JPH0613157B2 - Welding material for high Si austenitic stainless steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a welding material, particularly a welding material suitable for welding high Si austenitic stainless steel having excellent corrosion resistance to nitric acid.

(Prior art) A major component of a nitric acid production plant is a high temperature and high concentration nitric acid environment, and such an environment is extremely oxidative and stainless steel becomes overpassivated. Cannot be maintained. Moreover, it is also a severe corrosive environment with a great intergranular corrosiveness.

In such an environment with strong oxidizing properties, glass lining materials have been conventionally used because silicates have durability, but since such materials cannot be welded, it is difficult to manufacture large-scale equipment. In addition, there were serious problems in plant maintenance such as brittleness and weakness against impact.

Recently, about 4% (hereinafter, “%” means “% by weight” unless otherwise specified in the present specification) of Si as a metal material durable in this type of environment. A 17% Cr-14% Ni-4% Si-based austenitic stainless steel is used.This is because the oxide layer mainly composed of SiO ₂ is formed on the surface in an environment with strong oxidizing properties. We were able to dramatically improve the service life and reliability of the equipment.

For example, the one disclosed in Japanese Examined Patent Publication No. 57-37669 has Si: 2.5 to 5% and Ta or Zr in addition to Cr and Ni at 10 × C% to
It is a stainless steel for concentrated nitric acid with a composition of 2.5% (1 part Nb can be substituted). This type of nitric acid resistant stainless steel has been used extensively to date due to its excellent properties, and is a material that has been evaluated in the industry.

(Problems to be Solved by the Invention) However, as the material has been used for a long period of time, the problem of corrosion perforation at the weld bead portion has been highlighted even with such materials having excellent nitric acid resistance.

That is, particularly during use, the weld bead portion may undergo severe corrosion with unevenness as shown in FIG. 3, and the preferential corrosion portion may penetrate to cause leakage of the solution, resulting in a serious accident. Incidentally, FIGS. 3 (a) and 3 (b) are micrographs of the metal structure of the corroded portion in the conventional example, as described later. Therefore, at present, measures such as shortening the inspection period are taken.

However, such a method is symptomatic and has not been a fundamental solution, and the operation is often interrupted so that the inspection is costly.

Thus, an object of the present invention is to provide a welding material having the same nitric acid resistance as the base material, which prevents the occurrence of corrosion perforation in the weld bead when welding the nitric acid resistant stainless steel as described above. That is.

(Means for Solving Problems) As a result of various investigations and studies to solve the above-described problem of corrosion piercing of the weld bead portion, the following findings were obtained.

(1) The welding material that has been used for welding the above-mentioned nitric acid resistant stainless steel is, for example, as disclosed in JP-A-58-154491, C ≦ 0.015%, Si: 5 to 7%. , N ≦ 0.0
2%, C + N ≦ 0.03%, Nb + Ta ≧ 15 × (C + N)%,
It was low carbon, low nitrogen and contained a relatively large amount of Si. Further, it has been conventionally considered necessary to add a relatively large amount of Nb and Ta relative to the amount of (C + N).

(2) Therefore, when the above-mentioned welding materials were used to weld the previous nitrate-resistant stainless steel, the corrosion was found to be the intermetallic compound with a high Si content that was precipitated as a result of the solidification segregation of the weld.
It has become clear that it is due to the Ni-Si-Nb (Ta) system.

(3) That is, with the precipitation of the high Si-containing intermetallic compound as described above, the Si content in the surrounding matrix, which is essential for exhibiting excellent corrosion resistance in sulfuric acid, decreases, and the preferential corrosion of the matrix It causes pitting-like corrosion accompanied by.

Here, as a result of various studies on the preventive measures against such pitting corrosion, the present inventors have found that 17% Cr-14% Ni-4% Si.
A common metal-based welding material commonly used as a welding material
Nb + Ta Welding material (Nb + Ta: 0.5-1.2% added) Nb + Ta
While controlling the content to 0.3% or less (C + N)
It is possible to remarkably reduce the above-mentioned intermetallic compound precipitation amount by limiting it to the range of 2 × (C + N) to 6 × (C + N), which effectively prevents the pitting corrosion of the bead portion. The present invention has been completed based on the finding that it can be prevented.

Therefore, the gist of the present invention is that, by weight%, C ≦ 0.04%, Si: 2.5 to 5.0%, Mn ≦ 5.0%, Cr: 15 to 20%, Ni: 10 to 20%, Nb + Ta: 0.05 to 0.3%, N ≦ 0.02%, 2 × (% C +% N) ≦% (Nb + Ta) ≦ 6 × (% C +% N), the balance Fe and inevitable impurities, and bal. is -4 to -2
High Si that has excellent corrosion resistance to nitric acid and can prevent corrosion perforation of the weld bead
Welding material for austenitic stainless steel.

