JPS6219953B2 - - Google Patents

Description

[Detailed description of the invention] The present invention is applied when austenitic stainless steel pipes used in nuclear power equipment, boiler equipment, ships, etc. are butted against each other and welded on one side. The present invention relates to a multilayer welding method for austenitic stainless steel that can effectively prevent stress corrosion cracking.

When pipes a and a are butted together and welded on one side as shown in Figs. 1 and 2, the amount of shrinkage on the outer surface of the welded part c is generally larger than that on the inner surface of the pipe, so
The tubes a and a are deformed as shown by imaginary lines in Fig. 1,
Tensile stress is generated in the heat affected zone (or sensitized zone) k, k near the weld line j and on both sides of the weld, and in the case of austenitic stainless steel, if this tensile stress is large, stress is generated in the inner surface of the heat affected zone k, k. Corrosion cracking occurs. Therefore, in austenitic stainless steel, it is extremely important to reduce residual stress in welded joints, and various thermal or mechanical treatment methods have been used to reduce this residual stress. .

Figure 3 shows the so-called HSW method (Heat Sink Weld)
During welding work, water is passed through the inner surface of the pipes a to be connected (arrow b), or cooling water is sprayed on the inner surface d of the pipe near the right-turning part c (arrow e), The residual stress is reduced by utilizing the temperature difference between the inside and outside of the welded part c caused by these.

However, this method takes time to prepare for welding,
Another drawback is that it cannot be applied until at least the first layer has been welded.

Figure 4 shows the IHSI method (Induction Heating Stress
After welding, pipe a,
While cooling water is passed through a (arrow b), a high-frequency heating coil f is used to heat the outside of the weld c, and the resulting temperature difference between the inner and outer surfaces of the weld c is used to reduce residual stress. Reduce. However, this method has the drawbacks of high equipment costs and the need for advanced technology for construction.

In view of the above-mentioned drawbacks, the present invention aims to simplify and facilitate welding work by reducing the residual stress upon completion of welding by utilizing changes in the physical properties of the weld metal as it cools and solidifies. This was done to prevent the occurrence of stress corrosion cracking, and the method is as follows:
Pipe material made of austenitic stainless steel,
When welding one side of plates etc. against each other,
The layer closest to the deepest part of the groove is welded using an austenitic filler material, and at least one layer on the outside adjacent to the layer is welded using a martensitic filler material.

When the weld metal cools and solidifies, the amount of contraction of the martensitic weld metal outside this layer is smaller than the amount of contraction of the austenitic weld metal in the layer near the deepest part of the groove, so the interaction between both weld metals is Therefore, the amount of shrinkage is offset and averaged, and the residual stress on the inner surface of the heat affected zone is reduced.

Embodiments of the present invention will be described below with reference to the drawings. Figures 5 to 7 show one embodiment of the present invention. In the figures, numerals 1 and 1 are austenitic stainless steel pipes to be connected, 2 is a welded part, and filler metal in the first layer 3. An austenitic filler rod is used as the filler material for the second layer 4 and the third layer 5, a martensitic filler rod is used as the filler material for the fourth layer 6, and an austenitic filler rod is used as the filler material for the fourth layer 6.

Next, the effects of cooling and solidifying each weld metal will be explained with reference to FIG. 7. Figure 7 shows the shrinkage curve l of austenitic weld metal (the curve passing through point ABDC) and the shrinkage curve m of martensitic weld metal.
(curve passing through point ABDE), and the amount of shrinkage of austenitic weld metal (hereinafter referred to as austenitic) increases monotonically along curve l as the temperature decreases from state A at a certain temperature. (The expansion coefficient is always positive).

On the other hand, martensitic weld metal (hereinafter referred to as martensitic) shrinks in almost the same way as austenitic from state A to 450°C, but about 450°C.
martensitic transformation occurs in the temperature range between ℃ and approximately 150℃, and the amount of shrinkage decreases significantly (expansion coefficient is B-
becomes negative between D). Then, in the temperature range below 150°C, the amount of shrinkage increases again (see between D and E). As shown in the above temperature range of 150°C to 450°C, two types of weld metals with different shrinkage characteristics are adjacent to each other in a molten state (for example, if the second layer is placed outside the first layer, the first layer (The layer partially melts to create this state), and as it cools down to approximately 800°C, the difference in the amount of shrinkage causes mechanical interaction between the weld metals, and the amount of shrinkage cancels out. is averaged and the residual stress is reduced. The degree of the above-mentioned averaging increases as austenitic and martensitic systems are stacked alternately, but the combination of austenitic and martensitic systems near the deepest part of the groove reduces the residual stress on the inner surface of the heat affected zone. play a dominant role.

Figure 6 shows the method of the present invention (see Figure 5) and the conventional method (using only austenitic filler rods).
The distribution of residual stress on the inner surface 7 of the tube when the tubes are butted and welded on one side using a solid line g and a broken line h are shown, respectively. In addition, the horizontal axis of the figure shows the distance measured from the welding line o as a starting point along each tube inner surface 7, 7 in a direction perpendicular to the welding line o, and the vertical axis shows the tensile and compressive stress on the tube inner surface 7. As shown in the figure, the tensile stress near the weld line o and in the heat-affected zones 9, 9 is significantly reduced compared to conventional welded joints, and the occurrence of stress corrosion cracking in the heat-affected zones 9, 9 is prevented. Note that 10 indicates a weld bead.

In the above explanation, it was stated that an austenitic filler metal is used in the first layer and a martensitic filler metal is used in the second layer.
When welding welding materials (more than ), the first and second layers are austenitic, and the third layer is
A martensitic material is used for the layers, and the difference in shrinkage between the second and third layers is used to reduce residual stress.

Note that the present invention is not limited to the above-described embodiments, and may be applied to butt welded joints between stainless steel plates, for example, and various other modifications may be made without departing from the gist of the present invention. Of course, modifications can be made.

The multi-layer welding method for austenitic stainless steel of the present invention has the above-described configuration, and therefore exhibits the following excellent effects.

(i) Since an austenitic filler metal is used in the layer closest to the deepest part of the groove, and a martensitic filler metal is used in the layer adjacent to the outside of said layer, the amount of shrinkage when the weld metal cools and solidifies is reduced. canceled out, averaged out,
The residual stress on the inner surface of the heat-affected zone of the welded joint can be significantly reduced, and as a result, the occurrence of stress corrosion cracking can be prevented.

(ii) As a result of paragraph (i), there is no need to install expensive equipment for removing residual stress, spend time on preparatory work, or rely on advanced technology as in the past.

[Brief explanation of the drawing]

Figures 1 and 2 are diagrams for explaining the mechanism by which tensile stress is generated in the heat-affected zone of a welded joint. Figure 1 is a cross-sectional view of a pipe, Figure 2 is a cross-sectional view of a welded joint, and Figure 3 Both Fig. 4 and Fig. 4 are pipe cross-sectional views for explaining the conventional residual stress reduction method.
Figures 7 through 7 show examples of this method, Figure 5 is a cross-sectional view of a welded part, Figure 6 is a diagram showing the distribution of residual stress on the inner surface of a tube, and Figure 7 is a shrinkage curve of weld metal. FIG. In the figure, 1 indicates an austenitic stainless steel pipe, and 2 indicates a welded joint.

Claims (1)

[Claims] 1 When welding pipes, plates, etc. made of austenitic stainless steel against each other on one side, the layer closest to the deepest part of the groove is welded using an austenitic filler metal, and the outer layer adjacent to the layer A multilayer welding method for austenitic stainless steel, characterized in that at least one layer of the above is welded using a martensitic filler metal.