JPS6087992A - Build-up weld metal improving resistance to crystal boundary corrosion - Google Patents

Abstract

PURPOSE:To improve the resistance to crystal boundary corrosion by forming the component range of a weld metal of a SUS347 stainless steel which is a low alloy steel and limiting the contents of C, N and ferrite and Nb/C value. CONSTITUTION:The chemical components of a build up weld metal consisting of an SUS347 stainless steel are made to consist of about <=0.04% C, 0.5-0.9% Nb, 10-20Fn, ferrite and 13-26Nb/C value. The other chemical components may be of the component ratios to be resulted eventually by the SUS347 deposited metal. The weld metal having extremely high resistance to crystal boundary corrosion is obtd. by executing this invention and wide execution of the invention including a pressure vessel for an atomic reactor with which welding conditions are extremely severe is possible.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to overlay weld metal, and particularly to a weld metal that can significantly reduce intergranular corrosion when welding low alloy steel.

For example, to improve corrosion resistance, austenitic thread-based stainless steel is overlay welded to the inner surface of a pressure vessel used in a light water nuclear power reactor. Most of the build-up welding is carried out using a strip electrode build-up welding method, and a weld metal having a component of 5US308 thread is obtained as the weld metal. On the other hand, the mother that forms the pressure vessel body Wtj, I'
In A, since low alloy steel such as Mn-Ni-Mo steel is used, heat treatment is performed at about 620°C after welding.

However, this heat treatment reduces the O
Intergranular precipitation of r-carbides occurs, resulting in sensitization and deterioration of intergranular corrosion resistance. In order to improve this intergranular corrosion resistance, it is effective to lower the ail in the weld metal, and it is effective to use a metal with a low C content as the weld metal. However, during welding, the components of the welding material are diluted due to penetration of the base metal into the deposited metal, and this is a phenomenon that is affected by the components of the base ion.
There is a limit to the effectiveness of low carbon weld metal. Therefore, it has been confirmed that the weld metal is subject to intergranular corrosion even when using low C level stainless steel 2, such as 5US309 stainless steel shown in the table below, which is currently industrially acceptable.

Table 1 Meanwhile, other stainless steels shown in this table, 5US34
'7 thread metal is often used in chemical plants, and is known to have higher grain boundary + (is) corrosion resistance than the aforementioned 308.309 series stainless steel.However, it contains δ-ferrite. Since weld metal is easily embrittled by heat treatment, its use is limited to areas with a small amount of ferrite.

In Figure 1, the base 4 is made of 347 series stainless steel.
A weld metal 1 having penetration depth 3 was formed on the second weld metal, and a sulfuric acid/copper sulfate corrosion test was conducted on this weld metal.
Intergranular corrosion of about 0.5 mm occurred, and the intergranular corrosion resistance required for a nuclear reactor IE power container could not be met.

This invention was made in view of the above-mentioned problems, and provides an overlay weld metal with high intergranular corrosion resistance by limiting the ratio of predetermined components in the overlay weld metal to respective ranges. There is a particular thing.

In short, in this invention, the composition range of the weld metal is 347 series stainless steel, which is a low alloy steel, and in particular, Of! ＋”b
'It + Ferrite 7, the grain boundary corrosion resistance is improved by limiting the value of nψ to a certain range.

Examples of the present invention will be described below.

First, the inventors used submerged arc welding (SAW) and electroslag welding (GSW) to weld 347 series weld metal to the base metal, low alloy steel, by varying the Nb content. Overlay welding was performed, and intergranular corrosion tests were conducted on each weld metal, and tests were conducted on the effect of Nb on intergranular corrosion in each welding method. The opening in the figure is SAW, and ○ is E.
SW is shown, and the numbers inside and outside the box indicate the depth of intergranular corrosion (μm/h).

First, submerged arc welding involves inserting a bare electrode wire into granular flux and welding using the arc heat generated between it and the base metal, and the other involves continuously feeding a wire into molten slag to reduce the current flowing through the slag. Comparing electroslag welding, which uses resistance heat to weld wire and steel, submerged arc welding, which uses arc heat, penetrates the molten metal deeper into the base metal. In contrast, the penetration of electroslag welding, which does not generate an arc, is considerably shallower. For this reason, the dilution ratio in the weld metal is greater in submerged 7-1 welding with deep penetration, and as is clear from Figure 2, the weld metal produced by submerged arc welding has a higher overall c. It shows. Also, no matter which method welds, the higher the Nb content, the less intergranular corrosion will occur, and if it contains 1.0% or more of Nb, almost no intergranular corrosion will occur regardless of the welding method. I was able to confirm that there was no such thing. However, when looking at the relationship between the amount of C contained and the amount of N contained,
There is a clear correlation between the amount of b added, and the content of cl
If the amount of Nb is small, intergranular corrosion can be prevented with a small amount of Nb addition.In the case of electroslag welding, where the C content can be reduced to 0.35% or less, the Nb content should be 0.2 to 0.4%.
The purpose can be achieved by setting it as %.