Ni-bal. =% Ni + 30 (% C +% N) + 0.5% Mn-1.1 [% Cr + 1.5% Si + 0.5% (Nb + Ta)] + 8.2. In the following, since Nb and Ta are equal, some of them may be represented by Nb.

The amount of Nb added in normal austenitic Nb stabilized steel is
8 × (C + N)% or more is general and shows a tendency different from the result in the case of the present invention, which can be considered as follows.

That is, in the past, the stabilizing element Nb was
In order to suppress the precipitation of carbides as much as possible and to secure the amount of solid solution Cr, it was added to fix carbon and nitrogen, and it was added in a relatively large amount considering its yield and other factors. It was.

However, the positive segregation of Si is strongly recognized in the weld metal crystal grain boundaries of the steel type to which the present invention is applied for welding due to the concentration effect during weld solidification. Therefore, although undergoing multiple thermal cycles during welding, when the Nb addition amount is small as in the present invention, even if a Cr deficient region is formed along with the formation of Cr carbide in the weld metal grain boundary, the Si in the Cr deficient region is formed. It is considered that the nitric acid resistance does not decrease due to the high concentration.

The steel types that the present invention targets for welding are
It is an austenitic stainless steel of 17% Cr-14% Ni-4% Si system, preferably containing about 4% of Si and adding Nb, but it is not particularly limited thereto and generally The present invention is applicable to any high Si austenitic stainless steel.

Since such a steel type has a high Si content, it is generally intrinsically highly susceptible to welding hot cracking. In order to reduce this cracking susceptibility, it is effective to adjust the composition so that δ-ferrite is generated in the weld metal structure. It is necessary to reduce the hot cracking susceptibility. Therefore, in the present invention, the Ni-bal.
It is limited to 4 to -2.

(Operation) Next, the reasons for limiting each component in the welding material according to the present invention will be described below.

C: In order to improve the corrosion resistance, especially in order to prevent the precipitation of Cr carbide in a high temperature concentrated nitric acid environment where intergranular corrosion is large, C is preferably as low as possible, but in the case of a covered arc welding rod, Since C inevitably mixes during welding,
The upper limit of the C content is 0.04%. However, in the welding material according to the present invention, in order to prevent Ni-Si-Nb (Ta) -based precipitates peculiar to high Si steel from forming in the weld metal and lowering the corrosion resistance, Nb for stabilizing C is used. The content is kept low. Therefore, it is preferable that the C content is as small as possible. TIG, MIG
In the welding rod, there is almost no increase in the amount of C during the above-described welding using a coated rod, but since the amount of Nb is small as described above, it is desirable that C ≦ 0.015%.

Si: Si is a main element that gives a nitric acid resistance by forming a SiO ₂ -based film on the surface of steel in a highly oxidizing environment, and contains at least 2.5% or more of Si, which is about the same as the base metal. However, when adding Si: more than 5%, not only the productivity of the welding rod is deteriorated, but also Si: 5.87% is shown in FIG.
As shown in No. 18 of No. 18, sufficient results cannot be obtained in terms of absorbed energy and ear cracks.

Mn: contributes to austenitization of steel by balancing elements such as Cr, Si, and Nb as austenite-forming elements, and has the effect of fixing S, which is an impurity element in steel, to reduce weld cracking susceptibility, Used as a deoxidizer in industrial production. Usually, Mn of 2% or less is sufficient as a deoxidizer, but addition of 1 to 5% is effective in order to prevent weld cracking susceptibility, particularly weld reheat cracking. Generally 5.0
% Or less. The addition of Mn in excess of 5% deteriorates the resistance to concentrated nitric acid, so the upper limit is made 5%.

Cr: Generally, Cr of 15% or more is necessary to provide corrosion resistance and corrosion resistance to nitric acid of low and medium concentration. However, if it exceeds 20%, a large amount of ferrite is formed in the austenite matrix to present a two-phase structure, impairing workability and accelerating sigma brittleness.
Limited to 15-20%.

Ni: Ni is the main austenite forming element, and Cr, S
In order to obtain a welded joint with good workability by keeping the structure austenite in balance with i, Nb, etc., 10 to 20% is necessary.

Nb + Ta: Nb and Ta are elements that fix C in the steel and impart intergranular corrosion resistance to the steel.

