JPH0445575B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to overlay weld metal, and particularly to an alloy that can significantly reduce intergranular corrosion when welding low alloy steel. For example, austenitic stainless steel is overlay welded to the inner surface of a pressure vessel used in a light water nuclear power reactor to improve corrosion resistance. Most of the overlay welding is performed using the strip electrode overlay welding method, and SUS308 is used as the weld metal.
A weld metal having a composition of the following types has been obtained. On the other hand, the base material constituting the pressure vessel body is Mn−Ni−Mo
Since low alloy steel such as steel is used, it will take approximately
Heat treatment is performed at 620℃. However, this heat treatment causes intergranular precipitation of Cr carbides in the weld metal, resulting in sensitization and deterioration of intergranular corrosion resistance. In order to improve this intergranular corrosion resistance, it is effective to reduce the amount of C 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 base metal melts into the weld metal and the components of the welding material are diluted, resulting in a phenomenon that is influenced by the components of the base metal. There are limits to its effectiveness. Therefore, it has been confirmed that even when using industrially available low C level stainless steel, such as SUS309 stainless steel shown in the table below, the weld metal is subject to intergranular corrosion.

[Table] On the other hand, other stainless steels shown in this table,
SUS347 series metals are often used in chemical plants and are known to have higher intergranular corrosion resistance than the aforementioned 308 and 309 series stainless steels. However, weld metals containing δ-ferrite are easily embrittled by heat treatment and are therefore used only in areas where the amount of ferrite is small. In Fig. 1, a weld metal 1 having a penetration depth 3 was formed into a base metal 2 using 347 series stainless steel, and a sulfuric acid/copper sulfate corrosion test was conducted on this weld metal. Some degree of intergranular corrosion occurs,
It was not possible to meet the intergranular corrosion resistance required for nuclear reactor pressure vessels. This invention was made in view of the above-mentioned problems, and an object of the present invention is to provide a build-up weld metal with high intergranular corrosion resistance by limiting the ratio of predetermined components in the build-up weld metal to respective ranges. It is in. In short, this invention provides C0.04% or less, Si1.0% or less, Mn2.5% or less, P0.04% or less, S0.03% or less,
The alloy contains 9.0 to 11.0% Ni, 18.0 to 21.0% Cr, and 0.5 to 0.9% Nb, and contains Nb and C at a ratio of Nb/C = 13 to 26, with the remainder essentially consisting of Fe. , is an austenitic-ferritic stainless steel alloy characterized by containing a ferrite content FN = 10 to 20, which prevents heat treatment embrittlement and improves intergranular corrosion resistance. Examples of the present invention will be described below. First, the inventors used submerged arc welding (SAW) and electroslag welding (ESW) to overlay 347-series weld metal on base metal low-alloy steel by varying the Nb content. death,
Intergranular corrosion tests were conducted on each weld metal to examine the effect of Nb on intergranular corrosion in each welding method. In the figure, □ indicates SAW, ○ indicates ESW,
Further, the numbers inside □ and ○ 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 the electrode wire and the base metal, and second, submerged arc welding, in which a bare electrode wire is inserted into granular flux and welding is performed using the arc heat generated between it and the base metal. When electroslag welding, which uses resistance heat to weld a wire and base metal, is compared, 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 larger in submerged arc welding with deep penetration, and
As is clear from the figure, the weld metal produced by submerged arc welding has an overall higher C content. Also, regardless of the welding method, the higher the Nb content, the less intergranular corrosion will occur, and if the weld metal contains 1.0% or more of Nb, almost no intergranular corrosion will occur regardless of the welding method. It could be confirmed. However, when looking at the relationship between the content of C and the content of Nb, there is a clear correlation between the content of C and the Nb content.
If the amount is small, intergranular corrosion can be prevented by adding a small amount of Nb.In the case of electroslag welding, where the amount of C contained can be reduced to 0.35% or less, the purpose can be achieved by setting the Nb content to 0.2 to 0.4%. can. Next, regarding the amount of Nb added, if the amount exceeds 0.9%, the weld metal will become brittle during heat treatment. Therefore, it is necessary to prevent intergranular corrosion by adding Nb to 0.9% or less, and for this purpose, the C content of the weld metal must be reduced.
It needs to be 0.040% or less. However, as is clear from the figure, even in weld metals based on electroslag welding where the C content is 0.04% or less, intergranular corrosion occurs despite the addition of 0.5% Nb. It's been found. Therefore, simply determining the Nb content and C content is not necessarily sufficient to obtain high intergranular corrosion resistance, and it is necessary to consider other factors as well. FIG. 3 shows the results of testing the relationship between intergranular corrosion and the amount of ferrite in the weld metal, which is said to be closely related to intergranular corrosion, in addition to the amounts of Nb and C mentioned above. In other words, the ratio of Nb and C is Nb/C
The relationship between FN and the amount of ferrite (FN is an abbreviation of Ferrite Number indicating the amount of ferrite) is shown. Furthermore, 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, the Nb/C value is between about 13 and about 26, and the ferrite content is about 11 or more and 20 The following is true. Regarding the FN value, systematic embrittlement is observed at 20 or higher, and 25
In the above case, significant embrittlement such as cracking of the weld metal was observed in the bending test. On the other hand, regarding the Nb/C value
Embrittlement was confirmed at 26 or higher. From the above points, weld metals with high intergranular corrosion resistance are
It is SUS347 series stainless steel, with a C content of approximately 0.04% or less, a Nb content of approximately 0.5% or more and 0.9% or less, a ferrite content of approximately 11FN or more and 20FN or less, and a Nb/C value of approximately
It can be seen that the value should be between 13 and 26 (within the frame A in Figure 3). Note that this component range can be applied not only to build-up welding but also to welding stainless steels with a low C content. Note that the basis for numerically limiting the composition of the alloy in the present invention is based on FIGS. 2 and 3, in which intergranular corrosion was confirmed in experiments. In addition, regarding the numerical limitation of the compositional components, it is a SUS347 alloy, and when each component falls outside the numerical range specified in the "Claims" column, the phenomenon in the case of excess and the phenomenon in the case of deficiency will be considered. The phenomena are summarized in Table 3 attached, and these are also used as the reasons for limitation. The inventors formed a predetermined weld metal based on the invention described above, and conducted intergranular corrosion tests on this weld metal. The specific details of the test are as follows. (1) Welding materials (a) Weld metal SUS347 (b) Flux CaF ₂ −Al ₂ O ₃ −MgO sintered type (2) Welding conditions (a) 2500A, 25V, welding speed 14cm/min (b) Base material A533 (3) Heat treatment conditions (620°C ± 15°C) × 6 to 50 hours Weld metals shown in Table 2 below were obtained under the conditions (1) to (3) above.

