JPS6142155B2 - - Google Patents

Description

[Detailed Description of the Invention] Stainless steel pipes are used for piping in plants such as nuclear reactors and chemical machinery, and the pipes are filled with corrosive static or working fluids, and the present invention can be used in such applications. This article relates to a welding joint method for stainless steel pipes.

Piping welding joints include, for example, joints between straight pipes or joints between elbows and straight pipes, and conventionally, grooves were provided at each end of the pipes to be connected, and these were butted together and welded. However, if only butt welding is performed, the heat-affected zone becomes more sensitive due to welding heat, making it more susceptible to stress corrosion cracking.
Welded parts of SUS304 stainless steel pipes are sensitive to corrosion by high-temperature, high-pressure water containing oxygen and chloride ions, and cases of stress corrosion cracking have been reported in nuclear reactor piping. In actual equipment, it is often difficult to perform solution treatment, residual stress removal treatment, or other anticorrosion treatment after welding.

When placed in a corrosive atmosphere containing sulfides under tensile stress, so-called sulfide stress corrosion cracking occurs even if the stress is below the yield point of steel. It is known to cover the surface of a welded part with a metal that is less susceptible to cracking by overlay welding.

An object of the present invention is to provide a welding joint method for stainless steel pipes that can prevent stress corrosion cracking without particularly requiring treatment such as solution treatment or residual stress removal treatment after welding.

Welded parts of SUS304 stainless steel pipes used in nuclear reactor core cooling systems or recirculation systems require special measures to prevent stress corrosion cracking. The occurrence of stress corrosion cracking in welds is largely due to metallurgical sensitization of the weld heat-affected zone, generation of high tensile stress due to weld residual stress, high-temperature, high-pressure water in the pipe, thermal deformation, etc. attempts to prevent stress corrosion cracking by isolating the sensitized portion.

In the present invention, a low carbon austenitic stainless steel, which is a corrosion-resistant metal with lower stress corrosion cracking susceptibility than the steel pipe, is welded to the inner peripheral surface of the connecting part of the stainless steel pipe in advance, and then the welding is performed. The present invention provides a welding joint method for stainless steel pipes in which stress corrosion cracking of the welded part is prevented by solution-treating the raised part and butt-welding the end faces of the pipe in such a state with the corrosion-resistant metal. When performing the overlay welding, the thickness of the inner circumferential surface can be removed and overlay can be applied to the formed space.
By doing so, the fluid in the pipe flows smoothly without any turbulence, which is effective in preventing corrosion cracking.

The present invention provides a welded joint with excellent stress corrosion cracking resistance. That is, after overlaying the end of the inner surface of the tube with low carbon austenitic stainless steel having a carbon content lower than that of the steel pipe,
By solution-treating at least the weld heat-affected zone of the weld, and then butt-welding with low-carbon austenitic stainless steel in the same manner as described above, the effect of butt welding on the overlay weld on the inner surface is small, making it resistant to heat. Excellent stress corrosion cracking resistance. If the weld metal in butt welding has the same carbon content as the base metal, carbon dilution occurs in the overlay weld due to butt welding, resulting in an increase in carbon content and the welding being affected by the thermal effects of butt welding. Phenomena similar to thermal effects may occur and cause stress corrosion cracking. Therefore, it is necessary to use low carbon austenitic stainless steel for both overlay welding and butt welding.

In addition to SUS304, materials used for stainless steel piping include SUS304L, 321, 316, 316L,
309, 310, 347, 329, etc., and the present invention is applicable to these steel pipes.

Example 1 A method of welding SUS304 stainless steel pipes together by the method of the present invention will be described.

The welding of the present invention can be applied not only to straight pipes, but also to joints between container nozzles and pipes, and other pipe weld joints.

A SUS304 stainless steel pipe is preliminarily provided with a built-up portion of a predetermined thickness on the entire inner circumferential surface from a pipe end of the connecting portion to a predetermined distance from the pipe end of the connecting portion. For this built-up portion, low carbon austenitic stainless steel, which is a corrosion-resistant metal such as the aforementioned 304L or 316L, which is less susceptible to stress corrosion cracking than steel pipes, is selected. After this overlay welding is performed, the sensitized portion resulting from the overlay welding is subjected to solution treatment, and then a groove of a predetermined shape is formed.

The pipe ends having the grooves described above are butted against each other and connected by welding with low carbon stainless steel in the same manner as described above to obtain a pipe joint. According to the pipe joint made in this way, the metallurgically sensitized area near the heat-affected zone that occurs when forming the butt weld is covered with a built-up metal that has high stress corrosion resistance. Contact between the target sensitizer and the fluid within the tube is prevented. As a result, it has a sufficient function to prevent stress corrosion cracking.

In this way, since this build-up welding is performed before welding that is assembled into piping, it is possible to perform solution treatment immediately after build-up welding to restore the sensitized part to its original state, which significantly reduces stress corrosion in the welded part. Cracking is prevented.

Example 2 In this example, the surface shape of the toe of the overlay weld is smoothed by cutting or polishing, and then solution treatment is performed.
The rest is the same as in Example 1.

This eliminates unevenness on the surface of the built-up portion and improves the shape discontinuity of the toe, thereby alleviating stress concentration at the built-up toe, and as a result, is more effective in preventing stress corrosion cracking.

Example 3 In this example, after the inner surface of the tube is processed (removed of the thickness) to a thickness required for overlay welding, overlay welding is performed on that part and solution treatment is performed.

The rest is the same as in Example 1.

By this method, the toe of build-up welding in particular becomes completely smooth and no stress concentration occurs. By smoothing the built-up toe, when the fluid in the pipe flows, it can flow smoothly without turbulence.

According to the present invention, by a very simple method,
In addition to overlaying the sensitized part of the pipe joint with a corrosion-resistant metal that is less susceptible to stress corrosion cracking than steel pipe,
Similarly, by butt welding with a corrosion-resistant metal, stress corrosion cracking of the welded portion of stainless steel pipe can be prevented.

Claims (1)

[Claims] 1 In a method for butt welding austenitic stainless steel pipes, the inner peripheral surface of the steel pipe and at least the portion where carbides precipitate under the influence of heat due to the butt welding are preliminarily treated with a low carbon austenitic material that is less susceptible to stress corrosion cracking than the steel pipe. a step of overlay welding with stainless steel, a step of solution treating the steel pipe at least in the overlay welded portion after the overlay welding, and a step of solution treating the steel pipe in the overlay welded portion after the solution treatment A method for welding austenitic stainless steel pipes, the method comprising the step of butt welding carbon austenitic stainless steel pipes.