It is generally considered that Nb + Ta needs to be added in an amount of 8 × (C + N)% or more, and in the case of the base material, about 18 × (C + N)% is added. However, the material according to the present invention is a welding material for a high Si austenitic steel that imparts nitric acid resistance by forming a SiO ₂ film on the steel surface in a highly oxidizing environment, and occurs in ordinary austenitic stainless steel. It exhibits a tendency different from the intergranular corrosion phenomenon caused by the Cr-deficient layer at the crystal grain boundary. That is, in the case of the present invention, positive segregation of Si is strongly recognized in the bond grain boundaries of the weld metal due to the concentration phenomenon during weld solidification. Therefore, even if multiple heat cycles are generated during welding and the formation of Cr carbides in the weld metal grain boundaries and the associated Cr-deficient regions are formed in the surrounding matrix, the Si concentration in the Cr-deficient regions is high, and therefore the nitric acid resistance is high. No decrease occurs. To obtain this effect, Nb + Ta of 0.05% or more is required. Also,
When the amount of Nb + Ta added to the weld metal is large, a Ni—Si—Nb (Ta) -based intermetallic compound precipitates to lower the corrosion resistance and cause piercing corrosion of the bead portion. In the case of 17% Cr-14% Ni-4% Si system, the weld crack susceptibility increases rapidly when the amount of Nb + Ta exceeds 0.3%. Therefore, from the characteristics of the high Si weld metal, the lower limit of the Nb + Ta addition amount is set to 0.05% and the upper limit is set to 0.3%.

As shown in the graph in Figure 1 below, the maximum erosion depth is
Especially when the ratio of (Nb + Ta) / (C + N) is in the range of 2-6, a remarkable decrease is exhibited. In this way, one part may be replaced by Ta
If the amount of Nb is less than 2 × (C + N)%, the corrosion resistance is insufficient, while if it exceeds 6 × (C + N)%, the corrosion puncture resistance is insufficient. Therefore, in the present invention, 2 × (%
C +% N) ≦% (Nb + Ta) ≦ 6 × (% C +% N).

N: N has a strong affinity with Nb and is preferably as low as possible rather than inhibiting fixation of C by Nb, but the upper limit is made 0.02% in consideration of manufacturability and economy.

The welding material according to the present invention is austenitic steel made of Si.
Since the content is extremely high and contains Nb, the solidification cracking susceptibility at the time of welding is high. Therefore, in the present invention, in order to reduce the solidification cracking sensitivity, 5 to 15% of δ ferrite is generated in the weld metal. Ni-bal. To -4 to -2
Control to the range of. When the value of Ni-bal. Is more than -2, the amount of δ ferrite produced is small and the effect of reducing crack susceptibility is not sufficient. On the other hand, if it is less than −4, the amount of δ ferrite produced is too large, which deteriorates the toughness of the weld metal. Further, if it is less than -4, the hot workability is significantly deteriorated and the manufacturability is deteriorated.

Next, the present invention will be described more specifically by way of its examples.

Examples Table 1 shows the chemical composition of each welding material of the present invention and comparative examples. Using these series of materials, 17% Cr-14 was used.
The post-TIG corrosion test of high Ni austenitic steel of% Ni-4% Si system was conducted. A part of the test results is shown in Table 2 and FIG.

The relationship between the weld crack susceptibility of these welding materials and the amount of Nb is summarized in FIG.

That is, Table 2 shows that the weld metal (welding + 650 ° C x 2 hr sensitization) obtained as described above was used in an actual nitric acid production plant (98% HNO ₃ , boiling state and NO _X gas atmosphere).
Fig. 3 shows the results of the 3624-hour corrosion test in
FIG. 1 is a graph showing the results.

As can be seen from these results, it is understood that the range of (Nb + Ta) / (C + N) of 2 to 6 is the best when the C content is 0.04% or less. Further, when the amount of C is about 0.05%, intergranular corrosion occurs severely in addition to piercing corrosion, which is not practically sufficient regardless of (Nb + Ta) / (C + N).

Figure 2 shows the effect of Nb content on crack susceptibility during TIG welding. It can be seen that crack susceptibility is significantly increased when the Nb content exceeds 0.3%. The crack index is
0: no crack, 1: pit occurred, 2: slightly cracked, 3: slightly cracked, 4: severely cracked.

In the figure, the number in parentheses indicates the test number.

3 (a) and 3 (b) are test No. 1 in Table 2 which is a comparative example.
3 is a micrograph of a metal microstructure showing a state of piercing corrosion of a corrosion test piece under the above-mentioned environment for steel No. 0. Fig. 3 (b)
Is an enlarged version of that in Fig. 3 (a), which clearly shows the appearance of pitting corrosion.