[Table] Intergranular corrosion tests were conducted on the above weld metals, but no intergranular corrosion occurred, and no cracking occurred in the side bending tests. For the overlay weld metal, good results can be obtained if the C content, Nb content, ferrite content, and Nb/C value are maintained at specified values. It does not matter if it is a component ratio that naturally follows. That is, specifically Si: about 1.0% or less, Mn: about 2.5% or less,
P: approx. 0.04% or less, S: approx. 0.03% or less, Ni: approx. 9.0
% or more and 11.0% or less, Cr: approximately 18.0% or more and 20% or less. Next, the range limitation of the components constituting the stainless steel alloy of the present invention will be described. C is due to the formation of chromium carbides at grain boundaries
Since Cr depletion deteriorates intergranular corrosion resistance,
The content is set to 0.04% or less, and Nb is added within the above range to stabilize C, prevent Cr depletion, and prevent intergranular corrosion. The content of Si was set to 1.0% or less to prevent high-temperature cracking.
Mn is necessary as a deoxidizing agent for S etc.
This regulation is necessary because if the content exceeds 2.5%, the amount of ferrite decreases and the intergranular corrosion resistance deteriorates. It is better to have less P as an impurity. Also 0.04%
If it is in excess, high-temperature cracking is likely to occur. Like P, S also enters stainless steel alloys as an impurity, but it is kept to 0.03% or less in order to prevent hot cracking. The range of Ni is defined as 9 to 11%, but below this range the austenite structure will become unstable, and if it is in excess above this range, the amount of ferrite will decrease and intergranular corrosion resistance will deteriorate. . Similarly, the range for Cr is set at 18-21%,
Below this range, there is a problem that the austenite structure becomes unstable. If the amount exceeds this range, the amount of ferrite will increase, resulting in structural embrittlement and
This results in the problem of insufficient bending ductility. As mentioned above, the relationship between Nb and C is important. However, Nb itself is specified in a range of 0.5 to 0.9%; below this range, intergranular corrosion resistance deteriorates, and in excess of this range, embrittlement occurs during heat treatment. By carrying out this invention, it is possible to obtain an austenitic-ferritic stainless steel alloy with extremely high intergranular corrosion resistance, and it can be widely used in applications such as nuclear reactor pressure vessels where welding conditions are severe.

【table】

【table】 [Brief explanation of the drawing]

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

Claims (1)

[Claims] 1 C0.04% or less, Si1.0% or less, Mn2.5% or less,
P0.04% or less, S0.03% or less, Ni9.0~11.0%,
An alloy containing 18.0 to 21.0% Cr, 0.5 to 0.9% Nb, and containing Nb and C in a ratio of Nb/C = 13 to 26, with the balance essentially consisting of Fe, and the amount of ferrite in the alloy is FN. An austenitic/ferritic stainless steel alloy characterized by improved intergranular corrosion resistance including =10 to 20.