Next, regarding one company that added Nb, if the nail content exceeded 0.9%, the weld metal would become brittle during heat treatment. Therefore, Nb
It is necessary to prevent intergranular corrosion at an addition chamber of 0.9% or less.
For this purpose, the C content of the weld metal needs to be 0.04.0% or less. However, as is clear from the figure, C
Even in the weld metal based on the electroslag welding method where the Nb content is 0.04% or less, the Nb addition 1i is 0.5%.
It was discovered that intergranular corrosion occurred even though it was set as %. Therefore, simply determining Nb H (#Oljl:) is not necessarily sufficient to obtain high intergranular corrosion resistance, and other factors must also be considered.

Figure 3 shows the relationship between f, fl & of Nb and C mentioned above, as well as ferrite in the weld metal, which is said to be closely related to intergranular corrosion resistance. The results of testing the relationship are shown. That is, the relationship between the value Nb/C of the ratio of Nb and C and the amount of ferrite FN (1-FNJ is an abbreviation of Ferrite Number indicating ferrite i) is shown.

Further, welding was performed by electroslag welding, which keeps the C content low. In this case, the only area in which intergranular corrosion did not occur is within range A in the same figure, specifically N1)
10 value is between about 13 and about 26, the amount of ferrite is about 11
20 or less. Regarding the FN value, structural embrittlement is observed at 20 or higher, and significant embrittlement occurs when the weld metal cracks during bending tests at 25 or higher.

It was accepted. On the other hand, embrittlement was confirmed when the Nb/C value was 26 or higher.

From the above points, weld metal with high intergranular corrosion resistance is 5US3.
47 series stainless steel and has a C content of approximately 0.04.
% or less, Nb amount approximately 0.5% or less, 10.9% or less, ferrite amount approximately 11FN or more and 20FN or less, NbZC value approximately 1
It can be seen that it is sufficient to set the value to 3 or more and 26 or less (within the frame A in FIG. 3). Note that this component range can be applied not only to tit-overlay welding, but also to welding stainless steel with a low C content.

The inventors prepared a predetermined weld metal based on the invention described below, and conducted intergranular corrosion tests on this weld metal. The specific details of the test are as follows.

(1) Welding material (a) Weld metal 5US347 (b) Flux OaF2-Al 203-Mg0 system sintered type (2) Welding conditions (i) 250OA, 25V, welding speed 14cm/m
1n(b)UJ=Ishi(A 533) (3) Heat treatment conditions Weld metals shown in Table 2 below were obtained under the conditions of J2(-c) to (3).

Table 24 7= Intergranular corrosion tests were conducted on the above weld metals, but intergranular corrosion did not occur, and no cracking occurred at all in the side bending test.

In addition, the overlay welding metals are Cbare, Nbfij, Huerai)
For rats, good results can be obtained by keeping all the predetermined values for the Nb10 value, and for other chemical components, 5U
Naturally, the component ratio may be such that the S347 type weld metal has a 1'σ concentration. That is, specifically Sl:
Approximately 1.0% or less, Mn: approximately 2.5% or less, P: approximately 0.0
4% or less, S: about 0.03% or less, Nj, 2 about 9+0%
Cr: approximately 18.0% or more and 20% or less.

By carrying out this invention, a weld metal with extremely high intergranular corrosion resistance can be obtained, and it can be applied to a wide range of applications including nuclear reactor pressure vessels where welding conditions are severe.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of a test piece consisting of weld metal and base metal subjected to an intergranular corrosion test.
FIG. 3 is a distribution diagram showing the relationship between ferrite (pN) in the weld metal and the Nb//C value. 1...Weld metal 2...1U separation 3...Penetration

Claims (1)

[Claims] 1. 5US347 series stainless steel overlay (chemical composition of X metal: 0: about 0.04% or less, Nb: 0.5% to 0.9%, ferrite amount: 10FN to 20FN. Nb/C value: 13 to 2. An overlay weld metal with improved intergranular corrosion resistance, characterized in that the cladding weld metal has increased intergranular corrosion resistance. 2. Sl: about 1.2 for chemical components other than the above chemical components.
0% or less, Mn: about 2.5% or more F, P: about 0.04% or less, S: about 0.03% or less, Ni: about 9.0% or more about 1
1.0% or less, Cr: about 1860% or more and about 21.0% or less. 3. The build-up weld metal with improved intergranular corrosion resistance according to claim 1 or 2, wherein the build-up weld metal is formed using low alloy steel as a base material. 4. The overlay weld metal with improved intergranular corrosion resistance according to claim 3, wherein the overlay welding is performed by electroslag welding.