arc welding, from the viewpoint of work efficiency, workability, arc stability, customs, etc., 10,000 to 20,000 JIcrft is required for normal stainless steel welding work.
A certain amount of heat input is used. However, 5000JI
When welding with minimal human heat of about cTn, 14000J of mouth
Compared to the case of Icrf1, when the bead amount is the same, the base material sensitized region 3 in the bead cross section is significantly smaller as shown in FIG. Note that 5 is the base material solution region. FIG. 2 shows the procedure for heat lamination during butt welding of grooves by the welding method of the present invention in comparison with the conventional method.

In the butt welding according to the present invention, the sensitized region caused by welding the normal human heat bead 4 adjacent to the small heat bead 2 which is not in direct contact with the groove surface is generated within the small heat bead 2;
In this case, the stainless steel weld metal has a δ of about 5 to 15%.
Since it contains ferrite, there is no fear that the austenite grain boundaries of the weld metal will become sensitized even if subjected to a sensitization thermal cycle.
In addition, by placing the dwarf heat bead 2, the groove width becomes narrower than in the conventional method as shown in the figure, and can be made narrower than the bead width normally produced by human heat, making it possible to perform low heat input welding.
In Fig. 2, the base material sensitized area 3 of A by the method of the present invention is also shown.
is significantly smaller than the oral matrix sensitization zone 3 according to the conventional method. The material of the base material 1 is austenitic stainless steel. Figure 3 shows SUS3O4, 6B (6 inch outer diameter) pipe,
An embodiment of the austenitic steel welding method of the present invention and a conventional method in the case of butt welding a schedule 80 (thickness 11?) stainless steel pipe are shown under the welding conditions shown in FIG.

After welding, the precipitated region of intergranular carbides in the heat-affected zone of the bead cross section, that is, the sensitized region, was immersed in a 70°C aqueous solution of 10% nitric acid + 3% hydrofluoric acid for 2 hours to corrode it. As a result of comparison, in the case of the embodiment of the method of the present invention shown in A, the maximum thickness of the layer in the sensitized region 3 was 2.5 and 2.7? In contrast, the conventional method shown in the mouth is 9.3 and 9.1.
Tfn, which could be reduced to about 113 or less. In addition, the welding conditions in this case are as shown in the table, after first bus 1, 2, and 3 are welded with tungsten arc argon gas coating, the butt parts are fixed, and then the welding conditions are in the order indicated by the numbers in parentheses in Table 1. The test was carried out under the conditions shown in FIG. The dwarf heat bead shown in the welding conditions in Table 1 is 5300JIcm,
As a result of the experiment, if it is less than 8000J1d, the sensitized region of the heat affected zone is sufficiently smaller than that of the conventional method, and the plate thickness is 127n! In n, the maximum sensitization width can be kept to 3 frames or less.
In addition, 6 is a weld metal. As mentioned above, when welding austenitic stainless steel, in order to significantly reduce the sensitization of the heat-affected zone without using forced cooling such as water cooling, the bead in contact with the base metal is welded using a small diameter welding rod. This method uses normal welding heat input to weld the center part of the groove.

Then, the bead of the part adjacent to the groove is 5000~60.
By welding with dwarf heat of 00 JId, the sensitization width of the heat affected zone of the base metal could be significantly reduced, and the sensitization width could be reduced to 113 or less with a width vibe (schedule 80). In particular, when the wall thickness is thin, the effect of reducing heat input becomes even greater, so the effect of reducing sensitization further increases with small diameter and thin-walled pipes. In addition, in the case of conventional water cooling in pipes, the occurrence of welding defects due to dew point adhesion at the groove due to supercooling is eliminated, and it is possible to achieve the same effect as the water cooling method simply by devising and improving the welding method. No equipment is required, and the number of man-hours can be significantly reduced. As described above, according to the austenitic stainless steel welding method of the present invention, the sensitized region of the heat affected zone can be reduced without water cooling, which requires a large number of man-hours, and the number of man-hours is significantly reduced. This has the effect of reducing the

[Brief explanation of the drawing]

FIG. 1 shows a comparison of the sensitized area caused by the small heat bead at the base metal contact area by the austenitic stainless steel welding method of the present invention and the normal human heat bead at the base metal contact area by the conventional method. A sectional view of a welded area by the conventional method, a sectional view of a welded area by the conventional method, and FIG. Fig. 3 shows a comparison of the sensitized area of the welded part by the welding method of austenitic stainless steel of the present invention and the welded part by the conventional method. A sectional view of a welded part by the welding method of the present invention, the opening is a sectional view of a welded part by the conventional method, FIG. 4 shows the order in which beads are placed when implementing the welding method of FIG. 3, and A is an explanation of the case of the welding method of the present invention. Figure 1 is an explanatory diagram of conventional method 7. 1...Base material, 2...Dwarf heat bead, 3
...Base metal sensitization area, 4...Normal human heat bead.

Claims (1)

[Claims] 1. A method for welding austenitic stainless steel, characterized in that the welding heat input of the bead in the part that is in direct contact with the base metal or its groove is 8000 J/cm or less, and other bead parts are welded with the normal welding heat input. .