Next, in order to see the effect of Si concentration, Table 2 Test No. 5 and (Nb
EPMA (X-ray microanalyzer) in the vicinity of the weld metal corroded portion for each of the test materials of Experiment No. 9 in Table 2 in which a large amount of (+ Ta) exceeds the upper limit of the present invention.
X-ray intensity map by Secondary electron beam images of the test materials of the present invention example and the comparative example were respectively prepared and the distribution of each element and the concentrated state were examined. In the case of the comparative example, the Si-Ni-Nb concentrated portion The presence of a Si thin portion was clearly recognized, but in the case of the present invention example, it was as strong as in the comparative example.
No Si enrichment was observed.

Next, the weld crackability, toughness, and hot workability were evaluated for the significance of Ni-bal. Of the welding material according to the present invention.

First, each test piece with dimensions of 20t x 150w x 200l (mm) was heated to 1200 ° C and then hot-rolled to 8t by 4-pass rolling. At that time, visual observation of processing cracks generated on the mill edge Then, the ear cracking index was evaluated in the following 5 steps.

The test results are summarized in the graph in FIG. In the figure, the number in parentheses indicates the test number.

Cracking index 0: No cracking 1: Pit generation 2: Slight cracking 3: Slight cracking 4: Severe cracking Next, for each test material of 5 × 10 × 55 mm with a 2 mm V notch TIG Welding was performed, the weld crack index was determined in the manner described above, and a Charpy impact test was performed at 0 ° C. on each of the obtained weld metal parts. The test result is the fourth
The graphs are collectively shown in FIGS. In the figure, ()
The number inside shows the test No.

As is clear from the results shown in FIG. 4, the absorbed energy is remarkably lowered when the Ni-ba is less than 1.-4, and the ear cracking is aggravated.

Further, as is clear from the results shown in FIG. 5, the weld cracking becomes significantly worse when it exceeds -2.

(Effect of the Invention) According to the present invention, as can be seen from the above description, the Nb content is 0.3% or less and 2 × (C + N) ≦ Nb (+ Ta) ≦ 6 ×
By limiting to (C + N), the weld cracking property is greatly improved, and therefore, the welding and assembling of the equipment made of the nitric acid resistant material can be easily carried out, and the benefit of the present invention is great.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the relationship between the maximum erosion depth of weld metal and the amount of Nb + Ta / C + N; FIG. 2 shows the weld crack susceptibility of the welding material and the Nb (+ Ta) content. Fig. 3 is a graph showing the relationship; Fig. 3 is a photomicrograph of the microstructure showing the state of pitting corrosion observed in the conventional example; Figs. 4 and 5 are graphs showing the effect of Ni-bal. On the material properties. .

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Seiya Wada 2-12-1 Minatomachi, Joetsu City, Niigata Prefecture Naoetsu Research Institute, Japan Stainless Co., Ltd. (72) Hideki Uno 1-12-12 Minatomachi, Joetsu City, Niigata Prefecture Japan Stainless Steel Co., Ltd. Naoetsu Research Institute (72) Inventor Ryoyasu Ikeda 2-12-1 Minatomachi, Joetsu City, Niigata Prefecture Japan Stainless Steel Co., Ltd. Naoetsu Research Laboratory (72) Inventor Masanori Takahashi 12-12 Minatomachi, Joetsu City, Niigata Prefecture 1 Japan Stainless Steel Co., Ltd. Naoetsu Research Institute (72) Inventor Masayoshi Miki 5-1 Sokai-cho, Niihama-shi, Ehime Sumitomo Chemical Co., Ltd. (72) Inventor Katsuhiro Sakata 5-1 Sokai-cho, Niihama-shi, Ehime Sumitomo Within Kagaku Kogyo Co., Ltd. (56) Reference JP-A-58-154491 (JP, A)

Claims (1)

[Claims] 1. By weight%, C ≦ 0.04%, Si: 2.5 to 5.0%, Mn ≦ 5.0%, Cr: 15 to 20%, Ni: 10 to 20%, Nb + Ta: 0.05 to 0.3%, N ≦ 0.02 %, Further 2 × (% C +% N) ≦% (Nb + Ta) ≦ 6 × (% C +% N), the balance Fe and inevitable impurities, and the Ni-bal.
High Si that has excellent corrosion resistance to nitric acid and can prevent corrosion perforation of the weld bead
Welding material for austenitic stainless steel. Ni-bal. =% Ni + 30 (% C +% N) + 0.5% Mn-1.1 [% Cr + 1.5% Si + 0.5% (Nb + Ta)] + 8.2